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AX Enterprize

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Linköping University

IDA Linköping 58183 Sweden gurtov@acm.org

Internet DRIP RFC Request for Comments I-D Internet-Draft RID This document describes the use of 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.

Introduction describes 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). This document describes the use of Hierarchical HITs (HHITs) 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. HITs are statistically unique through the cryptographic hash feature of second-preimage resistance. The cryptographically-bound addition of the Hierarchy and thus 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.

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.

Notation

|

Signifies concatenation of information - e.g., X | Y is the concatenation of X and Y.

Definitions See for common DRIP terms.

cSHAKE (The customizable SHAKE function):

Extends the SHAKE scheme to allow users to customize their use of the function.

HI

Host Identity. The public key portion of an asymmetric keypair used in HIP.

HIP

Host Identity Protocol. The origin of HI, HIT, and HHIT, required for DRIP. Optional full use of HIP enables additional DRIP functionality.

HDA (Hierarchical HIT Domain Authority):

The 16 bit field identifying the HIT Domain Authority under an RAA.

HHIT

Hierarchical Host Identity Tag. A HIT with extra hierarchical information not found in a standard HIT.

HID (Hierarchy ID):

The 32 bit field providing the HIT Hierarchy ID.

HIT

Host Identity Tag. A 128 bit handle on the HI. HITs are valid IPv6 addresses.

Keccak (KECCAK Message Authentication Code):

The family of all sponge functions with a KECCAK-f permutation as the underlying function and multi-rate padding as the padding rule.

RAA (Registered Assigning Authority):

The 16 bit field identifying the Hierarchical HIT Assigning Authority.

RVS (Rendezvous Server):

The HIP Rendezvous Server for enabling mobility, as defined in .

SHAKE (Secure Hash Algorithm KECCAK):

A secure hash that allows for an arbitrary output length.

XOF (eXtendable-Output Function):

A function on bit strings (also called messages) in which the output can be extended to any desired length.

Hierarchical HITs as Remote ID Hierarchical HITs are a refinement on the Host Identity Tag (HIT) of HIPv2. HHITs require a new ORCHID mechanism as described in . HHITs for UAS ID also use the new EdDSA/SHAKE128 HIT suite defined in (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 specifies three UAS ID types:

1. A static, manufacturer assigned, hardware serial number per ANSI/CTA-2063-A "Small Unmanned Aerial System Serial Numbers" .
2. A CAA assigned (presumably static) ID.
3. A UTM system assigned UUID , which can but need not be dynamic.

For HHITs to be used effectively as UAS IDs, F3411-19 SHOULD add UAS ID type 4 as HHIT.

Remote ID as one class of Hierarchical HITs UAS Remote ID may be one of a number of uses of HHITs. As such these follow-on uses need to be considered in allocating the RAAs or HHIT prefix assignments .

Hierarchy in ORCHID Generation ORCHIDS, as defined in , do not cryptographically bind the IPv6 prefix nor the Orchid Generation Algorithm (OGA) ID (the HIT Suite 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 , 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.

Hierarchical HIT Registry HHITs are registered to Hierarchical HIT Domain Authorities (HDAs) as described in . 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.

Remote ID Authentication using HHITs The EdDSA25519 Host Identity (HI) [] underlying the HHIT is used for the Message Wrapper, Sec 4.2 (requirements GEN-2). It and the HDA's HI/HHIT are used for the Auth Certificate, sec 5.1 (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.

UAS ID HHIT in DNS There are 2 approaches for storing and retrieving the HHIT from DNS. These are:

• As FQDNs in the .aero TLD.
• Reverse DNS lookups as IPv6 addresses per .

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: The individual HHITs are potentially too numerous (e.g. 60 - 600M) 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: An HHIT reverse lookup would be to is:

Other UTM uses of HHITs HHITs can be used extensively within the UTM architecture beyond UA ID (and USS in UA ID registration and authentication). This includes a GCS HHIT ID. It could use this if it is the source of Network Remote ID for securing the transport and for secure C2 transport . 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, .

DRIP Requirements addressed This document provides solutions to GEN 1 - 3, ID 1 - 5, and REG 1 - 2.

ASTM Considerations ASTM will need to make the following changes to the "UA ID" in the Basic Message:

Type 4:

This document UA ID of Hierarchical HITs (see ).

IANA Considerations IANA will need to make the following changes to the "Host Identity Protocol (HIP) Parameters" registries:

Host ID:

This document defines the new EdDSA Host ID (see ).

