Internet Engineering Task Force (IETF) A. Wiethuechter, Ed.
Request for Comments: 9575 S. Card
Category: Standards Track AX Enterprize, LLC
ISSN: 2070-1721 R. Moskowitz
HTT Consulting
April
June 2024
DRIP Entity Tag (DET) Authentication Formats and Protocols for Broadcast
Remote Identification (RID)
Abstract
The Drone Remote Identification Protocol (DRIP), plus trust policies
and periodic access to registries, augments Unmanned Aircraft System
(UAS) Remote Identification (RID), enabling local real-time
assessment of trustworthiness of received RID messages and observed
UAS, even by Observers lacking Internet access. This document
defines DRIP message types and formats to be sent in Broadcast RID
Authentication Messages to verify that attached and recently detached
messages were signed by the registered owner of the DRIP Entity Tag
(DET) claimed.
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/rfc9575.
Copyright Notice
Copyright (c) 2024 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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. DRIP Entity Tag (DET) Authentication Goals for Broadcast
RID
2. Terminology
2.1. Required Terminology
2.2. Definitions
3. UAS RID Authentication Background and Procedures
3.1. DRIP Authentication Protocol Description
3.1.1. Usage of DNS
3.1.2. Providing UAS RID Trust
3.2. ASTM Authentication Message Framing
3.2.1. Authentication Page
3.2.2. Authentication Payload Field
3.2.3. Specific Authentication Method (SAM) SAM Data Format
3.2.4. ASTM Broadcast RID Constraints
4. DRIP Authentication Formats
4.1. UA Signed UA-Signed Evidence Structure
4.2. DRIP Link
4.3. DRIP Wrapper
4.3.1. Wrapped Count and Format Validation
4.3.2. Wrapper over Extended Transports
4.3.3. Wrapper Limitations
4.4. DRIP Manifest
4.4.1. Hash Count and Format Validation
4.4.2. Manifest Ledger Hashes
4.4.3. Hash Algorithms and Operation
4.5. DRIP Frame
5. Forward Error Correction
5.1. Encoding
5.2. Decoding
5.3. FEC Limitations
6. Requirements and Recommendations
6.1. Legacy Transports
6.2. Extended Transports
6.3. Authentication
6.4. Operational
6.4.1. DRIP Wrapper
6.4.2. UAS RID Trust Assessment
7. Summary of Addressed DRIP Requirements
8. IANA Considerations
8.1. IANA DRIP Registry
9. Security Considerations
9.1. Replay Attacks
9.2. Wrapper vs Manifest
9.3. VNA Timestamp Offsets for DRIP Authentication Formats
9.4. DNS Security in DRIP
10. Acknowledgments
11. References
11.1.
10.1. Normative References
11.2.
10.2. Informative References
Appendix A. Authentication States
A.1. None: Black
A.2. Partial: Gray
A.3. Unsupported: Brown
A.4. Unverifiable: Yellow
A.5. Verified: Green
A.6. Trusted: Blue
A.7. Questionable: Orange
A.8. Unverified: Red
A.9. Conflicting: Purple
Appendix B. Operational Recommendation Analysis
B.1. Page Counts vs Frame Counts
B.1.1. Special Cases
B.2. Full Authentication Example
B.2.1. Raw Example
Acknowledgments
Authors' Addresses
1. Introduction
The initial regulations (e.g., [FAA-14CFR]) and standards (e.g.,
[F3411]) for Unmanned Aircraft Systems (UAS) Remote Identification
(RID) and tracking do not address trust. However, this is a
requirement that needs to be addressed for various different parties
that have a stake in the safe operation of National Airspace Systems
(NAS). Drone Remote ID Protocol's (DRIP's) goal is to specify how
RID can be made trustworthy and available in both Internet and local-
only connected scenarios, especially in emergency situations.
UAS often operate in a volatile environment. A small Unmanned
Aircraft (UA) offers little capacity for computation and
communication. UAS RID must also be accessible with ubiquitous and
inexpensive devices without modification. This limits options. Most
current small UAS are Internet of Things (IoT) devices even if they
are not typically thought of as such. Thus many IoT considerations
apply here. Some DRIP work, currently strongly scoped to UAS RID, is
likely to be applicable to some other IoT use cases.
Generally, two communication schemes for UAS RID are considered:
Broadcast and Network. This document focuses on adding trust to
Broadcast RID (Section 3.2 of [RFC9153] and Section 1.2.2 of
[RFC9434]). As defined in [F3411] and outlined in [RFC9153] and
[RFC9434], Broadcast RID is a one-way Radio Frequency (RF)
transmission of Media Access Control (MAC) layer messages over
Bluetooth or Wi-Fi.
Senders can make any claims the RID message formats allow. Observers
have no standardized means to assess the trustworthiness of message
content, nor verify whether the messages were sent by the UA
identified therein, nor confirm that the UA identified therein is the
one they are visually observing. Indeed, Observers have no way to
detect whether the messages were sent by a UA or spoofed by some
other transmitter (e.g., a laptop or smartphone) anywhere in direct
wireless broadcast range. Authentication is the primary strategy for
mitigating this issue.
1.1. DRIP Entity Tag (DET) Authentication Goals for Broadcast RID
ASTM [F3411] Authentication Messages (Message Type 0x2), when used
with DET-based formats [RFC9374], enable a high level of trust that
the content of other ASTM Messages was generated by their claimed
registered source. These messages are designed to provide the
Observers with trustworthy and immediately actionable information.
Appendix A provides a high-level overview of the various states of
trustworthiness that may be used along with these formats.
This authentication approach also provides some error correction
(Section 5) as mandated by the United States (US) Federal Aviation
Administration (FAA) [FAA-14CFR], which is missing from [F3411] over
Legacy Transports (Bluetooth 4.x).
These DRIP enhancements to ASTM's specification for RID and tracking
[F3411] further support the important use case of Observers who may
be offline at the time of observation.
Section 7 summarizes the DRIP requirements [RFC9153] addressed
herein.
Note: The Endorsement (used in Section 4.2) that proves that a DET is
registered MUST come from its immediate parent in the registration
hierarchy, e.g., a DRIP Identity Management Entity (DIME) [DRIP-REG].
In the definitive hierarchy, the parent of the UA is its HHIT Domain
Authority (HDA), the parent of an HDA is its Registered Assigning
Authority (RAA), etc. It is also assumed that all DRIP-aware
entities use a DET as their identifier during interactions with other
DRIP-aware entities.
2. Terminology
2.1. Required 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.
2.2. Definitions
This document makes use of the terms (CAA, Observer, USS, UTM, etc.)
defined in [RFC9153]. Other terms (such as DIME) are from [RFC9434],
while others (HI, DET, RAA, HDA, etc.) are from [RFC9374].
In addition, the following terms are defined for this document:
Extended Transports: Use of extended advertisements (Bluetooth 5.x),
service info (Wi-Fi Neighbor Awareness Networking (NAN)), or IEEE
802.11 Beacons with the vendor-specific information element as
specified in [F3411]. Must use ASTM Message Pack (Message Type
0xF).
Legacy Transports: Use of broadcast frames (Bluetooth 4.x) as
specified in [F3411].
Manifest: An immutable list of items being transported (in this
specific case over wireless communication).
Observation Session: The period of time during which a given
Observer's receiver is processing (even if only intermittently) a
series of UAS RID messages, at least some of which use DRIP
extensions to [F3411], all nominally from the same UA executing a
single flight operation.
Note: For the remainder of this document, _Broadcast Endorsement:
Parent, Child_ will be abbreviated as _BE: Parent, Child_. For
example, _Broadcast Endorsement: RAA, HDA_ will be abbreviated as
_BE: RAA, HDA_.
3. UAS RID Authentication Background and Procedures
3.1. DRIP Authentication Protocol Description
[F3411] defines Authentication Message framing only. It does not
define authentication formats or methods. It explicitly anticipates
several signature options but does not fully define those. Annex A1
of [F3411] defines a Broadcast Authentication Verifier Service, which
has a heavy reliance on Observer real-time connectivity to the
Internet. Fortunately, [F3411] also allows third-party standard
Authentication Types using the Type 5 0x5 Specific Authentication
Method (SAM), several of which DRIP defines herein.
The standardization of specific formats to support the DRIP
requirements in UAS RID for trustworthy communications over Broadcast
RID is an important part of the chain of trust for a UAS ID. Per
Section 5 of [RFC9434], Authentication formats are needed to relay
information for Observers to determine trust. No existing formats
(defined in [F3411] or other organizations leveraging this feature)
provide functionality to satisfy this goal, resulting in the work
reflected in this document.
3.1.1. Usage of DNS
Like most aviation matters, the overall objectives here are security
and ultimately safety oriented. Since DRIP depends on DNS for some
of its functions, DRIP usage of DNS needs to be protected per best
security practices. Many participating nodes will have limited local
processing power and/or poor, low-bandwidth QoS paths. Appropriate
and feasible security techniques will be highly dependent on the UAS
and Observer situation. Therefore, specification of particular DNS
security options, transports, etc. is outside the scope of this
document (see also Section 9.4).
In DRIP, Observers MUST validate all signatures received. This
requires that the Host Identity (HI) correspond to a DET [RFC9374].
HI's MAY be retrieved from a local cache, if present. The local
cache is pre-configured with well-known HIs (such as those of CAA
DIMEs) and is further populated by received Broadcast Endorsements
(BEs) (Section 3.1.2.1) and DNS lookups (when available).
The Observer MUST perform a DNS query, when connectivity allows, to
obtain a previously unknown HI. If a query cannot be performed, the
message SHOULD be cached by the Observer to be validated once the HI
is obtained.
A more comprehensive specification of DRIP's use of DNS is out of
scope for this document and can be found in [DRIP-REG].
3.1.2. Providing UAS RID Trust
For DRIP, two actions together provide a mechanism for an Observer to
trust in UAS RID using Authentication Messages.
First is the transmission of an entire trust chain via Broadcast
Endorsements (Section 3.1.2.1). This provides a hierarchy of DIMEs
down to and including an individual UA's registration of a claimed
DET and corresponding HI (public key). This alone cannot be trusted
as having any relevance to the observed UA because replay attacks are
trivial.
