| Internet-Draft | auth-formats | June 2021 |
| Wiethuechter, et al. | Expires 20 December 2021 | [Page] |
This document describes how to include trust into the ASTM Remote ID specification defined in ASTM F3411-19 under a Broadcast Remote ID (RID) scenario. It defines a few different message schemes (based on the Authentication Message) that can be used to assure past messages sent by a UA and also act as an assurance for UA trustworthiness in the absence of Internet connectivity at the receiving node.¶
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UA 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 standard [F3411-19] focuses on two ways of communicating to a UAS for RID: Broadcast and Network.¶
This document will focus on adding trust to Broadcast RID in the current (and an expanded) Authentication Message format.¶
The following [drip-requirements] will be addressed:¶
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.¶
See [drip-requirements] for common DRIP terms.¶
The current standard for Remote ID (RID) 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.¶
The following subsections will provide a high level reference to the ASTM standard for Authentication Messages and how their current limitations effect trust in the Broadcast RID environment.¶
The ASTM Authentication Message has provisions in [F3411-19] to allow for other organizations to define (and standardize) Authentication formats. The standardization of special 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 by ASTM or others) was flexible enough to satisfy this goal resulting in the work reflected in this document.¶
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+---------------+-----------------------------------------------+
| Auth Header | |
+---------------+ ASTM Authentication Headers +---------------+
| | |
+-----------------------------------------------+ |
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| Authentication Data / Signature |
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+---------------------------------------------------------------+
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+---------------+-----------------------------------------------+
| Auth Header | |
+---------------+ |
| |
| |
| |
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| Authentication Data / Signature |
| |
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+---------------------------------------------------------------+
Auth Header (1 byte):
Contains Authentication Type (AuthType) and Page Number.
ASTM Authentication Headers: (6 bytes)
Contains other header information for the Authentication
Message from ASTM UAS RID Standard.
Authentication Data / Signature: (0 to 255 bytes)
Opaque authentication data.
The above diagram is the format defined by ASTM [F3411-19] that is the frame which everything this document fits into. The specific details of the ASTM headers are abstracted away as they are not necessarily required for this document.¶
There is a 25th byte exclude in the diagrams that comes before the Auth Header. This is the ASTM Header and consists of the Protocol Version and Message Type of the given message frame/page.¶
To keep consistent formatting across the different mediums (Bluetooth 4, Bluetooth 5 and WiFi NaN) and their independent restrictions the authentication data being sent is REQUIRED to fit within the first 9 pages (Page 0 through Page 8) of the Authentication Message (giving a max of 201 bytes). The rest of the pages of the message is reserved exclusively for Forward Error Correction bytes and is only present on Bluetooth 4.¶
The existing ASTM [F3411-19] Authentication Type 0x1 can be used to send a fresh Self-Attestation of the UA over 7 pages.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +---------------+---------------+---------------+---------------+ | | | UA Hierarchical | | Host Identity Tag | | | +---------------+---------------+---------------+---------------+ | | | | | | | UA Host Identity | | | | | | | | | +---------------+---------------+---------------+---------------+ | Trust Timestamp | +---------------+---------------+---------------+---------------+ | Signing Timestamp | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | UA Signature | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+
The existing ASTM [F3411-19] Authentication Type 0x2 can be used to send a static Self-Attestation of the Operator over 7 pages.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +---------------+---------------+---------------+---------------+ | | | Operator Hierarchical | | Host Identity Tag | | | +---------------+---------------+---------------+---------------+ | | | | | | | Operator Host Identity | | | | | | | | | +---------------+---------------+---------------+---------------+ | Trust Timestamp | +---------------+---------------+---------------+---------------+ | Signing Timestamp | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Operator Signature | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+
When running under Extended Advertisements, the existing ASTM [F3411-19] Authentication Type 0x3 can be used to sign over the adjacent ASTM Messages in the Message Pack (0xF).¶
The concatenation of all messages in the Message Pack (excluding Authentication) before signing MUST be in Message Type order and be placed between the UA HHIT and Signing Timestamp field.¶
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 Hierarchical | | Host Identity Tag | | | +---------------+---------------+---------------+---------------+ | Trust Timestamp | +---------------+---------------+---------------+---------------+ | Signing Timestamp | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | UA Signature | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+
Under Specific Method (Authentication Type 0x5) is where the main set of DRIP Authentication Formats are defined. These formats unlike the previous ones are more well defined and can include Forward Error Correction data.¶
This is specified when the SAM ID is DRIP Frame. It is encapsulated by the ASTM Authentication Message (Section 3.3) and fills the Authentication Data / Signature field in Figure 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
+---------------+---------------+---------------+---------------+
| SAM ID | |
+---------------+ |
. .
