DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote ID
draft-ietf-drip-auth-17
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draft-ietf-drip-auth-17
DRIP Working Group A. Wiethuechter (Editor)
Internet-Draft S. Card
Intended status: Standards Track AX Enterprize, LLC
Expires: 9 February 2023 R. Moskowitz
HTT Consulting
8 August 2022
DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote
ID
draft-ietf-drip-auth-17
Abstract
This document describes how to add trust into the Broadcast Remote ID
(RID) specification discussed in the DRIP Architecture. It defines
message types and associated formats (sent within the Authentication
Message) that can be used to authenticate past messages sent by an
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 9 February 2023.
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. UAS Observers and DRIP Authentication . . . . . . . . . . 4
1.1.1. UA Endorsement . . . . . . . . . . . . . . . . . . . 4
1.1.2. DIME Endorsement . . . . . . . . . . . . . . . . . . 4
1.1.3. Chain of DIMEs to Trust Root . . . . . . . . . . . . 5
1.1.4. Authentication Content Correlation . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Required Terminology . . . . . . . . . . . . . . . . . . 5
2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Problem Space and Focus . . . . . . . . . . . . . . . . . 6
3.1.1. Broadcast RID Radio Frequency Options . . . . . . . . 7
3.2. Reasoning for IETF DRIP Authentication . . . . . . . . . 7
3.3. ASTM Authentication Message . . . . . . . . . . . . . . . 7
3.3.1. Authentication Page . . . . . . . . . . . . . . . . . 8
3.3.2. Authentication Payload Field . . . . . . . . . . . . 8
3.3.3. ASTM Constraints . . . . . . . . . . . . . . . . . . 10
4. Forward Error Correction . . . . . . . . . . . . . . . . . . 10
4.1. General Encoding Rules . . . . . . . . . . . . . . . . . 11
4.2. General Decoding Rules . . . . . . . . . . . . . . . . . 11
4.3. Single Page . . . . . . . . . . . . . . . . . . . . . . . 11
4.3.1. Encoding . . . . . . . . . . . . . . . . . . . . . . 11
4.3.2. Decoding . . . . . . . . . . . . . . . . . . . . . . 12
4.4. Multiple Page . . . . . . . . . . . . . . . . . . . . . . 12
4.4.1. Encoding . . . . . . . . . . . . . . . . . . . . . . 13
4.4.2. Decoding . . . . . . . . . . . . . . . . . . . . . . 15
4.5. FEC Limitations . . . . . . . . . . . . . . . . . . . . . 16
5. DRIP Authentication Formats . . . . . . . . . . . . . . . . . 17
5.1. DRIP Authentication Field Definitions . . . . . . . . . . 17
5.1.1. Broadcast Attestation Structure . . . . . . . . . . . 18
5.1.2. SAM Data Format . . . . . . . . . . . . . . . . . . . 20
5.2. DRIP Link . . . . . . . . . . . . . . . . . . . . . . . . 21
5.3. DRIP Wrapper . . . . . . . . . . . . . . . . . . . . . . 21
5.3.1. Wrapper over Extended Transports . . . . . . . . . . 23
5.3.2. Wrapper Limitations . . . . . . . . . . . . . . . . . 24
5.4. DRIP Manifest . . . . . . . . . . . . . . . . . . . . . . 24
5.4.1. Hash Count . . . . . . . . . . . . . . . . . . . . . 26
5.4.2. Message Hash Algorithms and Operation . . . . . . . . 26
5.4.3. Pseudo-Blockchain Hashes . . . . . . . . . . . . . . 26
5.4.4. Manifest Limitations . . . . . . . . . . . . . . . . 27
5.5. DRIP Frame . . . . . . . . . . . . . . . . . . . . . . . 27
5.5.1. Frame Type . . . . . . . . . . . . . . . . . . . . . 28
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6. Requirements & Recommendations . . . . . . . . . . . . . . . 29
6.1. Legacy Transports . . . . . . . . . . . . . . . . . . . . 29
6.2. Extended Transports . . . . . . . . . . . . . . . . . . . 29
6.3. Authentication . . . . . . . . . . . . . . . . . . . . . 29
6.4. Operational . . . . . . . . . . . . . . . . . . . . . . . 30
6.4.1. DRIP Wrapper . . . . . . . . . . . . . . . . . . . . 31
7. Summary of Addressed DRIP Requirements . . . . . . . . . . . 31
8. ICAO Considerations . . . . . . . . . . . . . . . . . . . . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9.1. IANA DRIP Registry . . . . . . . . . . . . . . . . . . . 32
10. Security Considerations . . . . . . . . . . . . . . . . . . . 32
