DRIP Authentication Formats & Protocols for Broadcast Remote ID
draft-ietf-drip-auth-08
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| Last updated | 2022-04-26 | ||
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draft-ietf-drip-auth-08
DRIP Working Group A. Wiethuechter (Editor)
Internet-Draft S. Card
Intended status: Standards Track AX Enterprize, LLC
Expires: 28 October 2022 R. Moskowitz
HTT Consulting
26 April 2022
DRIP Authentication Formats & Protocols for Broadcast Remote ID
draft-ietf-drip-auth-08
Abstract
This document describes how to include trust into the ASTM Remote ID
specification defined in ASTM F3411 under Broadcast Remote ID (RID).
It defines a few message schemes (sent within the Authentication
Message) that can be used to authenticate past messages sent by a
unmanned aircraft (UA) and provide proof of UA trustworthiness even
in the absence of Internet connectivity at the receiving node.
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 of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 28 October 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. DRIP Requirements Addressed . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Required Terminology . . . . . . . . . . . . . . . . . . 4
2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Problem Space and Focus . . . . . . . . . . . . . . . . . 4
3.2. Reasoning for IETF DRIP Authentication . . . . . . . . . 4
3.3. ASTM Authentication Message . . . . . . . . . . . . . . . 5
3.3.1. Authentication Page . . . . . . . . . . . . . . . . . 5
3.3.2. ASTM Constraints . . . . . . . . . . . . . . . . . . 8
4. Forward Error Correction . . . . . . . . . . . . . . . . . . 8
4.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. Single Page FEC . . . . . . . . . . . . . . . . . . . 8
4.1.2. Multiple Page FEC . . . . . . . . . . . . . . . . . . 9
4.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.1. Single Page FEC . . . . . . . . . . . . . . . . . . . 12
4.2.2. Multiple Page FEC . . . . . . . . . . . . . . . . . . 12
4.3. FEC Limitations . . . . . . . . . . . . . . . . . . . . . 13
5. Broadcast Attestation Structure . . . . . . . . . . . . . . . 13
6. DRIP Authentication Formats . . . . . . . . . . . . . . . . . 15
6.1. Operator ID Signature . . . . . . . . . . . . . . . . . . 15
6.2. Message Set Signature . . . . . . . . . . . . . . . . . . 16
6.3. Specific Authentication Method . . . . . . . . . . . . . 18
6.3.1. SAM Data Format . . . . . . . . . . . . . . . . . . . 18
6.3.2. DRIP Link . . . . . . . . . . . . . . . . . . . . . . 19
6.3.3. DRIP Wrapper . . . . . . . . . . . . . . . . . . . . 20
6.3.4. DRIP Manifest . . . . . . . . . . . . . . . . . . . . 22
6.3.5. DRIP Frame . . . . . . . . . . . . . . . . . . . . . 25
7. Requirements & Recommendations . . . . . . . . . . . . . . . 27
7.1. Legacy Transports . . . . . . . . . . . . . . . . . . . . 27
7.2. Extended Transports . . . . . . . . . . . . . . . . . . . 28
7.3. Authentication . . . . . . . . . . . . . . . . . . . . . 28
7.4. Operational . . . . . . . . . . . . . . . . . . . . . . . 29
7.4.1. DRIP Wrapper . . . . . . . . . . . . . . . . . . . . 29
8. ICAO Considerations . . . . . . . . . . . . . . . . . . . . . 30
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
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10. Security Considerations . . . . . . . . . . . . . . . . . . . 30
10.1. Manifest Hash Length . . . . . . . . . . . . . . . . . . 30
10.2. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 31
10.3. Trust Timestamp Offsets . . . . . . . . . . . . . . . . 31
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
12.1. Normative References . . . . . . . . . . . . . . . . . . 32
12.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Authentication State Diagrams & Color Scheme . . . . 33
A.1. State Table . . . . . . . . . . . . . . . . . . . . . . . 33
A.2. State Diagrams . . . . . . . . . . . . . . . . . . . . . 34
A.2.1. Notations . . . . . . . . . . . . . . . . . . . . . . 34
A.2.2. General . . . . . . . . . . . . . . . . . . . . . . . 35
A.2.3. DRIP SAM . . . . . . . . . . . . . . . . . . . . . . 36
A.2.4. DRIP Link . . . . . . . . . . . . . . . . . . . . . . 37
A.2.5. DRIP Wrapper/Manifest/Frame . . . . . . . . . . . . . 38
Appendix B. HDA-UA Broadcast Attestation . . . . . . . . . . . . 40
Appendix C. Example TX/RX Flow . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
1. Introduction
Unmanned Aircraft Systems (UAS) are usually in a volatile environment
when it comes to communication. UA are generally small with little
computational (or flying) horsepower to carry standard communication
equipment. This limits the mediums of communication to few viable
options.
Observer systems (e.g. smartphones and tablets) place further
constraints on the communication options. The Remote ID Broadcast
messages MUST be available to applications on these platforms without
modifying the devices.
The ASTM [F3411] standard focuses on two ways of communicating to a
UAS for Remote ID (RID): Broadcast and Network.
This document will focus on adding trust to Broadcast RID via the
Authentication Message by combining dynamically signed data with an
Attestation of the UA's identity from a Registry.
1.1. DRIP Requirements Addressed
The following [drip-requirements] will be addressed:
GEN 1: Provable Ownership This will be addressed using the DRIP Link
and DRIP Wrapper or DRIP Manifest.
GEN 2: Provable Binding This requirement is addressed using the DRIP
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Wrapper or DRIP Manifest.
GEN 3: Provable Registration This requirement is addressed using the
DRIP Link.
See Section 7.3 for further clarification.
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
See [drip-requirements] for common DRIP terms.
Legacy Transports: uses broadcast frames (Bluetooth 4.x).
Extended Transports: uses the extended advertisements (Bluetooth
5.X), service info (Wi-Fi NaN) or vendor specific element
information (Wi-Fi BEACON). Must use ASTM [F3411] Message Pack
(Message Type 0xF).
3. Background
3.1. Problem Space and Focus
The current standard for Remote ID does not, in any meaningful
capacity, address the concerns of trust in the UA space with
communication in the Broadcast RID environment. This is a
requirement that will need to be addressed eventually for various
different parties that have a stake in the UA industry.
