TM-RID Authentication Formats

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. The ASTM standard 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 authentication message format, using the Host Identity Protocol Version 2 (HIPv2) Hierarchical HIT (HHIT) .

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 when, and only when, they appear in all capitals, as shown here.

Civil Aeronautics Administration. An example is the Federal Aviation Administration; (FAA) in the United States of America. Command and Control. Remote ID. Host Identity. The public key portion of an assymetric keypair from HIP. In this document it is assume that the HI is a EdDSA25519 keypair. This is supported by new crypto defined in . Host Identity Tag. Defined in HIPv2 . Hierarchical Host Identity Tag. Defined in . Unmanned Aircraft. In this document UAs are typically though of as drones of commerical or military variety. This is a very strict definition which can be relaxed to include any and all aircraft that are unmanned. Composed of Unmanned Aircraft and all required on-board subsystems, payload, control station, other required off-board subsystems, any required launch and recovery equipment, all required crew members, and command and control (C2) links between UA and the control station. USSs provide UTM services to support the UAS community, to connect Operators and other entities to enable information flow across the USS network, and to promote shared situational awareness among UTM participants. (From FAA UTM ConOps V1, May 2018).

The current draft 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 reference to the ASTM standard for authentication messages and how their current limitations effect trust in the Broadcast RID envirorment.

As per the ASTM drafts : "Non-magnitude values, strings, or IDs that may be or may not be numerical (such as the Unique ID) shall be expressed in Network Byte Order which reads in a left to right, most significant byte (MSB) to least significant byte (LSB) order. Magnitude values expressed as 16 or 32 bit integers (such as Latitude, Longitude, Altitude, etc.) shall be expressed as “little endian”, where the LSB is on the left and the MSB is on the right." In short; representation is network byte order but values are encoded little-endian.

The format proposed by the ASTM is designed with a few major considerations in mind, which the authors feel put significant limitations on 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 an "atomic message". The idea in the ASTM 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 an atomic message. The reasoning is then the authentication message is for the entire atomic message pack. The authors have no issues with this proposed approach; given the restraints and current technologies the ASTM did a good job. 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.

This document is assuming that the first two levels of TM-RID (Identification and Authentication) are implemented. This document serves as a expansion to these two levels, leveraging the abilities of the HHIT Registries to its fullest potenional.

The following section describes various methods that HIP can help enable more trustworthy communication using the Authentication Message as the base. Each diagram will show all 5 pages of the format filled out.

Page 0: 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 +---------------+---------------+---------------+---------------+ | Msg. Header | Page Count | Length | / +---------------+---------------+---------------+---------------+ / Timestamp | H-Alg | H-Len | +---------------------------------------------------------------+ | Hash of Previous Auth. Message | +---------------------------------------------------------------+ | Hash of Current Auth. Message | +---------------------------------------------------------------+ | Message Hash | +---------------------------------------------------------------+ | Message Hash | +---------------------------------------------------------------+ DataPage 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 +---------------+---------------+-------------------------------+ | Msg. Header | H-Alg | H-Len | RESERVED | +---------------+---------------+-------------------------------+ | Message Hash | +---------------------------------------------------------------+ | Message Hash | +---------------------------------------------------------------+ | Message Hash | +---------------------------------------------------------------+ | Message Hash | +---------------------------------------------------------------+ | Message Hash | +---------------------------------------------------------------+ Page 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 +---------------+---------------+-------------------------------+ | Msg. Header | RESERVED | Signature Length | +---------------+---------------+-------------------------------+ | Signature Algorithm | | +-------------------------------+ | | | | | | | | HHIT Signature | | | | | | | +---------------------------------------------------------------+ Page 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 +---------------+-----------------------------------------------+ | Msg. Header | | +---------------+ | | | | | | | | | | HHIT Signature | | | | | | | | | +---------------------------------------------------------------+ Page 4: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +---------------+-----------------------------------------------+ | Msg. Header | | +---------------+ | | | | | | | | | | HHIT Signature | | | | | | | | | +---------------------------------------------------------------+ H-Alg, H-Len: (4 bits), (4 bits) These are fields for relaying information of the Hash algorithm used for the messages and the Hash length (in octets). For this example of the format a length of 4 bytes is used. Hash of Previous Auth. Message: (4 bytes) A hash of the previously sent Authentication message. Hash of Current Auth. Message: (4 bytes) A hash of the current Authentication message. Message Hash: (4 bytes) A hash of a previously sent message. Signature Length: (2 bytes) Length of signature in octets, excluding Length, and Padding Signature Algorithm: (2 bytes) Self explanatory. HHIT Signature: (64 bytes) EdDSA25519 signature using an EdDSA25519-based HHIT from HIP. Spread across 3 pages. This format is designed to provide provenance to Broadcast RID messages sent by a give UAS. It should be noted that the HHIT is not provided in the format like others - instead it must be obtained via the Basic ID Message. By hashing previously sent messages and signing them we gain trust in the UAS's previous reports. An observer who has been listening for any length of time can hash received messages and cross check against listed hashes. The signature is signed across the list of hashes.

