Internet-Draft DRIP Entity Tag (DET) September 2021
Moskowitz, et al. Expires 18 March 2022 [Page]
7401, 7343 (if approved)
Intended Status:
Standards Track
R. Moskowitz
HTT Consulting
S. Card
AX Enterprize, LLC
A. Wiethuechter
AX Enterprize, LLC
A. Gurtov
Linköping University

DRIP Entity Tag (DET) for Unmanned Aircraft System Remote Identification (UAS RID)


This document describes the use of Hierarchical Host Identity Tags (HHITs) as self-asserting IPv6 addresses and thereby a trustable identifier for use as the Unmanned Aircraft System Remote Identification and tracking (UAS RID). Within the context of RID, HHITs will be called DRIP Entity Tags (DET). HHITs self-attest to the included explicit hierarchy that provides Registrar discovery for 3rd-party identifier attestation.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 18 March 2022.

Table of Contents

1. Introduction

[drip-requirements] describes an Unmanned Aircraft System Remote Identification and tracking (UAS ID) as 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 Host Identity Tags (HHITs) (Section 3) as self-asserting IPv6 addresses and thereby a trustable identifier for use as the UAS Remote ID. HHITs include explicit hierarchy to enable DNS HHIT queries (Host ID for authentication, e.g. [drip-authentication]) and for EPP Registrar discovery [RFC7484] for 3rd-party identification attestation (e.g. [drip-authentication]).

HHITs as used within the context of UAS will be labeled as DRIP Entity Tags (DET). Throughout this document HHIT and DET will be used appropriately. HHIT will be used when convering the technology, and DET for their context within UAS RID.

HITs are statistically unique through the cryptographic hash feature of second-preimage resistance. The cryptographically-bound addition of the Hierarchy and a HHIT registration process [drip-registries] provide complete, global HHIT uniqueness. This is in contrast to using general identifiers (e.g. a Universally Unique IDentifier (UUID) [RFC4122] or device serial number) as the subject in an X.509 [RFC5280] certificate.

In a multi-CA (multi Certificate Authority) PKI alternative to HHITs, a Remote ID as the Subject (Section of [RFC5280]) can occur in multiple CAs, possibly fraudulently. CAs within the PKI would need to implement an approach to enforce assurance of the uniqueness achieved with HHITs.

Hierarchical HITs provide self-attestation of the HHIT registry. A HHIT can only be in a single registry within a registry system (e.g. Extensible Provisioning Protocol (EPP) [RFC5730] and DNS).

Hierarchical HITs are valid, though non-routable, IPv6 addresses [RFC8200]. As such, they fit in many ways within various IETF technologies.

2. Terms and Definitions

2.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2.2. Notations

Signifies concatenation of information - e.g., X | Y is the concatenation of X and Y.
Form of a predicate (X is Y, X has property Y, and most importantly X owns Y).
A set of one or more claims. This definition is borrowed from JWT/CWT ([RFC7519]/[RFC8392]).
A signed claim. X attests to Y.
A claim or attestation, Y, signed exclusively by a third party, X, and are only over identities.

2.3. Definitions

This document uses the terms defined in [drip-requirements]. The following new terms are used in the document:

cSHAKE (The customizable SHAKE function [NIST.SP.800-185]):
Extends the SHAKE [NIST.FIPS.202] scheme to allow users to customize their use of the SHAKE function.
HDA (Hierarchical HIT Domain Authority):
The 16-bit field that identifies the HHIT Domain Authority under an Registered Assigning Authority (RAA).
Hierarchical Host Identity Tag. A HIT with extra hierarchical information not found in a standard HIT [RFC7401].
Host Identity. The public key portion of an asymmetric key pair used in HIP.
HID (Hierarchy ID):
The 32 bit field providing the HIT Hierarchy ID.
HIP (Host Identity Protocol)
The origin of HI, HIT, and HHIT, required for DRIP.
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. In particular all the functions referenced from [NIST.FIPS.202] and [NIST.SP.800-185].
KMAC (KECCAK Message Authentication Code [NIST.SP.800-185]):
A PRF and keyed hash function based on KECCAK.
RAA (Registered Assigning Authority):
The 16 bit field identifying the business or organization that manages a registry of HDAs.
RVS (Rendezvous Server):
The HIP Rendezvous Server for enabling mobility, as defined in [RFC8004].
SHAKE (Secure Hash Algorithm KECCAK [NIST.FIPS.202]):
A secure hash that allows for an arbitrary output length.
XOF (eXtendable-Output Function [NIST.FIPS.202]):
A function on bit strings (also called messages) in which the output can be extended to any desired length.

