Internet Engineering Task Force (IETF) G. Selander
Request for Comments: 8613 J. Mattsson
Updates: 7252 F. Palombini
Category: Standards Track Ericsson AB
ISSN: 2070-1721 L. Seitz
RISE
June
July 2019
Object Security for Constrained RESTful Environments (OSCORE)
Abstract
This document defines Object Security for Constrained RESTful
Environments (OSCORE), a method for application-layer protection of
the Constrained Application Protocol (CoAP), using CBOR Object
Signing and Encryption (COSE). OSCORE provides end-to-end protection
between endpoints communicating using CoAP or CoAP-mappable HTTP.
OSCORE is designed for constrained nodes and networks supporting a
range of proxy operations, including translation between different
transport protocols.
Although an optional functionality of CoAP, OSCORE alters CoAP
options processing and IANA registration. Therefore, this document
updates RFC 7252.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8613.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 5
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 7
2. The OSCORE Option . . . . . . . . . . . . . . . . . . . . . . 7 8
3. The Security Context . . . . . . . . . . . . . . . . . . . . 7 8
3.1. Security Context Definition . . . . . . . . . . . . . . . 8 9
3.2. Establishment of Security Context Parameters . . . . . . 10 11
3.3. Requirements on the Security Context Parameters . . . . . 12 14
4. Protected Message Fields . . . . . . . . . . . . . . . . . . 13 15
4.1. CoAP Options . . . . . . . . . . . . . . . . . . . . . . 14 16
4.2. CoAP Header Fields and Payload . . . . . . . . . . . . . 23 24
4.3. Signaling Messages . . . . . . . . . . . . . . . . . . . 24 25
5. The COSE Object . . . . . . . . . . . . . . . . . . . . . . . 25 26
5.1. ID Context and 'kid context' . . . . . . . . . . . . . . 26 27
5.2. AEAD Nonce . . . . . . . . . . . . . . . . . . . . . . . 27 28
5.3. Plaintext . . . . . . . . . . . . . . . . . . . . . . . . 28 29
5.4. Additional Authenticated Data . . . . . . . . . . . . . . 28 30
6. OSCORE Header Compression . . . . . . . . . . . . . . . . . . 30 31
6.1. Encoding of the OSCORE Option Value . . . . . . . . . . . 30 32
6.2. Encoding of the OSCORE Payload . . . . . . . . . . . . . 32 33
6.3. Examples of Compressed COSE Objects . . . . . . . . . . . 32 33
7. Message Binding, Sequence Numbers, Freshness, and Replay
Protection . . . . . . . . . . . . . . . . . . . . . . . . . 34 36
7.1. Message Binding . . . . . . . . . . . . . . . . . . . . . 34 36
7.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 34 36
7.3. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 35 36
7.4. Replay Protection . . . . . . . . . . . . . . . . . . . . 35 37
7.5. Losing Part of the Context State . . . . . . . . . . . . 36 38
8. Processing . . . . . . . . . . . . . . . . . . . . . . . . . 37 39
8.1. Protecting the Request . . . . . . . . . . . . . . . . . 37 39
8.2. Verifying the Request . . . . . . . . . . . . . . . . . . 38 40
8.3. Protecting the Response . . . . . . . . . . . . . . . . . 39 41
8.4. Verifying the Response . . . . . . . . . . . . . . . . . 41 43
9. Web Linking . . . . . . . . . . . . . . . . . . . . . . . . . 42 44
10. CoAP-to-CoAP Forwarding Proxy . . . . . . . . . . . . . . . . 43 45
11. HTTP Operations . . . . . . . . . . . . . . . . . . . . . . . 44 46
11.1. The HTTP OSCORE Header Field . . . . . . . . . . . . . . 44 46
11.2. CoAP-to-HTTP Mapping . . . . . . . . . . . . . . . . . . 45 47
11.3. HTTP-to-CoAP Mapping . . . . . . . . . . . . . . . . . . 46 48
11.4. HTTP Endpoints . . . . . . . . . . . . . . . . . . . . . 46 48
11.5. Example: HTTP Client and CoAP Server . . . . . . . . . . 46 48
11.6. Example: CoAP Client and HTTP Server . . . . . . . . . . 48 50
12. Security Considerations . . . . . . . . . . . . . . . . . . . 49 51
12.1. End-to-end Protection . . . . . . . . . . . . . . . . . 49 51
12.2. Security Context Establishment . . . . . . . . . . . . . 50 52
12.3. Master Secret . . . . . . . . . . . . . . . . . . . . . 50 52
12.4. Replay Protection . . . . . . . . . . . . . . . . . . . 51 53
12.5. Client Aliveness . . . . . . . . . . . . . . . . . . . . 51 53
12.6. Cryptographic Considerations . . . . . . . . . . . . . . 51 53
12.7. Message Segmentation . . . . . . . . . . . . . . . . . . 52 54
12.8. Privacy Considerations . . . . . . . . . . . . . . . . . 52 54
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53 55
13.1. COSE Header Parameters Registry . . . . . . . . . . . . 53 55
13.2. CoAP Option Numbers Registry . . . . . . . . . . . . . . 53 55
13.3. CoAP Signaling Option Numbers Registry . . . . . . . . . 54 56
13.4. Header Field Registrations . . . . . . . . . . . . . . . 54 57
13.5. Media Type Registration . . . . . . . . . . . . . . . . 55 57
13.6. CoAP Content-Formats Registry . . . . . . . . . . . . . 56 58
13.7. OSCORE Flag Bits Registry . . . . . . . . . . . . . . . 57 58
13.8. Expert Review Instructions . . . . . . . . . . . . . . . 58 59
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 58 60
14.1. Normative References . . . . . . . . . . . . . . . . . . 58 60
14.2. Informative References . . . . . . . . . . . . . . . . . 60 62
Appendix A. Scenario Examples . . . . . . . . . . . . . . . . . 62 65
A.1. Secure Access to Sensor . . . . . . . . . . . . . . . . . 63 65
A.2. Secure Subscribe to Sensor . . . . . . . . . . . . . . . 64 66
Appendix B. Deployment Examples . . . . . . . . . . . . . . . . 65 68
B.1. Security Context Derived Once . . . . . . . . . . . . . . 65 68
B.2. Security Context Derived Multiple Times . . . . . . . . . 67 70
Appendix C. Test Vectors . . . . . . . . . . . . . . . . . . . . 72 75
C.1. Test Vector 1: Key Derivation with Master Salt . . . . . 73 75
C.2. Test Vector 2: Key Derivation without Master Salt . . . . 74 77
C.3. Test Vector 3: Key Derivation with ID Context . . . . . . 76 78
C.4. Test Vector 4: OSCORE Request, Client . . . . . . . . . . 77 80
C.5. Test Vector 5: OSCORE Request, Client . . . . . . . . . . 78 81
C.6. Test Vector 6: OSCORE Request, Client . . . . . . . . . . 80 82
C.7. Test Vector 7: OSCORE Response, Server . . . . . . . . . 81 84
C.8. Test Vector 8: OSCORE Response with Partial IV, Server . 82 85
Appendix D. Overview of Security Properties . . . . . . . . . . 83 86
D.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 83 86
D.2. Supporting Proxy Operations . . . . . . . . . . . . . . . 84 87
D.3. Protected Message Fields . . . . . . . . . . . . . . . . 85 87
D.4. Uniqueness of (key, nonce) . . . . . . . . . . . . . . . 86 88
D.5. Unprotected Message Fields . . . . . . . . . . . . . . . 87 89
Appendix E. CDDL Summary . . . . . . . . . . . . . . . . . . . . 90 93
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 91 94
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 91 94
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a web
transfer protocol designed for constrained nodes and networks
[RFC7228]; CoAP may be mapped from HTTP [RFC8075]. CoAP specifies
the use of proxies for scalability and efficiency and references DTLS
[RFC6347] for security. CoAP-to-CoAP, HTTP-to-CoAP, and CoAP-to-HTTP
proxies require DTLS or TLS [RFC8446] to be terminated at the proxy.
Therefore, the proxy not only has access to the data required for
performing the intended proxy functionality, but is also able to
eavesdrop on, or manipulate any part of, the message payload and
metadata in transit between the endpoints. The proxy can also
inject, delete, or reorder packets since they are no longer protected
by (D)TLS.
This document defines the Object Security for Constrained RESTful
Environments (OSCORE) security protocol, protecting CoAP and CoAP-
mappable HTTP requests and responses end-to-end across intermediary
nodes such as CoAP forward proxies and cross-protocol translators
including HTTP-to-CoAP proxies [RFC8075]. In addition to the core
CoAP features defined in [RFC7252], OSCORE supports the Observe
[RFC7641], Block-wise [RFC7959], and No-Response [RFC7967] options,
as well as the PATCH and FETCH methods [RFC8132]. An analysis of
end-to-end security for CoAP messages through some types of
intermediary nodes is performed in [CoAP-E2E-Sec]. OSCORE
essentially protects the RESTful interactions: the request method,
the requested resource, the message payload, etc. (see Section 4),
where "RESTful" refers to the Representational State Transfer (REST)
Architecture [REST]. OSCORE protects neither the CoAP messaging
layer nor the CoAP Token, which may change between the endpoints;
therefore, those are processed as defined in [RFC7252].
Additionally, since the message formats for CoAP over unreliable
transport [RFC7252] and for CoAP over reliable transport [RFC8323]
differ only in terms of CoAP messaging layer, OSCORE can be applied
to both unreliable and reliable transports (see Figure 1).
OSCORE works in very constrained nodes and networks, thanks to its
small message size and the restricted code and memory requirements in
addition to what is required by CoAP. Examples of the use of OSCORE
are given in Appendix A. OSCORE may be used over any underlying
layer, such as UDP or TCP, and with non-IP transports (e.g.,
[CoAP-802.15.4]). OSCORE may also be used in different ways with
HTTP. OSCORE messages may be transported in HTTP, and OSCORE may
also be used to protect CoAP-mappable HTTP messages, as described
below.
+-----------------------------------+
| Application |
+-----------------------------------+
+-----------------------------------+ \
| Requests / Responses / Signaling | |
|-----------------------------------| |
| OSCORE | | CoAP
|-----------------------------------| |
| Messaging Layer / Message Framing | |
+-----------------------------------+ /
+-----------------------------------+
| UDP / TCP / ... |
+-----------------------------------+
Figure 1: Abstract Layering of CoAP with OSCORE
OSCORE is designed to protect as much information as possible while
still allowing CoAP proxy operations (Section 10). It works with
existing CoAP-to-CoAP forward proxies [RFC7252], but an OSCORE-aware
proxy will be more efficient. HTTP-to-CoAP proxies [RFC8075] and
CoAP-to-HTTP proxies can also be used with OSCORE, as specified in
Section 11. OSCORE may be used together with TLS or DTLS over one or
more hops in the end-to-end path, e.g., transported with HTTPS in one
hop and with plain CoAP in another hop. The use of OSCORE does not
affect the URI scheme; therefore, OSCORE can be used with any URI
scheme defined for CoAP or HTTP. The application decides the
conditions for which OSCORE is required.
OSCORE uses pre-shared keys that may have been established out-of-
band or with a key establishment protocol (see Section 3.2). The
technical solution builds on CBOR Object Signing and Encryption
(COSE) [RFC8152], providing end-to-end encryption, integrity, replay
protection, and binding of response to request. A compressed version
of COSE is used, as specified in Section 6. The use of OSCORE is
signaled in CoAP with a new option (Section 2), and in HTTP with a
new header field (Section 11.1) and content type (Section 13.5). The
solution transforms a CoAP/HTTP message into an "OSCORE message"
before sending, and vice versa after receiving. The OSCORE message
is a CoAP/HTTP message related to the original message in the
following way: the original CoAP/HTTP message is translated to CoAP
(if not already in CoAP) and protected in a COSE object. The
encrypted message fields of this COSE object are transported in the
CoAP payload/HTTP body of the OSCORE message, and the OSCORE option/
header field is included in the message. A sketch of an exchange of
OSCORE messages, in the case of the original message being CoAP, is
provided in Figure 2. The use of OSCORE with HTTP is detailed in
Section 11.
Client Server
| OSCORE request - POST example.com: |
| Header, Token, |
| Options: OSCORE, ..., |
| Payload: COSE ciphertext |
+--------------------------------------------->|
| |
|<---------------------------------------------+
| OSCORE response - 2.04 (Changed): |
| Header, Token, |
| Options: OSCORE, ..., |
| Payload: COSE ciphertext |
| |
Figure 2: Sketch of CoAP with OSCORE
An implementation supporting this specification MAY implement only
the client part, MAY implement only the server part, or MAY implement
only one of the proxy parts.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with the terms and concepts
described in CoAP [RFC7252], COSE [RFC8152], Concise Binary Object
Representation (CBOR) [RFC7049], Concise Data Definition Language
(CDDL) [RFC8610] as summarized in Appendix E, and constrained
environments [RFC7228]. Additional optional features include Observe
[RFC7641], Block-wise [RFC7959], No-Response [RFC7967] and CoAP over
reliable transport [RFC8323].
The term "hop" is used to denote a particular leg in the end-to-end
path. The concept "hop-by-hop" (as in "hop-by-hop encryption" or
"hop-by-hop fragmentation") opposed to "end-to-end", is used in this
document to indicate that the messages are processed accordingly in
the intermediaries, rather than just forwarded to the next node.
The term "stop processing" is used throughout the document to denote
that the message is not passed up to the CoAP request/response layer
(see Figure 1).
The terms Common Context, Sender Context, Recipient Context, Master
Secret, Master Salt, Sender ID, Sender Key, Recipient ID, Recipient
Key, ID Context, and Common IV are defined in Section 3.1.
2. The OSCORE Option
The OSCORE option defined in this section (see Figure 3, which
extends "Table 4: Options" of [RFC7252]) indicates that the CoAP
message is an OSCORE message and that it contains a compressed COSE
object (see Sections 5 and 6). The OSCORE option is critical, safe
to forward, part of the cache key, and not repeatable.
+------+---+---+---+---+----------------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+------+---+---+---+---+----------------+--------+--------+---------+
| 9 | x | | | | OSCORE | (*) | 0-255 | (none) |
+------+---+---+---+---+----------------+--------+--------+---------+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable
(*) See below.
Figure 3: The OSCORE Option
The OSCORE option includes the OSCORE flag bits (Section 6), the
Sender Sequence Number, the Sender ID, and the ID Context when these
fields are present (Section 3). The detailed format and length is
specified in Section 6. If the OSCORE flag bits are all zero (0x00),
the option value SHALL be empty (Option Length = 0). An endpoint
receiving a CoAP message without payload that also contains an OSCORE
option SHALL treat it as malformed and reject it.
A successful response to a request with the OSCORE option SHALL
contain the OSCORE option. Whether error responses contain the
OSCORE option depends on the error type (see Section 8).
For CoAP proxy operations, see Section 10.
3. The Security Context
OSCORE requires that client and server establish a shared security
context used to process the COSE objects. OSCORE uses COSE with an
Authenticated Encryption with Associated Data (AEAD, [RFC5116])
algorithm for protecting message data between a client and a server.
In this section, we define the security context and how it is derived
in client and server based on a shared secret and a key derivation
function.
3.1. Security Context Definition
The security context is the set of information elements necessary to
carry out the cryptographic operations in OSCORE. For each endpoint,
the security context is composed of a "Common Context", a "Sender
Context", and a "Recipient Context".
The endpoints protect messages to send using the Sender Context and
verify messages received using the Recipient Context; both contexts
being derived from the Common Context and other data. Clients and
servers need to be able to retrieve the correct security context to
use.
An endpoint uses its Sender ID (SID) to derive its Sender Context;
the other endpoint uses the same ID, now called Recipient ID (RID),
to derive its Recipient Context. In communication between two
endpoints, the Sender Context of one endpoint matches the Recipient
Context of the other endpoint, and vice versa. Thus, the two
security contexts identified by the same IDs in the two endpoints are
not the same, but they are partly mirrored. Retrieval and use of the
security context are shown in Figure 4.
.---------------------. .---------------------.
| Common Context | = | Common Context |
+---------------------+ +---------------------+
| Sender Context | = | Recipient Context |
+---------------------+ +---------------------+
| Recipient Context | = | Sender Context |
'---------------------' '---------------------'
Client Server
| |
Retrieve context for | OSCORE request: |
target resource | Token = Token1, |
Protect request with | kid = SID, ... |
Sender Context +---------------------->| Retrieve context with
| | RID = kid
| | Verify request with
| | Recipient Context
| OSCORE response: | Protect response with
| Token = Token1, ... | Sender Context
Retrieve context with |<----------------------+
Token = Token1 | |
Verify request with | |
Recipient Context | |
Figure 4: Retrieval and Use of the Security Context
The Common Context contains the following parameters:
o AEAD Algorithm. The COSE AEAD algorithm to use for encryption.
o HKDF Algorithm. An HMAC-based key derivation function (HKDF,
[RFC5869]) used to derive the Sender Key, Recipient Key, and
Common IV.
o Master Secret. Variable length, random byte string (see
Section 12.3) used to derive AEAD keys and Common IV.
o Master Salt. Optional variable-length byte string containing the
salt used to derive AEAD keys and Common IV.
o ID Context. Optional variable-length byte string providing
additional information to identify the Common Context and to
derive AEAD keys and Common IV. The use of ID Context is
described in Section 5.1.
o Common IV. Byte string derived from the Master Secret, Master
Salt, and ID Context. Used to generate the AEAD nonce (see
Section 5.2). Same length as the nonce of the AEAD Algorithm.
The Sender Context contains the following parameters:
o Sender ID. Byte string used to identify the Sender Context, to
derive AEAD keys and Common IV, and to contribute to the
uniqueness of AEAD nonces. Maximum length is determined by the
AEAD Algorithm.
o Sender Key. Byte string containing the symmetric AEAD key to
protect messages to send. Derived from Common Context and Sender
ID. Length is determined by the AEAD Algorithm.
o Sender Sequence Number. Non-negative integer used by the sender
to enumerate requests and certain responses, e.g., Observe
notifications. Used as "Partial IV" [RFC8152] to generate unique
AEAD nonces. Maximum value is determined by the AEAD Algorithm.
Initialization is described in Section 3.2.2.
