| Internet-Draft | UAS Remote ID | July 2020 |
| Moskowitz, et al. | Expires 14 January 2021 | [Page] |
This document describes using Hierarchical Host Identity Tags (HHITs) as a self-asserting and thereby trustable Identifier for use as the UAS Remote ID. HHITs include explicit hierarchy to provide Registrar discovery for 3rd-party ID attestation. Further, HHITs can also be used elsewhere in the UTM architecture to facilitate UAS communications.¶
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This document describes the use of Hierarchical HITs (HHITs) [hierarchical-hit] as self-asserting and thereby a trustable Identifier for use as the UAS Remote ID. HHITs include explicit hierarchy to provide Registrar discovery for 3rd-party ID attestation.¶
The Drip Requirements [drip-requirements] describe a UAS ID as a "unique (ID-4), non-spoofable (ID-5), and identify a registry where the ID is listed (ID-2)"; all within a 20 character Identifier (ID-1).¶
HITs are statistically unique through the cryptograhic hash feature of second-preimage resistance. The cryptograhically-bound addition of the Hierarchy and thus HHIT Registries [hhit-registries] provide complete, global HHIT uniqueness. This is in contrast to general IDs (e.g. a UUID or device serial number) as the subject in an X.509 certificate.¶
In a multi-CA PKI, a subject can occur in multiple CAs, possibly fraudulently. CAs within the PKI would need to implement an approach to enforce assurance of uniqueness.¶
Hierarchical HITs are valid, though non-routable, IPv6 addresses. As such, they fit in many ways within various IETF technologies.¶
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 [drip-requirements] for common DRIP terms.¶
Hierarchical HITs are a refinement on the Host Identity Tag (HIT) of HIPv2 [RFC7401]. HHITs require a new ORCHID mechanism as described in [new-orchid]. HHITs for UAS ID also use the new EdDSA/SHAKE128 HIT suite defined in [new-hip-crypto] (requirements GEN-2). This hierarchy, cryptographically embedded within the HHIT, provides the information for finding the UA's HHIT registry (ID-3).¶
The current ASTM [F3411-19] supports three types of UAS IDs, namely the [CTA2063A] serial number, CAA registration ID, and UTM-provided UUID session ID. For HHITs to be used effectively as UAS IDs, F3411-19 SHOULD add HHIT as the fourth UAS ID type.¶
ORCHIDS, as defined in [RFC7343], do not cryptographically bind the IPv6 prefix nor the Orchid Generation Algorithm (OGA) ID to the hash of the HI. The justification then was attacks against these fields are DOS attacks against protocols using them.¶
HHITs, as defined in [new-orchid], cryptographically bind all content in the ORCHID though the hashing function. Thus a recipient of a HHIT that has the underlying HI can directly act on all content in the HHIT. This is especially important to using the hierarchy to find the HHIT Registry.¶
HHITs are registered to Hierarchical HIT Domain Authorities (HDAs) as described in [hhit-registries]. This registration process ensures UAS ID global uniqueness (ID-4). It also provides the mechanism to create UAS Public/Private data associated with the HHIT UAS ID (REG-1 and REG-2).¶
The 2 levels of hierarchy within the HHIT allows for CAAs to have their own Registered Assigning Authority (RAA) for their National Air Space (NAS). Within the RAA, the CAAs can delegate HDAs as needed. There may be other RAAs allowed to operate within a given NAS; this is a policy decision by the CAA.¶
The EdDSA25519 Host Identity (HI) underlying the HHIT is used for the Message Wrapper, Sec 4.2 [drip-auth] (requirements GEN-2). It and the HDA's HI/HHIT are used for the Auth Certificate, sec 5.1 [drip-auth] (requirements GEN-3). These messages also establish that the UA owns the HHIT and that no other UA can assert ownership of the HHIT (GEN-1).¶
The number of HDAs authorized to register UAs within an NAS determines the size of the HDA credential cache a device processing the Offline Authentication. This cache contains the HDA's HI/HHIT and HDA meta-data; it could be very small.¶
There are 2 approaches for storing and retrieving the HHIT from DNS. These are:¶
The HHIT can be used to construct an FQDN that points to the USS that has the Public/Private information for the UA (REG-1 and REG-2). For example the USS for the HHIT could be found via the following. Assume that the RAA is 100 and the HDA is 50. The PTR record is constructed as:¶
100.50.hhit.uas.areo IN PTR foo.uss.areo.