3. 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 HIT construction is defined in Section 3.5. The input values for the Encoding rules are in Section 3.5.1.

A HHIT is built from the following fields:

  • IANA prefix (max 28 bit)
  • 32 bit Hierarchy ID (HID)
  • 4 (or 8) bit HIT Suite ID
  • ORCHID hash (96 - prefix length - Suite ID length bits, e.g. 64) See Section 3.5

The Context ID for the ORCHID hash is:

    Context ID :=  0x00B5 A69C 795D F5D5 F008 7F56 843F 2C40

A python script is available for generating HHITs [hhit-gen].

3.1. HHIT prefix

A unique IANA IPv6 prefix, no larger than 28 bit, for HHITs is recommended. It clearly separates the flat-space HIT processing from HHIT processing per Section 3.5.

Without a unique prefix, the first 4 bits of the RRA would be interpreted as the HIT Suite ID per HIPv2 [RFC7401].

3.2. HHIT Suite IDs

The HIT Suite IDs specifies the HI and hash algorithms. Any HIT Suite ID can be used for HHITs. The 8 bit format is supported (only when the first 4 bits are ZERO), but this reduces the ORCHID hash length.

3.2.1. 8 bit HIT Suite IDs

Support for 8 bit HIT Suite IDs is allowed in Section 5.2.10 of [RFC7401], but not specified in how ORCHIDs are generated with these longer OGAs. Section 3.5 provides the algorithmic flexibility, allowing for HDA custom HIT Suite IDs as follows:

     HIT Suite             Four-bit ID    Eight-bit encoding
     HDA Assigned 1            NA         TBD3 (suggested value 0x0E)
     HDA Assigned 2            NA         TBD4 (suggested value 0x0F)

This feature may be used for large-scale experimenting with post quantum computing hashes or similar domain specific needs. Note that currently there is no support for domain specific HI algorithms.

3.3. 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)

3.3.1. 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:   IN PTR

3.3.2. 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.

3.4. Edward Digital Signature Algorithm for HITs

Edwards-Curve Digital Signature Algorithm (EdDSA) [RFC8032] are specified here for use as Host Identities (HIs) per HIPv2 [RFC7401]. Further the HIT_SUITE_LIST is specified as used in [RFC7343].

See Section 3.2 for use of the HIT Suite for this document.

3.4.1. 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 [RFC7401].

     profiles    Values

     EdDSA       TBD1 (suggested value 13) [RFC8032]    (RECOMMENDED)

For hosts that implement EdDSA as the algorithm, the following EdDSA curves are available:

     Algorithm    Curve            Values

     EdDSA        RESERVED         0
     EdDSA        EdDSA25519       1 [RFC8032]
     EdDSA        EdDSA25519ph     2 [RFC8032]
     EdDSA        EdDSA448         3 [RFC8032]
     EdDSA        EdDSA448ph       4 [RFC8032]


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 [RFC7401].

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:

     HIT Suite        4-bit ID                   8-bit encoding
     RESERVED         0                          0x00
     EdDSA/cSHAKE128  TBD2 (suggested value 5)   0x50   (RECOMMENDED)

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 of cSHAKE128 is variable per the needs of a specific ORCHID construction. It is at most 96 bits long and is directly used in the ORCHID (without truncation).

Table 1: HIT Suites
Index Hash function HMAC Signature algorithm family Description
5 cSHAKE128 KMAC128 EdDSA EdDSA HI hashed with cSHAKE128, output is variable

3.5. ORCHIDs for Hierarchical HITs

This section improves on ORCHIDv2 [RFC7343] with three enhancements:

  • Optional Info field between the Prefix and OGA ID.
  • Increased flexibility on the length of each component in the ORCHID construction, provided the resulting ORCHID is 128 bits.
  • Use of cSHAKE, NIST SP 800-185 [NIST.SP.800-185], for the hashing function.