The Recipient Context contains the following parameters:
o Recipient ID. Byte string used to identify the Recipient Context,
to derive AEAD keys and Common IV, and to contribute to the
uniqueness of AEAD nonces. Maximum length is determined by the
AEAD Algorithm.
o Recipient Key. Byte string containing the symmetric AEAD key to
verify messages received. Derived from Common Context and
Recipient ID. Length is determined by the AEAD Algorithm.
o Replay Window (Server only). The replay window used to verify
requests received. Replay protection is described in Section 7.4
and Section 3.2.2.
All parameters except Sender Sequence Number and Replay Window are
immutable once the security context is established. An endpoint may
free up memory by not storing the Common IV, Sender Key, and
Recipient Key, deriving them when needed. Alternatively, an endpoint
may free up memory by not storing the Master Secret and Master Salt
after the other parameters have been derived.
Endpoints MAY operate as both client and server and use the same
security context for those roles. Independent of being client or
server, the endpoint protects messages to send using its Sender
Context, and verifies messages received using its Recipient Context.
The endpoints MUST NOT change the Sender/Recipient ID when changing
roles. In other words, changing the roles does not change the set of
AEAD keys to be used.
3.2. Establishment of Security Context Parameters
Each endpoint derives the parameters in the security context from a
small set of input parameters. The following input parameters SHALL
be preestablished:
o Master Secret
o Sender ID
o Recipient ID
The following input parameters MAY be preestablished. In case any of
these parameters is not preestablished, the default value indicated
below is used:
o AEAD Algorithm
* Default is AES-CCM-16-64-128 (COSE algorithm encoding: 10)
o Master Salt
* Default is the empty byte string
o HKDF Algorithm
* Default is HKDF SHA-256
o Replay Window
* The default mechanism is an anti-replay sliding window (see
Section 4.1.2.6 of [RFC6347] with a window size of 32
All input parameters need to be known and agreed on by both
endpoints, but the Replay Window may be different in the two
endpoints. The way the input parameters are preestablished is
application specific. Considerations of security context
establishment are given in Section 12.2 and examples of deploying
OSCORE in Appendix B.
3.2.1. Derivation of Sender Key, Recipient Key, and Common IV
The HKDF MUST be one of the HMAC-based HKDF [RFC5869] algorithms
defined for COSE [RFC8152]. HKDF SHA-256 is mandatory to implement.
The security context parameters Sender Key, Recipient Key, and Common
IV SHALL be derived from the input parameters using the HKDF, which
consists of the composition of the HKDF-Extract and HKDF-Expand steps
[RFC5869]:
output parameter = HKDF(salt, IKM, info, L)
where:
o salt is the Master Salt as defined above
o IKM is the Master Secret as defined above
o info is the serialization of a CBOR array consisting of (the
notation follows [RFC8610] as summarized in Appendix E):
info = [
id : bstr,
id_context : bstr / nil,
alg_aead : int / tstr,
type : tstr,
L : uint,
]
where:
o id is the Sender ID or Recipient ID when deriving Sender Key and
Recipient Key, respectively, and the empty byte string when
deriving the Common IV.
o id_context is the ID Context, or nil if ID Context is not
provided.
o alg_aead is the AEAD Algorithm, encoded as defined in [RFC8152].
o type is "Key" or "IV". The label is an ASCII string and does not
include a trailing NUL byte.
o L is the size of the key/nonce for the AEAD Algorithm used, in
bytes.
For example, if the algorithm AES-CCM-16-64-128 (see Section 10.2 in
[RFC8152]) is used, the integer value for alg_aead is 10, the value
for L is 16 for keys and 13 for the Common IV. Assuming use of the
default algorithms HKDF SHA-256 and AES-CCM-16-64-128, the extract
phase of HKDF produces a pseudorandom key (PRK) as follows:
PRK = HMAC-SHA-256(Master Salt, Master Secret)
and as L is smaller than the hash function output size, the expand
phase of HKDF consists of a single HMAC invocation; therefore, the
Sender Key, Recipient Key, and Common IV are the first 16 or 13 bytes
of
output parameter = HMAC-SHA-256(PRK, info || 0x01)
where different values of info are used for each derived parameter
and where || denotes byte string concatenation.
Note that [RFC5869] specifies that if the salt is not provided, it is
set to a string of zeros. For implementation purposes, not providing
the salt is the same as setting the salt to the empty byte string.
OSCORE sets the salt default value to empty byte string, which is
converted to a string of zeroes (see Section 2.2 of [RFC5869]).
3.2.2. Initial Sequence Numbers and Replay Window
The Sender Sequence Number is initialized to 0.
The supported types of replay protection and replay window size is
application specific and depends on how OSCORE is transported (see
Section 7.4). The default mechanism is the anti-replay window of
received messages used by IPsec AH/ESP and DTLS (see Section 4.1.2.6
of [RFC6347]) with a window size of 32.
3.3. Requirements on the Security Context Parameters
To ensure unique Sender Keys, the quartet (Master Secret, Master
Salt, ID Context, Sender ID) MUST be unique, i.e., the pair (ID
Context, Sender ID) SHALL be unique in the set of all security
contexts using the same Master Secret and Master Salt. This means
that Sender ID SHALL be unique in the set of all security contexts
using the same Master Secret, Master Salt, and ID Context; such a
requirement guarantees unique (key, nonce) pairs for the AEAD.
Different methods can be used to assign Sender IDs: a protocol that
allows the parties to negotiate locally unique identifiers, a trusted
third party (e.g., [ACE-OAuth]), or the identifiers can be assigned
out-of-band. The Sender IDs can be very short (note that the empty
string is a legitimate value). The maximum length of Sender ID in
bytes equals the length of the AEAD nonce minus 6, see Section 5.2.
For AES-CCM-16-64-128 the maximum length of Sender ID is 7 bytes.
To simplify retrieval of the right Recipient Context, the Recipient
ID SHOULD be unique in the sets of all Recipient Contexts used by an
endpoint. If an endpoint has the same Recipient ID with different
Recipient Contexts, i.e., the Recipient Contexts are derived from
different Common Contexts, then the endpoint may need to try multiple
times before verifying the right security context associated to the
Recipient ID.
The ID Context is used to distinguish between security contexts. The
methods used for assigning Sender ID can also be used for assigning
the ID Context. Additionally, the ID Context can be used to
introduce randomness into new Sender and Recipient Contexts (see
Appendix B.2). ID Context can be arbitrarily long.
4. Protected Message Fields
OSCORE transforms a CoAP message (which may have been generated from
an HTTP message) into an OSCORE message, and vice versa. OSCORE
protects as much of the original message as possible while still
allowing certain proxy operations (see Sections 10 and 11). This
section defines how OSCORE protects the message fields and transfers
them end-to-end between client and server (in any direction).
The remainder of this section and later sections focus on the
behavior in terms of CoAP messages. If HTTP is used for a particular
hop in the end-to-end path, then this section applies to the
conceptual CoAP message that is mappable to/from the original HTTP
message as discussed in Section 11. That is, an HTTP message is
conceptually transformed to a CoAP message and then to an OSCORE
message, and similarly in the reverse direction. An actual
implementation might translate directly from HTTP to OSCORE without
the intervening CoAP representation.
Protection of signaling messages (Section 5 of [RFC8323]) is
specified in Section 4.3. The other parts of this section target
request/response messages.
Message fields of the CoAP message may be protected end-to-end
between CoAP client and CoAP server in different ways:
o Class E: encrypted and integrity protected,
o Class I: integrity protected only, or
o Class U: unprotected.
The sending endpoint SHALL transfer Class E message fields in the
ciphertext of the COSE object in the OSCORE message. The sending
endpoint SHALL include Class I message fields in the AAD of the AEAD
algorithm, allowing the receiving endpoint to detect if the value has
changed in transfer. Class U message fields SHALL NOT be protected
in transfer. Class I and Class U message field values are
transferred in the header or options part of the OSCORE message,
which is visible to proxies.
Message fields not visible to proxies, i.e., transported in the
ciphertext of the COSE object, are called "Inner" (Class E). Message
fields transferred in the header or options part of the OSCORE
message, which is visible to proxies, are called "Outer" (Class I or
Class U). There are currently no Class I options defined.
An OSCORE message may contain both an Inner and an Outer instance of
a certain CoAP message field. Inner message fields are intended for
the receiving endpoint, whereas Outer message fields are used to
enable proxy operations.
4.1. CoAP Options
A summary of how options are protected is shown in Figure 5. Note
that some options may have both Inner and Outer message fields, which
are protected accordingly. Certain options require special
processing as is described in Section 4.1.3.
Options that are unknown or for which OSCORE processing is not
defined SHALL be processed as Class E (and no special processing).
Specifications of new CoAP options SHOULD define how they are
processed with OSCORE. A new COAP option SHOULD be of Class E unless
it requires proxy processing. If a new CoAP option is of class U,
the potential issues with the option being unprotected SHOULD be
documented (see Appendix D.5).
4.1.1. Inner Options
Inner option message fields (Class E) are used to communicate
directly with the other endpoint.
The sending endpoint SHALL write the Inner option message fields
present in the original CoAP message into the plaintext of the COSE
object (Section 5.3) and then remove the Inner option message fields
from the OSCORE message.
The processing of Inner option message fields by the receiving
endpoint is specified in Sections 8.2 and 8.4.
+------+-----------------+---+---+
| No. | Name | E | U |
+------+-----------------+---+---+
| 1 | If-Match | x | |
| 3 | Uri-Host | | x |
| 4 | ETag | x | |
| 5 | If-None-Match | x | |
| 6 | Observe | x | x |
| 7 | Uri-Port | | x |
| 8 | Location-Path | x | |
| 9 | OSCORE | | x |
| 11 | Uri-Path | x | |
| 12 | Content-Format | x | |
| 14 | Max-Age | x | x |
| 15 | Uri-Query | x | |
| 17 | Accept | x | |
| 20 | Location-Query | x | |
| 23 | Block2 | x | x |
| 27 | Block1 | x | x |
| 28 | Size2 | x | x |
| 35 | Proxy-Uri | | x |
| 39 | Proxy-Scheme | | x |
| 60 | Size1 | x | x |
| 258 | No-Response | x | x |
+------+-----------------+---+---+
E = Encrypt and Integrity Protect (Inner)
U = Unprotected (Outer)
Figure 5: Protection of CoAP Options
4.1.2. Outer Options
Outer option message fields (Class U or I) are used to support proxy
operations, see Appendix D.2.
The sending endpoint SHALL include the Outer option message field
present in the original message in the options part of the OSCORE
message. All Outer option message fields, including the OSCORE
option, SHALL be encoded as described in Section 3.1 of [RFC7252],
where the delta is the difference from the previously included
instance of Outer option message field.
The processing of Outer options by the receiving endpoint is
specified in Sections 8.2 and 8.4.
A procedure for integrity-protection-only of Class I option message
fields is specified in Section 5.4. Specifications that introduce
repeatable Class I options MUST specify that proxies MUST NOT change
the order of the instances of such an option in the CoAP message.
Note: There are currently no Class I option message fields defined.
4.1.3. Special Options
Some options require special processing as specified in this section.
4.1.3.1. Max-Age
An Inner Max-Age message field is used to indicate the maximum time a
response may be cached by the client (as defined in [RFC7252]), end-
to-end from the server to the client, taking into account that the
option is not accessible to proxies. The Inner Max-Age SHALL be
processed by OSCORE as a normal Inner option, specified in
Section 4.1.1.
An Outer Max-Age message field is used to avoid unnecessary caching
of error responses caused by OSCORE processing at OSCORE-unaware
intermediary nodes. A server MAY set a Class U Max-Age message field
with value zero to such error responses, described in Sections 7.4,
8.2, and 8.4, since these error responses are cacheable, but
subsequent OSCORE requests would never create a hit in the
intermediary node caching it. Setting the Outer Max-Age to zero
relieves the intermediary from uselessly caching responses.
Successful OSCORE responses do not need to include an Outer Max-Age
option. Except when the Observe option (see Section 4.1.3.5) is
used, responses appear to the OSCORE-unaware intermediary as 2.04
(Changed) responses, which are non-cacheable (see Section 4.2). For
Observe responses, which are cacheable, an Outer Max-Age option with
value 0 may be used to avoid unnecessary proxy caching.
The Outer Max-Age message field is processed according to
Section 4.1.2.
4.1.3.2. Uri-Host and Uri-Port
When the Uri-Host and Uri-Port are set to their default values (see
Section 5.10.1 [RFC7252]), they are omitted from the message
(Section 5.4.4 of [RFC7252]), which is favorable both for overhead
and privacy.
In order to support forward proxy operations, Proxy-Scheme, Uri-Host,
and Uri-Port need to be Class U. For the use of Proxy-Uri, see
Section 4.1.3.3.
Manipulation of unprotected message fields (including Uri-Host, Uri-
Port, destination IP/port or request scheme) MUST NOT lead to an
OSCORE message becoming verified by an unintended server. Different
servers SHALL have different security contexts.
4.1.3.3. Proxy-Uri
When Proxy-Uri is present, the client SHALL first decompose the
Proxy-Uri value of the original CoAP message into the Proxy-Scheme,
Uri-Host, Uri-Port, Uri-Path, and Uri-Query options according to
Section 6.4 of [RFC7252].
Uri-Path and Uri-Query are Class E options and SHALL be protected and
processed as Inner options (Section 4.1.1).
The Proxy-Uri option of the OSCORE message SHALL be set to the
composition of Proxy-Scheme, Uri-Host, and Uri-Port options as
specified in Section 6.5 of [RFC7252] and processed as an Outer
option of Class U (Section 4.1.2).
Note that replacing the Proxy-Uri value with the Proxy-Scheme and
Uri-* options works by design for all CoAP URIs (see Section 6 of
[RFC7252]). OSCORE-aware HTTP servers should not use the userinfo
component of the HTTP URI (as defined in Section 3.2.1 of [RFC3986]),
so that this type of replacement is possible in the presence of CoAP-
to-HTTP proxies (see Section 11.2). In future specifications of
cross-protocol proxying behavior using different URI structures, it
is expected that the authors will create Uri-* options that allow
decomposing the Proxy-Uri, and specifying the OSCORE processing.
An example of how Proxy-Uri is processed is given here. Assume that
the original CoAP message contains:
o Proxy-Uri = "coap://example.com/resource?q=1"
During OSCORE processing, Proxy-Uri is split into:
o Proxy-Scheme = "coap"
o Uri-Host = "example.com"
o Uri-Port = "5683" (default)
o Uri-Path = "resource"
o Uri-Query = "q=1"
Uri-Path and Uri-Query follow the processing defined in
Section 4.1.1; thus, they are encrypted and transported in the COSE
object:
o Uri-Path = "resource"
o Uri-Query = "q=1"
The remaining options are composed into the Proxy-Uri included in the
options part of the OSCORE message, which has value:
o Proxy-Uri = "coap://example.com"
See Sections 6.1 and 12.6 of [RFC7252] for more details.
4.1.3.4. The Block Options
Block-wise [RFC7959] is an optional feature. An implementation MAY
support CoAP [RFC7252] and the OSCORE option without supporting
block-wise transfers. The Block options (Block1, Block2, Size1,
Size2), when Inner message fields, provide secure message
segmentation such that each segment can be verified. The Block
options, when Outer message fields, enable hop-by-hop fragmentation
of the OSCORE message. Inner and Outer block processing may have
different performance properties depending on the underlying
transport. The end-to-end integrity of the message can be verified
both in case of Inner and Outer Block-wise transfers, provided all
blocks are received.
4.1.3.4.1. Inner Block Options
The sending CoAP endpoint MAY fragment a CoAP message as defined in
[RFC7959] before the message is processed by OSCORE. In this case,
the Block options SHALL be processed by OSCORE as normal Inner
options (Section 4.1.1). The receiving CoAP endpoint SHALL process
the OSCORE message before processing Block-wise as defined in
[RFC7959].
4.1.3.4.2. Outer Block Options
Proxies MAY fragment an OSCORE message using [RFC7959] by introducing
Block option message fields that are Outer (Section 4.1.2). Note
that the Outer Block options are neither encrypted nor integrity
protected. As a consequence, a proxy can maliciously inject block
fragments indefinitely, since the receiving endpoint needs to receive
the last block (see [RFC7959]) to be able to compose the OSCORE
message and verify its integrity. Therefore, applications supporting
OSCORE and [RFC7959] MUST specify a security policy defining a
maximum unfragmented message size (MAX_UNFRAGMENTED_SIZE) considering
the maximum size of message that can be handled by the endpoints.
Messages exceeding this size SHOULD be fragmented by the sending
endpoint using Inner Block options (Section 4.1.3.4.1).
An endpoint receiving an OSCORE message with an Outer Block option
SHALL first process this option according to [RFC7959], until all
blocks of the OSCORE message have been received or the cumulated
message size of the blocks exceeds MAX_UNFRAGMENTED_SIZE. In the
former case, the processing of the OSCORE message continues as
defined in this document. In the latter case, the message SHALL be
discarded.
Because of encryption of Uri-Path and Uri-Query, messages to the same
server may, from the point of view of a proxy, look like they also
target the same resource. A proxy SHOULD mitigate a potential mix-up
of blocks from concurrent requests to the same server, for example,
using the Request-Tag processing specified in Section 3.3.2 of
[CoAP-ECHO-REQ-TAG].
4.1.3.5. Observe
Observe [RFC7641] is an optional feature. An implementation MAY
support CoAP [RFC7252] and the OSCORE option without supporting
[RFC7641], in which case the Observe-related processing can be
omitted.
The support for Observe [RFC7641] with OSCORE targets the
requirements on forwarding of Section 2.2.1 of [CoAP-E2E-Sec], i.e.,
that observations go through intermediary nodes, as illustrated in
Figure 8 of [RFC7641].
Inner Observe SHALL be used to protect the value of the Observe
option between the endpoints. Outer Observe SHALL be used to support
forwarding by intermediary nodes.
The server SHALL include a new Partial IV (see Section 5) in
responses (with or without the Observe option) to Observe
registrations, except for the first response where Partial IV MAY be
omitted.
For cancellations, Section 3.6 of [RFC7641] specifies that all
options MUST be identical to those in the registration request except
for the Observe option and the set of ETag options. For OSCORE
messages, this matching is to be done to the options in the decrypted
message.
[RFC7252] does not specify how the server should act upon receiving
the same Token in different requests. When using OSCORE, the server
SHOULD NOT remove an active observation just because it receives a
request with the same Token.
Since POST with the Observe option is not defined, for messages with
the Observe option, the Outer Code MUST be set to 0.05 (FETCH) for
requests and to 2.05 (Content) for responses (see Section 4.2).