10.1. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 32
10.2. Trust Timestamp Offsets . . . . . . . . . . . . . . . . 33
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.1. Normative References . . . . . . . . . . . . . . . . . . 33
12.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Authentication State Diagrams & Color Scheme . . . . 35
A.1. State Colors . . . . . . . . . . . . . . . . . . . . . . 35
A.2. State Diagrams . . . . . . . . . . . . . . . . . . . . . 36
A.2.1. Notations . . . . . . . . . . . . . . . . . . . . . . 36
A.2.2. General . . . . . . . . . . . . . . . . . . . . . . . 37
A.2.3. DRIP SAM . . . . . . . . . . . . . . . . . . . . . . 38
A.2.4. DRIP Link . . . . . . . . . . . . . . . . . . . . . . 39
A.2.5. DRIP Wrapper/Manifest/Frame . . . . . . . . . . . . . 40
Appendix B. Broadcast Endorsement: DIME, UA . . . . . . . . . . 42
Appendix C. Example TX/RX Flow . . . . . . . . . . . . . . . . . 44
Appendix D. FEC Examples . . . . . . . . . . . . . . . . . . . . 44
D.1. Multiple Page: Page Recovery . . . . . . . . . . . . . . 44
D.2. Multiple Page: Frame Recovery . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46
1. Introduction
Unmanned Aircraft Systems (UAS) operate usually in a volatile
environment when it comes to communication. Unmanned Aircraft (UA)
are generally small with little computational (or flying) horsepower
to carry standard communication equipment. This limits the media of
communication to few viable options.
Observer systems (e.g., smartphones and tablets) place further
constraints on the communication options. The Broadcast Remote ID
(RID) messages must be available to applications on these platforms
without modifying the devices.
As discussed in [RFC9153] two communication schemes to a UAS for
Remote ID (RID) are considered: Broadcast and Network RID.
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This document focuses on adding trust to Broadcast RID (Section 3.2
of [RFC9153]) via the Authentication Message by combining dynamically
signed data with an endorsement of the UA's identity from a DRIP
Identity Management Entity (DIME).
This authentication approach also provides the missing, but United
States (US) Federal Aviation Administration (FAA) mandated, error
correction for the Bluetooth 4.x 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).
A summary of addressed DRIP requirements is provided in Section 7.
1.1. UAS Observers and DRIP Authentication
Without authentication, a UA Observer has no basis for trust. As the
messages are sent via wireless broadcast, they may be sourced
anywhere within wireless range and making any claims desired by the
sender. The DRIP Specific Authentication Methods, as defined herein,
when properly used enables a high level of trust on other ASTM
message content and source. These messages are designed to provide
the Observers with actionable information.
1.1.1. UA Endorsement
When an Observer receives a DRIP-based Authentication Message
(Section 5.3, Section 5.4, Section 5.5) that only contains the UA
DET, timestamps, and signature; it SHOULD use the DRIP Entity Tag
(DET) to retrieve the Host Identity (HI) from DNS (Section 5,
[drip-registries]) or a local cache to validate the signature. Once
the Observer has the DET/HI pair, all further (or cached previous)
DRIP Authentication Messages can be validated. The content signed
over can now be trusted to have been sent by the holder of the
private key corresponding to the HI and DET but not the context of
it.
1.1.2. DIME Endorsement
When an Observer receives a DRIP Link Authentication Message
(Section 5.2), that contains an endorsement of the UA DET DIME
registration (Appendix B); it SHOULD use the DET of the DIME to
retrieve the DIME HI from DNS (Section 5, [drip-registries]) or a
local cache to validate the signature. The UA DET/HI pair is now
known to be registered in a given DIME. As the HI is encapsulated in
the data being endorsed all further (or previously received and
cached) DRIP Authentication Messages using the UA DET can be
validated.