¶
The individual HHITs are potentially too numerous (e.g. 63M) and dynamic to actually store in a signed, DNS zone. Rather the USS would provide the HHIT detail response.¶
The HHIT reverse lookup can be a standard IPv6 reverse look up, or it can leverage off the HHIT structure. Assume that the RAA is 10 and the HDA is 20 and the HHIT is:¶
2001:14:28:14:a3ad:1952:ad0:a69e
¶
An HHIT reverse lookup would be to is:¶
a69e.ad0.1952.a3ad14.28.14.2001.20.10.hhit.arpa.
¶
HHITs can be used extensively within the UTM architecture beyond for UA ID (and USS in UA ID registration and authentication). The GCS SHOULD have its own HHIT as an ID. It could use this if it is the source of Network Remote ID for securing the transport and for secure C2 transport [drip-secure-nrid-c2].¶
Observers SHOULD have HHITs to facilitate UAS information retrieval (e.g. for authorization to private UAS data). They could also use their HHIT for establishing a HIP connection with the UA Pilot for direct communications per authorization. Further, they can be used by FINDER observers, [crowd-sourced-rid].¶
This document provides solutions to GEN 1 - 3, ID 1 - 5, and REG 1 - 2.¶
ASTM will need to make the following changes to the "UA ID" in the Basic Message:¶
The security considerations with Hierarchical HITs, most notably the short hash of the HI, are discussed in [hierarchical-hit]. The binding of the hierarchy to the hash of the HI is covered in [new-orchid].¶
Cryptographically Generated Addresses (CGAs) provide a unique assurance of uniqueness. This is two-fold. The address (in this case the UAS ID) is a hash of a public key and a Registry hierarchy naming. Collision resistance (more important that its implied second-preimage resistance) makes it statistically challenging to attacks. A registration process as in HHIT Registries [hhit-registries] provides a level of assured uniqueness unattainable without mirroring this approach.¶
The second aspect of assured uniqueness is the digital signing process of the HHIT by the HI private key and the further signing of the HI public key by the Registry's key. This completes the ownership process. The observer at this point does not know WHAT owns the HHIT, but is assured, other than the risk of theft of the HI private key, that this UAS ID is owned by something and is properly registered.¶
The HHIT UAS RID in the ASTM Basic Message (the actual Remote ID message) does not provide any assertion of trust. The best that might be done is 4 bytes truncated from a HI signing of the HHIT (the UA ID field is 20 bytes and a HHIT is 16). It is in the ASTM Authentication Messages as defined in [drip-auth] that provide all of the actual ownership proofs. These claims include timestamps to defend against replay attacks. But in themselves, they do not prove which UA actually sent the message. They could have been sent by a dog running down the street with a Broadcast Remote ID device strapped to its back.¶
Proof of UA transmission comes when the Authentication Message includes proofs for the Location/Vector Message and the observer can see the UA or that information is validated by ground multilateration [crowd-sourced-rid]. Only then does an observer gain full trust in the HHIT Remote ID.¶
HHIT Remote IDs obtained via the Network Remote ID path provides a different approach to trust. Here the UAS SHOULD be securely communicating to the USS (see [drip-secure-nrid-c2]), thus asserting HHIT RID trust.¶
EU is defining a future of airspace management known as U-space within the Single European Sky ATM Research (SESAR) undertaking. Concept of Operation for EuRopean UTM Systems (CORUS) project proposed low-level Concept of Operations [corus] for UAS in EU. It introduces strong requirements for UAS privacy based on European GDPR regulations. It suggests that UAs are identified with agnostic IDs, with no information about UA type, the operators or flight trajectory. Only authorized persons should be able to query the details of the flight with a record of access.¶
Due to the high privacy requirements, a casual observer can only query U-space if it is aware of a UA seen in a certain area. A general observer can use a public U-space portal to query UA details based on the UA transmitted "Remote identification" signal. Direct remote identification (DRID) is based on a signal transmitted by the UA directly. Network remote identification (NRID) is only possible for UAs being tracked by U-Space and is based on the matching the current UA position to one of the tracks.¶
The project lists "E-Identification" and "E-Registrations" services as to be developed. These services can follow the privacy mechanism proposed in this document. If an "agnostic ID" above refers to a completely random identifier, it creates a problem with identity resolution and detection of misuse. On the other hand, a classical HIT has a flat structure which makes its resolution difficult. The Hierarchical HITs provide a balanced solution by associating a registry with the UA identifier. This is not likely to cause a major conflict with U-space privacy requirements, as the registries are typically few at a country level (e.g. civil personal, military, law enforcement, or commercial).¶
Dr. Gurtov is an advisor on Cybersecurity to the Swedish Civil Aviation Administration.¶