ACE
Internet Engineering Task Force (IETF) M. Sahni, Ed.
Internet-Draft
Request for Comments: 9482 S. Tripathi, Ed.
Intended status:
Category: Standards Track Palo Alto Networks
Expires: 16 November
ISSN: 2070-1721 October 2023 15 May 2023
CoAP
Constrained Application Protocol (CoAP) Transfer for the Certificate
Management Protocol
draft-ietf-ace-cmpv2-coap-transport-10
Abstract
This document specifies the use of the Constrained Application
Protocol (CoAP) as a transfer mechanism for the Certificate
Management Protocol (CMP). CMP defines the interaction between
various PKI entities for the purpose of certificate creation and
management. CoAP is an HTTP-like client-server protocol used by
various constrained devices in the IoT Internet of Things space.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents an Internet Standards Track document.
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(IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum 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 six months this document, any errata,
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This Internet-Draft will expire on 16 November 2023.
https://www.rfc-editor.org/info/rfc9482.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 Requirements Language
2. CoAP Transfer Mechanism for CMP . . . . . . . . . . . . . . . 3
2.1. CoAP URI Format . . . . . . . . . . . . . . . . . . . . . 3
2.2. Discovery of CMP RA/CA . . . . . . . . . . . . . . . . . 4
2.3. CoAP Request Format . . . . . . . . . . . . . . . . . . . 4
2.4. CoAP Block-Wise Transfer Mode . . . . . . . . . . . . . . 4
2.5. Multicast CoAP . . . . . . . . . . . . . . . . . . . . . 5
2.6. Announcement PKIMessage . . . . . . . . . . . . . . . . . 5
3. Proxy Support . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1.
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2.
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The Certificate Management Protocol (CMP) [RFC4210] is used by the
PKI entities for the generation and management of certificates. One
of the requirements of Certificate Management Protocol CMP is to be independent of the transport
protocol in use. CMP has mechanisms to take care of required
transactions, error reporting reporting, and protection of messages.
The Constrained Application Protocol (CoAP) defined in [RFC7252],
[RFC7959]
[RFC7959], and [RFC8323] is a client-server protocol like HTTP. It
is designed to be used by constrained devices over constrained
networks. The recommended transport for CoAP is UDP, however UDP; however,
[RFC8323] specifies the support of CoAP over TCP, TLS TLS, and Websockets.
WebSockets.
This document specifies the use of CoAP over UDP as a transport
medium for the CMP version 2 [RFC4210], CMP version 3
[I-D.ietf-lamps-cmp-updates] designated [RFC9480]
(designated as CMP in this document document), and the Lightweight CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile].
This document, in
[RFC9483]. In general, this document follows the HTTP transfer for
CMP specifications defined in [RFC6712] and specifies the
requirements for using CoAP as a transfer mechanism for the CMP.
This document also provides guidance on how to use a "CoAP-to-HTTP"
proxy to ease adoption of a CoAP transfer mechanism by enabling the
interconnection with existing PKI entities already providing CMP over
HTTP.
1.1. Terminology Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY",and "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.
2. CoAP Transfer Mechanism for CMP
A CMP transaction consists of exchanging PKIMessages [RFC4210]
between PKI End Entities end entities (EEs), Registration Authorities registration authorities (RAs), and
Certification Authorities
certification authorities (CAs). If the EEs are constrained devices devices,
then they may prefer, as a CMP client, the use of CoAP instead of
HTTP as the transfer mechanism. The In general, the RAs and CAs, in general, CAs are not
constrained and can support both CoAP and HTTP Client client and Server server
implementations. This section specifies how to use CoAP as the
transfer mechanism for the Certificate Management Protocol. CMP.
2.1. CoAP URI Format
The CoAP URI format is described in section Section 6 of [RFC7252]. The CoAP
endpoints MUST support use of the path prefix "/.well-known/" as
defined in [RFC8615] and the registered name "cmp" to help with
endpoint discovery and interoperability. Optional path segments MAY
be added after the registered application name (i.e. (i.e., after "/.well-
known/cmp") to provide distinction. The path segment 'p' followed by
an arbitraryLabel <name> could could, for example example, support the
differentiation of specific CAs or certificate profiles. Further
path segments, e.g., for example, as specified in the Lightweight CMP Profile [I-
D.ietf-lamps-lightweight-cmp-profile],
[RFC9483], could indicate PKI management operations using an
operationLabel <operation>. A valid full CMP URI can look like this:
coap://www.example.com/.well-known/cmp
coap://www.example.com/.well-known/cmp/<operation>
coap://www.example.com/.well-known/cmp/p/<profileLabel>
coap://www.example.com/.well-known/cmp/p/<profileLabel>/<operation>
2.2. Discovery of CMP RA/CA
The EEs can be configured with enough information to form the CMP
server URI. The minimum information that can be configured is the
scheme i.e.