After an Observer has gathered such a complete key trust chain (from
pre-configured cache entries, Broadcast Endorsements received over
the air and/or DNS lookups) and verified all of its links, that
device can trust that the claimed DET and corresponding public key
are properly registered, but the UA has not yet been proven to
possess the corresponding private key.
It
Second is necessary for the UA to prove possession by dynamically signing data
that is unique and unpredictable but easily verified by the Observer
(Section 3.1.2.2). Verification of this signed data MUST be
performed by the Observer as part of the received UAS RID information
trust assessment (Section 6.4.2).
3.1.2.1. DIME Endorsements of Subordinate DETs
Observers receive DRIP Link Authentication Messages (Section 4.2)
containing Broadcast Endorsements by DIMEs of child DET
registrations. A series of these Endorsements confirms a path
through the hierarchy, defined in [DRIP-REG], from the DET Prefix
Owner all the way to an individual UA DET registration.
Note: For the remainder of this document, Broadcast Endorsement:
Parent, Child will be abbreviated as BE: Parent, Child. For example,
Broadcast Endorsement: RAA, HDA will be abbreviated as BE: RAA, HDA.
3.1.2.2. UA Signed UA-Signed Evidence
To prove possession of the private key associated with the DET, the
UA MUST sign and send data that is unique and unpredictable but
easily validated by the Observer, that is signed over. Observer. The data can be an ASTM Message
that fulfills the requirements to be unpredictable but easily
validated. An Observer receives this UA signed UA-signed Evidence from
DRIP-based DRIP-
based Authentication Messages (Sections 4.3 or 4.4). The Observer
must verify the signature (cryptographically, as specified in
Section 3.1.1) and validate the signed content (via non-cryptographic
means, as specified in Section 6.3).
Whether the content is true is a separate question that DRIP cannot
address, but validation performed using observable and/or out-of-band
data (Section 6) is possible and encouraged.
3.2. ASTM Authentication Message Framing
The Authentication Message (Message Type 0x2) is unique in the ASTM
[F3411] Broadcast standard, as it is the only message that can be
larger than the Legacy Transport size. To address this limitation
around transport size, it is defined as a set of "pages", each of
which fits into a single Legacy Transport frame. For Extended
Transports, pages are still used but they are all in a single frame.
| Informational Note: Message Pack (Message Type 0xF) is also
| larger than the Legacy Transport size but is limited for use
| only on Extended Transports where is it can be supported.
The following subsections are a brief overview of the Authentication
Message format defined in [F3411] for better context on how DRIP
Authentication fills and uses various fields already defined by ASTM
[F3411].
3.2.1. Authentication Page
This document leverages Authentication Type 0x5 (Specific
Authentication Method (SAM)) as the principal authentication
container, defining a set of SAM Types in Section 4. Authentication
Type is encoded in every Authentication Page in the Page Header. _Page Header_.
The SAM Type is defined as a field in the Authentication Payload _Authentication Payload_
(see Section 3.2.3.1). 3.2.3).
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
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| |
| |
| Authentication Payload |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 1: Standard ASTM Authentication Message Page
Page Header:
_Page Header_: (1 octet)
Authentication Type (4 bits) and Page Number (4 bits)
Authentication Payload:
_Authentication Payload_: (23 octets per page)
Authentication Payload, including headers. Null padded. See
Section 3.2.2.
The Authentication Message is structured as a set of pages per
Figure 1. There is a technical maximum of 16 pages (indexed 0 to 15)
that can be sent for a single Authentication Message, with each page
carrying a maximum 23-octet Authentication Payload. _Authentication Payload_. See
Section 3.2.4 for more details. Over Legacy Transports, these
messages are "fragmented", with each page sent in a separate Legacy
Transport frame.
Either as a single Authentication Message or a set of fragmented
Authentication Message Pages, the structure is further wrapped by
outer ASTM framing and the specific link framing.
3.2.2. Authentication Payload Field
Figure 2 is the source data view of the data fields found in the
Authentication Message as defined by [F3411]. This data is placed
into the Authentication Payload _Authentication Payload_ shown in Figure 1, which spans
multiple Authentication Pages. _Authentication Pages_.
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
+---------------+---------------+---------------+---------------+
| Authentication Headers |
| +---------------+---------------+
| | |
+---------------+---------------+ |
. .
. Authentication Data / Signature .
. .
| |
+---------------+---------------+---------------+---------------+
| ADL | |
+---------------+ |
. .
. Additional Data .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 2: ASTM Authentication Message Fields
Authentication Headers:
_Authentication Headers_: (6 octets)
As defined in [F3411].
Authentication
_Authentication Data / Signature: Signature_: (0 to 255 octets)
Opaque authentication data. The length of this payload is known
through a field in the Authentication Headers _Authentication Headers_ (defined in
[F3411]).
Additional
_Additional Data Length (ADL): (ADL)_: (1 octet - unsigned)
Length in octets of Additional Data. _Additional Data_. The value of ADL _ADL_ is
calculated as the minimum of 361 - Authentication Data / Signature
Length and 255. Only present with Additional Data.
Additional Data: (ADL _Additional Data_.
_Additional Data:_ (_ADL_ octets)
Data that follows the Authentication _Authentication Data / Signature Signature_ but is not
considered part of the Authentication Data, _Authentication Data_, and thus is not
covered by a signature. For DRIP, this field is used to carry
Forward Error Correction (FEC) generated by transmitters and
parsed by receivers as defined in Section 5.
3.2.3. Specific Authentication Method (SAM)
3.2.3.1. SAM Data Format
Figure 3 is the general format to hold authentication data when using
SAM and is placed inside the Authentication _Authentication Data / Signature Signature_ field
in Figure 2.
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
+---------------+---------------+---------------+---------------+
| SAM Type | |
+---------------+ |
. .
. SAM Authentication Data .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 3: SAM Data Format
SAM Type:
_SAM Type_: (1 octet)
The following SAM Types are allocated to DRIP:
+==========+=============================+
| SAM Type | Description |
+==========+=============================+
| 0x01 | DRIP Link (Section 4.2) |
+----------+-----------------------------+
| 0x02 | DRIP Wrapper (Section 4.3) |
+----------+-----------------------------+
| 0x03 | DRIP Manifest (Section 4.4) |
+----------+-----------------------------+
| 0x04 | DRIP Frame (Section 4.5) |
+----------+-----------------------------+
Table 1: DRIP SAM Types
| Note: ASTM International is the owner of these code points as
| they are defined in [F3411]. In accordance with Annex 5 of
| [F3411], the International Civil Aviation Organization (ICAO)
| has been selected by ASTM as the registrar to manage
| allocations of these code points. The list is available at
| [ASTM-Remote-ID].
SAM
_SAM Authentication Data: Data_: (0 to 200 octets)
Contains opaque authentication data formatted as defined by the
preceding SAM Type.
3.2.4. ASTM Broadcast RID Constraints
3.2.4.1. Wireless Frame Constraints
A UA has the option to broadcast using Bluetooth (4.x and 5.x), Wi-Fi
NAN, or IEEE 802.11 Beacon; see Section 6. With Bluetooth, FAA and
other Civil Aviation Authorities (CAA) mandate transmitting
simultaneously over both 4.x and 5.x. The same application-layer
information defined in [F3411] MUST be transmitted over all the
physical-layer interfaces performing RID, because Observer transports
may be limited. If an Observer can support multiple transports, it
should be assumed to use (display, report, etc.) the latest data regardless of the
transport
received over. over which that data was received.
Bluetooth 4.x presents a payload-size challenge in that it can only
transmit 25 octets of payload per frame, while other transports can
support larger payloads per frame. However, the As [F3411] message formats are
the same for all media, and their framing dictated by Bluetooth 4.x constraints is inherited by [F3411]
over other media.
It should be noted that was designed to fit within
these legacy constraints, Extended Transports by cannot send larger
messages; instead, the Message Pack format encapsulates multiple
messages (each of which fits within these legacy constraints).
By definition have Error
Correction built in, unlike Extended Transports provide FEC, but Legacy Transports. For Authentication
Messages, this means that Transports
lack FEC. Thus over Legacy Transport pages Transports, paged Authentication Messages
may not
received by Observers resulting suffer the loss of one or more pages. This would result in incomplete messages during
operation, although
delivery to the use Observer application of incomplete (typically
unusable) messages, so DRIP FEC (Section 5) reduces its
likelihood. is specified to enable
recovery of a single lost page and thereby reduce the likelihood of
receiving incompletely reconstructable Authentication Messages.
Authentication Messages sent using Extended Transports do not suffer
this issue, as the full message (all pages) is sent using a single
Message Pack. Furthermore, the use of one-way RF broadcasts
prohibits the use of any congestion-control or loss-
recovery loss-recovery schemes
that require ACKs or NACKs.
3.2.4.2. Paged Authentication Message Constraints
To keep consistent formatting across the different transports (Legacy
and Extended) and their independent restrictions, the authentication
data being sent is REQUIRED to fit within the page limit that the
most constrained existing transport can support. Under Broadcast
RID, the Extended Transport that can hold the least amount of
authentication data is Bluetooth 5.x at 9 pages.
As such, DRIP transmitters are REQUIRED to adhere to the following
when using the Authentication Message:
1. Authentication _Authentication Data / Signature Signature_ data MUST fit in the first 9
pages (Page Numbers 0 through 8).
2. The Length _Length_ field in the Authentication Headers _Authentication Headers_ (which encodes
the length in octets of Authentication _Authentication Data / Signature Signature_ only)
MUST NOT exceed the value of 201. This includes the SAM Type but
excludes Additional Data. _Additional Data_.
3.2.4.3. Timestamps
In ASTM [F3411], timestamps are a Unix-style timestamp with an epoch
of 2019-01-01 00:00:00 UTC. For DRIP, this format is adopted for
Authentication to keep a common time format in Broadcast payloads.
Under DRIP, there are two timestamps defined: Valid Not Before (VNB)
and Valid Not After (VNA).
Valid Not Before (VNB) Timestamp: (4 octets)
Timestamp denoting the recommended time at which to start trusting
data. MUST follow the format defined in [F3411] as described
above. MUST be set no earlier than the time the signature (across
a given structure) is generated.