. DRIP Authentication Data .
. .
| |
+---------------+---------------+---------------+---------------+
| |
. .
. Forward Error Correction .
. .
| |
+---------------+---------------+---------------+---------------+
SAM ID (1 byte):
The SAM ID (Specific Authentication Method ID) is
defined by ASTM under AuthType 0x5 and values allocated
by ICAO.
For DRIP there are four SAM IDs allocated:
SAM ID | Value
-----------------------------+-------
DRIP Frame | 0
DRIP Wrapper | 1
DRIP Manifest | 2
DRIP Link | 3
DRIP Authentication Data (0 to 200 bytes):
DRIP Authentication data.
Forward Error Correction (0 to 161 bytes):
Forward Error Correction data.
Defined by ASTM (only under AuthType 0x5), values are allocated by ICAO. For DRIP there are four SAM IDs: DRIP Frame, DRIP Wrapper, DRIP Manifest and DRIP Link.¶
This field has a maximum size of 200 bytes. If the data is less than the max and a page is only partially filled then the rest of the partially filled page must be null padded. Note that the Length field in the Authentication Message is set to the length of the DRIP Authentication Data and MUST NOT include the Forward Error Correction.¶
When possible the DRIP Broadcast Attestation Structure (Section 12) should be used in this space.¶
This field has a maximum size of 161 bytes and SHOULD be page aligned at start. The number of pages present after the data indicate the FEC scheme. When a single page of FEC is present an XOR operation MUST be used. When there are multiple pages of FEC (2 or more) a Reed Solomon method MUST be used.¶
See Section 13 for more.¶
This is specified when the SAM ID is DRIP Wrapper. It is encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast Attestation Structure (Section 12); filling the Attestation Data (Section 12.2) field with full (25-byte) ASTM Messages. The minimum number of ASTM Messages being 1 (Editors Note: Is this minimum 1 or 0?) and the max being 4. The encapsulated ASTM Messages MUST be in Message Type order as defined by ASTM. All message types except Authentication (0x2) and Message Pack (0xF) are allowed.¶
To determine the number of messages wrapped the receiver can check that the length of the Attestation Data (Section 12.2) field of the DRIP Broadcast Attestation (Section 12) is a multiple of 25-bytes.¶
This format is specified when SAM ID is set to DRIP Manifest. It is encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast Attestation Structure (Section 12); filling the Attestation Data (Section 12.2) field with 8-byte hashes of previous ASTM Messages.¶
By hashing previously sent messages and signing them we gain trust in UAs previous reports. An observer who has been listening for any considerable length of time can hash received messages and cross check against listed hashes. This is a way to evade the limitation of a maximum of 4 messages in the Wrapper Format and reduce overhead.¶
(Editors Note: Manifests MUST NOT be of a length multiple of 25-bytes or 48-bytes.)¶
The hash algorithm used for the Manifest Message is the same hash algorithm used in creation of the HHIT that is signing the Manifest.¶
A standard HHIT would be using cSHAKE128 from [NIST.SP.800-185]. With cSHAKE128, the hash is computed as follows:¶
cSHAKE128(Message, 128, "", "Remote ID Auth Hash")¶
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.¶
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. Without the original messages received no verification can be done. Examples of such scenarios include delivery or survey UA.¶
Another limitation is the length of hash, which is discussed in Section 8.¶
This format is specified when SAM ID is set to DRIP Link. It is encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast Attestation Structure (Section 12) but the attestation has already taken place, thus the UA need not dynamically sign the structure.¶
See Broadcast Attestation as defined in [drip-rid] and Section 11.6.¶
With Legacy Advertisements the goal is to attempt to bring reliable receipt of the paged Authentication Message. Forward Error Correction (Section 4.4.1.3) MUST be enabled when using Legacy Advertising methods (such as Bluetooth 4.X).¶
Under ASTM Bluetooth 4.X rules, transmission of dynamic messages are at least every 1 second while static messages (which is what Authentication is classified under) are sent at least every 3 seconds.¶
Under DRIP the Certificate Message MUST be transmitted to properly meet the GEN 1 and GEN 3 requirement.¶
The ASTM Message Wrapper and Manifest both satisfy the GEN 2 requirement. At least one MUST be implemented to comply with the GEN 2 requirement.¶