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1.1.3. Chain of DIMEs to Trust Root
An Observer can receive a series of DRIP Link Authentication Messages
(Section 5.2) each one pertaining to a DIMEs registration on the
chain. Similar to Section 1.1.2, each link can be validated. A
chain of DIME Endorsements (Section 1.1.2) can also be obtained via
DNS. This is done by decomposing the received DET and altering the
HID values and performing CERT lookups containing a copy of DIME
Endorsements.
1.1.4. Authentication Content Correlation
While the content of DRIP Authentication Messages can be validated
via their signature this does not resolves issues due to context of
that information (as noted in Section 1.1.1). After signature
validation the Observer MUST use other sources of information, for
example a visual confirmation of UA position, to correlate against
and provided context. When a correlation does not make sense it
SHOULD be rejected as if the signature failed to validate.
2. Terminology
2.1. Required Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. Definitions
This document makes use of the terms defined in [RFC9153]. In
addition, the following terms are defined:
DRIP Entity Tag (DET):
An HHIT that is used as an identifier in DRIP as specified in
[drip-rid].
DRIP Identity Management Entity:
Registry service for DETs and other information in DRIP as
specified in [drip-registries].
Legacy Transports:
use of broadcast frames (Bluetooth 4.x) as specified in [F3411].
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Extended Transports:
use of extended advertisements (Bluetooth 5.x), service info (Wi-
Fi NaN) or vendor specific element information (Wi-Fi BEACON) in
broadcast frames as specified in [F3411]. Must use ASTM Message
Pack (Message Type 0xF).
Hierarchial Host Identity Tag (HHIT):
A special-use, non-routable, IPv6 address constructed as specified
in [drip-rid].
HHIT Domain Authority (HDA):
A class of DIME usually associated with a USS in UTM.
Hierarchial ID (HID):
Encoding of the RAA and HDA into the HHIT structure as defined in
[drip-rid].
Host Identity (HI):
Public key have of an asymmetric keypair used in generating a HHIT
as specified in [drip-rid].
Registered Assigning Authority (RAA):
A class of DIME usually associated with a CAA such as the US FAA.
3. Background
3.1. Problem Space and Focus
The initial standards for RID ([FAA-14CFR], [F3411]) do not 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 for various different parties that have a stake in the
UA industry.
DRIP's goal as stated in the charter is:
to specify how RID can be made trustworthy and available in both
Internet and local-only connected scenarios, especially in
emergency situations.
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This document focuses on providing the first observable "link" of
this trust chain over Broadcast RID; with an importance of the
observer being offline. This first link is the primary stepping
stone for an observer to gain access and use "enhanced related
services".
3.1.1. Broadcast RID Radio Frequency 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
Civil Aviation Authorities (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 the CAA.
Bluetooth 4.x presents a payload size challenge in that it can only
transmit 25 bytes of payload where the others all can support larger
payloads.
3.2. Reasoning for IETF DRIP Authentication
[F3411] defines Authentication Message framing only. It does not
define authentication formats or methods. It explicitly anticipates
several signature options, but does not fully define even those.
[F3411] Annex A1 defines a Broadcast Authentication Verifier Service,
which has a heavy reliance on real-time connectivity to the Internet
(specifically into UTM) that is not always guaranteed. Fortunately,
[F3411] also allows third party standard Authentication Types,
several of which DRIP defines herein.
The standardization of specific formats to support the DRIP
requirements in UAS RID for trustworthy communications over Broadcast
RID is an important part of the chain of trust for a UAS ID. Per
[drip-arch] in Section 5, there is a need to have Authentication
formats to relay information for observers to determine trust. 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.x frame size. To address this, it is
defined as a set of "pages" that each fits into a single Bluetooth
4.x broadcast frame. For other media these pages are still used but
all in a single frame.
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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
This document leverages Authentication Type 0x5, Specific
Authentication Method (SAM), as the principal authentication
container, defining a set of SAM Types in Section 5. This is denoted
in every Authentication Page in the Page Header. The SAM Type is
denoted as a field in the Authentication Payload (see Section 5.1.2).