3.2. Reasoning for IETF DRIP Authentication
The ASTM Authentication Message has provisions in [F3411] to allow
for other organizations to standardize additional Authentication
formats beyond those explicitly in [F3411]. 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. No existing formats (defined in
[F3411] or other organizations leveraging this feature) provide the
functionality to satisfy this goal resulting in the work reflected in
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this document.
3.3. ASTM Authentication Message
The ASTM Authentication Message (Message Type 0x2) is a unique
message in the Broadcast [F3411] standard as it is the only one that
is larger than the Bluetooth 4 frame size. To address this, it is
defined as a set of "pages" that each fits into a single Bluetooth 4
broadcast frame. For other media these pages are still used but all
in a single frame.
3.3.1. Authentication Page
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 |
| |
| |
+---------------+---------------+---------------+---------------+
Page Header: (1 byte)
Authentication Type (4 bits)
Page Number (4 bits)
Authentication Payload: (23 bytes per page)
Authentication Payload, including headers. Null padded.
Figure 1: Standard ASTM Authentication Message Page
A single Authentication Page is akin to a UDP packet. For
Authentication Pages the structure is further wrapped by outer ASTM
framing and the specific link framing (Bluetooth or Wi-Fi).
3.3.1.1. Authentication Type
[F3411] has the following subset of Authentication Type's defined and
that can be used in the Page Header:
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+=====================+================================+
| Authentication Type | Description |
+=====================+================================+
| 0x2 | Operator ID Signature |
+---------------------+--------------------------------+
| 0x3 | Message Set Signature |
+---------------------+--------------------------------+
| 0x5 | Specific Authentication Method |
+---------------------+--------------------------------+
Table 1
3.3.1.1.1. Specific Authentication Method (SAM)
This document leverages Authentication Type 0x5, Specific
Authentication Method (SAM), defining a set of SAM Types in
Section 6.3. Other Authentication Types are also used in DRIP and
their use is defined in Section 6.
3.3.1.2. Page Number
There is a technical maximum of 16-pages (indexed 0 to 15 in the Page
Header) that can be sent for a single Authentication Message, with
each page carrying a max 23-byte Authentication Payload. See
Section 3.3.2 for more details.
3.3.1.3. Authentication Payload Field
The following is shown in its complete format.
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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 .
. .
| |
+---------------+---------------+---------------+---------------+
Authentication Headers: (6 bytes)
Contains other header information for the Authentication
Message as defined in F3411.
Authentication Data / Signature: (0 to 255 bytes)
Opaque authentication data.
Additional Data Length (ADL): (1 byte - unsigned)
Length in bytes of Additional Data.
Additional Data: (0 to 255 bytes):
Data that follows the Authentication Data / Signature but
is not considered part of the Authentication Data.
Figure 2: ASTM Authentication Message Fields
Figure 2 is the abstract view of the data fields found in the
Authentication Message as defined by [F3411]. This data is placed
into Figure 1's Authentication Payload, spanning multiple pages.
When Additional Data is being sent, a single unsigned byte
(Additional Data Length) directly follows the Authentication Data /
Signature and has the length, in bytes, of the following Additional
Data. For DRIP, this field is used to carry Forward Error Correction
as defined in Section 4.
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3.3.2. ASTM 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 of the most
constrained existing transport can support. Under Broadcast RID the
transport that can hold the least amount of authentication data is
Bluetooth 5 and Wi-Fi BEACON at 9-pages.
As such DRIP transmitters are REQUIRED to adhere to the following
when using the Authentication Message:
1. Authentication Data / Signature data MUST fit in a 9 pages (Page
Numbers 0 through 8).
2. The Length field in the Authentication Headers (which denotes the
length in bytes of Authentication Data / Signature only) MUST NOT
exceed the value of 201.
4. Forward Error Correction
For Broadcast RID, Forward Error Correction (FEC) is provided by the
lower layers in Extended Transports (Bluetooth 5.X, Wi-Fi NaN, and
Wi-Fi BEACON). Legacy Transports do not have supporting FEC so with
DRIP Authentication the following application level FEC scheme is
used.
4.1. Encoding
For any encoding the FEC data MUST start on new ASTM Authentication
Page. To do this null padding is add before the actual FEC data
starts and the length of the whole blob (null padding and FEC) is
used as the Additional Data Length. To properly fit FEC data into an
Authentication Page the number of parity-bytes is limited to 23 (or a
multiple thereof). This means that the Page Header (and anything
before it) is omitted in the FEC process.
4.1.1. Single Page FEC
To generate the parity a simple XOR operation using the previous and
current page is used. Only the last 23-bytes are used during the XOR
operation. For Page 0, a 23-byte null pad is used for the previous
page. The resulting parity fills the Additional Data field of
[F3411] with the Additional Data Length field being set to 23 or
greater (depending on number of null pad bytes are needed to get onto
the next page).
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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 3: Example Single Page FEC Encoding
4.1.2. Multiple Page FEC
For Multiple Page FEC there are two flavors: Frame Recovery and Page
Recovery. Both follow a similar process, but are offset at what data
is actually protected.
(Editor Note: to improve interop should we explicitly select a
polynomial for Reed Solomon that DRIP must use?)
4.1.2.1. Page Recovery
Take the following example of an Authentication Message that 3-pages
of parity are to be generated for:
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12 50 098960bf8c05 042001001000a00145aac6b00abba268b7
12 51 2001001000a0014579d8a404d48f2ef9bb9a4470ada5b4
12 52 ff1352c7402af9d9ebd20034e8d7a12920f4d7e91c1a73
12 53 dca7d04e776150825863c512c6eb075a206a95c59b297e
12 54 f2935fd416f27b1b42fd5d9dfaa0dec79f32287f41b454
12 55 7101415def153a770d3e6c0b17ae560809bc634a822c1f
12 56 3b1064b80a000000000000000000000000000000000000
For Page Recovery the first two columns are ignored (being the Page
Header and any data before it), the last 23 columns are extracted and
have Reed Solomon performed on it to produce parity bytes. For the
example the following 3-bytes of parity are generated:
dc6c2b = ReedSolomon.encoder(0920ffdcf2713b)
Each set of parity is the placed into a pseudo-frame as follows (each
byte in its own message in the same column):
00 00 dc00000000000000000000000000000000000000000000
00 00 6c00000000000000000000000000000000000000000000
00 00 2b00000000000000000000000000000000000000000000
The above data set produces the following full set of parity:
00 00 dc6657acd30b2ec4aa582049f52adf9f922e62c469563a
00 00 6c636a59145a55417a3895fd543f19e94200be4abc5e94
00 00 02bba5e28f5896d754caf50016a983993b149b5c9e6eeb
The last 23-bytes are then added into the Additional Data fields of
their respective pages:
12 57 dc6657acd30b2ec4aa582049f52adf9f922e62c469563a
12 58 6c636a59145a55417a3895fd543f19e94200be4abc5e94
12 59 02bba5e28f5896d754caf50016a983993b149b5c9e6eeb
4.1.2.2. Frame Recovery
Frame Recovery uses the full ASTM Message and performs Reed Solomon
over each byte. Below is an example of a number of messages.