4.1.3.5.1. Registrations and Cancellations
The Inner and Outer Observe options in the request MUST contain the
Observe value of the original CoAP request; 0 (registration) or 1
(cancellation).
Every time a client issues a new request with the Observe option, a
new Partial IV MUST be used (see Section 5), and so the payload and
OSCORE option are changed. The server uses the Partial IV of the new
request as the 'request_piv' of all associated notifications (see
Section 5.4).
Intermediaries are not assumed to have access to the OSCORE security
context used by the endpoints; thus, they cannot make requests or
transform responses with the OSCORE option that pass verification (at
the receiving endpoint) as having come from the other endpoint. This
has the following consequences and limitations for Observe
operations.
o An intermediary node removing the Outer Observe 0 option does not
change the registration request to a request without the Observe
option (see Section 2 of [RFC7641]). Instead other means for
cancellation may be used as described in Section 3.6 of [RFC7641].
o An intermediary node is not able to transform a normal response
into an OSCORE-protected Observe notification (see Figure 7 of
[RFC7641]) that verifies as coming from the server.
o An intermediary node is not able to initiate an OSCORE protected
Observe registration (Observe option with value 0) that verifies
as coming from the client. An OSCORE-aware intermediary SHALL NOT
initiate registrations of observations (see Section 10). If an
OSCORE-unaware proxy resends an old registration message from a
client, the replay protection mechanism in the server will be
triggered. To prevent this from resulting in the OSCORE-unaware
proxy canceling the registration, a server MAY respond to a
replayed registration request with a replay of a cached
notification. Alternatively, the server MAY send a new
notification.
o An intermediary node is not able to initiate an OSCORE-protected
Observe cancellation (Observe option with value 1) that verifies
as coming from the client. An application MAY decide to allow
intermediaries to cancel Observe registrations, e.g., to send the
Observe option with value 1 (see Section 3.6 of [RFC7641]);
however, that can also be done with other methods, e.g., by
sending a RST message. This is out of scope for this
specification.
4.1.3.5.2. Notifications
If the server accepts an Observe registration, a Partial IV MUST be
included in all notifications (both successful and error), except for
the first one where the Partial IV MAY be omitted. To protect
against replay, the client SHALL maintain a Notification Number for
each Observation it registers. The Notification Number is a non-
negative integer containing the largest Partial IV of the received
notifications for the associated Observe registration. Further
details of replay protection of notifications are specified in
Section 7.4.1.
For notifications, the Inner Observe option value MUST be empty (see
Section 3.2 of [RFC7252]). The Outer Observe option in a
notification is needed for intermediary nodes to allow multiple
responses to one request, and it MAY be set to the value of the
Observe option in the original CoAP message. The client performs
ordering of notifications and replay protection by comparing their
Partial IVs and SHALL ignore the Outer Observe option value.
If the client receives a response to an Observe request without an
Inner Observe option, then it verifies the response as a non-Observe
response, as specified in Section 8.4. If the client receives a
response to a non-Observe request with an Inner Observe option, then
it stops processing the message, as specified in Section 8.4.
A client MUST consider the notification with the highest Partial IV
as the freshest, regardless of the order of arrival. In order to
support existing Observe implementations, the OSCORE client
implementation MAY set the Observe option value to the three least
significant bytes of the Partial IV. Implementations need to make
sure that the notification without Partial IV is considered the
oldest.
4.1.3.6. No-Response
No-Response [RFC7967] is an optional feature used by the client to
communicate its disinterest in certain classes of responses to a
particular request. An implementation MAY support [RFC7252] and the
OSCORE option without supporting [RFC7967].
If used, No-Response MUST be Inner. The Inner No-Response SHALL be
processed by OSCORE as specified in Section 4.1.1. The Outer option
SHOULD NOT be present. The server SHALL ignore the Outer No-Response
option. The client MAY set the Outer No-Response value to 26
(suppress all known codes) if the Inner value is set to 26. The
client MUST be prepared to receive and discard 5.04 (Gateway Timeout)
error messages from intermediaries potentially resulting from
destination time out due to no response.
4.1.3.7. OSCORE
The OSCORE option is only defined to be present in OSCORE messages as
an indication that OSCORE processing has been performed. The content
in the OSCORE option is neither encrypted nor integrity protected as
a whole, but some part of the content of this option is protected
(see Section 5.4). Nested use of OSCORE is not supported: If OSCORE
processing detects an OSCORE option in the original CoAP message,
then processing SHALL be stopped.
4.2. CoAP Header Fields and Payload
A summary of how the CoAP header fields and payload are protected is
shown in Figure 6, including fields specific to CoAP over UDP and
CoAP over TCP (marked accordingly in the table).
+------------------+---+---+
| Field | E | U |
+------------------+---+---+
| Version (UDP) | | x |
| Type (UDP) | | x |
| Length (TCP) | | x |
| Token Length | | x |
| Code | x | |
| Message ID (UDP) | | x |
| Token | | x |
| Payload | x | |
+------------------+---+---+
E = Encrypt and Integrity Protect (Inner)
U = Unprotected (Outer)
Figure 6: Protection of CoAP Header Fields and Payload
4.2. CoAP Header Fields and Payload
A summary of how the CoAP header fields and payload are protected is
shown in Figure 6, including fields specific to CoAP over UDP and
CoAP over TCP (marked accordingly in the table).
Most CoAP header fields (i.e., the message fields in the fixed 4-byte
header) are required to be read and/or changed by CoAP proxies; thus,
they cannot, in general, be protected end-to-end from one endpoint to
the other. As mentioned in Section 1, OSCORE protects the CoAP
request/response layer only and not the CoAP messaging layer
(Section 2 of [RFC7252]), so fields such as Type and Message ID are
not protected with OSCORE.
The CoAP header field Code is protected by OSCORE. Code SHALL be
encrypted and integrity protected (Class E) to prevent an
intermediary from eavesdropping on or manipulating it (e.g., changing
from GET to DELETE).
The sending endpoint SHALL write the Code of the original CoAP
message into the plaintext of the COSE object (see Section 5.3).
After that, the sending endpoint writes an Outer Code to the OSCORE
message. With one exception (see Section 4.1.3.5), the Outer Code
SHALL be set to 0.02 (POST) for requests and to 2.04 (Changed) for
responses. The receiving endpoint SHALL discard the Outer Code in
the OSCORE message and write the Code of the COSE object plaintext
(Section 5.3) into the decrypted CoAP message.
The other currently defined CoAP header fields are Unprotected (Class
U). The sending endpoint SHALL write all other header fields of the
original message into the header of the OSCORE message. The
receiving endpoint SHALL write the header fields from the received
OSCORE message into the header of the decrypted CoAP message.
The CoAP Payload, if present in the original CoAP message, SHALL be
encrypted and integrity protected; thus, it is an Inner message
field. The sending endpoint writes the payload of the original CoAP
message into the plaintext (Section 5.3) input to the COSE object.
The receiving endpoint verifies and decrypts the COSE object, and it
recreates the payload of the original CoAP message.
4.3. Signaling Messages
Signaling messages (CoAP Code 7.00-7.31) were introduced to exchange
information related to an underlying transport connection in the
specific case of CoAP over reliable transports [RFC8323].
OSCORE MAY be used to protect signaling if the endpoints for OSCORE
coincide with the endpoints for the signaling message. If OSCORE is
used to protect signaling then:
o To comply with [RFC8323], an initial empty Capabilities and
Settings Message (CSM) SHALL be sent. The subsequent signaling
message SHALL be protected.
o Signaling messages SHALL be protected as CoAP request messages,
except in the case in which the signaling message is a response to
a previous signaling message; then it SHALL be protected as a CoAP
response message. For example, 7.02 (Ping) is protected as a CoAP
request and 7.03 (Pong) as a CoAP response.
o The Outer Code for signaling messages SHALL be set to 0.02 (POST),
unless it is a response to a previous signaling message, in which
case it SHALL be set to 2.04 (Changed).
o All signaling options, except the OSCORE option, SHALL be Inner
(Class E).
NOTE: Option numbers for signaling messages are specific to the CoAP
Code (see Section 5.2 of [RFC8323]).
If OSCORE is not used to protect signaling, Signaling messages SHALL
be unaltered by OSCORE.
5. The COSE Object
This section defines how to use COSE [RFC8152] to wrap and protect
data in the original message. OSCORE uses the untagged COSE_Encrypt0
structure (see Section 5.2 of [RFC8152]) with an AEAD algorithm. The
AEAD key lengths, AEAD nonce length, and maximum Sender Sequence
Number are algorithm dependent.
The AEAD algorithm AES-CCM-16-64-128 defined in Section 10.2 of
[RFC8152] is mandatory to implement. For AES-CCM-16-64-128, the
length of Sender Key and Recipient Key is 128 bits; the length of
AEAD nonce and Common IV is 13 bytes. The maximum Sender Sequence
Number is specified in Section 12.
As specified in [RFC5116], plaintext denotes the data that is to be
encrypted and integrity protected, and Additional Authenticated Data
(AAD) denotes the data that is to be integrity protected only.
The COSE object SHALL be a COSE_Encrypt0 object with fields defined
as follows:
o The 'protected' field is empty.
o The 'unprotected' field includes:
* The 'Partial IV' parameter. The value is set to the Sender
Sequence Number. All leading bytes of value zero SHALL be
removed when encoding the Partial IV, except in the case of
Partial IV value 0, which is encoded to the byte string 0x00.
This parameter SHALL be present in requests and will not
typically be present in responses (for two exceptions, see
Observe notifications (Section 4.1.3.5.2) and Replay Window
synchronization (Appendix B.1.2)).
* The 'kid' parameter. The value is set to the Sender ID. This
parameter SHALL be present in requests and will not typically
be present in responses. An example where the Sender ID is
included in a response is the extension of OSCORE to group
communication [Group-OSCORE].
* Optionally, a 'kid context' parameter (see Section 5.1). This
parameter MAY be present in requests and, if so, MUST contain
an ID Context (see Section 3.1). This parameter SHOULD NOT be
present in responses: an example of how 'kid context' can be
used in responses is given in Appendix B.2. If 'kid context'
is present in the request, then the server SHALL use a security
context with that ID Context when verifying the request.
o The 'ciphertext' field is computed from the secret key (Sender Key
or Recipient Key), AEAD nonce (see Section 5.2), plaintext (see
Section 5.3), and the AAD (see Section 5.4) following Section 5.2
of [RFC8152].
The encryption process is described in Section 5.3 of [RFC8152].
5.1. ID Context and 'kid context'
For certain use cases, e.g., deployments where the same Sender ID is
used with multiple contexts, it is possible (and sometimes necessary,
see Section 3.3) for the client to use an ID Context to distinguish
the security contexts (see Section 3.1). For example:
o If the client has a unique identifier in some namespace, then that
identifier can be used as ID Context.
o The ID Context may be used to add randomness into new Sender and
Recipient Contexts, see Appendix B.2.
o In the case of group communication [Group-OSCORE], a group
identifier is used as ID Context to enable different security
contexts for a server belonging to multiple groups.
The Sender ID and ID Context are used to establish the necessary
input parameters and in the derivation of the security context (see
Section 3.2).
While the 'kid' parameter is used to transport the Sender ID, the new
COSE header parameter 'kid context' is used to transport the ID
Context in requests, see Figure 7.
+----------+--------+------------+----------------+-----------------+
| Name | Label | Value Type | Value Registry | Description |
+----------+--------+------------+----------------+-----------------+
| kid | 10 | bstr | | Identifies the |
| context | | | | context for the |
| | | | | key identifier |
+----------+--------+------------+----------------+-----------------+
Figure 7: Common Header Parameter 'kid context' for the COSE Object
If ID Context is non-empty and the client sends a request without
'kid context' resulting in an error indicating that the server could
not find the security context, then the client could include the ID
Context in the 'kid context' when making another request. Note that
since the error is unprotected, it may have been spoofed and the real
response blocked by an on-path attacker.
5.2. AEAD Nonce
The high-level design of the AEAD nonce follows Section 4.4 of
[IV-GEN]. The detailed construction of the AEAD nonce is presented
here (see Figure 8):
1. left-pad the Partial IV (PIV) with zeroes to exactly 5 bytes,
2. left-pad the Sender ID of the endpoint that generated the Partial
IV (ID_PIV) with zeroes to exactly nonce length minus 6 bytes,
3. concatenate the size of the ID_PIV (a single byte S) with the
padded ID_PIV and the padded PIV,
4. and then XOR with the Common IV.
Note that in this specification, only AEAD algorithms that use nonces
equal or greater than 7 bytes are supported. The nonce construction
with S, ID_PIV, and PIV together with endpoint-unique IDs and
encryption keys makes it easy to verify that the nonces used with a
specific key will be unique, see Appendix D.4.
If the Partial IV is not present in a response, the nonce from the
request is used. For responses that are not notifications (i.e.,
when there is a single response to a request), the request and the
response should typically use the same nonce to reduce message
overhead. Both alternatives provide all the required security
properties, see Section 7.4 and Appendix D.4. Another non-Observe
scenario where a Partial IV is included in a response is when the
server is unable to perform replay protection, see Appendix B.1.2.
For processing instructions see Section 8.
<- nonce length minus 6 B -> <-- 5 bytes -->
+---+-------------------+--------+---------+-----+
| S | padding | ID_PIV | padding | PIV |----+
+---+-------------------+--------+---------+-----+ |
|
<---------------- nonce length ----------------> |
+------------------------------------------------+ |
| Common IV |->(XOR)
+------------------------------------------------+ |
|
<---------------- nonce length ----------------> |
+------------------------------------------------+ |
| Nonce |<---+
+------------------------------------------------+
Figure 8: AEAD Nonce Formation
5.3. Plaintext
The plaintext is formatted as a CoAP message with a subset of the
header (see Figure 9) consisting of:
o the Code of the original CoAP message as defined in Section 3 of
[RFC7252]; and
o all Inner option message fields (see Section 4.1.1) present in the
original CoAP message (see Section 4.1). The options are encoded
as described in Section 3.1 of [RFC7252], where the delta is the
difference from the previously included instance of Class E
option; and
o the Payload of original CoAP message, if present, and in that case
prefixed by the one-byte Payload Marker (0xff).
NOTE: The plaintext contains all CoAP data that needs to be encrypted
end-to-end between the endpoints.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Class E options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(only if there is payload)
Figure 9: Plaintext
5.4. Additional Authenticated Data
The external_aad SHALL be a CBOR array wrapped in a bstr object as
defined below, following the notation of [RFC8610] as summarized in
Appendix E:
external_aad = bstr .cbor aad_array
aad_array = [
oscore_version : uint,
algorithms : [ alg_aead : int / tstr ],
request_kid : bstr,
request_piv : bstr,
options : bstr,
]
where:
o oscore_version: contains the OSCORE version number.
Implementations of this specification MUST set this field to 1.
Other values are reserved for future versions.
o algorithms: contains (for extensibility) an array of algorithms,
according to this specification only containing alg_aead.
o alg_aead: contains the AEAD Algorithm from the security context
used for the exchange (see Section 3.1).
o request_kid: contains the value of the 'kid' in the COSE object of
the request (see Section 5).
o request_piv: contains the value of the 'Partial IV' in the COSE
object of the request (see Section 5).
o options: contains the Class I options (see Section 4.1.2) present
in the original CoAP message encoded as described in Section 3.1
of [RFC7252], where the delta is the difference from the
previously included instance of class I option.
The oscore_version and algorithms parameters are established out-of-
band; thus, they are not transported in OSCORE, but the external_aad
allows to verify that they are the same in both endpoints.
NOTE: The format of the external_aad is, for simplicity, the same for
requests and responses, although some parameters, e.g., request_kid,
need not be integrity protected in all requests.
The AAD is composed from the external_aad as described in Section 5.3
of [RFC8152] (the notation follows [RFC8610] as summarized in
Appendix E):
AAD = Enc_structure = [ "Encrypt0", h'', external_aad ]
The following is an example of AAD constructed using AEAD Algorithm =
AES-CCM-16-64-128 (10), request_kid = 0x00, request_piv = 0x25 and no
Class I options:
o oscore_version: 0x01 (1 byte)
o algorithms: 0x810a (2 bytes)
o request_kid: 0x00 (1 byte)
o request_piv: 0x25 (1 byte)
o options: 0x (0 bytes)
o aad_array: 0x8501810a4100412540 (9 bytes)
o external_aad: 0x498501810a4100412540 (10 bytes)
o AAD: 0x8368456e63727970743040498501810a4100412540 (21 bytes)
Note that the AAD consists of a fixed string of 11 bytes concatenated
with the external_aad.
6. OSCORE Header Compression
The Concise Binary Object Representation (CBOR) [RFC7049] combines
very small message sizes with extensibility. The CBOR Object Signing
and Encryption (COSE) [RFC8152] uses CBOR to create compact encoding
of signed and encrypted data. However, COSE is constructed to
support a large number of different stateless use cases and is not
fully optimized for use as a stateful security protocol, leading to a
larger than necessary message expansion. In this section, we define
a stateless header compression mechanism, simply removing redundant
information from the COSE objects, which significantly reduces the
per-packet overhead. The result of applying this mechanism to a COSE
object is called the "compressed COSE object".
The COSE_Encrypt0 object used in OSCORE is transported in the OSCORE
option and in the Payload. The Payload contains the ciphertext of
the COSE object. The headers of the COSE object are compactly
encoded as described in the next section.
6.1. Encoding of the OSCORE Option Value
The value of the OSCORE option SHALL contain the OSCORE flag bits,
the 'Partial IV' parameter, the 'kid context' parameter (length and
value), and the 'kid' parameter as follows:
0 1 2 3 4 5 6 7 <------------- n bytes -------------->
+-+-+-+-+-+-+-+-+--------------------------------------
|0 0 0|h|k| n | Partial IV (if any) ...
+-+-+-+-+-+-+-+-+--------------------------------------
<- 1 byte -> <----- s bytes ------>
+------------+----------------------+------------------+
| s (if any) | kid context (if any) | kid (if any) ... |
+------------+----------------------+------------------+
Figure 10: The OSCORE Option Value
o The first byte, containing the OSCORE flag bits, encodes the
following set of bits and the length of the 'Partial IV'
parameter:
* The three least significant bits encode the Partial IV length
n. If n = 0, then the Partial IV is not present in the
compressed COSE object. The values n = 6 and n = 7 are
reserved.
* The fourth least significant bit is the 'kid' flag, k. It is
set to 1 if 'kid' is present in the compressed COSE object.