The Authentication Message is structured as a set of pages. 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 3.3.3
for more details. Over Bluetooth 4.x, these pages are "fragmented"
into separate Bluetooth 4.x broadcast frames.
Either as a single Authentication Message or a set of fragmented
Authentication Message Pages the structure is further wrapped by
outer ASTM framing and the specific link framing (Bluetooth or Wi-
Fi).
3.3.2. Authentication Payload Field
Figure 2 is the source data view of the data fields found in the
Authentication Message as defined by [F3411]. This data is placed
into Figure 1's Authentication Payload, spanning multiple pages.
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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
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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3.3.3. 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 that the
most constrained existing transport can support. Under Broadcast RID
the transport that can hold the least amount of authentication data
is Bluetooth 5.x 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.x, Wi-Fi NaN, and
Wi-Fi BEACON). The Bluetooth 4.x 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.x transmission/reception.
The Bluetooth 4.x lower layers have error detection but not
correction. Any frame in which Bluetooth detects an error is dropped
and not delivered to higher layers (in our case, DRIP). Thus it can
be treated as an erasure.
DRIP supports 2 different FEC encodings. Single Page FEC is the
simpler scheme, but can correct for only a single erased page.
Multiple Page FEC is more complex, but can correct for multiple
erased pages.
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.
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4.1. General Encoding Rules
For DRIP the FEC data MUST start on a new ASTM Authentication Page.
To do this once the results of parity encoding is placed in the
Additional Data field of Figure 2 with null padding before it to line
up with the next page. The Additional Data Length field is set to
number of padding bytes + number of parity bytes.
4.2. General Decoding Rules
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 not exhausted
then the remaining pages should be all FEC. The Additional Data
Length byte can further confirm this; taking into account any null
padding needed for page alignment.
4.3. Single Page
4.3.1. Encoding
To generate the parity a simple XOR operation using the previous
parity page and current page is used. Only the 23-byte
Authentication Payload field of Figure 1 is used in the XOR
operations. For Page 0, a 23-byte null pad is used for the previous
parity page.
Figure 3 shows the last two pages (out of N) of an Authentication
Message using DRIP Single Page FEC. The Additional Data Length is
set to 33 as there is 23-bytes of FEC data and 10-bytes of padding to
line it up into Page N.
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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
4.3.2. Decoding
To decode Single Page FEC in DRIP a rolling XOR is used on each
Authentication Page received in the current Authentication Message.
A Message Counter, outside of the ASTM Message but specified in
[F3411] is used to signal a different Authentication Message and to
correlate pages to them. This Message Counter is only 1-byte in
length, so it will rollover (to 0x00) after reaching its maximum
value (0xFF). If only 1-page is missing in the Authentication
Message the resulting parity bytes should be the data of the erased
page.
4.4. Multiple Page
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4.4.1. Encoding
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 protected.
For DRIP the polynomial to use for Reed Solomon is: 1 + x^2 + x^3 +
x^4 + x^8. This polynomial was selected as it commonly used in Reed
Solomon implementations. A form of it was deployed by the National
Aeronautics and Space Administration (NASA) for the Voyager probes
[VOYAGER]; a problem space with far tighter constraints than RID.
Figure 4 is a generic example of Multiple Page FEC Authentication
Message where 3-pages out of N are used for Reed Solomon FEC. The
Additional Data Length is set to 72 as there are 10-bytes of padding
and 62-bytes of parity from Reed Solomon.
Page N-3:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| Authentication Data / Signature |
| |
| +---------------+---------------+---------------+
| | ADL=72 | |
+---------------+---------------+ |
| Null Padding |
| |
+---------------+---------------+---------------+---------------+
Page N-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
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| |
| Forward Error Correction |
| |
| |
| |
+---------------+---------------+---------------+---------------+
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
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+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| |
| Forward Error Correction |
| |
| |
| |
+---------------+---------------+---------------+---------------+
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 |
| |
| +---------------+---------------+---------------+
| | |
+---------------+ Null Padding |
| |
+---------------+---------------+---------------+---------------+
Figure 4: Example Multiple Page FEC Encoding
Informative Note: the last page in the example is padded to fill
the full page as specified by [F3411].