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10 42012001001000a0014579d8a404d48f2ef9000000000000
11 249600006efeb019ee111ed37a097a0948081c10ffff0000
12 50 098960bf8c05 042001001000a00145aac6b00abba268b7
12 51 2001001000a0014579d8a404d48f2ef9bb9a4470ada5b4
12 52 ff1352c7402af9d9ebd20034e8d7a12920f4d7e91c1a73
12 53 dca7d04e776150825863c512c6eb075a206a95c59b297e
12 54 f2935fd416f27b1b42fd5d9dfaa0dec79f32287f41b454
12 55 7101415def153a770d3e6c0b17ae560809bc634a822c1f
12 56 3b1064b80a000000000000000000000000000000000000
13 0052656372656174696f6e616c2054657374000000000000
14 02c2ffb019322d1ed3010000c008e40700fc080000000000
15 004e2e4f5031323334353600000000000000000000000000
Each column is extracted and has Reed Solomon performed on it to
produce parity bytes. In the below example 5-bytes of parity are
generated with Frame Recovery:
6c3f42b8a8 = ReedSolomon.encoder(101112121212121212131415)
Each set of parity is the placed into a pseudo-frame as follows (each
byte in its own message in the same column):
6c000000000000000000000000000000000000000000000000
3f000000000000000000000000000000000000000000000000
42000000000000000000000000000000000000000000000000
b8000000000000000000000000000000000000000000000000
a8000000000000000000000000000000000000000000000000
The above data set produces the following sets of parity:
6c86337bf7ab746f5d62bb7f8de954104b121585d3975f6e92
3f06c1bce165b0e25930d57a63c24f751145e1dd8dc115029b
42e9979580327a6a14d421c12a33aa2e1a2e517daaee581016
b8012a7b3964f7b2720d387bfa77e945556f1831cd477ef3a3
a85bb403aada89926fb8fc2a14a9caacb4ec2f3a6ed2d8e9f9
For Frame Recovery the above data would be placed into Authentication
Pages like below:
12 57 6c86337bf7ab746f5d62bb7f8de954104b121585d3975f
12 58 6e923f06c1bce165b0e25930d57a63c24f751145e1dd8d
12 59 c115029b42e9979580327a6a14d421c12a33aa2e1a2e51
12 5a 7daaee581016b8012a7b3964f7b2720d387bfa77e94555
12 5b 6f1831cd477ef3a3a85bb403aada89926fb8fc2a14a9ca
12 5c acb4ec2f3a6ed2d8e9f900000000000000000000000000
Up to 240 (255 minus 15 pages max of FEC data) messages can be
protected using Frame Recovery.
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4.2. Decoding
Due to the nature of Bluetooth 4 and the existing ASTM paging
structure an optimization can be used. If a Bluetooth frame fails
its CRC check, then the frame is dropped without notification to the
upper protocol layers. From the Remote ID perspective this means the
loss of a complete frame/message/page. In Authentication Messages,
each page is already numbered so the loss of a page allows the
receiving application to build a "dummy" page filling the entire page
with nulls.
If Page 0 is being reconstructed an additional check of the Last Page
Index to check against how many pages are actually present, MUST be
performed for sanity. An additional check on the Length field SHOULD
also be performed.
To determine if Single Page FEC or Multiple Page FEC has been used a
simple check of the Last Page Index can be used. If the number of
pages left after the Length of Authentication Data is exhausted than
it is clear that the remaining pages are all FEC. The Additional
Data Length byte can further confirm this; taking into account any
null padding needed for page alignment.
4.2.1. Single Page FEC
Using the same methods as encoding, an XOR operation is used between
the previous and current page (a 23-byte null pad is used as the
start). The resulting 23-bytes should be data of the missing page.
4.2.2. Multiple Page FEC
To determine if Page Recovery or Frame Recovery is used two modulo
checks with the ADL after the length of the null-pad is removed are
needed. One against the value of 23, and the other against the value
of 25. If 23 comes back with a value of 0 then Page Recovery is
being used. If 25 comes back with 0 then Frame Recovery is used.
Any other combination indicates an error.
4.2.2.1. Page Recovery
To decode Page Recovery, dummy pages (pages with nulls as the data)
are needed in the places no page was received. Then Reed Solomon can
decode across the columns of the 23-bytes of each page. Erasures can
be used as it is known which pages are missing and can improve the
Reed Solomon results by specifying them.
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4.2.2.2. Frame Recovery
To decode Frame Recovery, the receiver must first extract all FEC
data from the pages; concatenate them and then break into 25-byte
chunks. This will produce the pseudo-frames. Now Reed Solomon can
be used to decode columns, with dummy frames inserted where needed.
4.3. FEC Limitations
The worst case scenario is when the Authentication Data / Signature
ends perfectly on a page (Page N-1). This means the Additional Data
Length would start the next page (Page N) and have 22-bytes worth of
null padding to align the FEC in to the next page (Page N+1). In
this scenario an entire page (Page N) is being wasted just to carry
the Additional Data Length. This should be be avoided at all costs -
in an effort to maintain efficiency.
5. Broadcast Attestation Structure
To directly support Broadcast RID a variation of the Attestation
Structure format of [drip-registries] SHOULD be used when running
DRIP under the various Authentication Types (filling the
Authentication Data / Signature field of Figure 2) and SAM Types
(filling the SAM Authentication Data field (Section 6.3.1.2)). The
notable changes of the structure is that the timestamps are set by
the UA and the Attestor Identity Information is set to the DET of the
UA.