Valid Not After (VNA) Timestamp: (4 octets)
Timestamp denoting the recommended time at which to stop trusting
data. MUST follow the format defined in [F3411] as described
above. Has an additional offset to push a short time into the
future (relative to VNB) to avoid replay attacks.
The exact offset is not defined in this document. Best practice
for identifying an acceptable offset should be used and should
take into consideration the UA environment, propagation
characteristics of the messages being sent, and clock differences
between the UA and Observers. A For UA signatures in scenarios
typical as of 2024, a reasonable time offset would be to set VNA
approximately 2 minutes after VNB. VNB; see Appendix B for examples
that may aid in tuning this value.
4. DRIP Authentication Formats
All formats defined in this section are contained in the
Authentication
_Authentication Data / Signature Signature_ field in Figure 2 and use the
Specific Authentication Method (SAM, Authentication Type 0x5). The
first octet of the Authentication _Authentication Data / Signature Signature_ of Figure 2 is
used to multiplex among these various formats.
When sending data over a medium that does not have underlying FEC,
for example Legacy Transports, then FEC (per Section 5 5) MUST be used.
Examples of Link, Wrapper, and Manifest are shown as part of an
operational schedule in Appendix B.2.1.
4.1. UA Signed UA-Signed Evidence Structure
The UA Signed _UA-Signed Evidence Structure Structure_ (Figure 4) is used by the UA
during flight to sign over information elements using the private key
associated with the current UA DET. It is encapsulated by the SAM _SAM
Authentication Data Data_ field of Figure 3.
This structure is used by the DRIP Wrapper (Section 4.3), Manifest
Section
(Section 4.4), and Frame (Section 4.5). DRIP Link (Section 4.2) MUST
NOT use it, as it will not fit in the ASTM Authentication Message
with its intended content (i.e., a Broadcast Endorsement).
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
+---------------+---------------+---------------+---------------+
| VNB Timestamp by UA |
+---------------+---------------+---------------+---------------+
| VNA Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
. .
. Evidence .
. .
| |
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 4: Endorsement Structure for UA Signed UA-Signed Evidence
Valid
_Valid Not Before (VNB) Timestamp by UA: UA_: (4 octets)
See Section 3.2.4.3. Set by the UA.
Valid
_Valid Not After (VNA) Timestamp by UA: UA_: (4 octets)
See Section 3.2.4.3. Set by the UA.
Evidence:
_Evidence_: (0 to 112 octets)
The evidence section _Evidence_ field MUST be filled in with data in the form of an
opaque object specified in the DRIP Wrapper (Section 4.3),
Manifest (Section 4.4), or Frame (Section 4.5).
UA
_UA DRIP Entity Tag: Tag_: (16 octets)
This is the current a DET [RFC9374] currently being used by the UA for
authentication; it is assumed to be a Specific Session ID (a type
of UAS ID). ID typically also used by the UA Signature: in the Basic ID Message).
_UA Signature_: (64 octets)
Signature over the concatenation of preceding fields (VNB, VNA,
Evidence, (_VNB_,
_VNA_, _Evidence_, and UA DET) _UA DET_) using the keypair of the UA DET.
The signature algorithm is specified by the Hierarchical Host
Identity Tags (HHIT) Suite ID of the DET.
When using this structure, the UA is minimally self-endorsing its
DET. The HI of the UA DET can be looked up by mechanisms described
in [DRIP-REG] or by extracting it from a Broadcast Endorsement (see
Sections 4.2 and 6.3).
4.2. DRIP Link
This SAM Type (Figure 5) is used to transmit Broadcast Endorsements.
For example, the BE: _BE: HDA, UA UA_ is sent (see Section 6.3) as a DRIP
Link message.
DRIP Link is important as its contents are used to provide trust in
the DET/HI pair that the UA is currently broadcasting. This message
does not require Internet connectivity to perform signature
verification of the contents when the DIME DET/HI is in the
Observer's cache. It also provides the UA HI, when it is filled with
a BE: HDA, UA, so that connectivity is not required when performing
signature verification of other DRIP Authentication Messages.
Various Broadcast Endorsements are sent during each UAS flight
operation to ensure that the full Broadcast Endorsement chain is
available offline. See Section 6.3 for further details.
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
+---------------+---------------+---------------+---------------+
| VNB Timestamp by Parent |
+---------------+---------------+---------------+---------------+
| VNA Timestamp by Parent |
+---------------+---------------+---------------+---------------+
| |
| DET |
| of Child |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| HI of Child |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| |
| DET |
| of Parent |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| Signature by Parent |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 5: Broadcast Endorsement / DRIP Link
VNB
_VNB Timestamp by Parent: Parent_: (4 octets)
See Section 3.2.4.3. Set by Parent Entity.
VNA
_VNA Timestamp by Parent: Parent_: (4 octets)
See Section 3.2.4.3. Set by Parent Entity.
DET
_DET of Child: Child_: (16 octets)
DRIP Entity Tag of Child Entity.
HI
_HI of Child: Child_: (32 octets)
Host Identity of Child Entity.
DET
_DET of Parent: Parent_: (16 octets)
DRIP Entity Tag of Parent Entity in DIME Hierarchy.
Signature
_Signature by Parent: Parent_: (64 octets)
Signature over concatenation of preceding fields (VNB, VNA, DET (_VNB_, _VNA_,
_DET of
Child, HI Child_, _HI of Child, Child_, and DET _DET of Parent) Parent_) using the
keypair of the Parent DET.
This DRIP Authentication Message is used in conjunction with other
DRIP SAM Types (such as the Manifest or the Wrapper) that contain
data (e.g., the ASTM Location/Vector Message, Message Type 0x2) that
is guaranteed to be unique, unpredictable, and easily cross-checked
by the receiving device.
A hash of the final link (BE: HDA on UA) in the Broadcast Endorsement
chain MUST be included in each DRIP Manifest (Section 4.4).
Note: The Endorsement that proves a DET is registered MUST come from
its immediate parent in the registration hierarchy, e.g., a DRIP
Identity Management Entity (DIME) [DRIP-REG]. In the definitive
hierarchy, the parent of the UA is its HHIT Domain Authority (HDA),
the parent of an HDA is its Registered Assigning Authority (RAA),
etc. It is also assumed that all DRIP-aware entities use a DET as
their identifier during interactions with other DRIP-aware entities.
4.3. DRIP Wrapper
This SAM Type is used to wrap and sign over a list of other [F3411]
Broadcast RID messages.
The evidence section _Evidence_ field of the UA Signed _UA-Signed Evidence Structure Structure_
(Section 4.1) is populated with up to four ASTM Messages [F3411] in a
contiguous octet sequence. Only ASTM Message Types 0x0, 0x1, 0x3,
0x4, and 0x5 are allowed and must be in Message Type order as defined
by [F3411]. These messages MUST include the Message Type and
Protocol Version octet and MUST NOT include the Message Counter octet
(thus are fixed at 25 octets in length).
4.3.1. Wrapped Count and Format Validation
When decoding a DRIP Wrapper on a receiver, a calculation of the
number of messages wrapped and a validation MUST be performed by
using the number of octets (defined as wrapperLength) between the VNA
_VNA Timestamp by UA UA_ and the UA DET _UA DET_ as shown in Figure 6.
<CODE BEGINS>
if (wrapperLength MOD 25) != 0 {
return DECODE_FAILURE;
}
wrappedCount = wrapperLength / 25;
if (wrappedCount == 0) {
// DECODE_SUCCESS; treat as DRIP Wrapper over extended transport
}
else if (wrappedCount > 4) {
return DECODE_FAILURE;
} else {
// DECODE_SUCCESS; treat as standard DRIP Wrapper
}
<CODE ENDS>
Figure 6: Pseudocode for Wrapper Validation and Number of
Messages calculation Calculation
4.3.2. Wrapper over Extended Transports
When using Extended Transports, an optimization to DRIP Wrapper can
be made to sign over co-located data in an ASTM Message Pack (Message
Type 0xF).
To perform this optimization, the UA Signed _UA-Signed Evidence Structure Structure_ is
filled with the ASTM Messages to be in the ASTM Message Pack, the
signature is generated, and then the evidence _Evidence_ field is cleared,
leaving the encoded form shown in Figure 7.
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
+---------------+---------------+---------------+---------------+
| VNB Timestamp by UA |
+---------------+---------------+---------------+---------------+
| VNA Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 7: DRIP Wrapper over Extended Transports
To verify the signature, the receiver MUST concatenate all the
messages in the Message Pack (excluding the Authentication Message
found in the same Message Pack) in ASTM Message Type order and set
the evidence section _Evidence_ field of the UA Signed _UA-Signed Evidence Structure Structure_ before
performing signature verification.
The functionality of a Wrapper in this form is equivalent to Message
Set Signature (Authentication Type 0x3) when running over Extended
Transports. The Wrapper provides the same format but over both
Extended and Legacy Transports, which allows the transports to be
similar. Message Set Signature also implies using the ASTM validator
system architecture, which depends on Internet connectivity for
verification that the receiver may not have at the time an
Authentication Message is received. This is something the Wrapper,
and all DRIP Authentication Formats, avoid when the UA key is
obtained via a DRIP Link Authentication Message.
4.3.3. Wrapper Limitations
The primary limitation of the Wrapper is the bounding of up to four
ASTM Messages that can be sent within it. Another limitation is that
the format cannot be used as a surrogate for messages it is wrapping
due to the potential that an Observer on the ground does not support
DRIP. Thus, when a Wrapper is being used, the wrapped data must
effectively be sent twice, once as a single-framed message (as
specified in [F3411]) and again within the Wrapper.
4.4. DRIP Manifest
This SAM Type is used to create message manifests that contain hashes
of previously sent ASTM Messages.
By hashing previously sent messages and signing them, we gain trust
in a UA's previous reports without retransmitting them. This is a
way to evade the limitation of a maximum of four messages in the
Wrapper (Section 4.3.3) and greatly reduce overhead.
Observers MUST hash all received ASTM Messages and cross-check them
against hashes in received Manifests.
Judicious use of a Manifest enables an entire Broadcast RID message
stream to be strongly authenticated with less than 100% overhead
relative to a completely unauthenticated message stream (see
Section 6.3 and Appendix B).