HIT Suite ID:

This document defines the new HIT Suite of EdDSA/cSHAKE (see ).

Because HHIT use of ORCHIDv2 format is not compatible with , IANA is requested to allocated a new 28-bit prefix out of the IANA IPv6 Special Purpose Address Block, namely 2001:0000::/23, as per .

Security Considerations A 64 bit hash space presents a real risk of second pre-image attacks . The HHIT Registry services effectively block attempts to "take over" a HHIT. It does not stop a rogue attempting to impersonate a known HHIT. This attack can be mitigated by the receiver of the HHIT using DNS to find the HI for the HHIT. Another mitigation of HHIT hijacking is if the HI owner supplies an object containing the HHIT and signed by the HI private key of the HDA. The two risks with hierarchical HITs are the use of an invalid HID and forced HIT collisions. The use of a DNS zone (e.g. "hhit.arpa.") is a strong protection against invalid HIDs. Querying an HDA's RVS for a HIT under the HDA protects against talking to unregistered clients. The Registry service has direct protection against forced or accidental HIT hash collisions. 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 it implied second-preimage resistance) makes it statistically challenging to attacks. A registration process as in 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.

Hierarchical HIT Trust 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 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 . 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 ), thus asserting HHIT RID trust.

Collision risks with Hierarchical HITs The 64 bit hash size does have an increased risk of collisions over the 96 bit hash size used for the other HIT Suites. There is a 0.01% probability of a collision in a population of 66 million. The probability goes up to 1% for a population of 663 million. See for the collision probability formula. However, this risk of collision is within a single "Additional Information" value. Some registration process should be used to reject a collision, forcing the client to generate a new HI and thus HIT and reapplying to the registration process.

References ASTM International ANSI CORUS

Radboud University

STMicroelectronics

STMicroelectronics

STMicroelectronics

EU U-Space RID Privacy Considerations 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 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).

The Hierarchical Host Identity Tag (HHIT) The Hierarchical HIT (HHIT) is a small but important enhancement over the flat HIT space. By adding two levels of hierarchical administration control, the HHIT provides for device registration/ownership, thereby enhancing the trust framework for HITs. HHITs represent the HI in only a 64 bit hash and uses the other 32 bits to create a hierarchical administration organization for HIT domains. Hierarchical HITs are "Using cSHAKE in ORCHIDs". The input values for the Encoding rules are in . A HHIT is built from the following fields:

• 28 bit IANA prefix
• 4 bit HIT Suite ID
• 32 bit Hierarchy ID (HID)
• 64 bit ORCHID hash

HHIT prefix A unique 28 bit prefix for HHITs is recommended. It clearly separates the flat-space HIT processing from HHIT processing per "Using cSHAKE in ORCHIDs".

HHIT Suite IDs The HIT Suite IDs specifies the HI and hash algorithms. Any HIT Suite ID can be used for HHITs, provided that the prefix for HHITs is different from flat space HITs. Without a unique prefix, , additional HIT Suite IDs would be needed for HHITs. This would risk exhausting the limited Suite ID space of only 15 IDs.

The Hierarchy ID (HID) The Hierarchy ID (HID) provides the structure to organize HITs into administrative domains. HIDs are further divided into 2 fields:

• 16 bit Registered Assigning Authority (RAA)
• 16 bit Hierarchical HIT Domain Authority (HDA)

The Registered Assigning Authority (RAA) An RAA is a business or organization that manages a registry of HDAs. For example, the Federal Aviation Authority (FAA) could be an RAA. The RAA is a 16 bit field (65,536 RAAs) assigned by a numbers management organization, perhaps ICANN's IANA service. An RAA must provide a set of services to allocate HDAs to organizations. It must have a public policy on what is necessary to obtain an HDA. The RAA need not maintain any HIP related services. It must maintain a DNS zone minimally for discovering HID RVS servers. As HHITs may be used in many different domains, RAA should be allocated in blocks with consideration on the likely size of a particular usage. Alternatively, different Prefixes can be used to separate different domains of use of HHTs. This DNS zone may be a PTR for its RAA. It may be a zone in a HHIT specific DNS zone. Assume that the RAA is 100. The PTR record could be constructed:

The Hierarchical HIT Domain Authority (HDA) An HDA may be an ISP or any third party that takes on the business to provide RVS and other needed services for HIP enabled devices. The HDA is an 16 bit field (65,536 HDAs per RAA) assigned by an RAA. An HDA should maintain a set of RVS servers that its client HIP-enabled customers use. How this is done and scales to the potentially millions of customers is outside the scope of this document. This service should be discoverable through the DNS zone maintained by the HDA's RAA. An RAA may assign a block of values to an individual organization. This is completely up to the individual RAA's published policy for delegation.