A single Manifest can carry at most (using the full 10 page limit and 8 byte hashes) 12 unique hashes of previously sent messages (of any type). This results in a total of 22 (12 + 10) frames of Bluetooth data being transmitted over Bluetooth.¶
In comparison the Message Wrapper sends 6 pages (each a single frame) for each wrapped message. For backwards compatibility the implementation should also send the standard ASTM message that was wrapped for non-DRIP compliant receivers to obtain. This method results in 84 total Bluetooth frames (12 + (12 * 6)) sent.¶
The question of which is better suited is up to the implementation.¶
Under the ASTM specification, Bluetooth 5 or WiFi NaN transport of Remote ID is to use the Message Pack (Type 0xF) format for all transmissions. Under Message Pack all messages are sent together (in Message Type order) in a single Bluetooth frame (up to 9 single frame equivalent messages). 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 4.4.1.3) MUST NOT be used as it is impractical.¶
For DRIP it is RECOMMENDED the following Authentication Formats are sent:¶
DRIP requests the following SAM IDs to be allocated:¶
This document does not require any actions by IANA.¶
For DRIP Manifest an 8-byte hash length has been selected by the authors for a number of reasons.¶
The astute reader may note that the DRIP Link messages, which are recommended to be sent under DRIP, are static in nature and contain various timestamps. These Attestation Link message can easily be replayed by an attacker who has copied them from previous broadcasts. There are two things to mitigate this in DRIP:¶
Note the discussion of Trust Timestamp Offsets here is in context of the DRIP Wrapper (Section 4.4.2) and DRIP Manifest (Section 4.4.3) messages. For DRIP Link (Section 4.4.4) messages these offsets are set by the Attestor (typically a registry) and have their own set of considerations as seen in (TODO: link to registry draft security considerations here).¶
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.¶
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 WiFi protocols would not give access to the MAC Address, originally used in calculation of the hashes for DRIP Manifest. Also for confirming that Message Packs (0xF) can only carry up to 9 ASTM frames worth of data (9 Authentication pages) - this drove the requirement for max page length of Authentication Data itself.¶
(Editor Note: is this valid anymore to keep?)¶
The format standardized by the ASTM is designed with a few major considerations in mind, which the authors of this document feel put significant limitations on the expansion of the standard.¶
The primary consideration (in this context) is the use of the Bluetooth 5.X Extended Frame format. This method allows for a 255 byte payload to be sent in what the ASTM refers to as a "Message Pack".¶
The idea is to include up to five standard ASTM Broadcast RID messages (each of which are 25 bytes) plus a single authentication message (5 pages of 25 bytes each) in the Message Pack. The reasoning is then the Authentication Message is for the entire Message Pack.¶
The authors have no issues with this proposed approach; this is a valid format to use for the Authentication Message provided by the ASTM. However, by limiting the Authentication Message to ONLY five pages in the standard it ignores the possibility of other formatting options to be created and used.¶
Another issue with this format, not fully addressed in this document is fragmentation. Under Bluetooth 4.X, each page is sent separately which can result in lose of pages on the receiver. This is disastrous as the loss of even a single page means any signature is incomplete.¶
With the current limitation of 5 pages, Forward Error Correction (FEC) is nearly impossible without sacrificing the amount of data sent. More pages would allow FEC to be performed on the Authentication Message pages so loss of pages can be mitigated.¶
All these problems are further amplified by the speed at which UA fly and the Observer's position to receive transmissions. There is no guarantee that the Observer will receive all the pages of even a 5 page Authentication Message in the time it takes a UA to traverse across their line of sight. Worse still is that is not including other UA in the area, which congests the spectrum and could cause further confusion attempting to collate messages from various UA. This specific problem is out of scope for this document and our solutions in general, but should be noted as a design consideration.¶