The evidence section _Evidence_ field of the UA Signed _UA-Signed Evidence Structure Structure_
(Section 4.1) is populated with 8-octet hashes of [F3411] Broadcast
RID messages (up to 11) and three special hashes (Section 4.4.2).
All of these hashes MUST be concatenated to form a contiguous octet
sequence in the evidence section. _Evidence_ field. It is RECOMMENDED that the maximum
number of ASTM Message Hashes used be 10 (see Appendix B.1.1.2).
The Previous _Previous Manifest Hash, Current Hash_, _Current Manifest Hash, Hash_, and DRIP _DRIP Link
(BE: HDA, UA) Hash Hash_ MUST always come before the ASTM _ASTM Message Hashes Hashes_
as seen in Figure 8.
An Observer MUST use the Manifest to verify each ASTM Message hashed
therein that it has previously received. It can do this without
having received them all. A Manifest SHOULD typically encompass a
single transmission cycle of messages being sent; see Section 6.4 and
Appendix B.
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
+---------------+---------------+---------------+---------------+
| Previous Manifest |
| Hash |
+---------------+---------------+---------------+---------------+
| Current Manifest |
| Hash |
+---------------+---------------+---------------+---------------+
| DRIP Link (BE: HDA, UA) |
| Hash |
+---------------+---------------+---------------+---------------+
| |
. .
. ASTM Message Hashes .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 8: DRIP Manifest Evidence Structure
Previous
_Previous Manifest Hash: Hash_: (8 octets)
Hash of the previously sent Manifest Message.
Current
_Current Manifest Hash: Hash_: (8 octets)
Hash of the current Manifest Message.
DRIP
_DRIP Link (BE: HDA, UA): UA)_: (8 octets)
Hash of the DRIP Link Authentication Message carrying BE: HDA, UA
(see Section 4.2).
ASTM
_ASTM Message Hash: Hash_: (8 octets)
Hash of a single full ASTM Message using hash operations described
in Section 4.4.3.
4.4.1. Hash Count and Format Validation
When decoding a DRIP Manifest on a receiver, a calculation of the
number of hashes and a validation can be performed by using the
number of octets between the UA DET _UA DET_ and the VNB _VNB Timestamp by UA UA_
(defined as manifestLength) such as shown in Figure 9.
<CODE BEGINS>
if (manifestLength MOD 8) != 0 {
return DECODE_FAILURE
}
hashCount = (manifestLength / 8) - 3;
<CODE ENDS>
Figure 9: Pseudocode for Manifest Sanity Check and Number of
Hashes Calculation
4.4.2. Manifest Ledger Hashes
Three
The following three special hashes are included in all Manifests. The Previous Manifests:
* the _Previous Manifest Hash, Hash_ links to the previous Manifest, and Manifest.
* the Current _Current Manifest Hash Hash_ is of the Manifest in which it
appears. These two
* the _DRIP Link (BE: HDA, UA) Hash_ ties the endorsed UA key to the
Manifest chain.
The Previous and Current hashes act as a ledger of provenance to for the
Manifest that could chain, which should be traced back if the Observer was present and UA
were within Broadcast RID wireless range of each other for an
extended periods period of time.
The DRIP _DRIP Link (BE: HDA, UA) UA)_ is included so there is a direct
signature by the UA over the Broadcast Endorsement (see Section 4.2).
Typical operation would expect that the list of ASTM _ASTM Message Hashes Hashes_
contain nonce-like data. To enforce a binding between the BE: HDA,
UA and avoid trivial replay attack vectors (see Section 9.1), at
least one ASTM _ASTM Message Hash Hash_ MUST be from an [F3411] message that
satisfies the fourth requirement in Section 6.3. At least once per
Observation Session, the Observer must process that message as
specified in Section 6.3.
4.4.3. Hash Algorithms and Operation
The hash algorithm used for the Manifest is the same hash algorithm
used in creation of the DET [RFC9374] that is signing the Manifest.
This is encoded as part of the DET using the HHIT Suite ID.
DETs that use cSHAKE128 [NIST.SP.800-185] compute the hash as
follows:
cSHAKE128(ASTM Message, 64, "", "Remote ID Auth Hash")
For ORCHID Generation Algorithms (OGAs) other than "5" (EdDSA/
cSHAKE128) [RFC9374], use the construct appropriate for the
associated hash. For example, the hash for "2" (ECDSA/SHA-384) is
computed as follows:
Ltrunc( SHA-384( ASTM Message | "Remote ID Auth Hash" ), 8 )
When building the list of hashes, the Previous Manifest Hash is known
from the previous Manifest. For the first built a Manifest, this value process MUST be followed:
1. The _Previous Manifest Hash_
a. is filled with a random nonce. nonce if and only if this is the
first manifest being generated;
b. otherwise, it contains the previous manifest's _Current
Manifest Hash_.
2. The Current _Current Manifest Hash Hash_ is null filled while ASTM Messages are hashed and fill the ASTM Messaged
Hashes section. When all messages with null.
3. _ASTM Message Hashes_ are hashed, the Current Manifest
Hash filled per Section 4.4.3.1 or
Section 4.4.3.2.
4. A hash, as defined above in this section, is computed calculated over the Previous
_Previous Manifest Hash, Current Hash_, _Current Manifest
Hash Hash_ (null filled),
and ASTM _ASTM Message Hashes. This hash value
replaces the null-filled Current Manifest Hash and becomes the
Previous Hashes_.
5. The _Current Manifest Hash for Hash_ (null filled) is replaced with the next Manifest.
hash generated in Step r.
4.4.3.1. Legacy Transport Hashing
Under this transport, DRIP hashes the full ASTM Message being sent
over the Bluetooth Advertising frame. This is the 25-octet object
that starts with the Message Type and Protocol Version octet along
with the 24 octets of message data. The hash MUST NOT include the
Message Counter octet.
For paged ASTM Messages (currently only Authentication Messages), all
of the pages are concatenated together in Page Number order and
hashed as one object.
4.4.3.2. Extended Transport Hashing
Under this transport, DRIP hashes the full ASTM Message Pack (Message
Type 0xF) regardless of its content. The hash MUST NOT include the
Message Counter octet.
4.5. DRIP Frame
This SAM Type is defined to enable the use of Section 4.1 the _UA-Signed Evidence
Structure_ (Section 4.1) in the future beyond the previously defined
formats (Wrapper and Manifest) by the inclusion of a single octet to
signal the format of evidence _Evidence_ data (up to 111 octets).
The content format of Frame _Frame Evidence Data Data_ is not defined in this
document. Other specifications MUST define the contents and register
for a Frame Type. _Frame Type_. At the time of publication, publication (2024), there are no
defined Frame Types other than Types; only an Experimental range. range has been defined.
Observers MUST check the signature of the structure (Section 4.1) per
Section 3.1.2.2 and MAY, if the specification of Frame Type _Frame Type_ is
known, parse the content in Frame _Frame Evidence Data. Data_.
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
+---------------+---------------+---------------+---------------+
| Frame Type | |
+---------------+ .
. Frame Evidence Data .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 10: DRIP Frame
Frame Type:
_Frame Type_: (1 octet)
Byte to subtype for future different DRIP Frame formats. It takes
the first octet
As shown in Figure 10, leaving the _Frame Type_ takes the first octet,
which leaves 111 octets available for
Frame _Frame Evidence Data. Data_. See
Section 8.1 for Frame Type allocations.
5. Forward Error Correction
For Broadcast RID, FEC is provided by the lower layers in Extended
Transports. The Bluetooth 4.x Legacy Transport does not support FEC,
so the following application-level scheme is used with DRIP
Authentication to add some FEC. When sending data over a medium that
does not have underlying FEC, for example Bluetooth 4.x, this section
MUST be used.
The Bluetooth 4.x lower layers have error detection but not
correction. Any frame in which Bluetooth detects an error is dropped
and not delivered to higher layers (in our case, DRIP). Thus it can
be treated as an erasure.
DRIP standardizes a single page FEC scheme using XOR parity across
all page data of an Authentication Message. This allows the
correction of a single erased page in an Authentication Message. If
more than a single page is missing, then handling of an incomplete
Authentication Message is determined by higher layers.
Other FEC schemes, to protect more than a single page of an
Authentication Message or multiple [F3411] Messages, are left for
future standardization if operational experience proves it necessary
and/or practical.
The data added during FEC is not included in the Authentication _Authentication Data
/ Signature, Signature_, but instead in the Additional Data _Additional Data_ field of Figure 2.
This may cause the Authentication Message to exceed 9 pages, up to a
maximum of 16 pages.
5.1. Encoding
When encoding, two things are REQUIRED:
1. The FEC data MUST start on a new Authentication Page. To do
this, the results of parity encoding MUST be placed in the
Additional Data
_Additional Data_ field of Figure 2 with null padding before it
to line up with the next page. The Additional _Additional Data Length Length_
field MUST be set to number of padding octets + number of parity
octets.
2. The Last _Last Page Index Index_ field (in Page 0) MUST be incremented from
what it would have been without FEC by the number of pages
required for the Additional _Additional Data Length Length_ field, null padding,
and FEC.
To generate the parity, a simple XOR operation using the previous
parity page and current page is used. Only the 23-octet
Authentication Payload
_Authentication Payload_ field of Figure 1 is used in the XOR
operations. For Page 0, a 23-octet null pad is used for the previous
parity page.
Figure 11 shows an example of the last two pages (out of N) of an
Authentication Message using DRIP Single Page FEC. The Additional _Additional
Data Length Length_ is set to 33, as there are always 23 octets of FEC data
and there are 10 octets of padding in this example to line it up into
Page N.
Page N-1:
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
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| Authentication Data / Signature |
| |
| +---------------+---------------+---------------+
| | ADL=33 | |
+---------------+---------------+ |
| Null Padding |
| |
+---------------+---------------+---------------+---------------+
Page N:
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
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| |
| Forward Error Correction |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 11: Example Single Page FEC Encoding
5.2. Decoding
Frame decoding is independent of the transmit media. However, the
decoding process can determine from the first Authentication page Page
that there may be a Bluetooth 4.x FEC page at the end. The decoding
process MUST test for the presence of FEC and apply it as follows.
