DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote ID
draft-ietf-drip-auth-13
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| Last updated | 2022-06-14 | ||
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draft-ietf-drip-auth-13
DRIP Working Group A. Wiethuechter (Editor)
Internet-Draft S. Card
Intended status: Standards Track AX Enterprize, LLC
Expires: 16 December 2022 R. Moskowitz
HTT Consulting
14 June 2022
DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote
ID
draft-ietf-drip-auth-13
Abstract
This document describes how to add 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 16 December 2022.
Copyright Notice
Copyright (c) 2022 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
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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 . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Required Terminology . . . . . . . . . . . . . . . . . . 4
2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Problem Space and Focus . . . . . . . . . . . . . . . . . 4
3.1.1. Broadcast RID RF Options . . . . . . . . . . . . . . 5
3.2. Reasoning for IETF DRIP Authentication . . . . . . . . . 5
3.3. ASTM Authentication Message . . . . . . . . . . . . . . . 5
3.3.1. Authentication Page . . . . . . . . . . . . . . . . . 5
3.3.2. ASTM Constraints . . . . . . . . . . . . . . . . . . 9
4. Forward Error Correction . . . . . . . . . . . . . . . . . . 9
4.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.1. Single Page FEC . . . . . . . . . . . . . . . . . . . 10
4.1.2. Multiple Page FEC . . . . . . . . . . . . . . . . . . 10
4.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Single Page FEC . . . . . . . . . . . . . . . . . . . 13
4.2.2. Multiple Page FEC . . . . . . . . . . . . . . . . . . 13
4.3. FEC Limitations . . . . . . . . . . . . . . . . . . . . . 14
5. DRIP Authentication Formats . . . . . . . . . . . . . . . . . 14
5.1. DRIP Authentication Field Definitions . . . . . . . . . . 14
5.1.1. Broadcast Attestation Structure . . . . . . . . . . . 15
5.1.2. SAM Data Format . . . . . . . . . . . . . . . . . . . 16
5.2. DRIP Link . . . . . . . . . . . . . . . . . . . . . . . . 18
5.3. DRIP Wrapper . . . . . . . . . . . . . . . . . . . . . . 18
5.3.1. Wrapper over Extended Transports . . . . . . . . . . 20
5.3.2. Wrapper Limitations . . . . . . . . . . . . . . . . . 22
5.4. DRIP Manifest . . . . . . . . . . . . . . . . . . . . . . 22
5.4.1. Hash Count . . . . . . . . . . . . . . . . . . . . . 24
5.4.2. Message Hash Algorithms and Operation . . . . . . . . 24
5.4.3. Pseudo-Blockchain Hashes . . . . . . . . . . . . . . 24
5.4.4. Manifest Limitations . . . . . . . . . . . . . . . . 25
5.5. DRIP Frame . . . . . . . . . . . . . . . . . . . . . . . 25
5.5.1. Frame Type . . . . . . . . . . . . . . . . . . . . . 26
6. Requirements & Recommendations . . . . . . . . . . . . . . . 27
6.1. Legacy Transports . . . . . . . . . . . . . . . . . . . . 27
6.2. Extended Transports . . . . . . . . . . . . . . . . . . . 27
6.3. Authentication . . . . . . . . . . . . . . . . . . . . . 27
6.4. Operational . . . . . . . . . . . . . . . . . . . . . . . 28
6.4.1. DRIP Wrapper . . . . . . . . . . . . . . . . . . . . 29
7. ICAO Considerations . . . . . . . . . . . . . . . . . . . . . 29
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8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
8.1. Update IANA DRIP Registry . . . . . . . . . . . . . . . . 29
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
9.1. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 30
9.2. Trust Timestamp Offsets . . . . . . . . . . . . . . . . . 30
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
11.1. Normative References . . . . . . . . . . . . . . . . . . 31
11.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Authentication State Diagrams & Color Scheme . . . . 32
A.1. State Colors . . . . . . . . . . . . . . . . . . . . . . 33
A.2. State Diagrams . . . . . . . . . . . . . . . . . . . . . 33
A.2.1. Notations . . . . . . . . . . . . . . . . . . . . . . 33
A.2.2. General . . . . . . . . . . . . . . . . . . . . . . . 34
A.2.3. DRIP SAM . . . . . . . . . . . . . . . . . . . . . . 35
A.2.4. DRIP Link . . . . . . . . . . . . . . . . . . . . . . 36
A.2.5. DRIP Wrapper/Manifest/Frame . . . . . . . . . . . . . 37
Appendix B. HDA-UA Broadcast Attestation . . . . . . . . . . . . 39
Appendix C. Example TX/RX Flow . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
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.