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 +---------------+---------------+---------------+---------------+ | | | Hierarchical | | Host Identity Tag | | | +---------------+---------------+---------------+---------------+ | | | | | | | Host Identity | | | | | | | | | +---------------+---------------+---------------+---------------+ | Trust Timestamp | +---------------+---------------+---------------+---------------+ | Signing Timestamp | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Signature | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+ Length = 120-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 +---------------+---------------+---------------+---------------+ | | . . . Axx . . . | | +---------------+---------------+---------------+---------------+ | | . . . Ayy . . . | | +---------------+---------------+---------------+---------------+ | Trust Timestamp by X | +---------------+---------------+---------------+---------------+ | Signing Timestamp by X | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Signature by X | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+ Length = 312-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 +---------------+---------------+---------------+---------------+ | | | Hierarchical | | Host Identity Tag of X | | | +---------------+---------------+---------------+---------------+ | | | Hierarchical | | Host Identity Tag of Y | | | +---------------+---------------+---------------+---------------+ | Trust Timestamp by X | +---------------+---------------+---------------+---------------+ | Signing Timestamp by X | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Signature by X | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+ Length = 104-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 +---------------+---------------+---------------+---------------+ | | . . . Axy . . . | | +---------------+---------------+---------------+---------------+ | Trust Timestamp by Y | +---------------+---------------+---------------+---------------+ | Signing Timestamp by Y | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Signature by Y | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+ Length = 384-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 +---------------+---------------+---------------+---------------+ | | . . . C-Axy . . . | | +---------------+---------------+---------------+---------------+ | Trust Timestamp by Y | +---------------+---------------+---------------+---------------+ | Signing Timestamp by Y | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Signature by Y | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+ Length = 176-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 +---------------+---------------+---------------+---------------+ | | | Hierarchical | | Host Identity Tag of X | | | +---------------+---------------+---------------+---------------+ | | | Hierarchical | | Host Identity Tag of Y | | | +---------------+---------------+---------------+---------------+ | | | | | | | Host Identity of Y | | | | | | | | | +---------------+---------------+---------------+---------------+ | Trust Timestamp by X | +---------------+---------------+---------------+---------------+ | Signing Timestamp by X | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Signature by X | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+ Length = 136-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 +---------------+---------------+---------------+---------------+ | | | Hierarchical | | Host Identity Tag of Z | | | +---------------+---------------+---------------+---------------+ | | . . . L-Axy . . . | | +---------------+---------------+---------------+---------------+ | Trust Timestamp by Z | +---------------+---------------+---------------+---------------+ | Signing Timestamp by Z | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Signature by Z | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+ Length = 264-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 +---------------+---------------+---------------+---------------+ | | . . . Azz . . . | | +---------------+---------------+---------------+---------------+ | | . . . M-Axy . . . | | +---------------+---------------+---------------+---------------+ | Trust Timestamp by Z | +---------------+---------------+---------------+---------------+ | Signing Timestamp by Z | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | Signature by Z | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+ Length = 264-bytes
When possible the following format should be used in the DRIP Authentication Data (Section 4.4.1.2) field.¶
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
+---------------+---------------+---------------+---------------+
| |
| Attestor Hierarchical |
| Host Identity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
. .