To determine if FEC has been used, a check of the Last _Last Page Index Index_
is performed. In general, if the Last _Last Page Index Index_ field is one
greater than that necessary to hold Length _Length_ octets of Authentication
Data, then FEC has been used. Note that if Length _Length_ octets are
exhausted exactly at the end of an Authentication Page, the Additional
_Additional Data
Length Length_ field will occupy the first octet of the
following page. The remainder of this page will be null padded under
DRIP to align the FEC to its own page. In this case, the Last _Last Page Index
Index_ will have been incremented once for initializing the Additional
_Additional Data Length Length_ field and once for the FEC page, for a total
of two additional pages, as in the last row of Table 5.
To decode FEC in DRIP, a rolling XOR is used on each Authentication
Page _Authentication
Page_ received in the current Authentication Message. A Message
Counter, outside of the ASTM Message but specified in [F3411], is
used to signal a different Authentication Message and to correlate
pages to messages. This Message Counter is only a single octet in
length, so it will roll over (to 0x00) after reaching its maximum
value (0xFF). If only a single page is missing in the Authentication
Message the resulting parity octets should be the data of the erased
page.
Authentication Page 0 contains various important fields, only located
on that page, that help decode the full ASTM Authentication Message.
If Page 0 has been reconstructed, the Last _Last Page Index Index_ and Length _Length_
fields MUST be validated by DRIP. The pseudocode in Figure 12 can be
used for both checks.
<CODE BEGINS>
function decode_check(auth_pages[], decoded_lpi, decoded_length) {
// check decoded_lpi does not exceed maximum value
if (decoded_lpi >= 16) {
return DECODE_FAILURE
}
// check that decoded length does not exceed DRIP maximum value
if (decoded_length > 201) {
return DECODE_FAILURE
}
// grab the page at index where length ends and extract its data
auth_data = auth_pages[(decoded_length - 17) / 23].data
// find the index of last auth byte
last_auth_byte = (17 + (23 * last_auth_page)) - decoded_length
// look for non-nulls after the last auth byte
if (auth_data[(last_auth_byte + 2):] has non-nulls) {
return DECODE_FAILURE
}
// check that byte directly after last auth byte is null
if (auth_data[last_auth_byte + 1] equals null) {
return DECODE_FAILURE
}
// we set our presumed Additional Data Length (ADL)
presumed_adl = auth_data[last_auth_byte + 1]
// use the presumed ADL to calculate a presumed LPI
//Last Page Index (LPI, a field defined in [F3411])
presumed_lpi = (presumed_adl + decoded_length - 17) / 23
// check that presumed LPI and decoded LPI match
if (presumed_lpi not equal decoded_lpi) {
return DECODE_FAILURE
}
return DECODE_SUCCESS
}
<CODE ENDS>
Figure 12: Pseudocode for Decode Checks
5.3. FEC Limitations
The worst-case scenario is when the Authentication _Authentication Data / Signature Signature_
ends perfectly on a page boundary (Page N-1). This means the
Additional
_Additional Data Length Length_ would start the next page (Page N) and have
22 octets worth of null padding to align the FEC to begin at the
start of the next page (Page N+1). In this scenario, an entire page
(Page N) is being wasted just to carry the Additional _Additional Data Length. Length_.
6. Requirements and Recommendations
6.1. Legacy Transports
Under DRIP, the goal is to bring reliable receipt of the paged
Authentication Message using Legacy Transports. FEC (Section 5) MUST
be used, per mandated RID rules (for example, the US FAA RID Rules
[FAA-14CFR]), when using Legacy Transports (such as Bluetooth 4.x).
Under [F3411], Authentication Messages are transmitted at the static
rate (at least every 3 seconds). Any DRIP Authentication Messages
containing dynamic data (such as the DRIP Wrapper) MAY be sent at the
dynamic rate (at least every 1 second).
6.2. Extended Transports
Under the ASTM specification, Extended Transports of RID must use the
Message Pack (Message Type 0xF) format for all transmissions. Under
Message Pack, ASTM Messages are sent together (in Message Type order)
in a single frame (up to 9 single-frame equivalent messages under
Legacy Transports). Message Packs are required by [F3411] to be sent
at a rate of 1 per second (like dynamic messages).
Message Packs are sent only over Extended Transports that provide
FEC. Thus, the DRIP decoders will never be presented with a Message
Pack from which a constituent Authentication Page has been dropped;
DRIP FEC could never provide benefit to a Message Pack, only consume
its precious payload space. Therefore, DRIP FEC (Section 5) MUST NOT
be used in Message Packs.
6.3. Authentication
To fulfill the requirements in [RFC9153], a UA:
1. UA MUST:
1. send DRIP Link (Section 4.2) using the BE: _BE: Apex, RAA RAA_ (partially
satisfying GEN-3); at least once per 5 minutes. Apex in this
context is the DET prefix owner.
2. MUST: send DRIP Link (Section 4.2) using the BE: RAA, HDA (partially
satisfying GEN-3); at least once per 5 minutes.
3. MUST: send DRIP Link (Section 4.2) using the BE: HDA, UA (satisfying
ID-5, GEN-1 and partially satisfying GEN-3); at least once per
minute.
4. MUST: send any other DRIP Authentication Format (non-DRIP Link) where
the UA is dynamically signing data that is guaranteed to be
unique, unpredictable, and easily cross checked by the receiving
device (satisfying ID-5, GEN-1 and GEN-2); at least once per 5
seconds.
An Observer's receiver must verify the signature (cryptographically,
as specified in Section 3.1.1) on each of the 4 messages sent in the
operations specified immediately above and the Observer MUST validate
the signed content (via non-cryptographic means) of the 4th message
sent in the last operation immediately above (the non-DRIP Link
message).
These four transmission transmission, receiver verification, and Observer validation
requirements collectively satisfy GEN-3.
6.4. Operational
UAS operation may impact the frequency of sending DRIP Authentication
messages.
Messages. When a UA dwells at an approximate location, and the
channel is heavily used by other devices, less frequent message
authentication may be effective (to minimize RF packet collisions)
for an Observer. Contrast this with a UA transiting an area, where
authenticated messages SHOULD be sufficiently frequent for an
Observer to have a high probability of receiving an adequate number
for validation during the transit.
A RECOMMENDED operational configuration (in alignment with
Section 6.3) with rationale can be found in Appendix B. It consists
of
recommends the following recommendations for every once per second:
* Under Legacy Transport:
- Two sets of those ASTM Messages required by a CAA in its
jurisdiction (example: Basic ID, Location Location/Vector, and System)
and one set of other ASTM Messages (example: Self ID, Operator
ID)
- An FEC-protected DRIP Manifest enabling authentication of those
ASTM Messages sent
- A single page of an FEC-protected DRIP Link
* Under Extended Transport:
- A Message Pack of ASTM Messages (up to 4) and a DRIP Wrapper
(per Section 4.3.2)
- A Message Pack of a DRIP Link
6.4.1. DRIP Wrapper
If DRIP Wrappers are sent, they MUST be sent in addition to any
required ASTM Messages in a given jurisdiction. An implementation
MUST NOT send DRIP Wrappers in place of any required ASTM Messages it
may encapsulate. Thus, messages within a Wrapper are sent twice:
once in the clear and once authenticated within the Wrapper.
The DRIP Wrapper has a specific use case for DRIP-aware Observers.
For an Observer plotting Location Location/Vector Messages (Message Type 0x2)
on a map, display of an embedded Location Location/Vector Message in a DRIP
Wrapper can be marked differently (e.g., via color) to signify trust
in the Location Location/Vector data.
6.4.2. UAS RID Trust Assessment
As described in Section 3.1.2, the Observer MUST perform validation
of the data being received in Broadcast RID. This is because trust
in a key is different from trust that an observed UA possesses that
key.
A chain of DRIP Links provides trust in a key. A message message, signed by
that key, containing data that changes rapidly changing, and is not predictable
far in advance (relative to typical operational flight times) but
that can be validated by Observers, signed
by that key, provides trust that some agent
with access to that data also possesses that key. If the validation
involves correlating physical world observations of the UA with
claims in that data, then the probability is high that the observed
UA is (or is collaborating with or observed in real time by) the
agent with the key.
After
At least once per Observation session, after signature verification
of any DRIP Authentication Message containing UAS RID information
elements (e.g., DRIP Wrapper Wrapper, Section 4.3) 4.3), the Observer MUST must use
other sources of information to correlate against and perform validation.
validation (as specified in Section 6.3). An example of another
source of information is a visual confirmation of the UA position.
When correlation of these different data streams does not match in
acceptable thresholds, the data MUST be rejected as if the signature
failed to validate. Acceptable threshold limits and what happens
after such a rejection are out of scope for this document.
7. Summary of Addressed DRIP Requirements
The following requirements as defined in [RFC9153] are addressed in
this document:
ID-5: Non-spoofability
Addressed using the DRIP Wrapper (Section 4.3), DRIP Manifest
(Section 4.4), or DRIP Frame (Section 4.5).
GEN-1: Provable Ownership
Addressed using the DRIP Link (Section 4.2) and DRIP Wrapper
(Section 4.3), DRIP Manifest (Section 4.4), or DRIP Frame
(Section 4.5).
GEN-2: Provable Binding
Addressed using the DRIP Wrapper (Section 4.3), DRIP Manifest
(Section 4.4) or DRIP Frame (Section 4.5).
GEN-3: Provable Registration
Addressed using the DRIP Link (Section 4.2).
8. IANA Considerations
8.1. IANA DRIP Registry
IANA has created the "DRIP SAM Types" and "DRIP Frame Types"
registries within the "Drone Remote ID Protocol" registry group
(https://www.iana.org/assignments/drip).
DRIP SAM Types:
This registry is a mirror for SAM Types containing the subset of
allocations used by DRIP Authentication Messages. Future
additions MUST be done through ASTM's designated registrar, which
is ICAO [ASTM-Remote-ID] at the time of publication of this RFC.