4.4.1.1. Page Recovery
Page Recovery in Multiple Page FEC protects the same content, just
the Authentication Message, as Single Page FEC. The benefit is
increased protection from a maximum of one page, up to the page
maximum (minus pages being used for authentication).
In Page Recovery, the Authentication Payload field of Figure 1 for
each page is used. Reed Solomon is performed in a byte-wise fashion
across each Authentication Page to generate a number of parity bytes.
The number of these parity bytes directly corresponds to the number
of pages of FEC to be append to the Authentication Message. The
resulting parity bytes are placed to the corresponding bytes in the
FEC pages. This can be considered a form of interleaving that takes
advantage of the fixed page length.
See Appendix D.1 for a detailed example of encoding for Page
Recovery.
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4.4.1.2. Frame Recovery
Frame Recovery in Multiple Page FEC protects not just the
Authentication Message it is carried in but also other ASTM Messages
being sent. This is at the cost of much longer Authentication
Messages. Up to 240 messages (255 minus 15 pages maximum for FEC)
can be protected using Frame Recovery.
For Frame Recovery both transmitter and receiver need to agree on
what messages are being Reed Solomon'd over. It is RECOMMENDED that
the data is limited to a full transmission set of ASTM Messages. For
example in the European Union (EU) a full set of ASTM Messages would
include: 1x Basic ID, 1x Location/Vector, 1x System and 1x Operator
ID. With DRIP this would also included 1x Authentication that would
be carrying the FEC (along with DRIP Authentication).
Similar to Section 4.4.1.1, Reed Solomon is performed in a bytes-wise
fashion across messages to generate the desired number of parity
bytes. These messages MUST be in Message Type order when performing
the Reed Solomon operation. All 25-bytes of the ASTM Message are
used during this operation for Frame Recovery. After the computation
the new pseudo-frames formed by the parity are concatenated together.
The length of this data is used in the calculation of the Additional
Data Length with the amount of padding needed to align to a new
Authentication Page. The padding and parity data are then placed in
the Additional Data field.
See Appendix D.2 for a detailed example of encoding for Frame
Recovery.
4.4.2. Decoding
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.
As it is known which pages were not received in an Authentication
Message (or were erased by Bluetooth due to detected errors), the
Reed Solomon capacity can be dedicated exclusively to correction of
erasures, rather than to detection and correction of errors, thereby
doubling its effective capacity. This is accomplished by marking the
erasures, i.e., filling the dummy page(s) or frames with nulls.
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For either Page Recovery or Frame recovery the first step on the
receiver is to create empty (or dummy) Authentication Pages for any
pages missing in the Authentication Message. Then the Additional
Data can be extracted from the Authentication Message, have its null-
padding removed and further processed.
4.4.2.1. Page Recovery
To decode Page Recovery, the received FEC data along with the
Authentication Payload of each Authentication Page has Reed Solomon
performed using erasures byte-wise across the pages. The results
should have all pages of the Authentication Message recovered. The
receiver SHOULD validate the rebuilt message before decoding the
actual authentication.
4.4.2.2. Frame Recovery
To decode Frame Recovery, the receiver breaks the Additional Data
into 25-byte chunks. This will produce the pseudo-frames of parity
bytes from the Authentication Message.
To build the rest of the message set, static messages such as Basic
ID and Operator ID are constant and can be filled in using any
received copy that is cache by the receiver for that UA. Dynamic
messages in the set, such as Location/Vector or System can be nulled
out to have Reed Solomon alway recover them and the results checked
against cached copies from recent transmissions of the UA.
With all the frames in the set, Reed Solomon can be used in an
erasure mode to decode byte-wise across the frames to fill in the
erasures and rebuild the entire set of messages. Validation of the
Authentication Message SHOULD be performed before further processing
of authentication data.
4.5. 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 to begin at the start of the next page
(Page N+1). In this scenario an entire page (Page N) is being wasted
just to carry the Additional Data Length. This should be avoided
where possible in an effort to maintain efficiency.
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5. DRIP Authentication Formats
All formats defined in this section are the content for the
Authentication Data / Signature field in Figure 2 and use 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.x, then Section 4
MUST be used. Appendix A gives a high-level overview of a state
machine for decoding and determining a trustworthiness state.
Appendix C shows an example of using the formats defined in this
section.
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 5.