When using this structure the UA is always self-attesting its DRIP
Entity Tag (DET). The Host Identity of the UA DET can be looked up
by mechanisms described in [drip-registries] or by extracting it from
Broadcast Attestation (see Section 6.3.2 and Section 7.3).
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
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
. .
. Attestation Data .
. .
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
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+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
Attestation Data (0 to 112 bytes):
Opaque attestation data.
Not Before Timestamp by UA (4-bytes):
Timestamp denoting recommended time to start trusting data.
Not After Timestamp by UA (4 bytes):
Timestamp denoting recommended time to stop trusting data.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
Figure 4: Broadcast Attestation Structure
Attestation Data is a field with a maximum of 112-bytes, containing
data that the UA is attesting during its flight.
The Not After Timestamp and Not Before Timestamp MUST follow the
format defined in [F3411]. That is a Unix-style timestamp but with
an epoch of 01/01/2019 00:00:00. Not Before Timestamp MUST be set to
the time the structure is signed over. An additional offset is then
added to push the Not After Timestamp a short time into the future to
avoid replay attacks.
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The offset used against the Unix-style timestamp is not defined in
this document. Best practice identifying an acceptable offset should
be used taking into consideration the UA environment, and propagation
characteristics of the messages being sent and clock differences
between the UA and Observers. A reasonable time would be to set Not
After Timestamp 2 minutes ahead of Not Before Timestamp.
6. DRIP Authentication Formats
All formats defined in this section fill the Authentication Data /
Signature field in Figure 2.
When sending data over a medium that does not have underlying Forward
Error Correction (FEC), for example Bluetooth 4, then Section 4 MUST
be used.
6.1. Operator ID Signature
The existing ASTM [F3411] Authentication Type 0x2 can be used to send
a static Self-Attestation of the Operator.
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
+---------------+---------------+---------------+---------------+
| |
| Operator |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| Operator Host Identity |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by Operator |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by Operator |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
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| |
| Operator Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The Operator DET in byte form (network byte order).
Operator Host Identity (32-bytes):
HI of the Operator.
Not Before Timestamp by Operator (4 bytes):
Timestamp denoting recommended time to start trusting data.
Not After Timestamp by Operator (4 bytes):
Timestamp denoting recommended time to stop trusting data.
Operator Signature (64 bytes):
Signature over preceding fields using the keypair of
the Operator.
Figure 5: DRIP Operator ID Signature
6.2. Message Set Signature
When running under Extended Transports, the existing ASTM [F3411]
Authentication Type 0x3 can be used to sign over the adjacent ASTM
Messages in the Message Pack (Message Type 0xF).
The concatenation of all messages in the Message Pack (excluding
Authentication) before signing MUST be in Message Type order and be
placed between the UA DRIP Entity Tag and Not Before Timestamp field.
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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
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
Not Before Timestamp by UA (4-bytes):
Timestamp denoting recommended time to start trusting data.
Not After Timestamp by UA (4 bytes):
Timestamp denoting recommended time to stop trusting data.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
Figure 6: DRIP Message Set Signature
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6.3. Specific Authentication Method
For ASTM Specific Authentication Method (Authentication Type 0x5) a
special SAM Type field, specified as the first byte of the
Authentication Data / Signature by [F3411], is used to multiplex
between various formats.
6.3.1. SAM Data Format
Figure 7 is the general format to hold authentication data when using
SAM and is placed inside the Authentication Data / 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 .
. .
| |
+---------------+---------------+---------------+---------------+
SAM Type (1 byte):
Byte defined by F3411 to multiplex SAMs
SAM Authentication Data (0 to 200 bytes):
Opaque SAM authentication data.
Figure 7: SAM Data Format
6.3.1.1. SAM Type
The SAM Type field is maintained by the International Civil Aviation
Organization (ICAO) and for DRIP four are planned to be allocated:
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+==========+===============================+
| SAM Type | Description |
+==========+===============================+
| 0x01 | DRIP Link (Section 6.3.2) |
+----------+-------------------------------+
| 0x02 | DRIP Wrapper (Section 6.3.3) |
+----------+-------------------------------+
| 0x03 | DRIP Manifest (Section 6.3.4) |
+----------+-------------------------------+
| 0x04 | DRIP Frame (Section 6.3.5) |
+----------+-------------------------------+
Table 2
6.3.1.2. SAM Authentication Data
This field has a maximum size of 200-bytes, as defined by
Section 3.3.2. When possible the Broadcast Attestation Structure
(Section 5) should be used in this space.
6.3.2. DRIP Link
This SAM Type is used to transmit Broadcast Attestation's. The
Broadcast Attestation of the Registry (HDA) over the UA MUST be sent
(see Section 7.3). Its structure is defined in [drip-registries] and
an example of it can be found in 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
+---------------+---------------+---------------+---------------+
| |
. .
. Broadcast Attestation .
. .
| |
+---------------+---------------+---------------+---------------+
Broadcast Attestation: (136-bytes)
HDA over UA. Generated by a DRIP Registry during Session ID
registration.
Figure 8: DRIP Link
This DRIP format MUST be used in conjunction with another DRIP SAM
Type (such as Manifest or Wrapper) that contains data that is
guaranteed to be unique and easily cross checked by the receiving
device. A good candidate for this is using the DRIP Wrapper to
encapsulate a Location Message (Message Type 0x2).
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6.3.2.1. Link Limitations
See Section 10.2 for details on why this structure is not dynamically
signed.
6.3.3. DRIP Wrapper
This SAM Type is used to wrap and sign over a list of other [F3411]
Broadcast RID messages. It MUST use the Broadcast Attestation
Structure (Section 5).
The Attestation Data field is filled with full (25-byte) [F3411]
Broadcast RID messages. The minimum number being 1 and the maximum
being 4. The encapsulated messages MUST be in Message Type order as
defined by [F3411]. All message types except Authentication (Message
Type 0x2) and Message Pack (Message Type 0xF) are allowed.
To determine the number of messages wrapped the receiver can check
that the length of the Attestation Data field of the DRIP Broadcast
Attestation (Section 5) is a multiple of 25-bytes.
