TM-RID Authentication Formats
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Internet
TMRID
RFC
Request for Comments
I-D
Internet-Draft
HIP
TMRID
This document describes how to include trust into the proposed ASTM
Remote ID specification defined in WK65041 by the F38 Committee
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 a assurance for UA trustworthiness in the absence of Internet
connectivity at the receiving node.
Introduction
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 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) .
Terms and Definitions
Requirements 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 when, and only when, they
appear in all capitals, as shown here.
Definitions
- CAA
-
Civil Aeronautics Administration. An example is the Federal
Aviation Administration (FAA) in the United States of
America.
- C2
-
Command and Control. A set of organizational and technical
attributes and processes that employs human, physical, and
information resources to solve problems and accomplish
missions. Mainly used in military contexts.
- HI
-
Host Identity. The public key portion of an assymetric
keypair from HIP. In this document it is assumed that the
HI is based on a EdDSA25519 keypair. This is supported by
new crypto defined in .
- HIT
-
Host Identity Tag. A 128 bit handle on the HI. Defined in
HIPv2 .
- HHIT
-
Hierarchical Host Identity Tag. A 128 bit handle on the HI
contain extra information not found in a standard HIT.
Defined in .
- UA
-
Unmanned Aircraft. In this document UA's 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.
- UAS
-
Unmanned Aircraft System. 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 C2
links between UA and the control station.
- UTM
-
UAS Traffic Management. A "traffic management" ecosystem
for uncontrolled operations that is separate from, but
complementary to, the FAA's Air Traffic Management (ATM)
system.
- USS
-
UAS Service Supplier. 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).
- RID
-
Remote ID. A unique identifier found on all UA to be used
in communication and in regulation of UA operation.
- Observer
-
Referred to in other UAS documents as a "user", but there
are also other classes of RID users, so we prefer
"observer" to denote an individual who has observed an UA
and wishes to know something about it, starting with its
RID.
Background
Problem Space And Document Focus
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 a high level reference to
the ASTM standard for authentication messages and how their current
limitations effect trust in the Broadcast RID envirorment.
ASTM Authentication Message
Thoughts on ASTM Authentication Message
The format proposed by the ASTM is designed with a few major
considerations in mind, which the authors 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 an "atomic
message".
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 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; 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.
TM-RID Supporting Levels
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 potential.
HIP Based Extensions to the ASTM Authentication Message
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.
HIP Based Authentication Wrapper
This format is a way to authenticate a given UA using Level 1 and
Level 2 of the TM-RID architecture.
When this authentication format is recieved the HHIT (provided by
Level 1 TM-RID) is first looked up by mechanisms defined in Level
2. This lookup chain ultimately obtains, on the Observers device,
the full HI associated with the HHIT recieved. Once completed the
signature can then be verifed with the respective data it was
signed with. This data, at a minimum would be the payload in the
authentication message.
The payload can be anything that fits within the 23/25 byte limit.
Some examples of what could be done with this format are found in
Section 5.
Signed Hash Lists
This format is designed to provide provenance to Broadcast RID
messages sent by a given UAS. It should be noted that the HHIT is
not provided in the format like others specified here - instead it
must be obtained via the Basic ID Message in a detached fashion.
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. The signature is signed across the list of hashes.
Hash Operation
With cSHAKE128 NIST SP 800-185, the hash is computed as follows:
The message MAC is prepended to the message, as the MAC is the only
information that links a UA's messages from a specific UA.
Pseudo-blockchain 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.
Limitations
With the current format proposed 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 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.
HIP Based Offline Authentication
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 UA without
requiring the need for immediate reverse lookups online.
Example Use Cases
This section introduces potentional use cases of the HIP based
extensions to the proposed ASTM standard authentication message.
Trusted Messages
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.
Wrapped Signed Hashes
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.
Security Considerations
TBD
References
Normative References
Informative References