This authentication methodology also provides the missing, but US FAA
mandated, Error Correction for the Bluetooth 4 transmissions (see
Section 4). This is error correction not only for the authentication
message itself, but indirectly, to other messages authenticated via
the Manifest method (see Section 5.4).
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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
Wrapper or DRIP Manifest.
GEN 3: Provable Registration This requirement is addressed using the
DRIP Link.
See Section 6.3 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).
Extended Transports: uses the extended advertisements (Bluetooth 5),
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.
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3.1.1. Broadcast RID RF Options
A UA has the option of broadcasting using Bluetooth (4 and 5) or Wi-
Fi (BEACON or NAN), see Section 6. With Bluetooth, FAA and other CAA
mandate transmitting simultaneously over both 4 and 5. With Wi-Fi,
use of BEACON is recommended. Wi-Fi NAN is another option, depending
on CAA.
Bluetooth 4 presents a payload size challenge in that it can only
transmit 25 bytes of payload where the others all can support 252
byte payloads.
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] that require use of a
multi-party online validator system. This has a heavy reliance on
real-time connectivity onto the Internet (specifically into UTM) that
is not always guaranteed.
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 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
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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
+---------------+---------------+---------------+---------------+
| 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
The Authentication Message is structured as a set of up to 16 pages.
Over Bluetooth 4, these pages are "fragmented" into separate
Bluetooth 4 broadcast frames.
Either as a single Authentication Message or a set of fragmented
Authentication Message Pages the structure(s) 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 example subset of Authentication Type's
defined and that can be used in the Page Header:
+=====================+================================+
| Authentication Type | Description |
+=====================+================================+
| 0x3 | Message Set Signature |
+---------------------+--------------------------------+
| 0x5 | Specific Authentication Method |
+---------------------+--------------------------------+
Table 1
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3.3.1.1.1. Specific Authentication Method (SAM)
This document leverages Authentication Type 0x5, Specific
Authentication Method (SAM), as the principal authentication
container, defining a set of SAM Types in Section 5. Message Set
Signature (Authentication Type 0x3) is also used in parallel form to
its use in [F3411]. However, the SAM formats provide a more complete
authentication approach.
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 maximum 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)
As defined in F3411.
Authentication Data / Signature: (255-bytes max)
Opaque authentication data.
Additional Data Length (ADL): (1-byte - unsigned)
Length in bytes of Additional Data.
Additional Data: (255-bytes max):
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 source data 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 the first 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, Wi-Fi NaN, and Wi-
Fi BEACON). The Bluetooth 4 Legacy Transport does not have
supporting FEC so with DRIP Authentication the following application
level FEC scheme is used to add FEC. This section is only used for
Bluetooth 4 transmission/reception.
The data added during FEC is not included in the Authentication Data
/ Signature but instead in the Additional Data field of Figure 2.
This may cause the Authentication Message to exceed 9-pages, up to a
maximum of 16-pages.
4.1. Encoding
For any encoding the FEC data MUST start on a new ASTM Authentication
Page. To do this, null padding is added 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 (size of Authentication data per page). That is,
the Page Header (and anything before it) is omitted in the FEC
process.