The Keccak [Keccak] based cSHAKE XOF hash function 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. Further, 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.

This ORCHID construction includes the fields in the ORCHID in the hash to protect them against substitution attacks. It also provides for inclusion of additional information, in particular the hierarchical bits of the Hierarchical HIT, in the ORCHID generation. This should be viewed as an addendum to ORCHIDv2 [RFC7343], as it can produce ORCHIDv2 output.

3.5.1. Adding additional information to the ORCHID

ORCHIDv2 [RFC7343] is currently defined as consisting of three components:

ORCHID     :=  Prefix | OGA ID | Encode_96( Hash )


Prefix          : A constant 28-bit-long bitstring value
                  (IANA IPv6 assigned).

OGA ID          : A 4-bit long identifier for the Hash_function
                  in use within the specific usage context.  When
                  used for HIT generation this is the HIT Suite ID.

Encode_96( )    : An extraction function in which output is obtained
                  by extracting the middle 96-bit-long bitstring
                  from the argument bitstring.

This addendum will be constructed as follows:

ORCHID     :=  Prefix (p) | Info (n) | OGA ID (o) | Hash (m)


Prefix (p)      : An IANA IPv6 assigned prefix (max 28-bit-long).

Info (n)        : n bits of information that define a use of the
                  ORCHID.  n can be zero, that is no additional

OGA ID (o)      : A 4 or 8 bit long identifier for the Hash_function
                  in use within the specific usage context.  When
                  used for HIT generation this is the HIT Suite ID.

Hash (m)        : An extraction function in which output is m bits.

p + n + o + m = 128 bits

With a 28 bit IPv6 Prefix, the remaining 100 bits can be divided in any manner between the additional information, OGA ID, 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. Note that if a 8 bit OGA is used, the hash may be 4 bits shorter. This may result in a greater risk of pre-image attacks and a corresponding greater need to manage HHIT registration and require look up of the HI from a trusted source.

3.5.2. ORCHID Encoding

This addendum adds a different encoding process to that currently used in ORCHIDv2. The input to the hash function explicitly includes all the header content plus the Context ID. The header content consists of the Prefix, the Additional Information, and OGA ID (HIT Suite ID). Secondly, the length of the resulting hash is set by sum of the length of the ORCHID header fields. For example, a 28 bit Prefix with 32 bits for the HID and 4 bits for the OGA ID leaves 64 bits for the hash length.

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:

    cSHAKE128(Input, L, "", Context ID)

    Input      :=  Prefix | Additional Information | OGA ID | HOST_ID
    L          :=  Length in bits of hash portion of ORCHID

For full Suite ID support (those that use fixed length hashes like SHA256), the following hashing can be used (Note: this does NOT produce output Identical to ORCHIDv2 for Prefix of /28 and Additional Information of ZERO length):

    Hash[L](Context ID | Input)

    Input      :=  Prefix | Additional Information | OGA ID | HOST_ID
    L          :=  Length in bits of hash portion of ORCHID

    Hash[L]    :=  An extraction function in which output is obtained
                   by extracting the middle L-bit-long bitstring
                   from the argument bitstring.

Hierarchical HIT uses the same context as all other HIPv2 HIT Suites as they are clearly separated by the distinct HIT Suite ID. Encoding ORCHIDs for HITv2

This section is included to provide backwards compatibility for ORCHIDv2 [RFC7343] as used for HITv2 [RFC7401].

For HITv2s, the Prefix MUST be 2001:20::/28. Info is length ZERO (not included), and OGA ID is length 4. Thus the HI Hash is length 96. Further the Prefix and OGA ID are NOT included in the hash calculation. Thus the following ORCHID calculations for fixed output length hashes are used:

    Hash[L](Context ID | Input)

    Input      :=  HOST_ID
    L          :=  96
    Context ID :=  0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA

    Hash[L]    :=  An extraction function in which output is obtained
                   by extracting the middle L-bit-long bitstring
                   from the argument bitstring.