* The fifth least significant bit is the 'kid context' flag, h.
It is set to 1 if the compressed COSE object contains a 'kid
context' (see Section 5.1).
* The sixth-to-eighth least significant bits are reserved for
future use. These bits SHALL be set to zero when not in use.
According to this specification, if any of these bits are set
to 1, the message is considered to be malformed and
decompression fails as specified in item 2 of Section 8.2.
The flag bits are registered in the "OSCORE Flag Bits" registry
specified in Section 13.7.
o The following n bytes encode the value of the Partial IV, if the
Partial IV is present (n > 0).
o The following 1 byte encodes the length s of the 'kid context'
(Section 5.1), if the 'kid context' flag is set (h = 1).
o The following s bytes encode the 'kid context', if the 'kid
context' flag is set (h = 1).
o The remaining bytes encode the value of the 'kid', if the 'kid' is
present (k = 1).
Note that the 'kid' MUST be the last field of the OSCORE option
value, even in the case in which reserved bits are used and
additional fields are added to it.
The length of the OSCORE option thus depends on the presence and
length of Partial IV, 'kid context', 'kid', as specified in this
section, and on the presence and length of additional parameters, as
defined in the future documents registering those parameters.
6.2. Encoding of the OSCORE Payload
The payload of the OSCORE message SHALL encode the ciphertext of the
COSE object.
6.3. Examples of Compressed COSE Objects
This section covers a list of OSCORE Header Compression examples for
requests and responses. The examples assume the COSE_Encrypt0 object
is set (which means the CoAP message and cryptographic material is
known). Note that the full CoAP unprotected message, as well as the
full security context, is not reported in the examples, but only the
input necessary to the compression mechanism, i.e., the COSE_Encrypt0
object. The output is the compressed COSE object as defined in
Section 6, divided into two parts, since the object is transported in
two CoAP fields: the OSCORE option and payload.
1. Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
0x25, and Partial IV = 0x05
Before compression (24 bytes):
[
h'',
{ 4:h'25', 6:h'05' },
h'aea0155667924dff8a24e4cb35b9',
]
After compression (17 bytes):
Flag byte: 0b00001001 = 0x09 (1 byte)
Option Value: 0x090525 (3 bytes)
Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)
2. Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
empty string, and Partial IV = 0x00
Before compression (23 bytes):
[
h'',
{ 4:h'', 6:h'00' },
h'aea0155667924dff8a24e4cb35b9',
]
After compression (16 bytes):
Flag byte: 0b00001001 = 0x09 (1 byte)
Option Value: 0x0900 (2 bytes)
Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)
3. Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
empty string, Partial IV = 0x05, and kid context = 0x44616c656b
Before compression (30 bytes):
[
h'',
{ 4:h'', 6:h'05', 10:h'44616c656b' },
h'aea0155667924dff8a24e4cb35b9',
]
After compression (22 bytes):
Flag byte: 0b00011001 = 0x19 (1 byte)
Option Value: 0x19050544616c656b (8 bytes)
Payload: 0xae a0155667924dff8a24e4cb35b9 (14 bytes)
4. Response with ciphertext = 0xaea0155667924dff8a24e4cb35b9 and no
Partial IV
Before compression (18 bytes):
[
h'',
{},
h'aea0155667924dff8a24e4cb35b9',
]
After compression (14 bytes):
Flag byte: 0b00000000 = 0x00 (1 byte)
Option Value: 0x (0 bytes)
Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)
5. Response with ciphertext = 0xaea0155667924dff8a24e4cb35b9 and
Partial IV = 0x07
Before compression (21 bytes):
[
h'',
{ 6:h'07' },
h'aea0155667924dff8a24e4cb35b9',
]
After compression (16 bytes):
Flag byte: 0b00000001 = 0x01 (1 byte)
Option Value: 0x0107 (2 bytes)
Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)
7. Message Binding, Sequence Numbers, Freshness, and Replay Protection
7.1. Message Binding
In order to prevent response delay and mismatch attacks
[CoAP-Actuators] from on-path attackers and compromised
intermediaries, OSCORE binds responses to the requests by including
the 'kid' and Partial IV of the request in the AAD of the response.
Therefore, the server needs to store the 'kid' and Partial IV of the
request until all responses have been sent.
7.2. Sequence Numbers
An AEAD nonce MUST NOT be used more than once per AEAD key. The
uniqueness of (key, nonce) pairs is shown in Appendix D.4, and in
particular depends on a correct usage of Partial IVs (which encode
the Sender Sequence Numbers, see Section 5). If messages are
processed concurrently, the operation of reading and increasing the
Sender Sequence Number MUST be atomic.
7.2.1. Maximum Sequence Number
The maximum Sender Sequence Number is algorithm dependent (see
Section 12) and SHALL be less than 2^40. If the Sender Sequence
Number exceeds the maximum, the endpoint MUST NOT process any more
messages with the given Sender Context. If necessary, the endpoint
SHOULD acquire a new security context before this happens. The
latter is out of scope of this document.
7.3. Freshness
For requests, OSCORE provides only the guarantee that the request is
not older than the security context. For applications having
stronger demands on request freshness (e.g., control of actuators),
OSCORE needs to be augmented with mechanisms providing freshness (for
example, as specified in [CoAP-ECHO-REQ-TAG]).
Assuming an honest server (see Appendix D), the message binding
guarantees that a response is not older than its request. For
responses that are not notifications (i.e., when there is a single
response to a request), this gives absolute freshness. For
notifications, the absolute freshness gets weaker with time, and it
is RECOMMENDED that the client regularly re-register the observation.
Note that the message binding does not guarantee that a misbehaving
server created the response before receiving the request, i.e., it
does not verify server aliveness.
For requests and notifications, OSCORE also provides relative
freshness in the sense that the received Partial IV allows a
recipient to determine the relative order of requests or responses.
7.4. Replay Protection
In order to protect from replay of requests, the server's Recipient
Context includes a Replay Window. A server SHALL verify that the
Sender Sequence Number received in the 'Partial IV' parameter of the
COSE object (see Section 6.1) has not been received before. If this
verification fails, the server SHALL stop processing the message, and
it MAY optionally respond with a 4.01 (Unauthorized) error message.
Also, the server MAY set an Outer Max-Age option with value zero to
inform any intermediary that the response is not to be cached. The
diagnostic payload MAY contain the string "Replay detected". The
size and type of the Replay Window depends on the use case and the
protocol with which the OSCORE message is transported. In case of
reliable and ordered transport from endpoint to endpoint, e.g., TCP,
the server MAY just store the last received Partial IV and require
that newly received Partial IVs equal the last received Partial IV +
1. However, in the case of mixed reliable and unreliable transports
and where messages may be lost, such a replay mechanism may be too
restrictive and the default replay window may be more suitable (see
Section 3.2.2).
Responses (with or without Partial IV) are protected against replay
as they are bound to the request and the fact that only a single
response is accepted. In this case the Partial IV is not used for
replay protection of responses.
The operation of validating the Partial IV and updating the replay
protection MUST be atomic.
7.4.1. Replay Protection of Notifications
The following applies additionally when the Observe option is
supported.
The Notification Number (see Section 4.1.3.5.2) is initialized to the
Partial IV of the first successfully verified notification in
response to the registration request. A client MUST only accept at
most one Observe notification without Partial IV, and treat it as the
oldest notification received. A client receiving a notification
containing a Partial IV SHALL compare the Partial IV with the
Notification Number associated to that Observe registration. The
client MUST stop processing notifications with a Partial IV that has
been previously received. Applications MAY decide that a client only
processes notifications that have a greater Partial IV than the
Notification Number.
If the verification of the response succeeds, and the received
Partial IV was greater than the Notification Number, then the client
SHALL overwrite the corresponding Notification Number with the
received Partial IV.
7.5. Losing Part of the Context State
To prevent reuse of an AEAD nonce with the same AEAD key or the
acceptance of replayed messages, an endpoint needs to handle the
situation of losing rapidly changing parts of the context, such as
the Sender Sequence Number and Replay Window. These are typically
stored in RAM and therefore lost in the case of, e.g., an unplanned
reboot. There are different alternatives to recover, for example:
1. The endpoints can reuse an existing Security Context after
updating the mutable parts of the security context (Sender
Sequence Number and Replay Window). This requires that the
mutable parts of the security context are available throughout
the lifetime of the device or that the device can establish a
fresh security context after loss of mutable security context
data. Examples are given based on careful use of nonvolatile
memory, see Appendix B.1.1 and the use of the Echo option, see
Appendix B.1.2. If an endpoint makes use of a partial security
context stored in nonvolatile memory, it MUST NOT reuse a
previous Sender Sequence Number and MUST NOT accept previously
received messages.
2. The endpoints can reuse an existing shared Master Secret and
derive new Sender and Recipient Contexts, see Appendix B.2 for an
example. This typically requires a good source of randomness.
3. The endpoints can use a trusted third-party-assisted key
establishment protocol such as [OSCORE-PROFILE]. This requires
the execution of a three-party protocol and may require a good
source of randomness.
4. The endpoints can run a key exchange protocol providing forward
secrecy resulting in a fresh Master Secret, from which an
entirely new Security Context is derived. This requires a good
source of randomness, and additionally, the transmission and
processing of the protocol may have a non-negligible cost, e.g.,
in terms of power consumption.
The endpoints need to be configured with information about which
method is used. The choice of method may depend on capabilities of
the devices deployed and the solution architecture. Using a key
exchange protocol is necessary for deployments that require forward
secrecy.
8. Processing
This section describes the OSCORE message processing. Additional
processing for Observe or Block-wise are described in subsections.
Note that, analogously to [RFC7252] where the Token and source/
destination pair are used to match a response with a request, both
endpoints MUST keep the association (Token, {Security Context,
Partial IV of the request}), in order to be able to find the Security
Context and compute the AAD to protect or verify the response. The
association MAY be forgotten after it has been used to successfully
protect or verify the response, with the exception of Observe
processing, where the association MUST be kept as long as the
Observation is active.
The processing of the Sender Sequence Number follows the procedure
described in Section 3 of [IV-GEN].
8.1. Protecting the Request
Given a CoAP request, the client SHALL perform the following steps to
create an OSCORE request:
1. Retrieve the Sender Context associated with the target resource.
2. Compose the AAD and the plaintext, as described in Sections 5.3
and 5.4.
3. Encode the Partial IV (Sender Sequence Number in network byte
order) and increment the Sender Sequence Number by one. Compute
the AEAD nonce from the Sender ID, Common IV, and Partial IV as
described in Section 5.2.
4. Encrypt the COSE object using the Sender Key. Compress the COSE
object as specified in Section 6.
5. Format the OSCORE message according to Section 4. The OSCORE
option is added (see Section 4.1.2).
8.2. Verifying the Request
A server receiving a request containing the OSCORE option SHALL
perform the following steps:
1. Discard Code and all Class E options (marked in Figure 5 with 'x'
in column E) present in the received message. For example, an
If-Match Outer option is discarded, but an Uri-Host Outer option
is not discarded.
2. Decompress the COSE object (Section 6) and retrieve the Recipient
Context associated with the Recipient ID in the 'kid' parameter,
additionally using the 'kid context', if present. Note that the
Recipient Context MAY be retrieved by deriving a new security
context, e.g. as described in Appendix B.2. If either the
decompression or the COSE message fails to decode, or the server
fails to retrieve a Recipient Context with Recipient ID
corresponding to the 'kid' parameter received, then the server
SHALL stop processing the request.
* If either the decompression or the COSE message fails to
decode, the server MAY respond with a 4.02 (Bad Option) error
message. The server MAY set an Outer Max-Age option with
value zero. The diagnostic payload MAY contain the string
"Failed to decode COSE".
* If the server fails to retrieve a Recipient Context with
Recipient ID corresponding to the 'kid' parameter received,
the server MAY respond with a 4.01 (Unauthorized) error
message. The server MAY set an Outer Max-Age option with
value zero. The diagnostic payload MAY contain the string
"Security context not found".
3. Verify that the Partial IV has not been received before using the
Replay Window, as described in Section 7.4.
4. Compose the AAD, as described in Section 5.4.
5. Compute the AEAD nonce from the Recipient ID, Common IV, and the
Partial IV, received in the COSE object.
6. Decrypt the COSE object using the Recipient Key, as per
Section 5.3 of [RFC8152]. (The decrypt operation includes the
verification of the integrity.)
* If decryption fails, the server MUST stop processing the
request and MAY respond with a 4.00 (Bad Request) error
message. The server MAY set an Outer Max-Age option with
value zero. The diagnostic payload MAY contain the string
"Decryption failed".
* If decryption succeeds, update the Replay Window, as described
in Section 7.
7. Add decrypted Code, options, and payload to the decrypted
request. The OSCORE option is removed.
8. The decrypted CoAP request is processed according to [RFC7252].
8.2.1. Supporting Block-wise
If Block-wise is supported, insert the following step before any
other:
A. If Block-wise is present in the request, then process the Outer
Block options according to [RFC7959], until all blocks of the request
have been received (see Section 4.1.3.4).
8.3. Protecting the Response
If a CoAP response is generated in response to an OSCORE request, the
server SHALL perform the following steps to create an OSCORE
response. Note that CoAP error responses derived from CoAP
processing (step 8 in Section 8.2) are protected, as well as
successful CoAP responses, while the OSCORE errors (steps 2, 3, and 6
in Section 8.2) do not follow the processing below but are sent as
simple CoAP responses, without OSCORE processing.
1. Retrieve the Sender Context in the Security Context associated
with the Token.
2. Compose the AAD and the plaintext, as described in Sections 5.3
and 5.4.
3. Compute the AEAD nonce as described in Section 5.2:
* Either use the AEAD nonce from the request, or
* Encode the Partial IV (Sender Sequence Number in network byte
order) and increment the Sender Sequence Number by one.
Compute the AEAD nonce from the Sender ID, Common IV, and
Partial IV.
4. Encrypt the COSE object using the Sender Key. Compress the COSE
object as specified in Section 6. If the AEAD nonce was
constructed from a new Partial IV, this Partial IV MUST be
included in the message. If the AEAD nonce from the request was
used, the Partial IV MUST NOT be included in the message.
5. Format the OSCORE message according to Section 4. The OSCORE
option is added (see Section 4.1.2).
8.3.1. Supporting Observe
If Observe is supported, insert the following step between steps 2
and 3 of Section 8.3:
A. If the response is an Observe notification:
o If the response is the first notification:
* compute the AEAD nonce as described in Section 5.2:
+ Either use the AEAD nonce from the request, or
+ Encode the Partial IV (Sender Sequence Number in network
byte order) and increment the Sender Sequence Number by one.
Compute the AEAD nonce from the Sender ID, Common IV, and
Partial IV.
Then, go to 4.
o If the response is not the first notification:
* encode the Partial IV (Sender Sequence Number in network byte
order) and increment the Sender Sequence Number by one.
Compute the AEAD nonce from the Sender ID, Common IV, and
Partial IV, then go to 4.
8.4. Verifying the Response
A client receiving a response containing the OSCORE option SHALL
perform the following steps:
1. Discard Code and all Class E options (marked in Figure 5 with 'x'
in column E) present in the received message. For example, ETag
Outer option is discarded, as well as Max-Age Outer option.
2. Retrieve the Recipient Context in the Security Context associated
with the Token. Decompress the COSE object (Section 6). If
either the decompression or the COSE message fails to decode,
then go to 8.
3. Compose the AAD, as described in Section 5.4.
4. Compute the AEAD nonce
* If the Partial IV is not present in the response, the AEAD
nonce from the request is used.
* If the Partial IV is present in the response, compute the AEAD
nonce from the Recipient ID, Common IV, and the Partial IV,
received in the COSE object.
5. Decrypt the COSE object using the Recipient Key, as per
Section 5.3 of [RFC8152]. (The decrypt operation includes the
verification of the integrity.) If decryption fails, then go to
8.
6. Add decrypted Code, options and payload to the decrypted request.
The OSCORE option is removed.
7. The decrypted CoAP response is processed according to [RFC7252].
8. In case any of the previous erroneous conditions apply: the
client SHALL stop processing the response.
8.4.1. Supporting Block-wise
If Block-wise is supported, insert the following step before any
other:
A. If Block-wise is present in the response, then process the Outer
Block options according to [RFC7959], until all blocks of the
response have been received (see Section 4.1.3.4).
8.4.2. Supporting Observe
If Observe is supported:
Insert the following step between step 5 and step 6:
A. If the request was an Observe registration, then:
o If the Partial IV is not present in the response, and the Inner
Observe option is present, and the AEAD nonce from the request was
already used once, then go to 8.
o If the Partial IV is present in the response and the Inner Observe
option is present, then follow the processing described in
Section 4.1.3.5.2 and Section 7.4.1, then:
* initialize the Notification Number (if first successfully
verified notification), or
* overwrite the Notification Number (if the received Partial IV
was greater than the Notification Number).
Replace step 8 of Section 8.4 with:
B. In case any of the previous erroneous conditions apply: the
client SHALL stop processing the response. An error condition
occurring while processing a response to an observation request does
not cancel the observation. A client MUST NOT react to failure by
re-registering the observation immediately.
9. Web Linking
The use of OSCORE MAY be indicated by a target "osc" attribute in a
web link [RFC8288] to a resource, e.g., using a link-format document
[RFC6690] if the resource is accessible over CoAP.
The "osc" attribute is a hint indicating that the destination of that
link is only accessible using OSCORE, and unprotected access to it is
not supported. Note that this is simply a hint, it does not include
any security context material or any other information required to
run OSCORE.
A value MUST NOT be given for the "osc" attribute; any present value
MUST be ignored by parsers. The "osc" attribute MUST NOT appear more
than once in a given link-value; occurrences after the first MUST be
ignored by parsers.
The example in Figure 11 shows a use of the "osc" attribute: the
client does resource discovery on a server and gets back a list of
resources, one of which includes the "osc" attribute indicating that
the resource is protected with OSCORE. The link-format notation (see
Section 5 of [RFC6690]) is used.
REQ: GET /.well-known/core
RES: 2.05 Content
</sensors/temp>;osc,
</sensors/light>;if="sensor"
Figure 11: The Web Link
10. CoAP-to-CoAP Forwarding Proxy
CoAP is designed for proxy operations (see Section 5.7 of [RFC7252]).
OSCORE is designed to work with OSCORE-unaware CoAP proxies.