scheme, i.e., "coap:" or "coaps:" "coaps:", and the authority portion of the
URI,
e.g. e.g., "example.com:5683". If the port number is not specified
in the authority, then the default ports port numbers MUST be assumed for
the "coap:" and the "coaps:" scheme URIs. The default port for coap: "coap:"
scheme URIs is 5683 and the default port for coaps: "coaps:" scheme URIs is
5684 [RFC7252].
Optionally, in the environments where a Local Registration Authority
(LRA) RA or a Local CA is deployed,
EEs can also use the CoAP service discovery mechanism [RFC7252] to
discover the URI of the Local RA or CA. The CoAP CMP endpoints
supporting service discovery MUST also support resource discovery in
the CoRE Constrained RESTful Environments (CoRE) Link Format Format, as described
in [RFC6690]. The Link link MUST include the 'ct' attribute defined in
section
Section 7.2.1 of [RFC7252] with the value of "application/pkixcmp" "application/pkixcmp",
as defined in the CoAP Content-Formats "CoAP Content-Formats" IANA registry.
2.3. CoAP Request Format
The CMP PKIMessages MUST be DER encoded and sent as the body of the
CoAP POST request. A CMP client MUST send each CoAP requests request marked
as a Confirmable message [RFC7252]. If the CoAP request is
successful
successful, then the CMP RA or CA MUST return a Success 2.xx response
code otherwise
code; otherwise, the CMP RA or CA MUST return an appropriate Client
Error 4.xx or Server Error 5.xx response code. A CMP RA or CA may
choose to send a Piggybacked piggybacked response [RFC7252] to the client client, or it
MAY send a Separate separate response [RFC7252] in case it takes some time for CA or
the RA or CA to process the CMP transaction.
When transferring CMP PKIMesssage PKIMessage over CoAP CoAP, the content-format
"application/pkixcmp" MUST be used.
2.4. CoAP Block-Wise Transfer Mode
A CMP PKIMesssage PKIMessage consists of a header, body, protection, and
extraCerts structures structure, which may contain many optional and potentially
large fields. Thus, a CMP message can be much larger than the
Maximum Transmission Unit (MTU) of the outgoing interface of the
device. The EEs and RAs or CAs, CAs MUST use the Block-Wise block-wise transfer mode
[RFC7959] to transfer such large messages instead of relying on IP
fragmentation.
If a CoAP-to-HTTP proxy is in the path between EEs and CA an RA or EEs
and
RA then, a CA and if the server supports, then it MUST use the chunked
transfer encoding [RFC9112] to send data over the HTTP transport.
The proxy MUST try to reduce the number of packets sent by using an
optimal chunk length for the HTTP transport.
2.5. Multicast CoAP
CMP PKIMessages sent over CoAP MUST NOT use a Multicast destination
address.
2.6. Announcement PKIMessage
A CMP server may publish announcements, announcements that can be event triggered by an
event or periodic, periodicly for the other PKI entities. Here is the list of
CMP announcement messages prefixed by their respective ASN.1
identifier
(section (see Section 5.1.2 [RFC4210]) of [RFC4210]):
[15] CA Key Update Announcement
[16] Certificate Announcement
[17] Revocation Announcement
[18] CRL Announcement
An EE MAY use the CoAP Observe option Option [RFC7641] to register itself to
get any announcement messages from the RA or CA. The EE can send a
GET request to the server's URI suffixed by "/ann". For example example, a
path to register for announcement messages may look like this:
coap://www.example.com/.well-known/cmp/ann
coap://www.example.com/.well-known/cmp/p/<profileLabel>/ann
If the server supports CMP Announcements announcement messages, then it MUST send
an appropriate Success 2.xx response code, otherwise code; otherwise, it MUST send an
appropriate Client Error 4.xx or Server Error 5.xx response code. If
for some reason the server cannot add the client to its list of
observers for the announcements, it can omit the Observe option Option
[RFC7641] in the response to the client. A client on Upon receiving a Success
2.xx success response without the Observe option [RFC7641] MAY try Option [RFC7641], after some time time,
a client MAY try to register again for announcements from the CMP
server. Since a server can remove the EE from the list of observers
for announcement messages, an EE SHOULD periodically re-register reregister
itself for announcement messages.