Broadcast Endorsement (136-bytes):
DIME HI over UA DET/HI. Generated by a DIME during a UA DET,
being used as a Session ID, registration. Used in Section 5.2.
Current Manifest Hash (12-bytes):
Hash of the current Manifest Message (Section 5.4). See
Section 5.4.3.
Frame Type (1-byte):
Sub-type for future different DRIP Frame formats. See
Section 5.5.1.
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Not Before Timestamp by UA (4-bytes):
Timestamp denoting recommended time to start trusting data in
Figure 5. 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 no earlier than the time the signature is generated.
Not After Timestamp by UA (4-bytes):
Timestamp denoting recommended time to stop trusting data in
Figure 5. 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 after Not Before
Timestamp.
Previous Manifest Hash (12-bytes):
Hash of the previously sent Manifest Message (Section 5.4). See
Section 5.4.3.
UA DRIP Entity Tag (16-bytes):
The UA DET [drip-rid] in byte form (network byte order) and is
part of Figure 5.
UA Signature (64-bytes):
Signature over all 4 preceding fields of Figure 5 using the HI of
the UA.
5.1.1. Broadcast Attestation Structure
Variations of the Attestation Structure format of [drip-registries]
MUST be used when running DRIP Authentication under the DRIP SAM
Types (filling the SAM Authentication Data field (Section 5.1.2.2)).
The only differences are that the timestamps are set by the UA and
the Attestor Identity Information is set to the DET of the UA.
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When using this structure, the UA is minimally self-attesting its
DET. It may be attesting the DET registration in a specific HID (see
Appendix B). The HI of the UA DET can be looked up by mechanisms
described in [drip-registries] or by extracting it from a Broadcast
Endorsement (see Section 5.2 and Section 6.3).
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 |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 5: Broadcast Attestation Structure
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5.1.2. SAM Data Format
Figure 6 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):
Authentication data (opaque to baseline F3411 but parsed by DRIP).
Figure 6: 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 1
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5.1.2.2. SAM Authentication Data
This field has a maximum size of 200-bytes, as defined by
Section 3.3.3. The Broadcast Attestation Structure (Section 5.1.1)
MUST be used in this space.
5.2. DRIP Link
The DRIP Link SAM Type is used to transmit Broadcast Endorsements.
For example, the Broadcast Endorsement: DIME, UA is sent (see
Section 6.3) as a DRIP Link message. The 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 pair that the UA is currently broadcasting. This message
does not require Internet connectivity to perform signature
validations of the contents when the DIME DET/HI is in the receiver's
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 Endorsement .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 7: 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, unpredictable and easily cross checked by
the receiving device. The only [F3411] message type satisfying these
requirements is the Location/Vector Message (Message Type 0x2). The
hash of such a message MAY merely be included in a DRIP Manifest, but
an entire such message SHOULD be encapsulated in a DRIP Wrapper
periodically for stronger security.
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).
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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.
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(s) .
. .
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 8: DRIP Wrapper over Legacy Transports
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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 ATM
Message Pack (Message Type 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 6 and sent in the same Message Pack.
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 9: DRIP Wrapper over Extended Transports
To verify the signature the receiver must concatenate all the
messages in the Message Pack (excluding Authentication Message found
in the same Message Pack) in Message Type order and place them
between the UA DRIP Entity Tag and Not Before Timestamp before
performing signature verification.
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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, avoid when the UA key is obtained
via a DRIP Link Authentication Message.
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.
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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 |
| |
+---------------+---------------+---------------+---------------+
| |
| Previous Manifest Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| Current Manifest Hash |
| |
+---------------+---------------+---------------+---------------+
| |
. .
. ASTM Message Hash(es) .
. .
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by UA |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 10: 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")
Informative 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 11: 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 2
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 RID rules (for example the US FAA RID Rule
[FAA-14CFR]), when using Legacy Advertising methods (such as
Bluetooth 4.x).
Under ASTM Bluetooth 4.x rules, transmission of dynamic messages is
at least every 1 second. DRIP Authentication Messages typically
contain dynamic data (such as the DRIP Manifest or DRIP Wrapper) and
should be sent at the dynamic rate of 1 per second.