Security requirements for forwarding are listed in Section 2.2.1 of
[CoAP-E2E-Sec]. Proxy processing of the (Outer) Proxy-Uri option
works as defined in [RFC7252]. Proxy processing of the (Outer) Block
options works as defined in [RFC7959].
However, not all CoAP proxy operations are useful:
o Since a CoAP response is only applicable to the original CoAP
request, caching is in general not useful. In support of existing
proxies, OSCORE uses the Outer Max-Age option, see
Section 4.1.3.1.
o Proxy processing of the (Outer) Observe option as defined in
[RFC7641] is specified in Section 4.1.3.5.
Optionally, a CoAP proxy MAY detect OSCORE and act accordingly. An
OSCORE-aware CoAP proxy:
o SHALL bypass caching for the request if the OSCORE option is
present.
o SHOULD avoid caching responses to requests with an OSCORE option.
In the case of Observe (see Section 4.1.3.5), the OSCORE-aware CoAP
proxy:
o SHALL NOT initiate an Observe registration.
o MAY verify the order of notifications using Partial IV rather than
the Observe option.
11. HTTP Operations
The CoAP request/response model may be mapped to HTTP and vice versa
as described in Section 10 of [RFC7252]. The HTTP-CoAP mapping is
further detailed in [RFC8075]. This section defines the components
needed to map and transport OSCORE messages over HTTP hops. By
mapping between HTTP and CoAP and by using cross-protocol proxies,
OSCORE may be used end-to-end between, e.g., an HTTP client and a
CoAP server. Examples are provided in Sections 11.5 and 11.6.
11.1. The HTTP OSCORE Header Field
The HTTP OSCORE header field (see Section 13.4) is used for carrying
the content of the CoAP OSCORE option when transporting OSCORE
messages over HTTP hops.
The HTTP OSCORE header field is only used in POST requests and
responses with HTTP Status Code 200 (OK). When used, the HTTP header
field Content-Type is set to 'application/oscore' (see Section 13.5)
indicating that the HTTP body of this message contains the OSCORE
payload (see Section 6.2). No additional semantics are provided by
other message fields.
Using the Augmented Backus-Naur Form (ABNF) notation of [RFC5234],
including the following core ABNF syntax rules defined by that
specification: ALPHA (letters) and DIGIT (decimal digits), the HTTP
OSCORE header field value is as follows.
base64url-char = ALPHA / DIGIT / "-" / "_"
OSCORE = 2*base64url-char
The HTTP OSCORE header field is not appropriate to list in the
Connection header field (see Section 6.1 of [RFC7230]) since it is
not hop-by-hop. OSCORE messages are generally not useful when served
from cache (i.e., they will generally be marked Cache-Control: no-
cache) and so interaction with Vary is not relevant (Section 7.1.4 of
[RFC7231]). Since the HTTP OSCORE header field is critical for
message processing, moving it from headers to trailers renders the
message unusable in case trailers are ignored (see Section 4.1 of
[RFC7230]).
In general, intermediaries are not allowed to insert, delete, or
modify the OSCORE header. In general, changes to the HTTP OSCORE
header field will violate the integrity of the OSCORE message
resulting in an error. For the same reason the HTTP OSCORE header
field is generally not preserved across redirects.
Since redirects are not defined in the mappings between HTTP and CoAP
([RFC8075] [RFC7252]), a number of conditions need to be fulfilled
for redirects to work. For CoAP-client-to-HTTP-server redirects,
such conditions include:
o the CoAP-to-HTTP proxy follows the redirect, instead of the CoAP
client as in the HTTP case.
o the CoAP-to-HTTP proxy copies the HTTP OSCORE header field and
body to the new request.
o the target of the redirect has the necessary OSCORE security
context required to decrypt and verify the message.
Since OSCORE requires the HTTP body to be preserved across redirects,
the HTTP server is RECOMMENDED to reply with 307 (Temporary Redirect)
or 308 (Permanent Redirect) instead of 301 (Moved Permanently) or 302
(Found).
For the case of HTTP-client-to-CoAP-server redirects, although
redirect is not defined for CoAP servers [RFC7252], an HTTP client
receiving a redirect should generate a new OSCORE request for the
server it was redirected to.
11.2. CoAP-to-HTTP Mapping
Section 10.1 of [RFC7252] describes the fundamentals of the CoAP-to-
HTTP cross-protocol mapping process. The additional rules for OSCORE
messages are as follows:
o The HTTP OSCORE header field value is set to:
* AA if the CoAP OSCORE option is empty; otherwise,
* the value of the CoAP OSCORE option (Section 6.1) in base64url
(Section 5 of [RFC4648]) encoding without padding.
Implementation notes for this encoding are given in Appendix C
of [RFC7515].
o The HTTP Content-Type is set to 'application/oscore' (see
Section 13.5), independent of CoAP Content-Format.
11.3. HTTP-to-CoAP Mapping
Section 10.2 of [RFC7252] and [RFC8075] specify the behavior of an
HTTP-to-CoAP proxy. The additional rules for HTTP messages with the
OSCORE header field are as follows.
o The CoAP OSCORE option is set as follows:
* empty if the value of the HTTP OSCORE header field is a single
zero byte (0x00) represented by AA; otherwise,
* the value of the HTTP OSCORE header field decoded from
base64url (Section 5 of [RFC4648]) without padding.
Implementation notes for this encoding are given in Appendix C
of [RFC7515].
o The CoAP Content-Format option is omitted, the content format for
OSCORE (Section 13.6) MUST NOT be used.
11.4. HTTP Endpoints
Restricted to subsets of HTTP and CoAP supporting a bijective
mapping, OSCORE can be originated or terminated in HTTP endpoints.
The sending HTTP endpoint uses [RFC8075] to translate the HTTP
message into a CoAP message. The CoAP message is then processed with
OSCORE as defined in this document. The OSCORE message is then
mapped to HTTP as described in Section 11.2 and sent in compliance
with the rules in Section 11.1.
The receiving HTTP endpoint maps the HTTP message to a CoAP message
using [RFC8075] and Section 11.3. The resulting OSCORE message is
processed as defined in this document. If successful, the plaintext
CoAP message is translated to HTTP for normal processing in the
endpoint.
11.5. Example: HTTP Client and CoAP Server
This section gives an example of what a request and a response
between an HTTP client and a CoAP server could look like. The
example is not a test vector but intended as an illustration of how
the message fields are translated in the different steps.
Mapping and notation here is based on "Simple Form" (Section 5.4.1 of
[RFC8075]).
[HTTP request -- Before client object security processing]
GET http://proxy.url/hc/?target_uri=coap://server.url/orders
HTTP/1.1
[HTTP request -- HTTP Client to Proxy]
POST http://proxy.url/hc/?target_uri=coap://server.url/ HTTP/1.1
Content-Type: application/oscore
OSCORE: CSU
Body: 09 07 01 13 61 f7 0f d2 97 b1 [binary]
[CoAP request -- Proxy to CoAP Server]
POST coap://server.url/
OSCORE: 09 25
Payload: 09 07 01 13 61 f7 0f d2 97 b1 [binary]
[CoAP request -- After server object security processing]
GET coap://server.url/orders
[CoAP response -- Before server object security processing]
2.05 Content
Content-Format: 0
Payload: Exterminate! Exterminate!
[CoAP response -- CoAP Server to Proxy]
2.04 Changed
OSCORE: [empty]
Payload: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]
[HTTP response -- Proxy to HTTP Client]
HTTP/1.1 200 OK
Content-Type: application/oscore
OSCORE: AA
Body: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]
[HTTP response -- After client object security processing]
HTTP/1.1 200 OK
Content-Type: text/plain
Body: Exterminate! Exterminate!
Note that the HTTP Status Code 200 (OK) in the next-to-last message
is the mapping of CoAP Code 2.04 (Changed), whereas the HTTP Status
Code 200 (OK) in the last message is the mapping of the CoAP Code
2.05 (Content), which was encrypted within the compressed COSE object
carried in the Body of the HTTP response.
11.6. Example: CoAP Client and HTTP Server
This section gives an example of what a request and a response
between a CoAP client and an HTTP server could look like. The
example is not a test vector but intended as an illustration of how
the message fields are translated in the different steps.
[CoAP request -- Before client object security processing]
GET coap://proxy.url/
Proxy-Uri=http://server.url/orders
[CoAP request -- CoAP Client to Proxy]
POST coap://proxy.url/
Proxy-Uri=http://server.url/
OSCORE: 09 25
Payload: 09 07 01 13 61 f7 0f d2 97 b1 [binary]
[HTTP request -- Proxy to HTTP Server]
POST http://server.url/ HTTP/1.1
Content-Type: application/oscore
OSCORE: CSU
Body: 09 07 01 13 61 f7 0f d2 97 b1 [binary]
[HTTP request -- After server object security processing]
GET http://server.url/orders HTTP/1.1
[HTTP response -- Before server object security processing]
HTTP/1.1 200 OK
Content-Type: text/plain
Body: Exterminate! Exterminate!
[HTTP response -- HTTP Server to Proxy]
HTTP/1.1 200 OK
Content-Type: application/oscore
OSCORE: AA
Body: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]
[CoAP response -- Proxy to CoAP Client]
2.04 Changed
OSCORE: [empty]
Payload: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]
[CoAP response -- After client object security processing]
2.05 Content
Content-Format: 0
Payload: Exterminate! Exterminate!
Note that the HTTP Code 2.04 (Changed) in the next-to-last message is
the mapping of HTTP Status Code 200 (OK), whereas the CoAP Code 2.05
(Content) in the last message is the value that was encrypted within
the compressed COSE object carried in the Body of the HTTP response.
12. Security Considerations
An overview of the security properties is given in Appendix D.
12.1. End-to-end Protection
In scenarios with intermediary nodes such as proxies or gateways,
transport layer security such as (D)TLS only protects data hop-by-
hop. As a consequence, the intermediary nodes can read and modify
any information. The trust model where all intermediary nodes are
considered trustworthy is problematic, not only from a privacy
perspective, but also from a security perspective, as the
intermediaries are free to delete resources on sensors and falsify
commands to actuators (such as "unlock door", "start fire alarm",
"raise bridge"). Even in the rare cases where all the owners of the
intermediary nodes are fully trusted, attacks and data breaches make
such an architecture brittle.
(D)TLS protects hop-by-hop the entire message. OSCORE protects end-
to-end all information that is not required for proxy operations (see
Section 4). (D)TLS and OSCORE can be combined, thereby enabling end-
to-end security of the message payload, in combination with hop-by-
hop protection of the entire message, during transport between
endpoint and intermediary node. In particular, when OSCORE is used
with HTTP, the additional TLS protection of HTTP hops is RECOMMENDED,
e.g., between an HTTP endpoint and a proxy translating between HTTP
and CoAP.
Applications need to consider that certain message fields and
messages types are not protected end-to-end and may be spoofed or
manipulated. The consequences of unprotected message fields are
analyzed in Appendix D.5.
12.2. Security Context Establishment
The use of COSE_Encrypt0 and AEAD to protect messages as specified in
this document requires an established security context. The method
to establish the security context described in Section 3.2 is based
on a common Master Secret and unique Sender IDs. The necessary input
parameters may be preestablished or obtained using a key
establishment protocol augmented with establishment of Sender/
Recipient ID, such as a key exchange protocol or the OSCORE profile
of the Authentication and Authorization for Constrained Environments
(ACE) framework [OSCORE-PROFILE]. Such a procedure must ensure that
the requirements of the security context parameters for the intended
use are complied with (see Section 3.3) even in error situations.
While recipient IDs are allowed to coincide between different
security contexts (see Section 3.3), this may cause a server to
process multiple verifications before finding the right security
context or rejecting a message. Considerations for deploying OSCORE
with a fixed Master Secret are given in Appendix B.
12.3. Master Secret
OSCORE uses HKDF [RFC5869] and the established input parameters to
derive the security context. The required properties of the security
context parameters are discussed in Section 3.3; in this section, we
focus on the Master Secret. In this specification, HKDF denotes the
composition of the expand and extract functions as defined in
[RFC5869] and the Master Secret is used as Input Keying Material
(IKM).
Informally, HKDF takes as source an IKM containing some good amount
of randomness but not necessarily distributed uniformly (or for which
an attacker has some partial knowledge) and derive from it one or
more cryptographically strong secret keys [RFC5869].
Therefore, the main requirement for the OSCORE Master Secret, in
addition to being secret, is that it have a good amount of
randomness. The selected key establishment schemes must ensure that
the necessary properties for the Master Secret are fulfilled. For
pre-shared key deployments and key transport solutions such as
[OSCORE-PROFILE], the Master Secret can be generated offline using a
good random number generator. Randomness requirements for security
are described in [RFC4086].
12.4. Replay Protection
Replay attacks need to be considered in different parts of the
implementation. Most AEAD algorithms require a unique nonce for each
message, for which the Sender Sequence Numbers in the COSE message
field 'Partial IV' is used. If the recipient accepts any sequence
number larger than the one previously received, then the problem of
sequence number synchronization is avoided. With reliable transport,
it may be defined that only messages with sequence numbers that are
equal to the previous sequence number + 1 are accepted. An adversary
may try to induce a device reboot for the purpose of replaying a
message (see Section 7.5).
Note that sharing a security context between servers may open up for
replay attacks, for example, if the Replay Windows are not
synchronized.
12.5. Client Aliveness
A verified OSCORE request enables the server to verify the identity
of the entity who generated the message. However, it does not verify
that the client is currently involved in the communication, since the
message may be a delayed delivery of a previously generated request,
which now reaches the server. To verify the aliveness of the client
the server may use the Echo option in the response to a request from
the client (see [CoAP-ECHO-REQ-TAG]).
12.6. Cryptographic Considerations
The maximum Sender Sequence Number is dependent on the AEAD
algorithm. The maximum Sender Sequence Number is 2^40 - 1, or any
algorithm-specific lower limit, after which a new security context
must be generated. The mechanism to build the AEAD nonce
(Section 5.2) assumes that the nonce is at least 56 bits, and the
Partial IV is at most 40 bits. The mandatory-to-implement AEAD
algorithm AES-CCM-16-64-128 is selected for compatibility with CCM*.
AEAD algorithms that require unpredictable nonces are not supported.
In order to prevent cryptanalysis when the same plaintext is
repeatedly encrypted by many different users with distinct AEAD keys,
the AEAD nonce is formed by mixing the sequence number with a secret
per-context initialization vector (Common IV) derived along with the
keys (see Section 3.1 of [RFC8152]), and by using a Master Salt in
the key derivation (see [MF00] for an overview). The Master Secret,
Sender Key, Recipient Key, and Common IV must be secret, the rest of
the parameters may be public. The Master Secret must have a good
amount of randomness (see Section 12.3).
The ID Context, Sender ID, and Partial IV are always at least
implicitly integrity protected, as manipulation leads to the wrong
nonce or key being used and therefore results in decryption failure.
12.7. Message Segmentation
The Inner Block options enable the sender to split large messages
into OSCORE-protected blocks such that the receiving endpoint can
verify blocks before having received the complete message. The Outer
Block options allow for arbitrary proxy fragmentation operations that
cannot be verified by the endpoints but that can, by policy, be
restricted in size since the Inner Block options allow for secure
fragmentation of very large messages. A maximum message size (above
which the sending endpoint fragments the message and the receiving
endpoint discards the message, if complying to the policy) may be
obtained as part of normal resource discovery.
12.8. Privacy Considerations
Privacy threats executed through intermediary nodes are considerably
reduced by means of OSCORE. End-to-end integrity protection and
encryption of the message payload and all options that are not used
for proxy operations provide mitigation against attacks on sensor and
actuator communication, which may have a direct impact on the
personal sphere.
The unprotected options (Figure 5) may reveal privacy-sensitive
information, see Appendix D.5. CoAP headers sent in plaintext allow,
for example, matching of CON and ACK (CoAP Message Identifier),
matching of request and responses (Token) and traffic analysis.
OSCORE does not provide protection for HTTP header fields that are
not both CoAP-mappable and Class E. The HTTP message fields that are
visible to on-path entities are only used for the purpose of
transporting the OSCORE message, whereas the application-layer
message is encoded in CoAP and encrypted.
COSE message fields, i.e., the OSCORE option, may reveal information
about the communicating endpoints. For example, 'kid' and 'kid
context', which are intended to help the server find the right
context, may reveal information about the client. Tracking 'kid' and
'kid context' to one server may be used for correlating requests from
one client.
Unprotected error messages reveal information about the security
state in the communication between the endpoints. Unprotected
signaling messages reveal information about the reliable transport
used on a leg of the path. Using the mechanisms described in
Section 7.5 may reveal when a device goes through a reboot. This can
be mitigated by the device storing the precise state of Sender
Sequence Number and Replay Window on a clean shutdown.
The length of message fields can reveal information about the
message. Applications may use a padding scheme to protect against
traffic analysis.
13. IANA Considerations
13.1. COSE Header Parameters Registry
The 'kid context' parameter has been added to the "COSE Header
Parameters" registry:
o Name: kid context
o Label: 10
o Value Type: bstr
o Value Registry:
o Description: Identifies the context for the key identifier
o Reference: Section 5.1 of this document
13.2. CoAP Option Numbers Registry
The OSCORE option has been added to the "CoAP Option Numbers"
registry:
+--------+-----------------+-------------------+
| Number | Name | Reference |
+--------+-----------------+-------------------+
| 9 | OSCORE | [RFC8613] |
+--------+-----------------+-------------------+
Furthermore, the following existing entries in the "CoAP Option
Numbers" registry have been updated with a reference to the document
specifying OSCORE processing of that option:
+--------+-----------------+-------------------------------+
| Number | Name | Reference |
+--------+-----------------+-------------------------------+
| 1 | If-Match | [RFC7252] [RFC8613] |
| 3 | Uri-Host | [RFC7252] [RFC8613] |
| 4 | ETag | [RFC7252] [RFC8613] |
| 5 | If-None-Match | [RFC7252] [RFC8613] |
| 6 | Observe | [RFC7641] [RFC8613] |
| 7 | Uri-Port | [RFC7252] [RFC8613] |
| 8 | Location-Path | [RFC7252] [RFC8613] |
| 11 | Uri-Path | [RFC7252] [RFC8613] |
| 12 | Content-Format | [RFC7252] [RFC8613] |
| 14 | Max-Age | [RFC7252] [RFC8613] |
| 15 | Uri-Query | [RFC7252] [RFC8613] |
| 17 | Accept | [RFC7252] [RFC8613] |
| 20 | Location-Query | [RFC7252] [RFC8613] |
| 23 | Block2 | [RFC7959] [RFC8323] [RFC8613] |
| 27 | Block1 | [RFC7959] [RFC8323] [RFC8613] |
| 28 | Size2 | [RFC7959] [RFC8613] |
| 35 | Proxy-Uri | [RFC7252] [RFC8613] |
| 39 | Proxy-Scheme | [RFC7252] [RFC8613] |
| 60 | Size1 | [RFC7252] [RFC8613] |
| 258 | No-Response | [RFC7967] [RFC8613] |
+--------+-----------------+-------------------------------+
Future additions to the "CoAP Option Numbers" registry need to
provide a reference to the document where the OSCORE processing of
that CoAP Option is defined.