Alternatively, an EE MAY periodically poll for the current status of
the CA via the "PKI Information Request" message, message; see section Section 6.5 of
[RFC4210]. If supported, EEs MAY also use "Support Messages" "support messages" defined
in section Section 4.3 of Lightweight CMP Profile
[I-D.ietf-lamps-lightweight-cmp-profile] [RFC9483] to get
information about the CA status. These mechanisms will help
constrained devices, devices that are acting as EEs, EEs to conserve resources by
eliminating the need to create an endpoint for receiving
notifications from the RA or CA. It will also simplify the
implementation of a CoAP-to-HTTP proxy.
3. Proxy Support
This section provides guidance on using a CoAP-to-HTTP proxy between
EEs and RAs or CAs in order to avoid changes to the existing PKI
implementation.
Since the CMP payload is the same over CoAP and HTTP transfer
mechanisms, a CoAP-to-HTTP cross-protocol proxy can be implemented
based on
section Section 10 of [RFC7252]. The CoAP-to-HTTP proxy can either
be located closer to the EEs or closer to the RA or CA. The proxy
MAY support service discovery and resource discovery discovery, as described in
section
Section 2.2. The CoAP-to-HTTP proxy MUST function as a reverse
proxy, only permitting connections to a limited set of pre-configured preconfigured
servers. It is out of scope of this document to specify how a
reverse proxy can route CoAP client requests to one of the configured
servers. Some recommended mechanisms are as follows:
* Use the Uri-Path option to identify a server.
* Use separate hostnames for each of the configured servers and then
use the Uri-Host option for routing the CoAP requests.
* Use separate hostnames for each of the configured servers and then
use Server Name Indication [RFC8446] in case of the "coaps://"
scheme for routing CoAP requests.
4. Security Considerations
* If PKIProtection is used, the PKIHeader and PKIBody of the CMP
protocol are
cryptographically protected against malicious modifications. As
such, UDP can be used without compromising the security of the CMP protocol.
CMP. Security Considerations considerations for CoAP are defined in [RFC7252].
* The CMP protocol does not provide confidentiality of the CMP payloads. If
confidentiality is desired, CoAP over DTLS [RFC9147] SHOULD be
used to provide confidentiality for the CMP payloads,
although payloads; although, it
cannot conceal that the CMP protocol is used within the DTLS layer.
* Section 9.1 of [RFC7252] defines how to use DTLS [RFC9147] for
securing the CoAP. DTLS [RFC9147] associations SHOULD be kept alive
and re-used reused where possible to amortize on the additional overhead
of DTLS on constrained devices.
* An EE might not witness all of the Announcement announcement messages when
using the CoAP Observe option Option [RFC7641], since the Observe option Option
is a "best-effort" approach and the server might lose its state
for subscribers to its announcement messages. The EEs may use an
alternate method described in section Section 2.6 to obtain time critical
changes
changes, such as CRL Certificate Revocation List (CRL) [RFC5280]
updates.
* Implementations SHOULD use the available datagram size and avoid
sending small datagrams containing partial CMP PKIMessage data in
order to reduce memory usage for packet buffering.
* A CoAP-to-HTTP proxy can also protect the PKI entities by handling
UDP and CoAP messages. The proxy can mitigate attacks attacks, like denial
of service
denial-of-service attacks, replay attacks attacks, and resource-exhaustion attacks
attacks, by enforcing basic checks checks, like validating that the ASN.1
syntax is compliant to CMP messages and validating the PKIMessage
protection before sending them to PKI entities.
* Since the Proxy proxy may have access to the CMP-Level CMP-level metadata and
control over the flow of CMP messages therefore messages, proper role based role-based access
control should be in place. The proxy can be deployed at the edge
of the "End Entities" "end entities" network or in front of an RA and CA to
protect them. The However, the proxy however may itself be vulnerable to
resource-exhaustion attacks as it's required to buffer the CMP
messages received over CoAP transport before sending it to the
HTTP endpoint. This can be mitigated by using short timers for
discarding the buffered messages and rate limiting clients based
on the resource usage.
5. IANA Considerations
This document adds a new entry to the CoAP Content-Formats
IANA
Registry (https://www.iana.org/assignments/core-parameters/core-
parameters.xhtml#content-formats) for has registered "application/pkixcmp" (ID 259) in the code of content-type
"application/pkixcmp", for transferring "CoAP
Content-Formats" registry <https://www.iana.org/assignments/core-
parameters> to transfer CMP transactions over CoAP,
from the identifier range 256-9999 reserved for IETF specifications. CoAP.