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
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| ASTM Message |
| |
| |
| |
+ +---------------+---------------+---------------+
| | |
+---------------+ |
| |
| ASTM Message |
| |
| |
| |
+ +---------------+---------------+
| | |
+---------------+---------------+ |
| |
| |
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| ASTM Message |
| |
| |
+ +---------------+
| | |
+---------------+---------------+---------------+ |
| |
| |
| ASTM Message |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
ASTM Message (25 bytes):
Full ASTM Message.
Not Before Timestamp by UA (4-bytes):
Timestamp denoting recommended time to start trusting data.
Not After Timestamp by UA (4 bytes):
Timestamp denoting recommended time to stop trusting data.
UA Signature (64 bytes):
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Signature over preceding fields using the keypair of
the UA.
Figure 9: Example 4-Message DRIP Wrapper
6.3.3.1. Wrapper Limitations
The primary limitation of the Wrapper format is the bounding of up to
4 ASTM Messages that can be sent within it. Another limitation is
that the format can not be used as a surrogate for messages it is
wrapping. This is due to high potential a receiver on the ground
does not support DRIP. Thus when Wrapper is being used the wrapper
data must effectively be sent twice; once as a single framed message
(as specified in [F3411]) and then again wrapped within the Wrapper
format.
6.3.4. DRIP Manifest
This SAM Type is used to create message manifests. It MUST use the
Broadcast Attestation Structure (Section 5).
By hashing previously sent messages and signing them we gain trust in
UAs previous reports. An observer who has been listening for any
considerable length of time can hash received messages and cross-
check against listed hashes. This is a way to evade the limitation
of a maximum of 4 messages in the Wrapper Format and reduce overhead.
The Attestation Data field is filled with 12-byte hashes of previous
[F3411] Broadcast messages. A receiver does not need to have
received every message in the manifest to verify it. A manifest
SHOULD typically encompass a single transmission cycle of messages
being sent, see Section 7.4.
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
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
+---------------+---------------+---------------+---------------+
| |
| Previous Manifest Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| Current Manifest Hash |
| |
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+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
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| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
Previous Manifest Hash (12 bytes):
See Section 6.3.4.3.
Current Manifest Hash (12 bytes):
See Section 6.3.4.3.
ASTM Message Hash (12 bytes):
Hash of a single full ASTM Message. Multiple hashes should
be in Message Type order.
Not Before Timestamp by UA (4-bytes):
Timestamp denoting recommended time to start trusting data.
Not After Timestamp by UA (4 bytes):
Timestamp denoting recommended time to stop trusting data.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
Figure 10: Example DRIP Manifest
6.3.4.1. Message Hash Algorithms and Operation
The hash algorithm used for the Manifest Message is the same hash
algorithm used in creation of the DET [drip-rid] that is signing the
Manifest.
An DET using cSHAKE128 [NIST.SP.800-185] computes the hash as
follows:
cSHAKE128(ASTM Message, 96, "", "Remote ID Auth Hash")
Note: [drip-rid] specifies cSHAKE128 but is open for the
expansion of other OGAs.
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6.3.4.1.1. Legacy Transport Hashing
Under this transport DRIP hashes the full ASTM Message being sent
over the Bluetooth Advertising frame. For Authentication Messages
all the Authentication Message Pages are concatenated together and
hashed as one object. For all other Message Types the 25-byte
message is hashed.
6.3.4.1.2. Extended Transport Hashing
Under this transport DRIP hashes the full ASTM Message Pack (Message
Type 0xF) - regardless of its content.
6.3.4.2. Pseudo-Blockchain Hashes
Two special hashes are included in all Manifest messages; a previous
manifest hash, which links to the previous manifest message, as well
as a current manifest hash. This gives a pseudo-blockchain
provenance to the manifest message that could be traced back if the
observer was present for extended periods of time.
Creation: During creation and signing of this message format this
field MUST be set to 0. So the signature will be based on this
field being 0, as well as its own hash. It is an open question of
if we compute the hash, then sign or sign then compute.
Cycling: There a few different ways to cycle this message. We can
"roll up" the hash of 'current' to 'previous' when needed or to
completely recompute the hash. This mostly depends on the
previous note.
6.3.4.3. Manifest Limitations
A potential limitation to this format is dwell time of the UA. If
the UA is not sticking to a general area then most likely the
Observer will not obtain many (if not all) of the messages in the
manifest. Examples of such scenarios include delivery or survey UA.
Another limitation is the length of hash, which is discussed in
Section 10.1.
6.3.5. DRIP Frame
This SAM Type is for when the authentication data does not fit in
other defined formats under DRIP and is reserved for future expansion
under DRIP if required. This SAM Type SHOULD use the Broadcast
Attestation Structure (Section 5).
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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
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| Frame Type | |
+---------------+ .
. Attestation Data .
. .
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
Frame Type (1 byte):
Sub-type for future different DRIP Frame formats.
Attestation Data (0 to 111 bytes):
Opaque attestation data.
Not Before Timestamp by UA (4-bytes):
Timestamp denoting recommended time to start trusting data.
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Not After Timestamp by UA (4 bytes):
Timestamp denoting recommended time to stop trusting data.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
Figure 11: Example DRIP Frame
6.3.5.1. Frame Type
Byte to sub-type for future different DRIP Frame formats.
+============+==============+==================+
| Frame Type | Name | Description |
+============+==============+==================+
| 0x00 | Reserved | Reserved |
+------------+--------------+------------------+
| 0xC0-0xFF | Experimental | Experimental Use |
+------------+--------------+------------------+
Table 3
6.3.5.2. Frame Limitations
With the Broadcast Attestation Structure only 115-bytes of
Attestation Data are free for use.
7. Requirements & Recommendations
7.1. Legacy Transports
With Legacy Advertisements the goal is to attempt to bring reliable
receipt of the paged Authentication Message. Forward Error
Correction (Section 4) MUST be used when using Legacy Advertising
methods (such as Bluetooth 4.X).
Under ASTM Bluetooth 4.X rules, transmission of dynamic messages are
at least every 1 second. DRIP Authentication Messages typically
contain dynamic data (such as the DRIP Manifest or DRIP Wrapper) and
must be sent at the dynamic rate of 1 per second.