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4.1.1. Single Page FEC
To generate the parity a simple XOR operation using the previous and
current page is used. Only the 23-byte Authentication Page data is
used in the XOR operation. For Page 0, a 23-byte null pad is used
for the previous page. The resulting parity fills the last 23 bytes
of the Additional Data field of Figure 2 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).
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 variations: Frame Recovery and
Page Recovery. Both follow a similar process, but are offset at what
data is actually protected.
For DRIP the polynomial to use for Reed Solomon is: 1 + x^2 + x^3 +
x^4 + x^8.
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4.1.2.1. Page Recovery
Take the following example of an Authentication Message with 7 pages
that 3 pages of parity are to be generated for. The first column is
just the Page Header with a visual space here to show the boundary.
50 098960bf8c05042001001000a00145aac6b00abba268b7
51 2001001000a0014579d8a404d48f2ef9bb9a4470ada5b4
52 ff1352c7402af9d9ebd20034e8d7a12920f4d7e91c1a73
53 dca7d04e776150825863c512c6eb075a206a95c59b297e
54 f2935fd416f27b1b42fd5d9dfaa0dec79f32287f41b454
55 7101415def153a770d3e6c0b17ae560809bc634a822c1f
56 3b1064b80a000000000000000000000000000000000000
For Page Recovery the first column is ignored and the last 23-bytes
of each page are extracted to have Reed Solomon performed on them in
a column wise fashion to produce parity bytes. For the example the
following 3-bytes of parity are generated with the first byte of each
page:
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). Below is an example of
the full parity generated and each 23-bytes of parity added into the
additional pages as Additional Data:
57 dc6657acd30b2ec4aa582049f52adf9f922e62c469563a
58 6c636a59145a55417a3895fd543f19e94200be4abc5e94
59 02bba5e28f5896d754caf50016a983993b149b5c9e6eeb
4.1.2.2. Frame Recovery
Frame Recovery uses the full ASTM Message and performs Reed Solomon
over each byte. Up to 240 (255 minus 15 pages maximum of FEC data)
messages can be protected using Frame Recovery.
Below is an example of a number of messages. Here the first column
is an additional ASTM Header that contain the Message Type; with a
visual space for clarity. The last 24-bytes are the actual message
contents; be it location information or an Authentication Page.
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10 42012001001000a0014579d8a404d48f2ef9000000000000
11 249600006efeb019ee111ed37a097a0948081c10ffff0000
12 50098960bf8c05042001001000a00145aac6b00abba268b7
12 512001001000a0014579d8a404d48f2ef9bb9a4470ada5b4
12 52ff1352c7402af9d9ebd20034e8d7a12920f4d7e91c1a73
12 53dca7d04e776150825863c512c6eb075a206a95c59b297e
12 54f2935fd416f27b1b42fd5d9dfaa0dec79f32287f41b454
12 557101415def153a770d3e6c0b17ae560809bc634a822c1f
12 563b1064b80a000000000000000000000000000000000000
13 0052656372656174696f6e616c2054657374000000000000
14 02c2ffb019322d1ed3010000c008e40700fc080000000000
15 004e2e4f5031323334353600000000000000000000000000
A similar process is followed as in Section 4.1.2.1. Here every
column of bytes has parity generated for it (even the ASTM Header).
In the below example 5-bytes of parity are generated using the ASTM
Header column:
6c3f42b8a8 = ReedSolomon.encoder(101112121212121212131415)
After doing this to all columns the following pseudo-frames would
have been generated:
6c86337bf7ab746f5d62bb7f8de954104b121585d3975f6e92
3f06c1bce165b0e25930d57a63c24f751145e1dd8dc115029b
42e9979580327a6a14d421c12a33aa2e1a2e517daaee581016
b8012a7b3964f7b2720d387bfa77e945556f1831cd477ef3a3
a85bb403aada89926fb8fc2a14a9caacb4ec2f3a6ed2d8e9f9
These 25-byte chunks are now concatenated together and are placed in
Authentication Pages, using the Additional Data, 23-bytes at a time.