ORCHIDs for Hierarchical HITs This section adds the Keccak based cSHAKE XOF hash function from NIST SP 800-185 to ORCHIDv2. cSHAKE is a variable output length hash function. As such it does not use the truncation operation that other hashes need. The invocation of cSHAKE specifies the desired number of bits in the hash output. This ORCHID construction includes the Prefix in the hash to protect against Prefix subsitution attacks. It also provides for inclusion of additional information, in particular the hierarchical bits of the Hierarchical HIT, in the ORCHID generation. It should be viewed as an addendum to ORCHIDv2. cSHAKE is used, rather than SHAKE from NIST FIPS 202, as cSHAKE has a parameter 'S' as a customization bit string. This parameter will be used for including the ORCHID Context Identifier in a standard fashion.

Adding additional information to the ORCHID ORCHIDv2 is currently defined as consisting of three components: This addendum will be constructed as follows: With a 28 bit IPv6 Prefix, the 96 bits currently allocated to the Encode_96 function can be divided in any manner between the additional information and the hash output. Care must be taken in determining the size of the hash portion, taking into account risks like pre-image attacks. Thus 64 bits as used in Hierarchical HITs may be as small as is acceptable.

ORCHID Decoding With this addendum, the decoding of an ORCHID is determined by the Prefix and OGA ID (HIT Suite ID). ORCHIDv2 decoding is selected when the Prefix is: 2001:20::/28. For Heirarchical HITs, the decoding is determined by the presence of the HHIT Prefix as specified in the HHIT document.

ORCHID Encoding ORCHIDv2 has a number of inputs including a Context ID, some header bits, the hash algorithm, and the input bitstream, normally just the public key. The output is a 96 bit value. This addendum adds a different encoding process to that currently used. The input to the hash function explicitly includes all the fixed header content plus the Context ID. The fixed header content consists of the Prefix, OGA ID (HIT Suite ID), and the Additional Information. Secondly, the length of the resulting hash is set by the rules set by the Prefix/OGA ID. In the case of Hierarchical HITs, this is 64 bits. To achieve the variable length output in a consistent manner, the cSHAKE hash is used. For this purpose, cSHAKE128 is appropriate. The the cSHAKE function call for this addendum is: Hierarchical HIT uses the same context as all other HIPv2 HIT Suites as they are clearly separated by the distinct HIT Suite ID.

Edward Digital Signature Algorithm for HITs Edwards-Curve Digital Signature Algorithm (EdDSA) are specified here for use as Host Identities (HIs).

HOST_ID The HOST_ID parameter specifies the public key algorithm, and for elliptic curves, a name. The HOST_ID parameter is defined in Section 5.2.19 of . For hosts that implement EdDSA as the algorithm, the following ECC curves are available:

HIT_SUITE_LIST The HIT_SUITE_LIST parameter contains a list of the supported HIT suite IDs of the Responder. Based on the HIT_SUITE_LIST, the Initiator can determine which source HIT Suite IDs are supported by the Responder. The HIT_SUITE_LIST parameter is defined in Section 5.2.10 of . The following HIT Suite ID is defined, and the relationship between the four-bit ID value used in the OGA ID field and the eight-bit encoding within the HIT_SUITE_LIST ID field is clarified: The following table provides more detail on the above HIT Suite combinations. The input for each generation algorithm is the encoding of the HI as defined in this Appendix. The output is 96 bits long and is directly used in the ORCHID. HIT Suites

Index	Hash function	HMAC	Signature algorithm family	Description
5	cSHAKE128	KMAC128	EdDSA	EdDSA HI hashed with cSHAKE128, output is 96 bits

Calculating Collision Probabilities The accepted formula for calculating the probability of a collision is:

Acknowledgments Dr. Gurtov is an adviser on Cybersecurity to the Swedish Civil Aviation Administration. Quynh Dang of NIST gave considerable guidance on using Keccak and the NIST supporting documents. Joan Deamen of the Keccak team was especially helpful in many aspects of using Keccak.