Additions RFC
(2024). The registration procedure for DRIP (only) SAM Types is
Standards Action [RFC8126]. Requests for new DRIP SAM Type
registrations will be coordinated by IANA and the ASTM
designated ASTM-designated
registrar of all SAM Types before final publication as being documented in Standards
Track RFCs. The following values have been allocated to the IETF:
+==========+===========+=======================================+
| SAM Type | Name | Description |
+==========+===========+=======================================+
| 0x01 | DRIP Link | Format to hold Broadcast Endorsements |
+----------+-----------+---------------------------------------+
| 0x02 | DRIP | Authenticate full ASTM Messages |
| | Wrapper | |
+----------+-----------+---------------------------------------+
| 0x03 | DRIP | Authenticate hashes of ASTM Messages |
| | Manifest | |
+----------+-----------+---------------------------------------+
| 0x04 | DRIP | Format for future DRIP authentication |
| | Frame | |
+----------+-----------+---------------------------------------+
Table 2: DRIP SAM Types
DRIP Frame Types:
This 8-bit value registry is for Frame Types in DRIP Frame
Authentication Messages. Future additions to this registry are to
be made through Expert Review (Section 4.5 of [RFC8126]) for
values 0x01 to 0x9F and First Come First Served (Section 4.4 of
[RFC8126]) for values 0xA0 to 0xEF. The following values are
defined:
+=============+==============+===============================+
| Frame Type | Name | Description |
+=============+==============+===============================+
| 0x00 | Reserved | Reserved |
+-------------+--------------+-------------------------------+
| 0x01 - 0xEF | Unassigned | |
+-------------+--------------+-------------------------------+
| 0xF0-0xFF | Experimental | Reserved for Experimental Use |
+-------------+--------------+-------------------------------+
Table 3: DRIP Frame Types
Criteria that should be applied by the designated experts includes
determining whether the proposed registration duplicates existing
functionality and whether the registration description is clear and
fits the purpose of this registry.
Registration requests MUST be sent to drip-reg-review@ietf.org
(mailto:drip-reg-review@ietf.org) and be evaluated by one or more
designated experts within a three-week review period. Within that
review period, the designated experts will either approve or deny the
registration request, and communicate their decision to the review
list and IANA. Denials should include an explanation and, if
applicable, suggestions to successfully register the DRIP Frame Type.
Registration requests that are undetermined for a period longer than
28 days can be brought to the IESG's attention for resolution.
9. Security Considerations
9.1. Replay Attacks
[F3411] (regardless of transport) lacks replay protection, as it more
fundamentally lacks fully specified authentication. An attacker can
spoof the UA sender MAC address and UAS ID, replaying (with or
without modification) previous genuine messages, and/or crafting
entirely new messages. Using DRIP in [F3411] Authentication message Message
framing enables verification that messages were signed with
registered keys, but when naively used may be vulnerable to replay
attacks. Technologies such as Single Emitter Identification can
detect such attacks, but they are not readily available and can be
prohibitively expensive, especially for typical Observer devices such
as smartphones.
Replay attack detection using DRIP requires Observer devices to
combine information from multiple Broadcast RID messages and from
sources other than Broadcast RID. A complete chain of Link messages
(Section 4.2) from an Endorsement root of trust to the claimed sender
must be collected and verified by the Observer device to provide
trust in a key. Successful signature verification, using that public
key, of a Wrapper (Section 4.3) or Manifest (Section 4.4) message,
authenticating content that is nonce-like, nonce-like (see below), provides trust
that the sender actually possesses that the corresponding private key.
The term "nonce-like" means the that describes data that is unique, changes
frequently, is not accurately predictable long in advance, and readily is
easily validated (i.e., can be checked quickly at low computational
cost using readily available data) by the Observer. A Location/
Vector Message is an obvious choice. This is described in
Section 6.3 (requirement 4) 3.1.2.2 and Section 3.1.2.2.
The Location message 6.3 (requirement 4). A Location/Vector
Message [F3411] reporting precise UA position and velocity at a
precise and very recent time is to can be checked by the Observer against
visual observations of the UA within RF. Thus, both RF and Visual Line of Sight is typically the recommended form of this data. Sight.
For normative specification of the foregoing, see Sections 3.1.2 and
6.4.2. As non-normative clarification, the requirements are
satisfied as follows:
The public key corresponding to a given DET (i.e., the key attested
in the DRIP Link (BE: HDA, UA) that is the last link in the relevant
chain of DRIP Links) is used by an Observer's receiver to try to
authenticate some signed message.
If the signature check passes,
_and_ the message was a Wrapper or Manifest,
_and_ the wrapped or manifested message contained content that was
nonce-like,
_and_ the Observer validated that content by non-cryptographic
means (e.g., if the wrapped or manifested message was a Location/
Vector Message and the UA was visually observed to be in
approximately the claimed location at the reported time),
_only then_ can the Observer trust that the currently observed
sending UA actually possesses the corresponding private key (and thus
owns the corresponding DET).
Messages that pass signature verification with trusted keys could
still be replays if they contain only static information (e.g.,
Broadcast Endorsements (Section 4.2), [F3411] Basic ID ID, or [F3411]
Operator ID), or information that cannot be readily validated (e.g.,
[F3411] Self-ID). Replay of Link messages is harmless (unless sent
so frequently as to cause RF data link congestion) and indeed can
increase the likelihood of an Observer device collecting an entire
trust chain in a short time window. Replay of other messages
([F3411] Basic ID, [F3411] Operator ID, or [F3411] Self-ID) remains a
vulnerability, unless they are combined with messages containing
nonce-like data ([F3411] Location Location/Vector or [F3411] System) in a
Wrapper or Manifest. For specification of this last requirement, see
Section 4.4.2.
9.2. Wrapper vs Manifest
Implementations have a choice of using Wrapper (Section 4.3),
Manifest (Section 4.4), or a combination to satisfy the fourth
requirement in Section 6.3.
Wrapper is an attached signature on the full content of one or more
[F3411] messages, providing strong authentication. Wrapper is an
attached signature of the full content of one or more [F3411]
messages, providing strong authentication. However, the size
limitation means it cannot support such signatures over other
Authentication Messages; thus, it cannot provide a direct binding to
any part of the trust chain (Sections 3.1.2 and 6.4.2).
Manifest explicitly provides the binding of the last link in the
trust chain (with the inclusion of the hash of the Link containing
BE: HDA, UA). The use of hashes and their length also allows for a
larger number (11 vs 4) of [F3411] messages to be authenticated,
making it more efficient compared to the Wrapper. However, the
detached signature requires additional Observer overhead in storing
and comparing hashes of received messages (some of which may not be
received) with those in a Manifest.
Appendix B contains a breakdown of frame counts and an example of a
schedule using both Manifest and Wrapper. Typical operation may see
(as an example) 2x Basic ID, 2x Location, Location/Vector, 2x System, 1x
Operator ID and 1x Self ID broadcast per second to comply with
jurisdiction mandates. Each of these messages is a single frame in
size. A Link message is 8 frames long (including FEC). This is a
base frame count of *16 frames*.
When Wrapper is used, up to four of the previous messages (except the
Link) can be authenticated. For this comparison, we will sign all
the messages we can in two Wrappers. This results in _20 frames_
(with FEC). Due to size constraints, the Link message is left
unauthenticated. The total frame count using Wrappers is *36 frames*
(wrapper frame count + base frame count).
When Manifest is used, up to 10 previous messages can be
authenticated. For this example, all messages (8) are hashed
(including the Link) resulting in a single Manifest that is _9
frames_ (with FEC). The total frame count using Manifest is *25
frames* (manifest frame count + base frame count).
9.3. VNA Timestamp Offsets for DRIP Authentication Formats
Note the discussion of VNA Timestamp offsets here is in the context
of the DRIP Wrapper (Section 4.3), DRIP Manifest (Section 4.4), and
DRIP Frame (Section 4.5). For DRIP Link (Section 4.2), these offsets
are set by the DIME and have their own set of considerations in
[DRIP-REG].
The offset of the VNA _VNA Timestamp by UA UA_ is one that needs careful
consideration for any implementation. The offset should be shorter
than any given flight duration (typically less than an hour) but be
long enough to be received and processed by Observers (larger than a
few seconds). It is recommended that 3-5 minutes should be
sufficient to serve this purpose in any scenario, but it is not
limited by design.
9.4. DNS Security in DRIP
As stated in Section 3.1 specification of particular DNS security
options, transports, etc. is outside the scope of this document.
[DRIP-REG] is the The
main specification for DNS operations in DRIP and
as such [DRIP-REG] will specify DRIP usage of
applicable best common practices for security
(such as practices (e.g., from [RFC9364]).
10. Acknowledgments
* Ryan Quigley, James Mussi, and Joseph Stanton of AX Enterprize,
LLC for early prototyping to find holes in earlier drafts of this
specification
* Carsten Bormann for the simple approach of using bit-column-wise
parity for erasure (dropped frame) FEC
* Soren Friis for pointing out that Wi-Fi implementations would not
always give access to the MAC Address, as was originally used in
calculation of the hashes for DRIP Manifest. Also, for confirming
that Message Packs (0xF) can only carry up to 9 ASTM frames worth
of data (9 Authentication pages)
* Gabriel Cox (chair of the working group that produced [F3411]) for
reviewing the specification for the SAM Type request as the ASTM
Designated Expert
* Mohamed Boucadair (Document Shepherd) for his many patches and
comments
* Eric Vyncke (DRIP AD) for his guidance regarding the document's
path to publication
* Thanks to the following reviewers:
- Rick Salz (secdir)
- Matt Joras (genart)
- Di Ma (dnsdir)
- Gorry Fairhurst (tsvart)
- Carlos Bernardos (intdir)
- Behcet Sarikaya (iotdir)
- Martin Duke (IESG)
- Roman Danyliw (IESG)
- Murray Kucherawy (IESG)
- Erik Kline (IESG)
- Warren Kumari (IESG)
- Paul Wouters (IESG)
11. References
11.1.
10.1. Normative References
[F3411] ASTM International, "Standard Specification for Remote ID
and Tracking", ASTM F3411-22A, DOI 10.1520/F3411-22A, July
2022, <https://www.astm.org/f3411-22a.html>.
[NIST.SP.800-185]
Kelsey, J., Chang, S., and R. Perlner, "SHA-3 Derived
Functions: cSHAKE, KMAC, TupleHash and ParallelHash", NIST
Special Publication 800-185, DOI 10.6028/NIST.SP.800-185,
December 2016,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-185.pdf>.
[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 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9153] Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A.