13.3. CoAP Signaling Option Numbers Registry
The OSCORE option has been added to the "CoAP Signaling Option
Numbers" registry:
+------------+--------+---------------------+-------------------+
| Applies to | Number | Name | Reference |
+------------+--------+---------------------+-------------------+
| 7.xx (all) | 9 | OSCORE | [RFC8613] |
+------------+--------+---------------------+-------------------+
13.4. Header Field Registrations
The HTTP OSCORE header field has been added to the "Message Headers"
registry:
+-------------------+----------+----------+---------------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+---------------------+
| OSCORE | http | standard | [RFC8613], |
| | | | Section 11.1 |
+-------------------+----------+----------+---------------------+
13.5. Media Type Registration
This section registers the 'application/oscore' media type in the
"Media Types" registry. This media type is used to indicate that the
content is an OSCORE message. The OSCORE body cannot be understood
without the OSCORE header field value and the security context.
Type name: application
Subtype name: oscore
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section
of [RFC8613].
Interoperability considerations: N/A
Published specification: [RFC8613]
Applications that use this media type: IoT applications sending
security content over HTTP(S) transports.
Fragment identifier considerations: N/A
Additional information:
* Deprecated alias names for this type: N/A
* Magic number(s): N/A
* File extension(s): N/A
* Macintosh file type code(s): N/A
Person & email address to contact for further information:
IESG <iesg@ietf.org>
Intended usage: COMMON
Restrictions on usage: N/A
Author: Goeran Selander <goran.selander@ericsson.com>
Change Controller: IESG
Provisional registration? No
13.6. CoAP Content-Formats Registry
This section registers the media type 'application/oscore' media type
in the "CoAP Content-Formats" registry. This Content-Format for the
OSCORE payload is defined for potential future use cases and SHALL
NOT be used in the OSCORE message. The OSCORE payload cannot be
understood without the OSCORE option value and the security context.
+----------------------+----------+----------+-------------------+
| Media Type | Encoding | ID | Reference |
+----------------------+----------+----------+-------------------+
| application/oscore | | 10001 | [RFC8613] |
+----------------------+----------+----------+-------------------+
13.7. OSCORE Flag Bits Registry
This document defines a subregistry for the OSCORE flag bits within
the "CoRE Parameters" registry. The name of the subregistry is
"OSCORE Flag Bits". The registry has been created with the Expert
Review policy [RFC8126]. Guidelines for the experts are provided in
Section 13.8.
The columns of the registry are as follows:
o Bit Position: This indicates the position of the bit in the set of
OSCORE flag bits, starting at 0 for the most significant bit. The
bit position must be an integer or a range of integers, in the
range 0 to 63.
o Name: The name is present to make it easier to refer to and
discuss the registration entry. The value is not used in the
protocol. Names are to be unique in the table.
o Description: This contains a brief description of the use of the
bit.
o Reference: This contains a pointer to the specification defining
the entry.
The initial contents of the registry are in the table below. The
reference column for all rows is this document. The entries with Bit
Position of 0 and 1 are marked as 'Reserved'. The entry with Bit
Position of 1 will be specified in a future document and will be used
to expand the space for the OSCORE flag bits in Section 6.1, so that
entries 8-63 of the registry are defined.
+--------------+-------------+-----------------------------+-----------+
| Bit Position | Name | Description | Reference |
+--------------+-------------+-----------------------------+-----------+
| 0 | Reserved | | |
+--------------+-------------+-----------------------------+-----------+
| 1 | Reserved | | |
+--------------+-------------+-----------------------------+-----------+
| 2 | Unassigned | | |
+--------------+-------------+-----------------------------+-----------+
| 3 | Kid Context | Set to 1 if kid context | [RFC8613] |
| | Flag | is present in the | |
| | | compressed COSE object | |
+--------------+-------------+-----------------------------+-----------+
| 4 | Kid Flag | Set to 1 if kid is present | [RFC8613] |
| | | in the compressed COSE | |
| | | object | |
+--------------+-------------+-----------------------------+-----------+
| 5-7 | Partial IV | Encodes the Partial IV | [RFC8613] |
| | Length | length; can have value | |
| | | 0 to 5 | |
+--------------+-------------+-----------------------------+-----------+
| 8-63 | Unassigned | | |
+--------------+-------------+-----------------------------+-----------+
13.8. Expert Review Instructions
The expert reviewers for the registry defined in this document are
expected to ensure that the usage solves a valid use case that could
not be solved better in a different way, that it is not going to
duplicate one that is already registered, and that the registered
point is likely to be used in deployments. They are furthermore
expected to check the clarity of purpose and use of the requested
code points. Experts should take into account the expected usage of
entries when approving point assignment, and the length of the
encoded value should be weighed against the number of code points
left that encode to that size and the size of device it will be used
on. Experts should block registration for entries 8-63 until these
points are defined (i.e., until the mechanism for the OSCORE flag
bits expansion via bit 1 is specified).
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/info/rfc8075>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
14.2. Informative References
[ACE-OAuth]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", Work in Progress, draft-ietf-ace-
oauth-authz-24, March 2019.
[CoAP-802.15.4]
Bormann, C., "Constrained Application Protocol (CoAP) over
IEEE 802.15.4 Information Element for IETF", Work in
Progress, draft-bormann-6lo-coap-802-15-ie-00, April 2016.
[CoAP-Actuators]
Mattsson, J., Fornehed, J., Selander, G., Palombini, F.,
and C. Amsuess, "Controlling Actuators with CoAP", Work in
Progress, draft-mattsson-core-coap-actuators-06, September
2018.
[CoAP-E2E-Sec]
Selander, G., Palombini, F., and K. Hartke, "Requirements
for CoAP End-To-End Security", Work in Progress, draft-
hartke-core-e2e-security-reqs-03, July 2017.
[CoAP-ECHO-REQ-TAG]
Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
Request-Tag, and Token Processing", Work in Progress,
draft-ietf-core-echo-request-tag-04, March 2019.
[Group-OSCORE]
Tiloca, M., Selander, G., Palombini, F., and J. Park,
"Group OSCORE - Secure Group Communication for CoAP", Work
in Progress, draft-ietf-core-oscore-groupcomm-04, March
2019.
[IV-GEN] McGrew, D., "Generation of Deterministic Initialization
Vectors (IVs) and Nonces", Work in Progress, draft-mcgrew-
iv-gen-03, October 2013.
[MF00] McGrew, D. and S. Fluhrer, "Attacks on Additive Encryption
of Redundant Plaintext and Implications on Internet
Security", Proceedings of the Seventh Annual Workshop on
Selected Areas in Cryptography (SAC 2000) Springer-
Verlag., pp. 14-28, 2000.
[OSCORE-PROFILE]
Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
"OSCORE profile of the Authentication and Authorization
for Constrained Environments Framework", Work in
Progress, draft-ietf-ace-oscore-profile-07, February 2019.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", Ph.D.
Dissertation, University of California, Irvine, 2010.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Scenario Examples
This section gives examples of OSCORE, targeting scenarios in
Section 2.2.1.1 of [CoAP-E2E-Sec]. The message exchanges are made,
based on the assumption that there is a security context established
between client and server. For simplicity, these examples only
indicate the content of the messages without going into detail of the
(compressed) COSE message format.
A.1. Secure Access to Sensor
This example illustrates a client requesting the alarm status from a
server.
Client Proxy Server
| | |
+------>| | Code: 0.02 (POST)
| POST | | Token: 0x8c
| | | OSCORE: [kid:5f, Partial IV:42]
| | | Payload: {Code:0.01,
| | | Uri-Path:"alarm_status"}
| | |
| +------>| Code: 0.02 (POST)
| | POST | Token: 0x7b
| | | OSCORE: [kid:5f, Partial IV:42]
| | | Payload: {Code:0.01,
| | | Uri-Path:"alarm_status"}
| | |
| |<------+ Code: 2.04 (Changed)
| | 2.04 | Token: 0x7b
| | | OSCORE: -
| | | Payload: {Code:2.05, "0"}
| | |
|<------+ | Code: 2.04 (Changed)
| 2.04 | | Token: 0x8c
| | | OSCORE: -
| | | Payload: {Code:2.05, "0"}
| | |
Square brackets [ ... ] indicate content of compressed COSE object.
Curly brackets { ... } indicate encrypted data.
Figure 12: Secure Access to Sensor
The CoAP request/response Codes are encrypted by OSCORE and only
dummy Codes (POST/Changed) are visible in the header of the OSCORE
message. The option Uri-Path ("alarm_status") and payload ("0") are
encrypted.
The COSE header of the request contains an identifier (5f),
indicating which security context was used to protect the message and
a Partial IV (42).
The server verifies the request as specified in Section 8.2. The
client verifies the response as specified in Section 8.4.
A.2. Secure Subscribe to Sensor
This example illustrates a client requesting subscription to a blood
sugar measurement resource (GET /glucose), first receiving the value
220 mg/dl and then a second value 180 mg/dl.
Client Proxy Server
| | |
+------>| | Code: 0.05 (FETCH)
| FETCH | | Token: 0x83
| | | Observe: 0
| | | OSCORE: [kid:ca, Partial IV:15]
| | | Payload: {Code:0.01,
| | | Observe:0,
| | | Uri-Path:"glucose"}
| | |
| +------>| Code: 0.05 (FETCH)
| | FETCH | Token: 0xbe
| | | Observe: 0
| | | OSCORE: [kid:ca, Partial IV:15]
| | | Payload: {Code:0.01,
| | | Observe:0,
| | | Uri-Path:"glucose"}
| | |
| |<------+ Code: 2.05 (Content)
| | 2.05 | Token: 0xbe
| | | Observe: 7
| | | OSCORE: -
| | | Payload: {Code:2.05,
| | | Observe:-,
| | | Content-Format:0, "220"}
| | |
|<------+ | Code: 2.05 (Content)
| 2.05 | | Token: 0x83
| | | Observe: 7
| | | OSCORE: -
| | | Payload: {Code:2.05,
| | | Observe:-,
| | | Content-Format:0, "220"}
... ... ...
| | |
| |<------+ Code: 2.05 (Content)
| | 2.05 | Token: 0xbe
| | | Observe: 8
| | | OSCORE: [Partial IV:36]
| | | Payload: {Code:2.05,
| | | Observe:-,
| | | Content-Format:0, "180"}
| | |
|<------+ | Code: 2.05 (Content)
| 2.05 | | Token: 0x83
| | | Observe: 8
| | | OSCORE: [Partial IV:36]
| | | Payload: {Code:2.05,
| | | Observe:-,
| | | Content-Format:0, "180"}
| | |
Square brackets [ ... ] indicate content of compressed COSE object
header. Curly brackets { ... } indicate encrypted data.
Figure 13: Secure Subscribe to Sensor
The dummy Codes (FETCH/Content) are used to allow forwarding of
Observe messages. The options Content-Format (0) and the payload
("220" and "180") are encrypted.
The COSE header of the request contains an identifier (ca),
indicating the security context used to protect the message and a
Partial IV (15). The COSE header of the second response contains the
Partial IV (36). The first response uses the Partial IV of the
request.
The server verifies that the Partial IV has not been received before.
The client verifies that the responses are bound to the request and
that the Partial IVs are greater than any Partial IV previously
received in a response bound to the request, except for the
notification without Partial IV, which is considered the oldest.
Appendix B. Deployment Examples
For many Internet of Things (IoT) deployments, a 128-bit uniformly
random Master Key is sufficient for encrypting all data exchanged
with the IoT device throughout its lifetime. Two examples are given
in this section. In the first example, the security context is only
derived once from the Master Secret. In the second example, security
contexts are derived multiple times using random inputs.
B.1. Security Context Derived Once
An application that only derives the security context once needs to
handle the loss of mutable security context parameters, e.g., due to
reboot.
B.1.1. Sender Sequence Number
In order to handle loss of Sender Sequence Numbers, the device may
implement procedures for writing to nonvolatile memory during normal
operations and updating the security context after reboot, provided
that the procedures comply with the requirements on the security
context parameters (Section 3.3). This section gives an example of
such a procedure.
There are known issues related to writing to nonvolatile memory. For
example, flash drives may have a limited number of erase operations
during its lifetime. Also, the time for a write operation to
nonvolatile memory to be completed may be unpredictable, e.g., due to
caching, which could result in important security context data not
being stored at the time when the device reboots.
However, many devices have predictable limits for writing to
nonvolatile memory, are physically limited to only send a small
amount of messages per minute, and may have no good source of
randomness.
To prevent reuse of Sender Sequence Number, an endpoint may perform
the following procedure during normal operations:
o Before using a Sender Sequence Number that is evenly divisible by
K, where K is a positive integer, store the Sender Sequence Number
(SSN1) in nonvolatile memory. After booting, the endpoint
initiates the new Sender Sequence Number (SSN2) to the value
stored in persistent memory plus K plus F: SSN2 = SSN1 + K + F,
where F is a positive integer.
* Writing to nonvolatile memory can be costly; the value K gives
a trade-off between frequency of storage operations and
efficient use of Sender Sequence Numbers.
* Writing to nonvolatile memory may be subject to delays, or
failure; F MUST be set so that the last Sender Sequence Number
used before reboot is never larger than SSN2.
If F cannot be set so SSN2 is always larger than the last Sender
Sequence Number used before reboot, the method described in this
section MUST NOT be used.
B.1.2. Replay Window
In case of loss of security context on the server, to prevent
accepting replay of previously received requests, the server may
perform the following procedure after booting:
o The server updates its Sender Sequence Number as specified in
Appendix B.1.1 to be used as Partial IV in the response containing
the Echo option (next bullet).
o For each stored security context, the first time after booting,
the server receives an OSCORE request, the server responds with an
OSCORE protected 4.01 (Unauthorized), containing only the Echo
option [CoAP-ECHO-REQ-TAG] and no diagnostic payload. The server
MUST use its Partial IV when generating the AEAD nonce and MUST
include the Partial IV in the response (see Section 5). If the
server with use of the Echo option can verify a second OSCORE
request as fresh, then the Partial IV of the second request is set
as the lower limit of the Replay Window of that security context.
B.1.3. Notifications
To prevent the acceptance of replay of previously received
notifications, the client may perform the following procedure after
booting:
o The client forgets about earlier registrations and removes all
Notification Numbers. The client then registers again using the
Observe option.
B.2. Security Context Derived Multiple Times
An application that does not require forward secrecy may allow
multiple security contexts to be derived from one Master Secret. The
requirements on the security context parameters MUST be fulfilled
(Section 3.3) even if the client or server is rebooted,
recommissioned, or in error cases.
This section gives an example of a protocol that adds randomness to
the ID Context parameter and uses that together with input parameters
preestablished between client and server, in particular Master
Secret, Master Salt, and Sender/Recipient ID (see Section 3.2), to
derive new security contexts. The random input is transported
between client and server in the 'kid context' parameter. This
protocol MUST NOT be used unless both endpoints have good sources of
randomness.
During normal requests, the ID Context of an established security
context may be sent in the 'kid context', which, together with 'kid',
facilitates for the server to locate a security context.
Alternatively, the 'kid context' may be omitted since the ID Context
is expected to be known to both client and server; see Section 5.1.
The protocol described in this section may only be needed when the
mutable part of security context is lost in the client or server,
e.g., when the endpoint has rebooted. The protocol may additionally
be used whenever the client and server need to derive a new security
context. For example, if a device is provisioned with one fixed set
of input parameters (including Master Secret, Sender and Recipient
Identifiers), then a randomized ID Context ensures that the security
context is different for each deployment.
Note that the server needs to be configured to run this protocol when
it is not able to retrieve an existing security context, instead of
stopping processing the message as described in step 2 of
Section 8.2.
The protocol is described below with reference to Figure 14. The
client or the server may initiate the protocol, in the latter case
step 1 is omitted.
Client Server
| |
1. Protect with | request #1 |
ID Context = ID1 |--------------------->| 2. Verify with
| kid_context = ID1 | ID Context = ID1
| |
| response #1 | Protect with
3. Verify with |<---------------------| ID Context = R2||ID1
ID Context = R2||ID1 | kid_context = R2 |
| |
Protect with | request #2 |
ID Context = R2||R3 |--------------------->| 4. Verify with
| kid_context = R2||R3 | ID Context = R2||R3
| |
| response #2 | Protect with
5. Verify with |<---------------------| ID Context = R2||R3
ID Context = R2||R3 | |
Figure 14: Protocol for Establishing a New Security Context
1. (Optional) If the client does not have a valid security context
with the server, e.g., because of reboot or because this is the
first time it contacts the server, then it generates a random
string R1 and uses this as ID Context together with the input
parameters shared with the server to derive a first security
context. The client sends an OSCORE request to the server
protected with the first security context, containing R1 wrapped
in a CBOR bstr as 'kid context'. The request may target a
special resource used for updating security contexts.
2. The server receives an OSCORE request for which it does not have
a valid security context, either because the client has generated
a new security context ID1 = R1 or because the server has lost
part of its security context, e.g., ID Context, Sender Sequence
Number or Replay Window. If the server is able to verify the
request (see Section 8.2) with the new derived first security
context using the received ID1 (transported in 'kid context') as
ID Context and the input parameters associated to the received
'kid', then the server generates a random string R2 and derives a
second security context with ID Context = ID2 = R2 || ID1. The
server sends a 4.01 (Unauthorized) response protected with the
second security context, containing R2 wrapped in a CBOR bstr as
'kid context', and caches R2. R2 MUST NOT be reused as that may
lead to reuse of key and nonce in response #1. Note that the
server may receive several requests #1 associated with one
security context, leading to multiple parallel protocol runs.
Multiple instances of R2 may need to be cached until one of the
protocol runs is completed, see Appendix B.2.1.