Two special hashes are included; a previous authentication hash, which links to the previous signed hash list message, as well as a current hash. This gives a pseudo-blockchain provenance to the authentication message that could be traced back if the observer was present for extended periods of time. In regards to the creation and use of the current authentication hash field: 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. 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.

With the current format defined by ASTM only 7 messages can be hashed reasonably in the above format. RESERVED padding, the Signature Algorithm, Signature Length and redundant H-Alg, H-Len fields (of page 1) could be removed. This would increase the total list of hashes to 9 while losing word alignment of the hashes in each page. To address this problem properly the authors feel that the Authentication Messages needs to have a max bound of 10 pages, instead of 5.

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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | DEV HHIT | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TIMESTAMP | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | DEV HHIT | | SIG | . . . . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | DEV HI | | | | | | | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AUTH TIMESTAMP | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | AUTH HHIT | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | AUTH | | SIG | . . . . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / PAYLOAD / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ DEV HHIT 16 byte Dev HHIT of EdDSA25519 HI TIMESTAMP 4 byte packet trust until timestamp DEV HHIT SIG 64 byte Signature of whole packet DEV HI 32 byte Device HI of EdDSA25519 HI AUTH TIMESTAMP 4 byte Dev HHIT trust until timestamp AUTH HHIT 16 byte Authorizer's HHIT of EdDSA25519 HI AUTH SIG 64 byte Signature of Device HHIT-HI PAYLOAD 0 to n bytes of payload Length 200 + n bytes This specific format does not currently fit within the ASTM specification. Requiring a minimum of 200 bytes, this would require the Authentication Message to have 10 pages, instead of the current 5 page limit. What this will grant, if attainable in future revisions of the ASTM specification, is the ability to authenticate UA information when the receiving device of the observer (e.g. a smartphone with a dedicated RID application) has no Internet service (e.g. LTE signal). By including the device HI along with a signature from the registry the UA is under, we can assert trust of a given drone without requiring the need for immediate reverse lookups online.

This section introduces potentional use cases of the HIP based extensions to the ASTM standard authentication message.

Using the HIP Based Authentication Wrapper any single Broadcast RID message defined by ASTM can become what the authors refer to as a "Trusted Message". One specific use case that is useful in the UAS RID space is the creation of a "Trusted Vector Message". By placing a previous [or new] vector message into the Payload section of this format a verifiable broadcast can be created. Due to being signed this creates an authentic vector that is hard to spoof, which can confirm flight paths in near real time. The figure below is a example of a "Trusted Vector Message". Note that the padding (RESERVED) byte are now gone. The "Trust Timestamp" and "Vector Message" fields now span multiple pages instead of being aligned to pages.

Using the HIP Based Authentication Wrapper a [short] list of hashes can be signed. These hashes are of previous individual RID messages. This follows the format of the Signed Hash List, excluding the psuedo-blockchain hashes and various other fields enabling it to fit within the 23 byte limit of the final page. To the authors, this format has limited use due to numerous concerns of replay attacks. It is suggested to instead use the full Signed Hash List format.

TBD