. Attestation Data .
. .
| |
+---------------+---------------+---------------+---------------+
| Trust Timestamp |
+---------------+---------------+---------------+---------------+
| Signing Timestamp |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| Attestor Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Attestor Hierarchial Host Identity Tag (16 bytes):
The Attestors HHIT in byte form (network byte order).
Attestation Data (0 to 112 bytes):
Opaque attestation data.
Trust Timestamp (4 bytes):
Timestamp denoting recommended time to trust data to.
Signing Timestamp (4 bytes):
Current time at signing.
Attestor Signature (64 bytes):
Signature over preceding fields using the keypair of
the Attestor.
The HHIT is an enhancement of the Host Identity Tag (HIT) [RFC7401] introducing hierarchy and how they are used in UAS RID as defined in [drip-rid].¶
This field has a maximum of 112 bytes in length. It is nominally filled with data as defined by the SAM ID being set or other sub-multiplexer in the authentication payload.¶
The Trust Timestamp is of the format defined in [F3411-19]. That is a UNIX timestamp offset by 01/01/2019 00:00:00. An additional offset is then added to push the timestamp a short time into the future to avoid replay attacks.¶
The offset used against the UNIX timestamp is not defined in this document. Best practices to identify a acceptable offset should be used taking into consideration the UA environment, and propagation characteristics of the messages being sent.¶
Follows the format defined in [F3411-19]. That is a UNIX timestamp offset by 01/01/2019 00:00:00.¶
The signature is generated over all the preceding data. ASTM/DRIP Headers are exclude from this operation only information within the Broadcast Attestation Structure (Section 12) is signed.¶
(Editors Note: move specifics of FEC (everything below) into its own draft for titled Integrity Protection)¶
Remote ID data can be sent across many different broadcast link media, all with different characteristics. To enable robustness in Remote ID transmission media that has Forward Error Correction capability SHOULD be used.¶
In cases where FEC is not available below the equivalent of the transport layer (known as Legacy Advertisements) DRIP Authentication REQUIRES that an application level FEC scheme is used. In cases where FEC is available below the equivalent of the transport layer (known as Extended Advertisements) DRIP MUST NOT use any application level FEC and instead SHALL rely on the lower layers FEC functionality.¶
For current Remote ID the media options are the following:¶
(Editors Note: add in self-protecting and more-than-self-protecting options, with their justifications)¶
When generating the parity the first byte of every Authentication Page MUST be exclude from the XOR operation. For pages 1 through N this leaves the data portion of the page while page 0 will include a number of headers along with 17 bytes of data.¶
To generate the parity a simple XOR operation using the previous and current page is used. For page 0, a 23 byte null pad is used for the previous page. The resulting 23 bytes of parity is appended in one full page (always the last) allowing for recovery when any single page is lost in transmission.¶
TODO (Reed Solomon)¶
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 Authentication Data field (and ASTM Authentication Headers fields if page 0) with nulls.¶
If page 0 is being reconstructed an additional check of the Page Count, to check against how many pages are actually present, MUST be performed for sanity. An additional check on the Data Length field SHOULD also be performed.¶
Using the same methods as encoding, an XOR operation is used between the previous and current page (a 23 byte null pad is used when page 0 is the current page). The resulting 23 bytes is the data of the missing page.¶
TODO (Reed Solomon)¶
If more than one page is lost (>1/5 for 5 page messages, >1/10 for 10 page messages) than the error rate of the link is already beyond saving and the application has more issues to deal with.¶
(Editors Note: Is this valid anymore, for XOR yes but for multi-page FEC?)¶