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7.2. Extended Transports
Under the ASTM specification, Bluetooth 5.X Wi-Fi NaN, and Wi-Fi
BEACON transport of Remote ID is to use the Message Pack (Message
Type 0xF) format for all transmissions. Under Message Pack messages
are sent together (in Message Type order) in a single Bluetooth 5
extended frame (up to 9 single frame equivalent messages under
Bluetooth 4.X). Message Packs are required by ASTM to be sent at a
rate of 1 per second (like dynamic messages).
Without any fragmentation or loss of pages with transmission Forward
Error Correction (Section 4) MUST NOT be used as it is impractical.
7.3. Authentication
It is REQUIRED that a UA send the following Authentication Formats to
fulfill the [drip-requirements]:
1. DRIP Link using the Broadcast Attestation of HDA and the UA
(satisfying GEN-1 and GEN-3)
2. Any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest
or DRIP Wrapper) where the UA is dynamically signing data
(satisfying GEN-1 and GEN-2)
It is RECOMMENDED the following set of Authentication Formats are
sent for support of offline Observers:
1. DRIP Link using the Broadcast Attestation of HID Root and the RAA
(CAA) (satisfies GEN-3)
2. DRIP Link using the Broadcast Attestation of RAA (CAA) and the
HDA (USS) (satisfies GEN-3)
3. DRIP Link using the Broadcast Attestation of HDA (USS) and the UA
(satisfies GEN-1 and GEN-3)
4. Any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest
or DRIP Wrapper) where the UA is dynamically signing data
(satisfies GEN-1 and GEN-2)
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7.4. Operational
UAS operation may impact the frequency of sending DRIP Authentication
messages. Where a UA is dwelling in one location, and the channel is
heavily used by other devices, "occasional" message authentication
may be sufficient for an observer. Contrast this with a UA
traversing an area, and then every message should be authenticated as
soon as possible for greatest success as viewed by the receiver.
Thus how/when these DRIP authentication messages are sent is up to
each implementation. Further complication comes in contrasting
Legacy and Extended Transports. In Legacy, each message is a
separate hash within the Manifest. So, again in dwelling, may lean
toward occasional message authentication. In Extended Transports,
the hash is over the Message Pack so only few hashes need to be in a
Manifest. A single Manifest can handle a potential two Message Packs
(for a full set of messages) and a DRIP Link Authentication Message
for the HDA UA assertion.
A separate issue is the frequency of transmitting the DRIP Link
Authentication Message for the HDA UA assertion when using a Manifest
Message. This message content is static; its hash never changes
radically. The only change is the 4-byte timestamp in the
Authentication Message headers. Thus, potentially, in a dwelling
operation it can be sent once per minute, where its hash is in every
Manifest. A receiver can cache all DRIP Link Authentication Message
for the HDA UA assertion to mitigate potential packet loss.
The preferred mode of operation is to send the HDA UA assertion every
3 seconds and Manifest messages immediately after a set of UA
operation messages (e.g. Basic, Location, and System messages).
7.4.1. DRIP Wrapper
The DRIP Wrapper MUST NOT be used in place of sending the ASTM
messages as is. All receivers MUST be able to process all the
messages specified in [F3411]. Only sending them within the DRIP
Wrapper will make them opaque to receivers lacking support for DRIP
authentication messages. Thus messages within a Wrapper are sent
twice: in the clear, and authenticated within the Wrapper. The DRIP
Manifest format would seem to be a more efficient use of the
transport channel.
The DRIP Wrapper has a specific use case for DRIP aware receivers.
For receiver plotting received Location Messages (Message Type 0x2)
on a map display an embedded Location Message in a DRIP Wrapper can
be colored differently to signify trust in the Location data - be it
current or previous Location reports that are wrapped.
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8. ICAO Considerations
DRIP requests the following SAM Type's to be allocated:
1. DRIP Link
2. DRIP Wrapper
3. DRIP Manifest
4. DRIP Frame
9. IANA Considerations
This document requests a new number field for Frame Type with initial
values as defined in Section 6.3.5.1.
10. Security Considerations
10.1. Manifest Hash Length
For DRIP Manifest an 12-byte hash length has been selected by the
authors for a number of reasons.
1. Hash lengths smaller than 8-bytes (for example 4-bytes) were
originally contemplated but ruled out by comments by various
cryptographers. The main concern raised in this forum was that
the length of hash would not provide strong resistance against
collision rate. The authors also after further review agreed
with this and also realized operationally it was not necessarily
viable. While 4-byte hashes would allow more messages to be
filled into a single DRIP Manifest payload (up to 22 individual
hashes) the length of time for the UA to stay in a single place
where the Observer would receive all the originally messages to
rehash to verify such a message was impractical.
2. Hash lengths larger than 8-bytes (for example 12 or 16-bytes)
were also considered by the authors. These got the approval of
the cryptographers but the number of hashes to send became much
lower (only 5 individual hashes). While this lower number is a
more reasonable number of original messages the Observer would
have to capture it would also mean that potentially more DRIP
Manifests would need to be sent. Overall the increase length of
the hash did not operationally justify the cost.
3. Simplifying the current design and locking it into using the same
hash as the HHIT instead of allowing for agility in either hash
algorithm or length seemed more realistic to the authors today.
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10.2. Replay Attacks
The astute reader may note that the DRIP Link messages, which are
recommended to be sent, are static in nature and contain various
timestamps. These Attestation Link messages can easily be replayed
by an attacker who has copied them from previous broadcasts. There
are two things to mitigate this in DRIP:
1. If an attacker (who is smart and spoofs more than just the UAS
ID/data payloads) willing replays an Attestation Link message
they have in principle actually helped by ensuring the message is
sent more frequently and be received by potential Observers.
2. It is RECOMMENDED to send more than just DRIP Link messages,
specifically those that sign over changing data using the current
session keypair, and those messages are sent more frequently. An
UA beaconing these messages then actually signing other messages
using the keypair validates the data receiver by an Observer. An
UA who does not either run DRIP themselves or does not have
possession of the same private key, would be clearly exposed upon
signature verification.
10.3. Trust Timestamp Offsets
Note the discussion of Trust Timestamp Offsets here is in context of
the DRIP Wrapper (Section 6.3.3) and DRIP Manifest (Section 6.3.4)
messages. For DRIP Link (Section 6.3.2) messages these offsets are
set by the Attestor (typically a registry) and have their own set of
considerations as seen in [drip-registries].