For variable output length hashes use:

    Hash[L](Context ID | Input)

    Input      :=  HOST_ID
    L          :=  96
    Context ID :=  0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA

    Hash[L]    :=  The L bit output from the hash function

Then the ORCHID is constructed as follows:

    Prefix | OGA ID | Hash Output

3.5.3. ORCHID Decoding

With this addendum, the decoding of an ORCHID is determined by the Prefix and OGA ID (HIT Suite ID). ORCHIDv2 [RFC7343] decoding is selected when the Prefix is: 2001:20::/28.

For Hierarchical HITs, the decoding is determined by the presence of the HHIT Prefix as specified in the HHIT document.

3.5.4. Decoding ORCHIDs for HITv2

This section is included to provide backwards compatibility for ORCHIDv2 [RFC7343] as used for HITv2 [RFC7401].

HITv2s are identified by a Prefix of 2001:20::/28. The next 4 bits are the OGA ID. is length 4. The remaining 96 bits are the HI Hash.

4. Hierarchical HITs as Remote ID DRIP Entity Tags (DET)

Hierarchical HITs are a refinement on the Host Identity Tag (HIT) of HIPv2 [RFC7401]. HHITs require a new Overlay Routable Cryptographic Hash Identifier (ORCHID [RFC7343]) mechanism as described in Section 3.5. HHITs for UAS ID (DET) also use the new EdDSA/SHAKE128 HIT suite defined in Section 3.4 (GEN-2 in [drip-requirements]). This hierarchy, cryptographically embedded within the HHIT, provides the information for finding the UA's HHIT registry (ID-3 in [drip-requirements]).

ASTM Standard Specification for Remote ID and Tracking [F3411-19] specifies three UAS ID types:

A static, manufacturer assigned, hardware serial number per ANSI/CTA-2063-A "Small Unmanned Aerial System Serial Numbers" [CTA2063A].
A CAA assigned (presumably static) ID.
A UTM system assigned UUID [RFC4122]. These can be dynamic, but do not need to be.

For HHITs to be used effectively as UAS IDs, F3411 should add UAS ID type 4 as DET.

4.1. Nontransferablity of HHITs

A HI and its HHIT SHOULD NOT be transferable between UA or even between replacement electronics (e.g. replacement of damaged controller CPU) for a UA. The private key for the HI SHOULD be held in a cryptographically secure component.

4.2. Encoding HHITs in CTA 2063-A Serial Numbers

In some cases it is advantageous to encode HHITs as a CTA 2063-A Serial Number [CTA2063A]. For example, the FAA Remote ID Rules [FAA_RID] state that a Remote ID Module (i.e. not integrated with UA controller) must only use "the serial number of the unmanned aircraft"; CTA 2063-A meets this requirement.

Encoding a HHIT within the 2063-A format is not simple. There is no place for the HID; there will need to be a mapping service from Manufacturer Code to HID. The HIT Suite ID and ORCHID hash will take 14 characters (see below), leaving only 1 character for the Manufacturer's use of other information.

A character in a CTA 2063-A Serial Number "shall include any combination of digits and uppercase letters, except the letters O and I, but may include all digits". This would allow for a Base34 encoding of the binary HIT Suite ID and ORCHID hash. Although, programatically, such a conversion is not hard, other technologies (e.g. credit card payment systems) that have used such odd base encoding have had performance challenges. Thus here a Base32 encoding will be used by also excluding the letters Z and S (too similar to the digits 2 and 5).

The low-order 68 bits (HIT Suite ID | ORCHID hash) of the HHIT SHALL be left-padded with 2 bits of ZERO. This 70 bit number will be encoded into 14 characters using the digit/letters above. The Manufacturer MAY use a Length Code of 14 or 15. If 15, the first character after the Length Code is set by the Manufacturer with the low order 14 characters for the encoded HIT Suite ID and ORCHID hash.

A mapping service (e.g. DNS) MUST provide a trusted (e.g. via DNSSEC) conversion of the 4 character Manufacturer Code to high-order 60 bits (Prefix | HID) of the HHIT. Definition of this mapping service is currently out of scope of this document.