3. The client receives a response with 'kid context' containing a
CBOR bstr wrapping R2 to an OSCORE request it made with ID
Context = ID1. The client derives a second security context
using ID Context = ID2 = R2 || ID1. If the client can verify the
response (see Section 8.4) using the second security context,
then the client makes a request protected with a third security
context derived from ID Context = ID3 = R2 || R3, where R3 is a
random byte string generated by the client. The request includes
R2 || R3 wrapped in a CBOR bstr as 'kid context'.
4. If the server receives a request with 'kid context' containing a
CBOR bstr wrapping ID3, where the first part of ID3 is identical
to an R2 sent in a previous response #1, which it has not
received before, then the server derives a third security context
with ID Context = ID3. The server MUST NOT accept replayed
request #2 messages. If the server can verify the request (see
Section 8.2) with the third security context, then the server
marks the third security context to be used with this client and
removes all instances of R2 associated to this security context
from the cache. This security context replaces the previous
security context with the client, and the first and the second
security contexts are deleted. The server responds using the
same security context as in the request.
5. If the client receives a response to the request with the third
security context and the response verifies (see Section 8.4),
then the client marks the third security context to be used with
this server. This security context replaces the previous
security context with the server, and the first and second
security contexts are deleted.
If verification fails in any step, the endpoint stops processing that
message.
The length of the nonces R1, R2, and R3 is application specific. The
application needs to set the length of each nonce such that the
probability of its value being repeated is negligible; typically, at
least 8 bytes long. Since R2 may be generated as the result of a
replayed request #1, the probability for collision of R2s is impacted
by the birthday paradox. For example, setting the length of R2 to 8
bytes results in an average collision after 2^32 response #1
messages, which should not be an issue for a constrained server
handling on the order of one request per second.
Request #2 can be an ordinary request. The server performs the
action of the request and sends response #2 after having successfully
completed the operations related to the security context in step 4.
The client acts on response #2 after having successfully completed
step 5.
When sending request #2, the client is assured that the Sender Key
(derived with the random value R3) has never been used before. When
receiving response #2, the client is assured that the response
(protected with a key derived from the random value R3 and the Master
Secret) was created by the server in response to request #2.
Similarly, when receiving request #2, the server is assured that the
request (protected with a key derived from the random value R2 and
the Master Secret) was created by the client in response to response
#1. When sending response #2, the server is assured that the Sender
Key (derived with the random value R2) has never been used before.
Implementation and denial-of-service considerations are made in
Appendix B.2.1 and Appendix B.2.2.
B.2.1. Implementation Considerations
This section add some implementation considerations to the protocol
described in the previous section.
The server may only have space for a few security contexts or only be
able to handle a few protocol runs in parallel. The server may
legitimately receive multiple request #1 messages using the same
immutable security context, e.g., because of packet loss. Replays of
old request #1 messages could be difficult for the server to
distinguish from legitimate. The server needs to handle the case
when the maximum number of cached R2s is reached. If the server
receives a request #1 and is not capable of executing it then it may
respond with an unprotected 5.03 (Service Unavailable) error message.
The server may clear up state from protocol runs that never complete,
e.g., set a timer when caching R2, and remove R2 and the associated
security contexts from the cache at timeout. Additionally, state
information can be flushed at reboot.
As an alternative to caching R2, the server could generate R2 in such
a way that it can be sent (in response #1) and verified (at reception
of request #2) as the value of R2 it had generated. Such a procedure
MUST NOT lead to the server accepting replayed request #2 messages.
One construction described in the following is based on using a
secret random HMAC key K_HMAC per set of immutable security context
parameters associated with a client. This construction allows the
server to handle verification of R2 in response #2 at the cost of
storing the K_HMAC keys and a slightly larger message overhead in
response #1. Steps below refer to modifications to Appendix B.2:
o In step 2, R2 is generated in the following way. First, the
server generates a random K_HMAC (unless it already has one
associated with the security context), then it sets R2 = S2 ||
HMAC(K_HMAC, S2) where S2 is a random byte string, and the HMAC is
truncated to 8 bytes. K_HMAC may have an expiration time, after
which it is erased. Note that neither R2, S2, nor the derived
first and second security contexts need to be cached.
o In step 4, instead of verifying that R2 coincides with a cached
value, the server looks up the associated K_HMAC and verifies the
truncated HMAC, and the processing continues accordingly depending
on verification success or failure. K_HMAC is used until a run of
the protocol is completed (after verification of request #2), or
until it expires (whatever comes first), after which K_HMAC is
erased. (The latter corresponds to removing the cached values of
R2 in step 4 of Appendix B.2 and makes the server reject replays
of request #2.)
The length of S2 is application specific and the probability for
collision of S2s is impacted by the birthday paradox. For example,
setting the length of S2 to 8 bytes results in an average collision
after 2^32 response #1 messages, which should not be an issue for a
constrained server handling on the order of one request per second.
Two endpoints sharing a security context may accidentally initiate
two instances of the protocol at the same time, each in the role of
client, e.g., after a power outage affecting both endpoints. Such a
race condition could potentially lead to both protocols failing, and
both endpoints repeatedly reinitiating the protocol without
converging. Both endpoints can detect this situation, and it can be
handled in different ways. The requests could potentially be more
spread out in time, for example, by only initiating this protocol
when the endpoint actually needs to make a request, potentially
adding a random delay before requests immediately after reboot or if
such parallel protocol runs are detected.
B.2.2. Attack Considerations
An on-path attacker may inject a message causing the endpoint to
process verification of the message. A message crafted without
access to the Master Secret will fail to verify.
Replaying an old request with a value of 'kid_context' that the
server does not recognize could trigger the protocol. This causes
the server to generate the first and second security context and send
a response. But if the client did not expect a response, it will be
discarded. This may still result in a denial-of-service attack
against the server, e.g., because of not being able to manage the
state associated with many parallel protocol runs, and it may prevent
legitimate client requests. Implementation alternatives with less
data caching per request #1 message are favorable in this respect;
see Appendix B.2.1.
Replaying response #1 in response to some request other than request
#1 will fail to verify, since response #1 is associated to request
#1, through the dependencies of ID Contexts and the Partial IV of
request #1 included in the external_aad of response #1.
If request #2 has already been well received, then the server has a
valid security context, so a replay of request #2 is handled by the
normal replay protection mechanism. Similarly, if response #2 has
already been received, a replay of response #2 to some other request
from the client will fail by the normal verification of binding of
response to request.
Appendix C. Test Vectors
This appendix includes the test vectors for different examples of
CoAP messages using OSCORE. Given a set of inputs, OSCORE defines
how to set up the Security Context in both the client and the server.
Note that in Appendix C.4 and all following test vectors the Token
and the Message ID of the OSCORE-protected CoAP messages are set to
the same value of the unprotected CoAP message to help the reader
with comparisons.
C.1. Test Vector 1: Key Derivation with Master Salt
In this test vector, a Master Salt of 8 bytes is used. The default
values are used for AEAD Algorithm and HKDF.
C.1.1. Client
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Master Salt: 0x9e7ca92223786340 (8 bytes)
o Sender ID: 0x (0 byte)
o Recipient ID: 0x01 (1 byte)
From the previous parameters,
o info (for Sender Key): 0x8540f60a634b657910 (9 bytes)
o info (for Recipient Key): 0x854101f60a634b657910 (10 bytes)
o info (for Common IV): 0x8540f60a6249560d (8 bytes)
Outputs:
o Sender Key: 0xf0910ed7295e6ad4b54fc793154302ff (16 bytes)
o Recipient Key: 0xffb14e093c94c9cac9471648b4f98710 (16 bytes)
o Common IV: 0x4622d4dd6d944168eefb54987c (13 bytes)
From the previous parameters and a Partial IV equal to 0 (both for
sender and recipient):
o sender nonce: 0x4622d4dd6d944168eefb54987c (13 bytes)
o recipient nonce: 0x4722d4dd6d944169eefb54987c (13 bytes)
C.1.2. Server
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Master Salt: 0x9e7ca92223786340 (8 bytes)
o Sender ID: 0x01 (1 byte)
o Recipient ID: 0x (0 byte)
From the previous parameters,
o info (for Sender Key): 0x854101f60a634b657910 (10 bytes)
o info (for Recipient Key): 0x8540f60a634b657910 (9 bytes)
o info (for Common IV): 0x8540f60a6249560d (8 bytes)
Outputs:
o Sender Key: 0xffb14e093c94c9cac9471648b4f98710 (16 bytes)
o Recipient Key: 0xf0910ed7295e6ad4b54fc793154302ff (16 bytes)
o Common IV: 0x4622d4dd6d944168eefb54987c (13 bytes)
From the previous parameters and a Partial IV equal to 0 (both for
sender and recipient):
o sender nonce: 0x4722d4dd6d944169eefb54987c (13 bytes)
o recipient nonce: 0x4622d4dd6d944168eefb54987c (13 bytes)
C.2. Test Vector 2: Key Derivation without Master Salt
In this test vector, the default values are used for AEAD Algorithm,
HKDF, and Master Salt.
C.2.1. Client
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Sender ID: 0x00 (1 byte)
o Recipient ID: 0x01 (1 byte)
From the previous parameters,
o info (for Sender Key): 0x854100f60a634b657910 (10 bytes)
o info (for Recipient Key): 0x854101f60a634b657910 (10 bytes)
o info (for Common IV): 0x8540f60a6249560d (8 bytes)
Outputs:
o Sender Key: 0x321b26943253c7ffb6003b0b64d74041 (16 bytes)
o Recipient Key: 0xe57b5635815177cd679ab4bcec9d7dda (16 bytes)
o Common IV: 0xbe35ae297d2dace910c52e99f9 (13 bytes)
From the previous parameters and a Partial IV equal to 0 (both for
sender and recipient):
o sender nonce: 0xbf35ae297d2dace910c52e99f9 (13 bytes)
o recipient nonce: 0xbf35ae297d2dace810c52e99f9 (13 bytes)
C.2.2. Server
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Sender ID: 0x01 (1 byte)
o Recipient ID: 0x00 (1 byte)
From the previous parameters,
o info (for Sender Key): 0x854101f60a634b657910 (10 bytes)
o info (for Recipient Key): 0x854100f60a634b657910 (10 bytes)
o info (for Common IV): 0x8540f60a6249560d (8 bytes)
Outputs:
o Sender Key: 0xe57b5635815177cd679ab4bcec9d7dda (16 bytes)
o Recipient Key: 0x321b26943253c7ffb6003b0b64d74041 (16 bytes)
o Common IV: 0xbe35ae297d2dace910c52e99f9 (13 bytes)
From the previous parameters and a Partial IV equal to 0 (both for
sender and recipient):
o sender nonce: 0xbf35ae297d2dace810c52e99f9 (13 bytes)
o recipient nonce: 0xbf35ae297d2dace910c52e99f9 (13 bytes)
C.3. Test Vector 3: Key Derivation with ID Context
In this test vector, a Master Salt of 8 bytes and an ID Context of 8
bytes are used. The default values are used for AEAD Algorithm and
HKDF.
C.3.1. Client
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Master Salt: 0x9e7ca92223786340 (8 bytes)
o Sender ID: 0x (0 byte)
o Recipient ID: 0x01 (1 byte)
o ID Context: 0x37cbf3210017a2d3 (8 bytes)
From the previous parameters,
o info (for Sender Key): 0x85404837cbf3210017a2d30a634b657910 (17
bytes)
o info (for Recipient Key): 0x8541014837cbf3210017a2d30a634b657910
(18 bytes)
o info (for Common IV): 0x85404837cbf3210017a2d30a6249560d (16
bytes)
Outputs:
o Sender Key: 0xaf2a1300a5e95788b356336eeecd2b92 (16 bytes)
o Recipient Key: 0xe39a0c7c77b43f03b4b39ab9a268699f (16 bytes)
o Common IV: 0x2ca58fb85ff1b81c0b7181b85e (13 bytes)
From the previous parameters and a Partial IV equal to 0 (both for
sender and recipient):
o sender nonce: 0x2ca58fb85ff1b81c0b7181b85e (13 bytes)
o recipient nonce: 0x2da58fb85ff1b81d0b7181b85e (13 bytes)
C.3.2. Server
Inputs:
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
o Master Salt: 0x9e7ca92223786340 (8 bytes)
o Sender ID: 0x01 (1 byte)
o Recipient ID: 0x (0 byte)
o ID Context: 0x37cbf3210017a2d3 (8 bytes)
From the previous parameters,
o info (for Sender Key): 0x8541014837cbf3210017a2d30a634b657910 (18
bytes)
o info (for Recipient Key): 0x85404837cbf3210017a2d30a634b657910 (17
bytes)
o info (for Common IV): 0x85404837cbf3210017a2d30a6249560d (16
bytes)
Outputs:
o Sender Key: 0xe39a0c7c77b43f03b4b39ab9a268699f (16 bytes)
o Recipient Key: 0xaf2a1300a5e95788b356336eeecd2b92 (16 bytes)
o Common IV: 0x2ca58fb85ff1b81c0b7181b85e (13 bytes)
From the previous parameters and a Partial IV equal to 0 (both for
sender and recipient):
o sender nonce: 0x2da58fb85ff1b81d0b7181b85e (13 bytes)
o recipient nonce: 0x2ca58fb85ff1b81c0b7181b85e (13 bytes)
C.4. Test Vector 4: OSCORE Request, Client
This section contains a test vector for an OSCORE-protected CoAP GET
request using the security context derived in Appendix C.1. The
unprotected request only contains the Uri-Path and Uri-Host options.
Unprotected CoAP request:
0x44015d1f00003974396c6f63616c686f737483747631 (22 bytes)
Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
o Key Derivation Function: HKDF SHA-256
o Common IV: 0x4622d4dd6d944168eefb54987c (13 bytes)
Sender Context:
o Sender ID: 0x (0 byte)
o Sender Key: 0xf0910ed7295e6ad4b54fc793154302ff (16 bytes)
o Sender Sequence Number: 20
The following COSE and cryptographic parameters are derived:
o Partial IV: 0x14 (1 byte)
o kid: 0x (0 byte)
o aad_array: 0x8501810a40411440 (8 bytes)
o AAD: 0x8368456e63727970743040488501810a40411440 (20 bytes)
o plaintext: 0x01b3747631 (5 bytes)
o encryption key: 0xf0910ed7295e6ad4b54fc793154302ff (16 bytes)
o nonce: 0x4622d4dd6d944168eefb549868 (13 bytes)
From the previous parameter, the following is derived:
o OSCORE option value: 0x0914 (2 bytes)
o ciphertext: 0x612f1092f1776f1c1668b3825e (13 bytes)
From there:
o Protected CoAP request (OSCORE message): 0x44025d1f00003974396c6f6
3616c686f7374620914ff612f1092f1776f1c1668b3825e (35 bytes)
C.5. Test Vector 5: OSCORE Request, Client
This section contains a test vector for an OSCORE-protected CoAP GET
request using the security context derived in Appendix C.2. The
unprotected request only contains the Uri-Path and Uri-Host options.
Unprotected CoAP request:
0x440171c30000b932396c6f63616c686f737483747631 (22 bytes)
Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
o Key Derivation Function: HKDF SHA-256
o Common IV: 0xbe35ae297d2dace910c52e99f9 (13 bytes)
Sender Context:
o Sender ID: 0x00 (1 bytes)
o Sender Key: 0x321b26943253c7ffb6003b0b64d74041 (16 bytes)
o Sender Sequence Number: 20
The following COSE and cryptographic parameters are derived:
o Partial IV: 0x14 (1 byte)
o kid: 0x00 (1 byte)
o aad_array: 0x8501810a4100411440 (9 bytes)
o AAD: 0x8368456e63727970743040498501810a4100411440 (21 bytes)
o plaintext: 0x01b3747631 (5 bytes)
o encryption key: 0x321b26943253c7ffb6003b0b64d74041 (16 bytes)
o nonce: 0xbf35ae297d2dace910c52e99ed (13 bytes)
From the previous parameter, the following is derived:
o OSCORE option value: 0x091400 (3 bytes)
o ciphertext: 0x4ed339a5a379b0b8bc731fffb0 (13 bytes)
From there:
o Protected CoAP request (OSCORE message): 0x440271c30000b932396c6f6
3616c686f737463091400ff4ed339a5a379b0b8bc731fffb0 (36 bytes)
C.6. Test Vector 6: OSCORE Request, Client
This section contains a test vector for an OSCORE-protected CoAP GET
request for an application that sets the ID Context and requires it
to be sent in the request, so 'kid context' is present in the
protected message. This test vector uses the security context
derived in Appendix C.3. The unprotected request only contains the
Uri-Path and Uri-Host options.
Unprotected CoAP request:
0x44012f8eef9bbf7a396c6f63616c686f737483747631 (22 bytes)
Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
o Key Derivation Function: HKDF SHA-256
o Common IV: 0x2ca58fb85ff1b81c0b7181b85e (13 bytes)
o ID Context: 0x37cbf3210017a2d3 (8 bytes)
Sender Context:
o Sender ID: 0x (0 bytes)
o Sender Key: 0xaf2a1300a5e95788b356336eeecd2b92 (16 bytes)
o Sender Sequence Number: 20
The following COSE and cryptographic parameters are derived:
o Partial IV: 0x14 (1 byte)
o kid: 0x (0 byte)
o kid context: 0x37cbf3210017a2d3 (8 bytes)
o aad_array: 0x8501810a40411440 (8 bytes)
o AAD: 0x8368456e63727970743040488501810a40411440 (20 bytes)
o plaintext: 0x01b3747631 (5 bytes)
o encryption key: 0xaf2a1300a5e95788b356336eeecd2b92 (16 bytes)
o nonce: 0x2ca58fb85ff1b81c0b7181b84a (13 bytes)
From the previous parameter, the following is derived:
o OSCORE option value: 0x19140837cbf3210017a2d3 (11 bytes)
o ciphertext: 0x72cd7273fd331ac45cffbe55c3 (13 bytes)
From there:
o Protected CoAP request (OSCORE message):
0x44022f8eef9bbf7a396c6f63616c686f73746b19140837cbf3210017a2d3ff
72cd7273fd331ac45cffbe55c3 (44 bytes)
C.7. Test Vector 7: OSCORE Response, Server
This section contains a test vector for an OSCORE-protected 2.05
(Content) response to the request in Appendix C.4. The unprotected
response has payload "Hello World!" and no options. The protected
response does not contain a 'kid' nor a Partial IV. Note that some
parameters are derived from the request.