Gurtov, "Drone Remote Identification Protocol (DRIP)
Requirements and Terminology", RFC 9153,
DOI 10.17487/RFC9153, February 2022,
<https://www.rfc-editor.org/info/rfc9153>.
[RFC9374] Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
"DRIP Entity Tag (DET) for Unmanned Aircraft System Remote
ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, March 2023,
<https://www.rfc-editor.org/info/rfc9374>.
[RFC9434] Card, S., Wiethuechter, A., Moskowitz, R., Zhao, S., Ed.,
and A. Gurtov, "Drone Remote Identification Protocol
(DRIP) Architecture", RFC 9434, DOI 10.17487/RFC9434, July
2023, <https://www.rfc-editor.org/info/rfc9434>.
11.2.
10.2. Informative References
[ASTM-Remote-ID]
International Civil Aviation Organization (ICAO), "Remote
ID Number Registration", December 2023,
<https://www.icao.int/airnavigation/IATF/Pages/ASTM-
Remote-ID.aspx>.
[DRIP-REG] Wiethuechter, A. and J. Reid, "DRIP Entity Tag (DET)
Identity Management Architecture", Work in Progress,
Internet-Draft, draft-ietf-drip-registries-15, 1 April draft-ietf-drip-registries-16, 31 May
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
drip-registries-15>.
drip-registries-16>.
[FAA-14CFR]
Federal Aviation Administration (FAA), "Remote
Identification of Unmanned Aircraft", January 2021,
<https://www.govinfo.gov/content/pkg/FR-2021-01-15/
pdf/2020-28948.pdf>.
[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>.
[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/info/rfc9364>.
Appendix A. Authentication States
ASTM Authentication has only three states: None, Invalid, and Valid.
This is because, under ASTM, the authentication is done by an
external service hosted somewhere on the Internet so it is assumed an
authoritative response will always be returned. This classification
becomes more complex in DRIP with the support of "offline" scenarios
where an Observer does not have Internet connectivity. With the use
of asymmetric cryptography, this means that the public key (PK) must
somehow be obtained. [DRIP-REG] provides more detail on how these
keys are stored on the DNS and how DRIP Authentication messages Messages can
be used to send PK's over Broadcast RID.
There are a few keys of interest: the PK of the UA and the PKs of
relevant DIMEs. This document describes how to send the PK of the UA
over the Broadcast RID messages. The keys of DIMEs are sent over
Broadcast RID using the same mechanisms (see Sections 4.2 and 6.3)
but MAY be sent at a far lower rate due to potential operational
constraints (such as saturation of limited bandwidth). As such,
there are scenarios where part of the key-chain may be unavailable at
the moment a full Authentication Message is received and processed.
The intent of this informative appendix is to recommend a way to
classify these various states and convey it to the user through
colors and state names/text. These states can apply to either a
single authentication message, Authentication Message, a DET (and its associated public key),
and/or a sender.
The table below lays out the recommended colors to associate with
Table 4 briefly describes each state and a brief description of each. recommends an associated
color.
+==============+========+===================================+
| State | Color | Details |
+==============+========+===================================+
| None | Black | No Authentication being received has been or is |
| | | being received (as yet) |
+--------------+--------+-----------------------------------+
| Partial | Gray | Authentication being received but |
| | | missing pages |
+--------------+--------+-----------------------------------+
| Unsupported | Brown | Authentication Type / SAM Type of |
| | | received message not supported |
+--------------+--------+-----------------------------------+
| Unverifiable | Yellow | Data needed for signature |
| | | verification is missing |
+--------------+--------+-----------------------------------+
| Verified | Green | Valid signature verification and |
| | | content validation |
+--------------+--------+-----------------------------------+
| Trusted | Blue | Evidence of Verified and DIME is |
| | | marked as only registering DETs |
| | | for trusted entities |
+--------------+--------+-----------------------------------+
| Unverified | Red | Invalid signature verification or |
| | | content validation |
+--------------+--------+-----------------------------------+
| Questionable | Orange | Evidence of both"Verified and |
| | | Unverified for the same claimed |
| | | sender |
+--------------+--------+-----------------------------------+
| Conflicting | Purple | Evidence of both Trusted and |
| | | Unverified for the same claimed |
| | | sender |
+--------------+--------+-----------------------------------+
Table 4: Authentication State Names, Colors, and Descriptions
A.1. None: Black
The default state where no authentication information has not yet been
received and is not currently being received.
A.2. Partial: Gray
A pending state where authentication pages Authentication Pages are being received, but a
full authentication message Authentication Message has yet to be compiled.
A.3. Unsupported: Brown
A state wherein authentication data is being or has been received but
cannot be used, as the Authentication Type or SAM Type is not
supported by the Observer.
A.4. Unverifiable: Yellow
A pending state where a full authentication message Authentication Message has been received
but other information, such as public keys to verify signatures, is
missing.
A.5. Verified: Green
A state where all authentication messages Authentication Messages that have been received
from that claimed sender up to that point pass signature verification
and the requirement of Section 6.4.2 has been met.
A.6. Trusted: Blue
A state where all authentication messages Authentication Messages that have been received
from that claimed sender up to that point have passed signature
verification, the requirement of Section 6.4.2 has been met, and the
public key of the sending UA has been marked as trusted.
The sending UA key will have been marked as trusted if the relevant
DIMEs only register DETs (of subordinate DIMEs, UAS operators, and
UA) that have been vetted as per their published registration
policies, and those DIMEs have been marked, by the owner (individual
or organizational) of the Observer, as per that owner's policy, as
trusted to register DETs only for trusted parties.
A.7. Questionable: Orange
A state where there is a mix of authentication messages Authentication Messages received that
are Verified (Appendix A.5) and Unverified (Appendix A.8).
State transitions from Verified to Questionable if a subsequent
message fails verification, so it would have otherwise been marked
Unverified. State transitions from Unverified to Questionable if a
subsequent message passes verification or validation, so it would
otherwise have been marked Verified. It may transition from either
of those states upon mixed results on the requirement of
Section 6.4.2.
A.8. Unverified: Red
A state where all authentication messages Authentication Messages that have been received
from that claimed sender up to that point failed signature
verification or the requirement of Section 6.4.2.
A.9. Conflicting: Purple
A state where there is a mix of authentication messages Authentication Messages received that
are Trusted (Appendix A.6) and Unverified (Appendix A.8) and the
public key of the aircraft is marked as trusted.
State transitions from Trusted to Conflicting if a subsequent message
fails verification, so it would have otherwise been marked
Unverified. State transitions from Unverified to Conflicting if a
subsequent message passes verification or validation and policy
checks, so it would otherwise have been marked Trusted. It may
transition from either of those states upon mixed results on the
requirement of Section 6.4.2.
Appendix B. Operational Recommendation Analysis
The recommendations in Section 6.4 may seem heavy-handed and
specific. This informative appendix lays out the math and
assumptions made that resulted in those recommendations and provides
an example.
In many jurisdictions, all jurisdictions known to the required authors of this document as of its
publication (2024), at least the following ASTM Messages are required
to be transmitted
every second are: at least once per second:
* Basic ID (0x1), (0x1)
* Location (0x2), and (0x2)
* System (0x4).
Typical implementations will (0x4)
Europe also requires:
* Operator ID Message (0x5)
Japan requires not one but two Basic ID messages:
* one carrying a manufacturer assigned serial number
* one carrying a CAA assigned registration number
Japan also requires:
* Authentication (0x2) using their own unique scheme
In all jurisdictions, one further message is optional, but highly
recommended for carriage of additional information on the nature of
the emergency if the Emergency value is sent in the Operational
Status field of the Location/Vector Message:
* Self ID (0x3)
To improve the likelihood of successful timely receipt of regulator
required RID data elements, most likely implementations send at a higher rate (2x
sets per cycle) resulting
rate, whether by repeating the same messages in 6 frames sent per cycle. Transmitting
this set of the same one second
interval, or updating message content and sending messages more
frequently than once a second is not discouraged, but
awareness is needed to avoid congesting per second. Excessive sending rate, however,
could congest the RF spectrum, causing
further issues.
Informational Note: In Europe, the Operator ID Message (0x5) is also
required. In Japan, two Basic ID (0x0), Location (0x1), leading to collisions and
Authentication (0x2) are required. Self ID (0x3) is optional but can
carry Emergency Status information. counter-
intuitively actually reducing the likelihood of timely receipt of RID
data.
B.1. Page Counts vs Frame Counts
There are two formulas to determine the number of Authentication
Pages required. The following formula is for Wrapper:
<CODE BEGINS>
wrapper_struct_size = 89 + (25 * num_astm_messages)
wrapper_page_count = ceiling((wrapper_struct_size - 17) / 23) + 1
<CODE ENDS>
The following formula is for Manifest:
<CODE BEGINS>
manifest_struct_size = 89 + (8 * (num_astm_hashes + 3))
manifest_page_count = ceiling((manifest_struct_size - 17) / 23) + 1
<CODE ENDS>
A similar formula can be applied to Links, as they are of fixed size:
<CODE BEGINS>
link_page_count = ceiling((137 - 17) / 23) + 1 = 7
<CODE ENDS>
Comparing Wrapper and Manifest Authentication Message page counts
against total frame counts, we have the following:
+==========+=========+==========+=================+===============+
| ASTM | Wrapper | Manifest | ASTM Messages + | ASTM Messages |
| Messages | (w/FEC) | (w/FEC) | Wrapper (w/FEC) | + Manifest |
| | | | | (w/FEC) |
+==========+=========+==========+=================+===============+
| 0 | 5 (6) | 6 (7) | 5 (6) | 6 (7) |
+----------+---------+----------+-----------------+---------------+
| 1 | 6 (7) | 6 (7) | 7 (8) | 7 (8) |
+----------+---------+----------+-----------------+---------------+
| 2 | 7 (8) | 6 (7) | 9 (10) | 8 (9) |
+----------+---------+----------+-----------------+---------------+
| 3 | 8 (9) | 7 (8) | 11 (12) | 10 (11) |
+----------+---------+----------+-----------------+---------------+
| 4 | 9 (10) | 7 (8) | 13 (14) | 11 (12) |
+----------+---------+----------+-----------------+---------------+
| 5 | N/A | 7 (8) | N/A | 12 (13) |
+----------+---------+----------+-----------------+---------------+
| 6 | N/A | 8 (9) | N/A | 14 (15) |
+----------+---------+----------+-----------------+---------------+
| 7 | N/A | 8 (9) | N/A | 15 (16) |
+----------+---------+----------+-----------------+---------------+
| 8 | N/A | 8 (9) | N/A | 16 (17) |
+----------+---------+----------+-----------------+---------------+
| 9 | N/A | 9 (10) | N/A | 18 (19) |
+----------+---------+----------+-----------------+---------------+
| 10 | N/A | 9 (10) | N/A | 19 (20) |
+----------+---------+----------+-----------------+---------------+
| 11 | N/A | 9 (11) | N/A | 20 (22) |
+----------+---------+----------+-----------------+---------------+
Table 5: Page and Frame Counts
Link shares the same page counts as Manifest with 5 ASTM Messages.