In the below figure the first column is the ASTM Header as before,
the second column is the Page Header for each Authentication Page and
then last column is the 23-bytes of data for each page.
12 57 6c86337bf7ab746f5d62bb7f8de954104b121585d3975f
12 58 6e923f06c1bce165b0e25930d57a63c24f751145e1dd8d
12 59 c115029b42e9979580327a6a14d421c12a33aa2e1a2e51
12 5a 7daaee581016b8012a7b3964f7b2720d387bfa77e94555
12 5b 6f1831cd477ef3a3a85bb403aada89926fb8fc2a14a9ca
12 5c acb4ec2f3a6ed2d8e9f900000000000000000000000000
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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. DRIP Authentication Formats
All formats defined in this section are the content for the
Authentication Data / Signature field in Figure 2 and uses the
Specific Authentication Method (SAM, Authentication Type 0x5). The
first byte of the Authentication Data / Signature of Figure 2, is
used to multiplex between these various formats.
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.
5.1. DRIP Authentication Field Definitions
ASTM Message (25-bytes): Full ASTM Message as defined in [F3411]
specifically Message Types 0x0, 0x1, 0x3, 0x4, and 0x5
ASTM Message Hash (12-bytes): Hash of a single full ASTM Message
using hash operations described in (Section 5.4.2). Multiple
hashes MUST be in Message Type order.
Attestation Data (0 to 112 bytes): Opaque attestation data that the
UA is attesting during its flight in Figure 4.
Broadcast Attestation (136-bytes): HDA HI over UA DET/HI. Generated
by a DRIP Registry during Session ID registration. Used in
Section 5.2.
Current Manifest Hash (12-bytes): See Section 5.4.3.
Frame Type (1-byte): Sub-type for future different DRIP Frame
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formats. See Section 5.5.1.
Not Before Timestamp by UA (4-bytes): Timestamp denoting recommended
time to start trusting data in Figure 4. MUST follow the format
defined in [F3411]. That is a Unix-style timestamp but with an
epoch of 01/01/2019 00:00:00. MUST be set to the time the
signature is generated.
Not After Timestamp by UA (4-bytes): Timestamp denoting recommended
time to stop trusting data in Figure 4. MUST follow the format
defined in [F3411]. That is a Unix-style timestamp but with an
epoch of 01/01/2019 00:00:00 with an additional offset is then
added to push a short time into the future (relative to Not Before
Timestamp) to avoid replay attacks. 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.
Previous Manifest Hash (12-bytes): See Section 5.4.3.
UA DRIP Entity Tag (16-bytes): The UA DET in byte form (network byte
order) and is part of Figure 4.
UA Signature (64-bytes): Signature over preceding fields of Figure 4
using the HI of the UA.
5.1.1. Broadcast Attestation Structure
Variations of the Attestation Structure format of [drip-registries]
SHOULD be used when running DRIP Authentication under the DRIP SAM
Types (filling the SAM Authentication Data field (Section 5.1.2.2)).
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 minimally self-attesting its DRIP
Entity Tag (DET). It may be attesting the DET registration in a
specific HID (see Appendix B). 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 5.2 and
Section 6.3).
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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 |
| |
+---------------+---------------+---------------+---------------+
| |
. .
. Attestation Data .
. .
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 4: Broadcast Attestation Structure
5.1.2. SAM Data Format
Figure 5 is the general format to hold authentication data when using
SAM and is placed inside the Authentication Data / Signature field in
Figure 2.