Unprotected CoAP response:
0x64455d1f00003974ff48656c6c6f20576f726c6421 (21 bytes)
Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
o Key Derivation Function: HKDF SHA-256
o Common IV: 0x4622d4dd6d944168eefb54987c (13 bytes)
Sender Context:
o Sender ID: 0x01 (1 byte)
o Sender Key: 0xffb14e093c94c9cac9471648b4f98710 (16 bytes)
o Sender Sequence Number: 0
The following COSE and cryptographic parameters are derived:
o aad_array: 0x8501810a40411440 (8 bytes)
o AAD: 0x8368456e63727970743040488501810a40411440 (20 bytes)
o plaintext: 0x45ff48656c6c6f20576f726c6421 (14 bytes)
o encryption key: 0xffb14e093c94c9cac9471648b4f98710 (16 bytes)
o nonce: 0x4622d4dd6d944168eefb549868 (13 bytes)
From the previous parameter, the following is derived:
o OSCORE option value: 0x (0 bytes)
o ciphertext: 0xdbaad1e9a7e7b2a813d3c31524378303cdafae119106 (22
bytes)
From there:
o Protected CoAP response (OSCORE message):
0x64445d1f0000397490ffdbaad1e9a7e7b2a813d3c31524378303cdafae119106
(32 bytes)
C.8. Test Vector 8: OSCORE Response with Partial IV, Server
This section contains a test vector for an OSCORE protected 2.05
(Content) response to the request in Appendix C.4. The unprotected
response has payload "Hello World!" and no options. The protected
response does not contain a 'kid', but contains a Partial IV. Note
that some parameters are derived from the request.
Unprotected CoAP response:
0x64455d1f00003974ff48656c6c6f20576f726c6421 (21 bytes)
Common Context:
o AEAD Algorithm: 10 (AES-CCM-16-64-128)
o Key Derivation Function: HKDF SHA-256
o Common IV: 0x4622d4dd6d944168eefb54987c (13 bytes)
Sender Context:
o Sender ID: 0x01 (1 byte)
o Sender Key: 0xffb14e093c94c9cac9471648b4f98710 (16 bytes)
o Sender Sequence Number: 0
The following COSE and cryptographic parameters are derived:
o Partial IV: 0x00 (1 byte)
o aad_array: 0x8501810a40411440 (8 bytes)
o AAD: 0x8368456e63727970743040488501810a40411440 (20 bytes)
o plaintext: 0x45ff48656c6c6f20576f726c6421 (14 bytes)
o encryption key: 0xffb14e093c94c9cac9471648b4f98710 (16 bytes)
o nonce: 0x4722d4dd6d944169eefb54987c (13 bytes)
From the previous parameter, the following is derived:
o OSCORE option value: 0x0100 (2 bytes)
o ciphertext: 0x4d4c13669384b67354b2b6175ff4b8658c666a6cf88e (22
bytes)
From there:
o Protected CoAP response (OSCORE message): 0x64445d1f00003974920100
ff4d4c13669384b67354b2b6175ff4b8658c666a6cf88e (34 bytes)
Appendix D. Overview of Security Properties
D.1. Threat Model
This section describes the threat model using the terms of [RFC3552].
It is assumed that the endpoints running OSCORE have not themselves
been compromised. The attacker is assumed to have control of the
CoAP channel over which the endpoints communicate, including
intermediary nodes. The attacker is capable of launching any passive
or active on-path or off-path attacks; including eavesdropping,
traffic analysis, spoofing, insertion, modification, deletion, delay,
replay, man-in-the-middle, and denial-of-service attacks. This means
that the attacker can read any CoAP message on the network and
undetectably remove, change, or inject forged messages onto the wire.
OSCORE targets the protection of the CoAP request/response layer
(Section 2 of [RFC7252]) between the endpoints, including the CoAP
Payload, Code, Uri-Path/Uri-Query, and the other Class E option
instances (Section 4.1).
OSCORE does not protect the CoAP messaging layer (Section 2 of
[RFC7252]) or other lower layers involved in routing and transporting
the CoAP requests and responses.
Additionally, OSCORE does not protect Class U option instances
(Section 4.1), as these are used to support CoAP forward proxy
operations (see Section 5.7.2 of [RFC7252]). The supported proxies
(forwarding, cross-protocol, e.g., CoAP to CoAP-mappable protocols
such as HTTP) must be able to change certain Class U options (by
instruction from the Client), resulting in the CoAP request being
redirected to the server. Changes caused by the proxy may result in
the request not reaching the server or reaching the wrong server.
For cross-protocol proxies, mappings are done on the Outer part of
the message so these protocols are essentially used as transport.
Manipulation of these options may thus impact whether the protected
message reaches or does not reach the destination endpoint.
Attacks on unprotected CoAP message fields generally causes denial-
of-service attacks which are out of scope of this document, more
details are given in Appendix D.5.
Attacks against the CoAP request-response layer are in scope. OSCORE
is intended to protect against eavesdropping, spoofing, insertion,
modification, deletion, replay, and man-in-the middle attacks.
OSCORE is susceptible to traffic analysis as discussed later in
Appendix D.
D.2. Supporting Proxy Operations
CoAP is designed to work with intermediaries reading and/or changing
CoAP message fields to perform supporting operations in constrained
environments, e.g., forwarding and cross-protocol translations.
Securing CoAP on the transport layer protects the entire message
between the endpoints, in which case CoAP proxy operations are not
possible. In order to enable proxy operations, security on the
transport layer needs to be terminated at the proxy; in which case,
the CoAP message in its entirety is unprotected in the proxy.
Requirements for CoAP end-to-end security are specified in
[CoAP-E2E-Sec], in particular, forwarding is detailed in
Section 2.2.1. The client and server are assumed to be honest, while
proxies and gateways are only trusted to perform their intended
operations.
By working at the CoAP layer, OSCORE enables different CoAP message
fields to be protected differently, which allows message fields
required for proxy operations to be available to the proxy while
message fields intended for the other endpoint remain protected. In
the remainder of this section, we analyze how OSCORE protects the
protected message fields and the consequences of message fields
intended for proxy operation being unprotected.
D.3. Protected Message Fields
Protected message fields are included in the plaintext (Section 5.3)
and the AAD (Section 5.4) of the COSE_Encrypt0 object and encrypted
using an AEAD algorithm.
OSCORE depends on a preestablished random Master Secret
(Section 12.3) used to derive encryption keys, and a construction for
making (key, nonce) pairs unique (Appendix D.4). Assuming this is
true, and the keys are used for no more data than indicated in
Section 7.2.1, OSCORE should provide the following guarantees:
o Confidentiality: An attacker should not be able to determine the
plaintext contents of a given OSCORE message or determine that
different plaintexts are related (Section 5.3).
o Integrity: An attacker should not be able to craft a new OSCORE
message with protected message fields different from an existing
OSCORE message that will be accepted by the receiver.
o Request-response binding: An attacker should not be able to make a
client match a response to the wrong request.
o Non-replayability: An attacker should not be able to cause the
receiver to accept a message that it has previously received and
accepted.
In the above, the attacker is anyone except the endpoints, e.g., a
compromised intermediary. Informally, OSCORE provides these
properties by AEAD-protecting the plaintext with a strong key and
uniqueness of (key, nonce) pairs. AEAD encryption [RFC5116] provides
confidentiality and integrity for the data. Response-request binding
is provided by including the 'kid' and Partial IV of the request in
the AAD of the response. Non-replayability of requests and
notifications is provided by using unique (key, nonce) pairs and a
replay protection mechanism (application dependent, see Section 7.4).
OSCORE is susceptible to a variety of traffic analysis attacks based
on observing the length and timing of encrypted packets. OSCORE does
not provide any specific defenses against this form of attack, but
the application may use a padding mechanism to prevent an attacker
from directly determining the length of the padding. However,
information about padding may still be revealed by side-channel
attacks observing differences in timing.
D.4. Uniqueness of (key, nonce)
In this section, we show that (key, nonce) pairs are unique as long
as the requirements in Sections 3.3 and 7.2.1 are followed.
Fix a Common Context (Section 3.1) and an endpoint, called the
encrypting endpoint. An endpoint may alternate between client and
server roles, but each endpoint always encrypts with the Sender Key
of its Sender Context. Sender Keys are (stochastically) unique since
they are derived with HKDF using unique Sender IDs, so messages
encrypted by different endpoints use different keys. It remains to
be proven that the nonces used by the fixed endpoint are unique.
Since the Common IV is fixed, the nonces are determined by PIV, where
PIV takes the value of the Partial IV of the request or of the
response, and by the Sender ID of the endpoint generating that
Partial IV (ID_PIV). The nonce construction (Section 5.2) with the
size of the ID_PIV (S) creates unique nonces for different (ID_PIV,
PIV) pairs. There are two cases:
A. For requests, and responses with Partial IV (e.g., Observe
notifications):
o ID_PIV = Sender ID of the encrypting endpoint
o PIV = current Partial IV of the encrypting endpoint
Since the encrypting endpoint steps the Partial IV for each use, the
nonces used in case A are all unique as long as the number of
encrypted messages is kept within the required range (Section 7.2.1).
B. For responses without Partial IV (e.g., single response to a
request):
o ID_PIV = Sender ID of the endpoint generating the request
o PIV = Partial IV of the request
Since the Sender IDs are unique, ID_PIV is different from the Sender
ID of the encrypting endpoint. Therefore, the nonces in case B are
different compared to nonces in case A, where the encrypting endpoint
generated the Partial IV. Since the Partial IV of the request is
verified for replay (Section 7.4) associated to this Recipient
Context, PIV is unique for this ID_PIV, which makes all nonces in
case B distinct.
D.5. Unprotected Message Fields
This section analyzes attacks on message fields that are not
protected by OSCORE according to the threat model Appendix D.1.
D.5.1. CoAP Header Fields
o Version. The CoAP version [RFC7252] is not expected to be
sensitive to disclosure. Currently, there is only one CoAP
version defined. A change of this parameter is potentially a
denial-of-service attack. Future versions of CoAP need to analyze
attacks to OSCORE-protected messages due to an adversary changing
the CoAP version.
o Token/Token Length. The Token field is a client-local identifier
for differentiating between concurrent requests [RFC7252]. CoAP
proxies are allowed to read and change Token and Token Length
between hops. An eavesdropper reading the Token can match
requests to responses that can be used in traffic analysis. In
particular, this is true for notifications, where multiple
responses are matched to one request. Modifications of Token and
Token Length by an on-path attacker may become a denial-of-service
attack, since it may prevent the client to identify to which
request the response belongs or to find the correct information to
verify integrity of the response.
o Code. The Outer CoAP Code of an OSCORE message is POST or FETCH
for requests with corresponding response codes. An endpoint
receiving the message discards the Outer CoAP Code and uses the
Inner CoAP Code instead (see Section 4.2). Hence, modifications
from attackers to the Outer Code do not impact the receiving
endpoint. However, changing the Outer Code from FETCH to a Code
value for a method that does not work with Observe (such as POST)
may, depending on proxy implementation since Observe is undefined
for several Codes, cause the proxy to not forward notifications,
which is a denial-of-service attack. The use of FETCH rather than
POST reveals no more than what is revealed by the presence of the
Outer Observe option.
o Type/Message ID. The Type/Message ID fields [RFC7252] reveal
information about the UDP transport binding, e.g., an eavesdropper
reading the Type or Message ID gain information about how UDP
messages are related to each other. CoAP proxies are allowed to
change Type and Message ID. These message fields are not present
in CoAP over TCP [RFC8323] and do not impact the request/response
message. A change of these fields in a UDP hop is a denial-of-
service attack. By sending an ACK, an attacker can make the
endpoint believe that it does not need to retransmit the previous
message. By sending a RST, an attacker may be able to cancel an
observation. By changing a NON to a CON, the attacker can cause
the receiving endpoint to ACK messages for which no ACK was
requested.
o Length. This field contains the length of the message [RFC8323],
which may be used for traffic analysis. This message field is not
present in CoAP over UDP and does not impact the request/response
message. A change of Length is a denial-of-service attack similar
to changing TCP header fields.
D.5.2. CoAP Options
o Max-Age. The Outer Max-Age is set to zero to avoid unnecessary
caching of OSCORE error responses. Changing this value thus may
cause unnecessary caching. No additional information is carried
with this option.
o Proxy-Uri/Proxy-Scheme. These options are used in CoAP forward
proxy deployments. With OSCORE, the Proxy-Uri option does not
contain the Uri-Path/Uri-Query parts of the URI. The other parts
of Proxy-Uri cannot be protected because forward proxies need to
change them in order to perform their functions. The server can
verify what scheme is used in the last hop, but not what was
requested by the client or what was used in previous hops.
o Uri-Host/Uri-Port. In forward proxy deployments, the Uri-Host/
Uri-Port may be changed by an adversary, and the application needs
to handle the consequences of that (see Section 4.1.3.2). The
Uri-Host may either be omitted, reveal information equivalent to
that of the IP address, or reveal more privacy-sensitive
information, which is discouraged.
o Observe. The Outer Observe option is intended for a proxy to
support forwarding of Observe messages, but it is ignored by the
endpoints since the Inner Observe option determines the processing
in the endpoints. Since the Partial IV provides absolute ordering
of notifications, it is not possible for an intermediary to spoof
reordering (see Section 4.1.3.5). The absence of Partial IV,
since only allowed for the first notification, does not prevent
correct ordering of notifications. The size and distributions of
notifications over time may reveal information about the content
or nature of the notifications. Cancellations (Section 4.1.3.5.1)
are not bound to the corresponding registrations in the same way
responses are bound to requests in OSCORE (see Appendix D.3).
However, that does not make attacks based on mismatched
cancellations possible, since for cancellations to be accepted,
all options in the decrypted message except for ETag options MUST
be the same (see Section 4.1.3.5).
o Block1/Block2/Size1/Size2. The Outer Block options enable
fragmentation of OSCORE messages in addition to segmentation
performed by the Inner Block options. The presence of these
options indicates a large message being sent, and the message size
can be estimated and used for traffic analysis. Manipulating
these options is a potential denial-of-service attack, e.g.,
injection of alleged Block fragments. The specification of a
maximum size of message, MAX_UNFRAGMENTED_SIZE
(Section 4.1.3.4.2), above which messages will be dropped, is
intended as one measure to mitigate this kind of attack.
o No-Response. The Outer No-Response option is used to support
proxy functionality, specifically to avoid error transmissions
from proxies to clients, and to avoid bandwidth reduction to
servers by proxies applying congestion control when not receiving
responses. Modifying or introducing this option is a potential
denial-of-service attack against the proxy operations, but since
the option has an Inner value, its use can be securely agreed upon
between the endpoints. The presence of this option is not
expected to reveal any sensitive information about the message
exchange.
o OSCORE. The OSCORE option contains information about the
compressed COSE header. Changing this field may cause OSCORE
verification to fail.
D.5.3. Error and Signaling Messages
Error messages occurring during CoAP processing are protected end-to-
end. Error messages occurring during OSCORE processing are not
always possible to protect, e.g., if the receiving endpoint cannot
locate the right security context. For this setting, unprotected
error messages are allowed as specified to prevent extensive
retransmissions. Those error messages can be spoofed or manipulated,
which is a potential denial-of-service attack.
This document specifies OPTIONAL error codes and specific diagnostic
payloads for OSCORE processing error messages. Such messages might
reveal information about how many and which security contexts exist
on the server. Servers MAY want to omit the diagnostic payload of
error messages, use the same error code for all errors, or avoid
responding altogether in case of OSCORE processing errors, if that is
a security concern for the application. Moreover, clients MUST NOT
rely on the error code or the diagnostic payload to trigger specific
actions, as these errors are unprotected and can be spoofed or
manipulated.
Signaling messages used in CoAP over TCP [RFC8323] are intended to be
hop-by-hop; spoofing signaling messages can be used as a denial-of-
service attack of a TCP connection.
D.5.4. HTTP Message Fields
In contrast to CoAP, where OSCORE does not protect header fields to
enable CoAP-CoAP proxy operations, the use of OSCORE with HTTP is
restricted to transporting a protected CoAP message over an HTTP hop.
Any unprotected HTTP message fields may reveal information about the
transport of the OSCORE message and enable various denial-of-service
attacks. It is RECOMMENDED to additionally use TLS [RFC8446] for
HTTP hops, which enables encryption and integrity protection of
headers, but still leaves some information for traffic analysis.
Appendix E. CDDL Summary
Data structure definitions in the present specification employ the
CDDL language for conciseness and precision [RFC8610]. This appendix
summarizes the small subset of CDDL that is used in the present
specification.
Within the subset being used here, a CDDL rule is of the form "name =
type", where "name" is the name given to the "type". A "type" can be
one of:
o a reference to another named type, by giving its name. The
predefined named types used in the present specification are as
follows: "uint", an unsigned integer (as represented in CBOR by
major type 0); "int", an unsigned or negative integer (as
represented in CBOR by major type 0 or 1); "bstr", a byte string
(as represented in CBOR by major type 2); "tstr", a text string
(as represented in CBOR by major type 3);
o a choice between two types, by giving both types separated by a
"/";
o an array type (as represented in CBOR by major type 4), where the
sequence of elements of the array is described by giving a
sequence of entries separated by commas ",", and this sequence is
enclosed by square brackets "[" and "]". Arrays described by an
array description contain elements that correspond one-to-one to
the sequence of entries given. Each entry of an array description
is of the form "name : type", where "name" is the name given to
the entry and "type" is the type of the array element
corresponding to this entry.
Acknowledgments
The following individuals provided input to this document: Christian
Amsuess, Tobias Andersson, Carsten Bormann, Joakim Brorsson, Ben
Campbell, Esko Dijk, Jaro Fietz, Thomas Fossati, Martin Gunnarsson,
Klaus Hartke, Rikard Hoeglund, Mirja Kuehlewind, Kathleen Moriarty,
Eric Rescorla, Michael Richardson, Adam Roach, Jim Schaad, Peter van
der Stok, Dave Thaler, Martin Thomson, Marco Tiloca, William Vignat,
and Malisa Vucinic.
Ludwig Seitz and Goeran Selander worked on this document as part of
the CelticPlus project CyberWI, with funding from Vinnova. Ludwig
Seitz had additional funding from the SSF project SEC4Factory under
the grant RIT17-0032.
Authors' Addresses
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
John Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
Francesca Palombini
Ericsson AB
Email: francesca.palombini@ericsson.com
Ludwig Seitz
RISE
Email: ludwig.seitz@ri.se