6.2. Extended Transports
Under the ASTM specification, Bluetooth 5.x, Wi-Fi NaN, and Wi-Fi
BEACON transport of RID 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.x 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 FEC
(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 requirements in [RFC9153]:
1. SHOULD: send DRIP Link using the Broadcast Endorsement:
DIME:Apex, DIME:RAA (satisfying GEN-3); at last once per 5
minutes
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2. MUST: send DRIP Link using the Broadcast Endorsement: DIME:RAA,
DIME:HDA(satisfying GEN-3); at least once per 5 minutes
3. MUST: send DRIP Link using the Broadcast Endorsement: DIME:HDA,
UA (satisfying ID-5, GEN-1 and GEN-3); at least once per minute
4. MUST: send any other DRIP Authentication Format (RECOMMENDED:
DRIP Manifest or DRIP Wrapper) where the UA is dynamically
signing data that is guaranteed to be unique, unpredictable and
easily cross checked by the receiving device (satisfying ID-5,
GEN-1 and GEN-2); at least once per 5 seconds
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 Broadcast Endorsement: DIME, UA.
A separate issue is the frequency of transmitting the DRIP Link
Authentication Message for the Broadcast Endorsement: DIME, UA 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 Broadcast Endorsement: DIME, UA to mitigate potential packet
loss.
The preferred mode of operation is to send the Broadcast Endorsement:
DIME, UA every 10 seconds and Manifest messages immediately after a
set of UA operation messages (e.g. Basic, Location, and System
messages).
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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. Summary of Addressed DRIP Requirements
The following [RFC9153] are addressed in this document:
ID-5: Non-spoofability
Addressed using the DRIP Wrapper (Section 5.3), DRIP Manifest
(Section 5.4) or DRIP Frame (Section 5.5).
GEN-1: Provable Ownership
Addressed using the DRIP Link (Section 5.2) and DRIP Wrapper
(Section 5.3), DRIP Manifest (Section 5.4) or DRIP Frame
(Section 5.5).
GEN-2: Provable Binding
Addressed using the DRIP Wrapper (Section 5.3), DRIP Manifest
(Section 5.4) or DRIP Frame (Section 5.5).
GEN-3: Provable Registration
Addressed using the DRIP Link (Section 5.2).
8. ICAO Considerations
DRIP requests the following SAM Types to be allocated:
1. DRIP Link
2. DRIP Wrapper
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3. DRIP Manifest
4. DRIP Frame
9. IANA Considerations
9.1. IANA DRIP Registry
This document requests a new subregistry for Frame Type under the
DRIP registry (https://datatracker.ietf.org/doc/html/draft-ietf-drip-
rid-28#section-8.2).
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 |
10. Security Considerations
10.1. 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 DRIP 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 DRIP 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 REQUIRED to send more than 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.
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10.2. 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 DIME) 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.
* 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.
* Many thanks to Rick Salz for the secdir review.
12. References
12.1. Normative References
[drip-arch]
Card, S. W., Wiethuechter, A., Moskowitz, R., Zhao, S.,
and A. Gurtov, "Drone Remote Identification Protocol
(DRIP) Architecture", Work in Progress, Internet-Draft,
draft-ietf-drip-arch-27, 4 August 2022,
<https://www.ietf.org/archive/id/draft-ietf-drip-arch-
27.txt>.
[F3411] "F3411-22a: Standard Specification for Remote ID and
Tracking", July 2022.
[NIST.SP.800-185]
Kelsey, J., Change, S., Perlner, R., and NIST, "SHA-3
derived functions: cSHAKE, KMAC, TupleHash and
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ParallelHash", NIST Special Publications
(General) 800-185, DOI 10.6028/NIST.SP.800-185, December
2016,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-185.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9153] Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A.
Gurtov, "Drone Remote Identification Protocol (DRIP)
Requirements and Terminology", RFC 9153,
DOI 10.17487/RFC9153, February 2022,
<https://www.rfc-editor.org/info/rfc9153>.
12.2. Informative References
[drip-registries]
Wiethuechter, A., Card, S., Moskowitz, R., and J. Reid,
"DRIP Entity Tag (DET) Registration & Lookup", Work in
Progress, Internet-Draft, draft-ietf-drip-registries-05,
11 July 2022, <https://www.ietf.org/archive/id/draft-ietf-
drip-registries-05.txt>.