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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
+---------------+---------------+---------------+---------------+
| 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 5: SAM Data Format
5.1.2.1. SAM Type
The SAM Type field is maintained by the International Civil Aviation
Organization (ICAO) and for DRIP four are planned to be allocated:
+==========+=============================+
| SAM Type | Description |
+==========+=============================+
| 0x01 | DRIP Link (Section 5.2) |
+----------+-----------------------------+
| 0x02 | DRIP Wrapper (Section 5.3) |
+----------+-----------------------------+
| 0x03 | DRIP Manifest (Section 5.4) |
+----------+-----------------------------+
| 0x04 | DRIP Frame (Section 5.5) |
+----------+-----------------------------+
Table 2
5.1.2.2. SAM Authentication Data
This field has a maximum size of 200-bytes, as defined by
Section 3.3.2. The Broadcast Attestation Structure (Section 5.1.1)
SHOULD be used in this space.
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5.2. DRIP Link
This SAM Type is used to transmit Broadcast Attestations. For
example, the Broadcast Attestation of the Registry (HDA) over the UA
is sent (see Section 6.3) as a DRIP Link message. Its structure is
defined in [drip-registries] and an example of it can be found in
Appendix B.
DRIP Link is important as its contents are used to provide trust in
the DET/HI that the UA is currently broadcasting. This message does
not require internet connectivity to perform signature validations of
the contents when the registry DET/HI is in the receivers cache. It
also provides the UA HI so that connectivity is not required when
performing validation of other DRIP Authentication Messages.
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 .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 6: DRIP Link
This DRIP Authentication Message is used in conjunction with other
DRIP SAM Types (such as Manifest or Wrapper) that contain 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).
5.3. 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 5.1.1).
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 5.1.1) is a multiple of 25-bytes.
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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 |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| ASTM Message |
| |
| |
| |
+ +---------------+---------------+---------------+
| | |
+---------------+ |
| |
| ASTM Message |
| |
| |
| |
+ +---------------+---------------+
| | |
+---------------+---------------+ |
| |
| |
| ASTM Message |
| |
| |
+ +---------------+
| | |
+---------------+---------------+---------------+ |
| |
| |
| ASTM Message |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
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| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 7: Example 4-Message DRIP Wrapper
5.3.1. Wrapper over Extended Transports
To send the DRIP Wrapper over Extended Transports the messages being
wrapped are co-located with the Authentication Message in a Message
Pack (0xF). The ASTM Messages are removed from the DRIP Wrapper
after signing (as they are redundant) leaving the following structure
that is placed into the SAM Authentication Data of Figure 5 and sent
in the same Message Pack.
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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 |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 8: DRIP Wrapper under Extended Transports
To verify the signature the receiver must concatenate all of the
messages in the Message Pack (excluding Authentication Message found
in the same Message Pack) in Message Type order and place the blob
between the UA DRIP Entity Tag and Not Before Timestamp before
performing signature verification.
The functionality of Wrapper in this form is identical to
Authentication Type 0x3 (Message Set Signature) when running over
Extended Transports. What Wrapper provides is the same format but
over both Extended and Legacy Transports allowing the transports to
be similar. Message Set Signature also implies using the ASTM
validator system architecture which relies on Internet connectivity
for verification which the receiver may not have at the time of
receipt of an Authentication Message. This is something Wrapper, and
all DRIP Authentication Formats, avoids when the UA key is obtained
via a DRIP Link Authentication Message.
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5.3.2. 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.
5.4. DRIP Manifest
This SAM Type is used to create message manifests. It MUST use the
Broadcast Attestation Structure (Section 5.1.1).
By hashing previously sent messages and signing them we gain trust in
UAs previous reports. An observer who has been listening for any
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 6.4.
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 |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
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| |
| 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 |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 9: Example DRIP Manifest
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5.4.1. Hash Count
The number of hashes in the Manifest can be variable (3-9). An easy
way to determine the number of hashes is to take the length of the
data between the end of the UA DRIP Entity Tag and Not Before
Timestamp by UA and divide it by the hash length (12). If this value
is not rational, the message is invalid.
5.4.2. 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.
5.4.2.1. 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.
5.4.2.2. Extended Transport Hashing
Under this transport DRIP hashes the full ASTM Message Pack (Message
Type 0xF) - regardless of its content.
5.4.3. 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.
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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.
5.4.4. 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.