4.3. Remote ID DET as one class of Hierarchical HITs

UAS Remote ID DET may be one of a number of uses of HHITs. However, it is out of the scope of the document to elaborate on other uses of HHITs. As such these follow-on uses need to be considered in allocating the RAAs Section 3.3.1 or HHIT prefix assignments Section 10.

4.4. Hierarchy in ORCHID Generation

ORCHIDS, as defined in [RFC7343], do not cryptographically bind an IPv6 prefix nor the Orchid Generation Algorithm (OGA) ID (the HIT Suite ID) to the hash of the HI. The rational at the time of developing ORCHID was attacks against these fields are DoS attacks against protocols using ORCHIDs and thus up to those protocols to address the issue.

HHITs, as defined in Section 3.5, cryptographically bind all content in the ORCHID through the hashing function. A recipient of a DET that has the underlying HI can directly trust and act on all content in the HHIT. This provides a strong, self-attestation for using the hierarchy to find the DET Registry based on the HID.

4.5. DRIP Entity Tag (DET) Registry

DETs are registered to Hierarchical HIT Domain Authorities (HDAs). A registration process, [drip-registries], ensures DET global uniqueness (ID-4 in [drip-requirements]). It also provides the mechanism to create UAS Public/Private data that are associated with the DET (REG-1 and REG-2 in [drip-requirements]).

The two levels of hierarchy within the DET 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.

4.6. Remote ID Authentication using DETs

The EdDSA25519 Host Identity (HI) [Section 3.4] underlying the DET can be used in an 84-byte self proof attestation as shown in Appendix B to provide proof of Remote ID ownership (GEN-1 in [drip-requirements]). A lookup service like DNS can provide the HI and registration proof (GEN-3 in [drip-requirements]).

Similarly the 200-byte offline self-attestation shown in Appendix B.1 provides the same proofs without Internet access and with a small cache that contains the HDA's HI/HHIT and HDA meta-data. These self-attestations are carried in the ASTM Authentication Message (Msg Type 0x2).

Hashes of previously sent ASTM messages can be placed in a signed "Manifest" Authentication Message (GEN-2 in [drip-requirements]). This can be either a standalone Authentication Message, or an enhanced self attestation Authentication Message. Alternatively the ASTM Message Pack (Msg Type 0xF) can provide this feature, but only over Bluetooth 5 or WiFi BEACON or NAN broadcasts.

5. DRIP Entity Tags (DET) in DNS

There are two approaches for storing and retrieving the DET using DNS. These are:

  • As FQDNs in the .aero TLD.
  • Reverse DNS lookups as IPv6 addresses per [RFC8005].

A DET 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 in [drip-requirements]). For example, the USS for the HHIT could be found via the following: Assume the RAA is 100 and the HDA is 50. The PTR record is constructed as:   IN PTR

The individual DETs are potentially too numerous (e.g. 60 - 600M) and dynamic (new DETs every minute for some HDAs) to actually store in a signed, DNS zone. The HDA SHOULD provide DNS service for its zone and provide the HHIT detail response. A secure connection (e.g. DNS over TLS) to the authoritative zone may be a viable alternative to DNSSEC.

The DET reverse lookup can be a standard IPv6 reverse look up, or it can leverage off the HHIT structure. Assume a prefix of 2001:30::/28, the RAA is 10 and the HDA is 20 and the DET is:


A DET reverse lookup could be to:

A 'standard' RR has the advantage of only one Registry service supported.

    e.9.6.a.0.d.a.    IN   PTR

6. Other UTM uses of HHITs beyond DET

HHITs might be used within the UTM architecture beyond DET (and USS in UA ID registration and authentication). This includes a GCS HHIT ID. The GCS may use its HIIT if it is the source of Network Remote ID for securing the transport and for secure C2 transport (e.g. [drip-secure-nrid-c2]).

Observers may have their own 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 (this use is currently outside the scope). Further, they can be used by FINDER observers, (e.g. [crowd-sourced-rid]).

7. DRIP Requirements addressed

This document in the previous sections provides the details to solutions for GEN 1 - 3, ID 1 - 5, and REG 1 - 2 as describled in [drip-requirements].