Type name: application
Subtype name: pkixcmp
Encoding: Content may contain arbitrary octet values. The octet
values are the ASN.1 DER encoding of a PKI message, as defined in the
[RFC4210] specifications.
Reference: This document and RFC 9482 [RFC4210]
This document
IANA has also adds registered a new path segment "ann" to in the CMP "CMP Well-
Known URI Path Segments (https://www.iana.org/assignments/cmp/
cmp.xhtml#cmp-well-known-uri) IANA Segments" registry <https://www.iana.org/assignments/
cmp> for the EEs to register themselves for the announcement
messages.
Path Segment: ann
Description: The path to send a GET request with the CoAP Observer Observe
Option to register for CMP announcement messages.
Reference: This document.
This RFC 9482
IANA has added this document references as a reference for the cmp, "cmp" entry in
the Well-Known URIs
(https://www.iana.org/assignments/well-known-uris/well-known-
uris.xhtml) "Well-Known URIs" registry <https://www.iana.org/assignments/
well-known-uris>.
IANA registry. Please add a reference of has also added this document
to the Well-Known URIs (https://www.iana.org/assignments/well-known-
uris/well-known-uris.xhtml) IANA registry as a reference for that entry.
This document also refers the path segment "p" entry in
the CMP "CMP Well-Known URI Path Segments (https://www.iana.org/assignments/cmp/
cmp.xhtml#cmp-well-known-uri) IANA registry. Please add a reference
of this document to the CMP Well-Known URI Path Segments
(https://www.iana.org/assignments/cmp/cmp.xhtml#cmp-well-known-uri)
for that path segment.
[Note RFC Editor]: This document should be published together or
after the CMP version 3 [I-D.ietf-lamps-cmp-updates] as it references
IANA entries created by that Internet draft. Segments" registry
<https://www.iana.org/assignments/cmp/>.
6. Acknowledgments
The authors would like to thank Hendrik Brockhaus, David von Oheimb,
and Andreas Kretschmer for their guidance in writing the content of
this document and providing valuable feedback.
7. References
7.1.
6.1. 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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Words", BCP 14, Infrastructure Certificate
Management Protocol (CMP)", RFC 8174, 4210,
DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. 10.17487/RFC4210, September 2005,
<https://www.rfc-editor.org/info/rfc4210>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6712] Kause, T. and M. Peylo, "Internet X.509 Public Key
Infrastructure -- HTTP Transfer for the Certificate
Management Protocol (CMP)", RFC 6712,
DOI 10.17487/RFC6712, September 2012,
<https://www.rfc-editor.org/info/rfc6712>.
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210,
DOI 10.17487/RFC4210, September 2005,
<https://www.rfc-editor.org/info/rfc4210>.
[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>.
[I-D.ietf-lamps-cmp-updates]
Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
Management Protocol (CMP) Updates", Work in Progress,
Internet-Draft, draft-ietf-lamps-cmp-updates-23, 29 June
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
lamps-cmp-updates-23>.
[I-D.ietf-lamps-lightweight-cmp-profile]
Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
Certificate Management Protocol (CMP) Profile", Work
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in
Progress, Internet-Draft, draft-ietf-lamps-lightweight-
cmp-profile-21, 17 February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
lightweight-cmp-profile-21>. RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)",
[RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 7641, 9112, DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>. 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/info/rfc9112>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
[RFC9112] Fielding, R., Ed., Nottingham, M.,
[RFC9480] Brockhaus, H., Ed., von Oheimb, D., and J. Reschke,
Ed., "HTTP/1.1", STD 99, Gray,
"Certificate Management Protocol (CMP) Updates", RFC 9112, 9480,
DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/info/rfc9112>.
7.2. 10.17487/RFC9480, October 2023,
<https://www.rfc-editor.org/info/rfc9480>.
[RFC9483] Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
Certificate Management Protocol (CMP) Profile", RFC 9483,
DOI 10.17487/RFC9483, October 2023,
<https://www.rfc-editor.org/info/rfc9483>.
6.2. Informative References
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[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>.
[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>.
Acknowledgements
The authors would like to thank Hendrik Brockhaus, David von Oheimb,
and Andreas Kretschmer for their guidance in writing the content of
this document and providing valuable feedback.
Authors' Addresses
Mohit Sahni (editor)
Palo Alto Networks
3000 Tannery Way
Santa Clara, CA 95054
United States of America
Email: msahni@paloaltonetworks.com
Saurabh Tripathi (editor)
Palo Alto Networks
3000 Tannery Way
Santa Clara, CA 95054
United States of America
Email: stripathi@paloaltonetworks.com