5.5. 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 MUST use the Broadcast
Attestation Structure (Section 5.1.1).
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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 | |
+---------------+ .
. Frame Attestation Data .
. .
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 10: Example DRIP Frame
5.5.1. Frame Type
Byte to sub-type for future different DRIP Frame formats. It takes
the first byte of Attestation Data in Section 5.1.1 leaving 111-bytes
for Frame Attestation Data.
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+============+==============+==================+
| Frame Type | Name | Description |
+============+==============+==================+
| 0x00 | Reserved | Reserved |
+------------+--------------+------------------+
| 0xC0-0xFF | Experimental | Experimental Use |
+------------+--------------+------------------+
Table 3
6. Requirements & Recommendations
6.1. Legacy Transports
With Legacy Advertisements the goal is to attempt to bring reliable
receipt of the paged Authentication Message. FEC (Section 4) MUST be
used, per mandated Remote ID rules (for example the US FAA Remote ID
Rule [faa-rid]), when using Legacy Advertising methods (such as
Bluetooth 4).
Under ASTM Bluetooth 4 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.
6.2. Extended Transports
Under the ASTM specification, Bluetooth 5, 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). 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.
6.3. 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)
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2. Any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest
or DRIP Wrapper) where the UA is dynamically signing data that is
guaranteed to be unique and easily cross checked by the receiving
device (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 that is
guaranteed to be unique and easily cross checked by the receiving
device (satisfying GEN-1 and GEN-2)
6.4. 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.
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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).
6.4.1. 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]. Sending them within the DRIP Wrapper
makes 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.
7. ICAO Considerations
DRIP requests the following SAM Type's to be allocated:
1. DRIP Link
2. DRIP Wrapper
3. DRIP Manifest
4. DRIP Frame
8. IANA Considerations
8.1. Update IANA DRIP Registry
This document requests a new subregistry for Frame Type.
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DRIP Frame Type: This 8-bit valued subregistry is for Frame Types in
DRIP Frame Authentication Messages. Future additions to this
subregistry are to be made through Expert Review (Section 4.5 of
[RFC8126]). The following values are defined:
+============+==============+==================+
| Frame Type | Name | Description |
+============+==============+==================+
| 0x00 | Reserved | Reserved |
+------------+--------------+------------------+
| 0xC0-0xFF | Experimental | Experimental Use |
+------------+--------------+------------------+
Table 4
9. Security Considerations
9.1. 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.
9.2. Trust Timestamp Offsets
Note the discussion of Trust Timestamp Offsets here is in context of
the DRIP Wrapper (Section 5.3) and DRIP Manifest (Section 5.4)
messages. For DRIP Link (Section 5.2) messages these offsets are set
by the Attestor (typically a registry) and have their own set of
considerations as seen in [drip-registries].
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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.
10. Acknowledgments
Ryan Quigley and James Mussi of AX Enterprize, LLC for early
prototyping to find holes in the draft specifications.
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 maximum page
length of Authentication Data itself.
11. References
11.1. 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>.
11.2. Informative References
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[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>.
[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>.
[faa-rid] United States Federal Aviation Administration (FAA),
"Remote Identification of Unmanned Aircraft", 2021,
<https://www.govinfo.gov/content/pkg/FR-2021-01-15/
pdf/2020-28948.pdf>.
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. The key of the HDA can be
sent over Broadcast RID using the same mechanisms (see Section 5.2
and Section 6.3) but is not required due to potential operational
constraints of sending multiple DRIP Link messages. As such there
are scenarios where you may have part of the key-chain but not all of
it.
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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 Colors
The table below lays out the RECOMMENDED colors to associate with
state.
+==============+========+===================================+
| 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 5
A.2. State Diagrams
This section gives some RECOMMENDED state flows that DRIP should
follow. Note that the state diagrams do not have all error
conditions mapped.
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 11: 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 12: 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 6
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 13: 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 7
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 14: 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 8
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 15: 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 9
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 16: 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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