The offset of the Trust Timestamp (defined as a very short Expiration
Timestamp) 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
recommended that 3-5 minutes should be sufficient to serve this
purpose in any scenario, but is not limited by design.
11. Acknowledgments
Ryan Quigley and James Mussi of AX Enterprize, LLC for early
prototyping to find holes in the draft specifications.
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Soren Friis for pointing out that Wi-Fi implementations would not
always give access to the MAC Address, 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) - this drove the requirement for max page
length of Authentication Data itself.
12. References
12.1. Normative References
[F3411] "Standard Specification for Remote ID and Tracking",
February 2020.
[NIST.SP.800-185]
Kelsey, J., Change, S., and R. Perlner, "SHA-3 Derived
Functions: cSHAKE, KMAC, TupleHash and ParallelHash", NIST
Special Publication SP 800-185,
DOI 10.6028/nist.sp.800-185, December 2016,
<http://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>.
12.2. Informative References
[drip-registries]
Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP
Registries", Work in Progress, Internet-Draft, draft-
wiethuechter-drip-registries-01, 22 October 2021,
<https://www.ietf.org/archive/id/draft-wiethuechter-drip-
registries-01.txt>.
[drip-requirements]
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>.
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[drip-rid] Moskowitz, R., Card, S. W., Wiethuechter, A., and A.
Gurtov, "UAS Remote ID", Work in Progress, Internet-Draft,
draft-ietf-drip-uas-rid-01, 9 September 2020,
<https://www.ietf.org/archive/id/draft-ietf-drip-uas-rid-
01.txt>.
Appendix A. Authentication State Diagrams & Color Scheme
ASTM Authentication has only 3 states: None, Invalid or Valid. This
is because under ASTM the idea is that Authentication is done by an
external service hosted somewhere on the Internet so it is assumed
you will always get some sort of answer back. With DRIP this
classification becomes more complex with the support of "offline"
scenarios where the receiver does not have Internet connectivity.
With the use of asymmetric keys this means the public key (PK) must
somehow be obtained - [drip-registries] gets more into detail how
these keys are stored on DNS and one reason for DRIP Authentication
is to send PK's over Broadcast RID.
There are two keys of interest: the PK of the UA and the PK of the
HDA (or Registry). This document gives a clear way to send the PK of
the UA over the Broadcast RID messages - however the PK of the
Registry is not. It can be using the same mechanism but is not
required to do so due to potential operational constraints and
implementation of a given UA transmitter. As such there are
scenarios where you may have part of the key-chain but not all of it.
The intent of this appendix is to give some kind of recommended way
to classify these various states and convey it to the user through
colors and state names/text.
A.1. State Table
The table below lays out the RECOMMENDED colors to associate with
state.
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+==============+========+===================================+
| State | Color | Details |
+==============+========+===================================+
| None | Black | No Authentication being received |
+--------------+--------+-----------------------------------+
| Partial | Gray | Authentication being received but |
| | | missing pages |
+--------------+--------+-----------------------------------+
| Unsupported | Brown | Authentication Type/SAM Type of |
| | | received message not supported |
+--------------+--------+-----------------------------------+
| Unverifiable | Yellow | Data needed for verification |
| | | missing |
+--------------+--------+-----------------------------------+
| Verified | Green | Valid verification results |
+--------------+--------+-----------------------------------+
| Trusted | Blue | Valid verification results and |
| | | HDA is marked as trusted |
+--------------+--------+-----------------------------------+
| Questionable | Orange | Inconsistent verification results |
+--------------+--------+-----------------------------------+
| Unverified | Red | Invalid verification results |
+--------------+--------+-----------------------------------+
| Conflicting | Purple | Inconsistent verification results |
| | | and HDA is marked as trusted |
+--------------+--------+-----------------------------------+
Table 4
A.2. State Diagrams
This section gives some RECOMMENDED state flows that DRIP should
follow.
A.2.1. Notations
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o--------------o
| PROCESS |
o--------------o
+--------------+
| STATE |
+--------------+
ooooo
o N o Transition N
ooooo
+-----> Transition Option False/No
-----> Transition Option True/Yes
Figure 12: Diagram Notations
A.2.2. General
o---------------------o ooooo +------+
| Start |---->o 1 o+----->| None |
o---------------------o ooooo +------+
|
v
ooooo +-------------+
o 2 o+----->| Unsupported |
ooooo +-------------+
| ^
v |
+---------+ ooooo |
| Partial |<-----+o 3 o |
+---------+ ooooo |
| |
v +
ooooo ooooo o-------------o
o 4 o------>o 5 o------>| SAM Decoder |
ooooo ooooo o-------------o
+
|
v
o------------------o
| AuthType Decoder |
o------------------o
Figure 13: Standard Authentication Colors/State
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+============+=============================+======================+
| Transition | Transition Query | Next State/Process/ |
| | | Transition (Yes, No) |
+============+=============================+======================+
| 1 | Receiving Authentication | 2, None |
| | Pages? | |
+------------+-----------------------------+----------------------+
| 2 | Authentication Type | 3, Unsupported |
| | Supported? | |
+------------+-----------------------------+----------------------+
| 3 | All Pages of Authentication | 4, Partial |
| | Message Received? | |
+------------+-----------------------------+----------------------+
| 4 | Is Authentication Type | 5, AuthType Decoder |
| | received 5? | |
+------------+-----------------------------+----------------------+
| 5 | Is SAM Type Supported? | SAM Decoder, |
| | | Unsupported |
+------------+-----------------------------+----------------------+
Table 5
A.2.3. DRIP SAM
o-------------o ooooo o-----------------------------o
| SAM Decoder |---->o 6 o------>| DRIP Wrapper/Manifest/Frame |
o-------------o ooooo o-----------------------------o
+ | ^
| | |
v v |
o-----------o o--------------------o |
| DRIP Link |--->| Update State Cache | |
o-----------o o--------------------o |
| |
v |
o--------------o ooooo o----------------------o
| NOP / Return |<------+o 7 o----->| Extract Message from |
o--------------o ooooo | Verification Queue |