8. DET Privacy

There is no expectation of privacy for DETs; it is not part of the Privacy Normative Requirements, Section 4.3.1, of [drip-requirements]. DETs are broadcast in the clear over the open air via Bluetooth and WiFi. They will be collected and collated with other public information about the UAS. This will include DET registration information and location and times of operations for a DET. A DET can be for the life of a UA if there is no concern about DET/UA activity harvesting.

Further, the MAC address of the wireless interface used for Remote ID broadcasts are a target for UA operation aggregation that may not be mitigated through address randomization. For Bluetooth 4 Remote ID messaging, the MAC address is used by observers to link the Basic Message that contains the RID with other Remote ID messages, thus must be constant for a UA operation. This message linkage use of MAC addresses may not be needed with the Bluetooth 5 or WiFi PHYs. These PHYs provide for a larger message payload and can use the Message Pack (Msg Type 0xF) and the Authentication Message to transmit the RID with other Remote ID messages. However it is not manditory to send the RID in a Message Pack or Authentication Message, so allowance for using the MAC address for UA message linking must be maintained. That is, the MAC address should be stable for at least a UA operation.

Finally, it is not adequate to simply change the DET and MAC for a UA per operation to defeat historically tracking a UA's activity.

Any changes to the UA MAC may have impacts to C2 setup and use. A constant GCS MAC may well defeat any privacy gains in UA MAC and RID changes. UA/GCS binding is complicated with changing MAC addresses; historically UAS design assumed these to be "forever" and made setup a one-time process. Additionally, if IP is used for C2, a changing MAC may mean a changing IP address to further impact the UAS bindings. Finally an encryption wrapper's identifier (such as ESP [RFC4303] SPI) would need to change per operation to insure operation tracking separation.

Creating and maintaining UAS operational privacy is a multifaceted problem. Many communication pieces need to be considered to truly create a separation between UA operations. Simply changing the UAS RID only starts the changes that need to be implemented.

9. ASTM Considerations

ASTM will need to make the following additional value to the "UA ID" in the Basic Message (Msg Type 0x0):

Type 4:
This document UA ID of DRIP Entity Tags, DET (see Section 4).

The DET authors will participate in ASTM to enact this change.

10. IANA Considerations

This document requests IANA to make the following changes to the IANA "Host Identity Protocol (HIP) Parameters" registry:

Host ID:
This document defines the new EdDSA Host ID with value TBD1 (suggested: 13) (see Section 3.4.1) in the "HI Algorithm" subregistry of the "Host Identity Protocol (HIP) Parameters" registry.
EdDSA Curve Label:
This document specifies a new algorithm-specific subregistry named "EdDSA Curve Label". The values for this subregistry are defined in Section 3.4.1.
HIT Suite ID:
This document defines the new HIT Suite of EdDSA/cSHAKE with value TBD2 (suggested: 5) (see Section 3.4.2) in the "HIT Suite ID" subregistry of the "Host Identity Protocol (HIP) Parameters" registry.
HIT Suite ID eight-bit encoding:
This document defines the first eight-bit encoded HIT Suite IDs as defined in Section 5.2.10 of [RFC7401]. These are the new HDA domain HIT Suites with values TBD3 and TBD4 (suggested: 0x0E and 0x0F) (see Section 3.2.1). IANA is requested to expand the "HIT Suite ID" subregistry of the "Host Identity Protocol (HIP) Parameters" registry to show both the four-bit and eight-bit values as shown in Section 5.2.10 of [RFC7401] and add these new values that only have eight bit representations.

10.1. New IPv6 prefix needed for HHITs

Because HHIT format is not compatible with [RFC7343], 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 [RFC6890] (suggested: 2001:30::/28).

11. Security Considerations

The 64-bit hash in HHITs presents a real risk of second pre-image cryptographic hash attack Section 11.2. There are no known (to the authors) studies of hash size to cryptographic hash attacks. A PYTHON script is available to randomly generate 1M HHITs that did not produce a hash collision which is a simpler attack than a first or second pre-image attack.