[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-14CFR]
"Remote Identification of Unmanned Aircraft", January
2021, <https://www.govinfo.gov/content/pkg/FR-2021-01-15/
pdf/2020-28948.pdf>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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[VOYAGER] "Reed-Solomon Codes and the Exploration of the Solar
System", August 1993, <https://trs.jpl.nasa.gov/bitstream/
handle/2014/34531/94-0881.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
DIME. This document gives a clear way to send the PK of the UA over
the Broadcast RID messages. The key of the DIME 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.
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.
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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 |
| | | DIME is marked as trusted |
+--------------+--------+-----------------------------------+
| Questionable | Orange | Inconsistent verification results |
+--------------+--------+-----------------------------------+
| Unverified | Red | Invalid verification results |
+--------------+--------+-----------------------------------+
| Conflicting | Purple | Inconsistent verification results |
| | | and DIME is marked as trusted |
+--------------+--------+-----------------------------------+
Table 3
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 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 4
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 5
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 | DIME DET/PK in Key | 10, 9 |
| | Cache? | |
+------------+-----------------------+---------------------------+
| 9 | DIME PK found Online? | 10, Unverifiable |
+------------+-----------------------+---------------------------+
| 10 | DIME Signature | Add UA DET/PK to Key |
| | Verified? | Cache, Unverified |
+------------+-----------------------+---------------------------+
| 11 | DIME DET/PK marked as | Mark UA DET/PK as Trusted |
| | Trusted in Key Cache? | in Key Cache, Verified |
+------------+-----------------------+---------------------------+
Table 6
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 7
Appendix B. Broadcast Endorsement: DIME, UA
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 DIME |
| |
+---------------+---------------+---------------+---------------+
| |
| DRIP |
| Entity Tag of UA |
| |
+---------------+---------------+---------------+---------------+
| |
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| |
| |
| Host Identity of UA |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| Not Before Timestamp by DIME |
+---------------+---------------+---------------+---------------+
| Not After Timestamp by DIME |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| Signature by DIME |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
DRIP Entity Tag of DIME: (16-bytes)
DET of DIME.
DRIP Entity Tag of UA: (16-bytes)
DET of UA.
Host Identity of UA: (32-bytes)
HI of UA
Expiration Timestamp by DIME (4 bytes):
Timestamp denoting recommended time to trust data to.
Signing Timestamp by DIME (4 bytes):
Current time at signing.
DIME Signature (64 bytes):
Signature over preceding fields using the keypair of
the DIME.
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Figure 17: Example DRIP Broadcast Endorsement: DIME, UA
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 (as described
in 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.
Appendix D. FEC Examples
D.1. Multiple Page: Page Recovery
The following example is 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.
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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. This can be
considered a form of interleaving that takes advantage of the fixed
page length. For the example the following 3-bytes of parity are
generated with the first byte of each page:
dc6c02 = 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
D.2. Multiple Page: Frame Recovery
Below is an example of a number of messages. 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.
10 42012001001000a0014579d8a404d48f2ef9000000000000
11 249600006efeb019ee111ed37a097a0948081c10ffff0000
12 50098960bf8c05042001001000a00145aac6b00abba268b7
12 512001001000a0014579d8a404d48f2ef9bb9a4470ada5b4
12 52ff1352c7402af9d9ebd20034e8d7a12920f4d7e91c1a73
12 53dca7d04e776150825863c512c6eb075a206a95c59b297e
12 54f2935fd416f27b1b42fd5d9dfaa0dec79f32287f41b454
12 557101415def153a770d3e6c0b17ae560809bc634a822c1f
12 563b1064b80a000000000000000000000000000000000000
13 0052656372656174696f6e616c2054657374000000000000
14 02c2ffb019322d1ed3010000c008e40700fc080000000000
15 004e2e4f5031323334353600000000000000000000000000
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A similar process is followed as in Section 4.4.1.1. 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
Authors' Addresses
Adam Wiethuechter
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Email: adam.wiethuechter@axenterprize.com
Stuart Card
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Email: stu.card@axenterprize.com
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Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America
Email: rgm@labs.htt-consult.com
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