o----------------------o
Figure 14: DRIP SAM Decoder
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+============+=====================+========================+
| Transition | Transition Query | Next State/Process/ |
| | | Transition (Yes, No) |
+============+=====================+========================+
| 6 | Is SAM Type DRIP | DRIP Link, DRIP |
| | Link? | Wrapper/Manifest/Frame |
+------------+---------------------+------------------------+
| 7 | Messages in | Extract Message from |
| | Verification Queue? | Verification Queue, |
| | | NOP / Return |
+------------+---------------------+------------------------+
Table 6
A.2.4. DRIP Link
o-----------o ooooo ooooo +--------------+
| DRIP Link |----->o 8 o+----->o 9 o+----->| Unverifiable |
o-----------o ooooo ooooo +--------------+
| |
|-------------'
v
ooooo +------------+
o 10 o+----->| Unverified |
ooooo +------------+
|
v
o---------------------o
| Add UA DET/PK |
| to Key Cache |
o---------------------o
|
v
ooooo +----------+
o 11 o+------>| Verified |
ooooo +----------+
| ^
v |
o-------------------------o
| Mark UA DET/PK |
| as Trusted in Key Cache |
o-------------------------o
Figure 15: DRIP Link State Decoder
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+============+==========================+===========================+
| Transition | Transition Query | Next State/Process/ |
| | | Transition (Yes, No) |
+============+==========================+===========================+
| 8 | Registry DET/PK in Key | 10, 9 |
| | Cache? | |
+------------+--------------------------+---------------------------+
| 9 | Registry PK found | 10, Unverifiable |
| | Online? | |
+------------+--------------------------+---------------------------+
| 10 | Registry Signature | Add UA DET/PK to Key |
| | Verified? | Cache, Unverified |
+------------+--------------------------+---------------------------+
| 11 | Registry DET/PK marked | Mark UA DET/PK as |
| | as Trusted in Key Cache? | Trusted in Key |
| | | Cache, Verified |
+------------+--------------------------+---------------------------+
Table 7
A.2.5. DRIP Wrapper/Manifest/Frame
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o-----------------------------o +--------------+
| DRIP Wrapper/Manifest/Frame | | Unverifiable |
o-----------------------------o +--------------+
| ^
v |
ooooo ooooo o--------------------o
o 12 o+----->o 13 o+----->| Add Message to |
ooooo ooooo | Verification Queue |
| | o--------------------o
| |
|-------------'
v
ooooo ooooo ooooo +------------+
o 14 o+----->o 15 o+----->o 16 o+----->| Unverified |
ooooo ooooo ooooo +------------+
| | |
v v |
ooooo +-------------+ |
o 17 o+----->| Conflicting | |
ooooo +-------------+ |
| |
v v
ooooo +--------------+
o 18 o---------------->| Questionable |
ooooo +--------------+
+
|
v
ooooo +----------+
o 19 o+----->| Verified |
ooooo +----------+
|
v
+---------+
| Trusted |
+---------+
Figure 16: DRIP Wrapper/Manifest/Frame State Decoder
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+============+==============================+======================+
| Transition | Transition Query | Next State/Process/ |
| | | Transition (Yes, No) |
+============+==============================+======================+
| 12 | UA DET/PK in Key Cache? | 14, 13 |
+------------+------------------------------+----------------------+
| 13 | UA PK found Online? | 14, Add Message to |
| | | Verification Queue |
+------------+------------------------------+----------------------+
| 14 | UA Signature Verified? | 17, 15 |
+------------+------------------------------+----------------------+
| 15 | Has past Messages of this | Conflicting, 16 |
| | type been marked as Trusted? | |
+------------+------------------------------+----------------------+
| 16 | Has past Messages of this | Questionable, |
| | type been marked as | Unverified |
| | Questionable or Verified? | |
+------------+------------------------------+----------------------+
| 17 | Has past Messages of this | Conflicting, 18 |
| | type been marked as | |
| | Conflicting? | |
+------------+------------------------------+----------------------+
| 18 | Has past Messages of this | Questionable, 19 |
| | type been marked as | |
| | Questionable or Unverified? | |
+------------+------------------------------+----------------------+
| 19 | Is UA DET/PK marked as | Trusted, Verified |
| | Trusted in Key Cache? | |
+------------+------------------------------+----------------------+
Table 8
Appendix B. HDA-UA Broadcast Attestation
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
+---------------+---------------+---------------+---------------+
| |
| DRIP |
| Entity Tag of HDA |
| |
+---------------+---------------+---------------+---------------+
| |
| DRIP |
| Entity Tag of UA |
| |
+---------------+---------------+---------------+---------------+
| |
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| |
| |
| Host Identity of UA |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by HDA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by HDA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| Signature by HDA |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
DRIP Entity Tag of HDA: (16-bytes)
DET of HDA.
DRIP Entity Tag of UA: (16-bytes)
DET of UA.
Host Identity of UA: (32-bytes)
HI of UA
Expiration Timestamp by HDA (4 bytes):
Timestamp denoting recommended time to trust data to.
Signing Timestamp by HDA (4 bytes):
Current time at signing.
HDA Signature (64 bytes):
Signature over preceding fields using the keypair of
the HDA.
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Figure 17: Example DRIP HDA-UA Broadcast Attestation
Appendix C. Example TX/RX Flow
In this example the UA is sending all DRIP Authentication Message
formats (DRIP Link, DRIP Wrapper and DRIP Manifest) during flight,
along with standard ASTM Messages. The objective is to show the
combinations of messages that must be received to properly validate a
DRIP equipped UA and examples of their various states (Appendix A).
+-------------------+
.-----| Unmanned Aircraft |-----.
| +-------------------+ |
| 1 | 2 | 3 | 4
| | | |
O O O O
--|-- --|-- --|-- --|--
/ \ / \ / \ / \
A B C D
Broadcast Paths: Messages Received
1: DRIP Link
2: DRIP Link and DRIP Wrapper or DRIP Manifest
3: DRIP Wrapper or DRIP Manifest
4: None
Observers: Authentication State
A: Unverifiable
B: Verified, Trusted, Unverified, Questionable, or Conflicting
C: Unverifiable
D: None
As the above example shows to properly authenticate both a DRIP Link
and a DRIP Wrapper or DRIP Manifest are required.
Authors' Addresses
Adam Wiethuechter
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Email: adam.wiethuechter@axenterprize.com
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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
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