The DET Registry services effectively block attempts to "take over" or "hijack" a DET. It does not stop a rogue attempting to impersonate a known DET. This attack can be mitigated by the receiver of the DET using DNS to find the HI for the DET. As such, use of DNSSEC and DNS over TLS by the DET registries is recommended.

The 60 bit hash for DETs with 8 bit OGAs may well have hash attack risks. As such its use should be restricted to testing and to small, well managed UAS.

Another mitigation of HHIT hijacking is if the HI owner (UA) supplies an object containing the HHIT and signed by the HI private key of the HDA such as Appendix B.1 as discussed in Section 4.6.

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. "") 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 [drip-registries], through its HHIT uniqueness enforcement, provides against forced or accidental HHIT hash collisions.

Cryptographically Generated Addresses (CGAs) provide an 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 ([drip-registries]) within the HDA provides a level of assured uniqueness unattainable without mirroring this approach.

The second aspect of assured uniqueness is the digital signing (attestation) process of the DET by the HI private key and the further signing (attestation) 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 DET, 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.

11.1. DET Trust

The DET in the ASTM Basic Message (Msg Type 0x0, the actual Remote ID message) does not provide any assertion of trust. The best that might be done within this Basic Message is 4 bytes truncated from a HI signing of the HHIT (the UA ID field is 20 bytes and a HHIT is 16). This is not trustable; that is, too open to a hash attack. Minimally, it takes 84 bytes, Appendix B, to prove ownership of a DET with a full EdDSA signature. Thus no attempt has been made to add DET trust directly within the very small Basic Message.

The ASTM Authentication Message (Msg Type 0x2) as shown in Section 4.6 can provide practical actual ownership proofs. These attestations 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 module strapped to its back.

Proof of UA transmission comes when the Authentication Message includes proofs for the ASTM Location/Vector Message (Msg Type 0x1) 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 DET of the UA.

DETs 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 DET trust.

11.2. Collision risks with DETs

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 Appendix C for the collision probability formula.

However, this risk of collision is within a single "Additional Information" value, i.e. a RAA/HDA domain. The UAS/USS registration process should include registering the DET and MUST reject a collision, forcing the UAS to generate a new HI and thus HHIT and reapplying to the DET registration process.

12. References

12.1. Normative References

ASTM International, "Standard Specification for Remote ID and Tracking", , <>.
Dworkin, M., "SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions", National Institute of Standards and Technology report, DOI 10.6028/nist.fips.202, , <>.
Kelsey, J., Change, S., and R. Perlner, "SHA-3 derived functions: cSHAKE, KMAC, TupleHash and ParallelHash", National Institute of Standards and Technology report, DOI 10.6028/nist.sp.800-185, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman, "Special-Purpose IP Address Registries", BCP 153, RFC 6890, DOI 10.17487/RFC6890, , <>.
Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

12.2. Informative References

CORUS, "U-space Concept of Operations", , <>.
Moskowitz, R., Card, S. W., Wiethuechter, A., Zhao, S., and H. Birkholz, "Crowd Sourced Remote ID", Work in Progress, Internet-Draft, draft-moskowitz-drip-crowd-sourced-rid-06, , <>.
ANSI/CTA, "Small Unmanned Aerial Systems Serial Numbers", , <>.
Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP Authentication Formats", Work in Progress, Internet-Draft, draft-ietf-drip-auth-01, , <>.
Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP Registries", Work in Progress, Internet-Draft, draft-wiethuechter-drip-registries-00, , <>.
Card, S. W., Wiethuechter, A., Moskowitz, R., and A. Gurtov, "Drone Remote Identification Protocol (DRIP) Requirements", Work in Progress, Internet-Draft, draft-ietf-drip-reqs-18, , <>.
Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov, "Secure UAS Network RID and C2 Transport", Work in Progress, Internet-Draft, draft-moskowitz-drip-secure-nrid-c2-03, , <>.
United States Federal Aviation Administration (FAA), "Remote Identification of Unmanned Aircraft", , <>.
"Python script to generate HHITs", , <>.
Bertoni, G., Daemen, J., Peeters, M., Van Assche, G., and R. Van Keer, "The Keccak Function", <>.
Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI 10.17487/RFC4122, , <>.
Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, , <>.
Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, , <>.
Hollenbeck, S., "Extensible Provisioning Protocol (EPP)", STD 69, RFC 5730, DOI 10.17487/RFC5730, , <>.
Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay Routable Cryptographic Hash Identifiers Version 2 (ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, , <>.
Moskowitz, R., Ed., Heer, T., Jokela, P., and T. Henderson, "Host Identity Protocol Version 2 (HIPv2)", RFC 7401, DOI 10.17487/RFC7401, , <>.
Blanchet, M., "Finding the Authoritative Registration Data (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, , <>.
Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, , <>.
Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004, , <>.
Laganier, J., "Host Identity Protocol (HIP) Domain Name System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005, , <>.
Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <>.
Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, , <>.