B.1.1. Special Cases
B.1.1.1. Zero ASTM Messages
Zero ASTM Messages (see Table 5) is where Extended Wrapper
(Section 4.3.2) without FEC is used in Message Packs. With a maximum
of nine "message slots" in a Message Pack, an Extended Wrapper fills
five slots; thus it can authenticate up to four ASTM Messages co-
located in the same Message Pack.
B.1.1.2. Eleven ASTM Messages
Eleven ASTM Messages (see Table 5) is where a Manifest with FEC
invokes the situation mentioned in Section 5.3.
Eleven is the maximum number of ASTM Message Hashes that can be
supported resulting in 14 total hashes. This completely fills the
evidence section
_Evidence_ field of the structure _UA-Signed Evidence Structure_ making its
total size 200 octets. This fits on exactly 9 Authentication Pages
((201 - 17) / 23 == 8), so when the ADL is added, it is placed on the
next page (Page 10). Per rule 1 in Section 5.1, this means that all
of Page 10 is null padded (expect the ADL octet) and FEC data fills
Page 11, resulting in a plus-two page count when FEC is applied.
This drives the recommendation is Section 4.4 to only use up to 10
ASTM Message Hashes, not 11.
B.2. Full Authentication Example
This example (Figure 13) is focused on showing that 100% of ASTM
Messages can be authenticated over Legacy Transports with up to 125%
overhead in Authentication Pages. Extended Transport is Transports are not shown as
in this example, because, for those, Authentication with DRIP in that case is covered
achieved using Extended Wrapper (Section 4.3.2). Two ASTM Message
Packs are sent in a given cycle: one containing up to four ASTM
Messages and an Extended Wrapper (authenticating the pack), and one
containing a Link message with a Broadcast Endorsement and up to two
other ASTM Messages.
This example transmit scheme covers and meets every known regulatory
case enabling manufacturers to use the same firmware worldwide.
+------------------------------------------------------+
| Frame Slots |
| 00 - 04 | 05 - 07 | 08 - 16 | 17 |
+-------------------+---------------+---------+--------+
| {A|B|C|D},V,S,I,O | {A|B|C|D},V,S | M[0,8] | L/W[0] |
+-------------------+---------------+---------+--------+
| {A|B|C|D},V,S,I,O | {A|B|C|D},V,S | M[0,8] | L/W[1] |
+-------------------+---------------+---------+--------+
| {A|B|C|D},V,S,I,O | {A|B|C|D},V,S | M[0,8] | L/W[2] |
+-------------------+---------------+---------+--------+
| {A|B|C|D},V,S,I,O | {A|B|C|D},V,S | M[0,8] | L/W[3] |
+-------------------+---------------+---------+--------+
| {A|B|C|D},V,S,I,O | {A|B|C|D},V,S | M[0,8] | L/W[4] |
+-------------------+---------------+---------+--------+
| {A|B|C|D},V,S,I,O | {A|B|C|D},V,S | M[0,8] | L/W[5] |
+-------------------+---------------+---------+--------+
| {A|B|C|D},V,S,I,O | {A|B|C|D},V,S | M[0,8] | L/W[6] |
+-------------------+---------------+---------+--------+
| {A|B|C|D},V,S,I,O | {A|B|C|D},V,S | M[0,8] | L/W[7] |
+-------------------+---------------+---------+--------+
A = Basic ID Message (0x0) ID Type 1
B = Basic ID Message (0x0) ID Type 2
C = Basic ID Message (0x0) ID Type 3
D = Basic ID Message (0x0) ID Type 4
V = Location/Vector Message (0x1)
I = Self ID Message (0x3)
S = System Message (0x4)
O = Operator ID Message (0x5)
L[y,z] = DRIP Link Authentication Message (0x2)
W[y,z] = DRIP Wrapper Authentication Message (0x2)
M[y,z] = DRIP Manifest Authentication Message (0x2)
y = Start Page
z = End Page
# = Empty Frame Slot
* = Message in DRIP Manifest Authentication Message
Figure 13: Example of a Fully Authenticated Legacy Transport
Transmit Schedule
Every common required message (Basic ID, Location, Location/Vector, and System)
is sent twice along with Operator ID and Self ID in a single second.
The Manifest is over all messages (8) in slots 00 - 04 and 05 - 07.
In two seconds, either a Link or Wrapper are is sent. The content and
order of Links and Wrappers runs as follows:
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Link: Apex on RAA
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Wrapper: Location Location/Vector (0x1), System (0x4)
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Link: Apex on RAA
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Wrapper: Location Location/Vector (0x1), System (0x4)
Link: IANA on UAS RID Apex
With
After perfect receipt of all messages, messages for a period of 8 seconds, all
messages (up to sent during that point
then all in future) are period have been authenticated within 8 seconds using the
Manifest.
Manifest (except for the Authentication Messages themselves). Within
136 seconds, the entire Broadcast Endorsement chain is received and
can be validated; interspersed with four validated. Interspersed in this schedule are 4 messages
directly signed over via Wrapper. sent
not only in their basic [F3411] form, but also in DRIP Wrapper
messages, together with their attached signatures, to defend against
the possibility of attack against the detached signatures provided by
the Manifest messages.
B.2.1. Raw Example
Assuming the following DET and HI:
2001:3f:fe00:105:a29b:3ff4:2226:c04e
b5fef530d450dedb59ebafa18b00d7f5ed0ac08a81975034297bea2b00041813
The following ASTM Messages are to be sent in a single second:
0240012001003ffe000105a29b3ff42226c04e000000000000
12000000000000000000000000000000000000000060220000
32004578616d706c652053656c662049440000000000000000
420000000000000000000100000000000000000010ea510900
52004578616d706c65204f70657261746f7220494400000000
0240012001003ffe000105a29b3ff42226c04e000000000000
12000000000000000000000000000000000000000060220000
420000000000000000000100000000000000000010ea510900
This is a Link with FEC that would be spread out over 8 seconds:
2250078910ea510904314b8564b17e66662001003ffe000105
2251a29b3ff42226c04eb5fef530d450dedb59ebafa18b00d7
2252f5ed0ac08a81975034297bea2b000418132001003ffe00
22530105b82bf1c99d87273103fc83f6ecd9b91842f205c222
2254dd71d8e165ad18ca91daf9299a73eec850c756a7e9be46
2255f51dddfa0f09db7bfdde14eec07c7a6dd1061c1d5ace94
2256d9ad97940d280000000000000000000000000000000000
2257a03b0f7a6feb0d198167045058cfc49f73129917024d22
This is a Wrapper with FEC that would be spread out over 8 seconds:
2250078b10ea510902e0dd7c6560115e671200000000000000
22510000000000000000000000000060220000420000000000
2252000000000100000000000000000010ea5109002001003f
2253fe000105a29b3ff42226c04ef0ecad581a030ca790152a
22542f08df5762a463e24a742d1c530ec977bbe0d113697e2b
2255b909d6c7557bdaf1227ce86154b030daadda4a6b8474de
22569a62f6c375020826000000000000000000000000000000
2257f5e8eebcb04f8c2197526053e66c010d5d7297ff7c1fe0
This is the Manifest with FEC sent in the same second as the original
messages:
225008b110ea510903e0dd7c6560115e670000000000000000
2251d57594875f8608b4d61dc9224ecf8b842bd4862734ed01
22522ca2e5f2b8a3e61547b81704766ba3eeb651be7eafc928
22538884e3e28a24fd5529bc2bd4862734ed012ca2e5f2b8a3
2254e61547b81704766ba3eeb62001003ffe000105a29b3ff4
22552226c04efb729846e7d110903797066fd96f49a77c5a48
2256c4c3b330be05bc4a958e9641718aaa31aeabad368386a2
22579ed2dce2769120da83edbcdc0858dd1e357755e7860317
2258e7c06a5918ea62a937391cbfe0983539de1b2e688b7c83
Acknowledgments
The authors acknowledge the following individuals:
* Ryan Quigley, James Mussi, and Joseph Stanton of AX Enterprize,
LLC for early prototyping to find holes in earlier drafts of this
specification.
* Carsten Bormann for the simple approach of using bit-column-wise
parity for erasure (dropped frame) FEC.
* Soren Friis for pointing out that Wi-Fi implementations would not
always give access to the MAC Address, as was originally used in
calculation of the hashes for DRIP Manifest. Also, for confirming
that Message Packs (0xF) can only carry up to 9 ASTM frames worth
of data (9 Authentication Pages).
* Gabriel Cox (chair of the working group that produced [F3411]) for
reviewing the specification for the SAM Type request as the ASTM
Designated Expert.
* Mohamed Boucadair (Document Shepherd) for his many patches and
comments.
* Eric Vyncke (DRIP AD) for his guidance regarding the document's
path to publication.
The authors also thank the following reviewers:
* Rick Salz (secdir)
* Matt Joras (genart)
* Di Ma (dnsdir)
* Gorry Fairhurst (tsvart)
* Carlos Bernardos (intdir)
* Behcet Sarikaya (iotdir)
* Martin Duke (IESG)
* Roman Danyliw (IESG)
* Murray Kucherawy (IESG)
* Erik Kline (IESG)
* Warren Kumari (IESG)
* Paul Wouters (IESG)
Authors' Addresses
Adam Wiethuechter (editor)
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Email: adam.wiethuechter@axenterprize.com
Stuart Card
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Email: stu.card@axenterprize.com
Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America
Email: rgm@labs.htt-consult.com