Appendix A. 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 [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).

Appendix B. Example HHIT Self Attestation

This section shows example uses of HHIT RID to prove trustworthiness of the RID and attestation of registration to the RAA|HDA. These are examples only and other documents will provide fully specified attestations. Care has been taken in the example design to minimize the risk of replay attacks.

This ownership/attestation of a HHIT can be proved in 84 bytes via the following HHIT Self Attestation following [drip-authentication] format:

  • 4 byte Signing Timestamp
  • 16 byte HHIT
  • 64 byte Signature (EdDSA25519 signature)

The Timestamp MAY be the standard UNIX time at the time of signing. A protocol specific timestamp may be used to avoid programming complexities. For example, [F3411-19] uses a 00:00:00 01/01/2019 offset.

To minimize the risk of replay, the UA SHOULD create a new Self Attestation, with a new timestamp, at least once a minute. The UA MAY precompute these attestations and transmit during the appropriate 1 minute window. 1 minute is chosen as a balance between attestation compute time against risk. A shorter window of use lessens the risk of replay.

The signature is over the 20 byte Timestamp + HHIT.

The receiver of such an attestation would need access to the underlying public key (HI) to validate the signature. This may be obtained via a DNS query using the HHIT. A larger (116 bytes) Self Attestation could include the EdDSA25519 HI. This larger 116 attestation allows for signature validation before HHIT lookup to prove registration attestation.

B.1. HHIT Offline Self Attestation

Ownership and RAA|HDA registration of a HHIT can be proved in 200 bytes without Internet access and a small cache via the following HHIT Offline Self Attestation [drip-authentication] format:

  • 16 byte UA HHIT
  • 32 byte UA EdDSA25519 HI
  • 4 byte HDA Signing Expiry Timestamp
  • 16 byte HDA HHIT
  • 64 byte HDA Signature (EdDSA25519 signature)
  • 4 byte UA Signing Timestamp
  • 64 byte UA Signature (EdDSA25519 signature)

The Timestamps MAY be the standard UNIX time at the time of signing. A protocol specific timestamp may be used to avoid programming complexities. For example, [F3411-19] uses a 00:00:00 01/01/2019 offset.

The HDA signature is over the 68 byte UA HHIT + UA HI + HDA Expiry Timestamp + HDA HHIT. During the UA Registration process, the UA would provide a Self Attestation to the HDA. The HDA would construct its attestation of registry with an Expiry Timestamp, its own HHIT, and its signature, returning a 132 byte HDA Registry Attestation to the UA. The UA would use this much the same way as its HHIT only in the Self Attestation above, creating a 200 byte Offline Self Attestation.

The receiver of such an attestation would need a cache of RAA ID, HDA ID, HDA HHIT, and HDA HI (min 80 bytes per RAA/HDA).

Appendix C. Calculating Collision Probabilities

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

    p = 1 - e^{-k^2/(2n)}

    P   Collision Probability
    n   Total possible population
    k   Actual population

The following table provides the approximate population size for a collision for a given total population.

                       Deployed Population
     Total            With Collision Risk of
     Population         .01%            1%

     2^96                 4T           42T
     2^72                 1B           10B
     2^68               250M          2.5B
     2^64                66M          663M
     2^60                16M          160M


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.

Authors' Addresses

Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America
Stuart W. Card
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Adam Wiethuechter
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Andrei Gurtov
Linköping University
SE-58183 Linköping