Internet-Draft DRIP Entity Tag (DET) May 2022
Moskowitz, et al. Expires 18 November 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 ID (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).

This document updates RFC7401 and RFC7343.

Within the context of RID, HHITs will be called DRIP Entity Tags (DETs). HHITs self-attest to the included explicit hierarchy that provides registry (via, e.g., DNS, EPP) 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 November 2022.

Table of Contents

1. Introduction

Drone Remote ID Protocol (DRIP) Requirements [RFC9153] describe an Unmanned Aircraft System Remote ID (UAS ID) as unique (ID-4), non-spoofable (ID-5), and identify a registry where the ID is listed (ID-2); all within a 19-character identifier (ID-1).

This document describes (per Section 3 of [drip-architecture]) 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 add explicit hierarchy to the 128-bit HITs, enabling DNS HHIT queries (Host ID for authentication, e.g., [drip-authentication]) and for Extensible Provisioning Protocol (EPP) Registrar discovery [RFC9224] for 3rd-party identification attestation (e.g., [drip-authentication]).

This addition of hierarchy to HITs is an extension to [RFC7401] and requires an update to [RFC7343]. As this document also adds EdDSA (Section 3.4) for Host Identities (HIs), a number of Host Identity Protocol (HIP) parameters in [RFC7401] are updated, but these should not be needed in a DRIP implementation that does not use HIP.

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

Hierarchical HITs provide self-attestation of the HHIT registry. A HHIT can only be in a single registry within a registry system (e.g., EPP and DNS).

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

1.1. HHIT Statistical Uniqueness different from UUID or X.509 Subject

HHITs 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. If the HHITs cannot be looked up with services provided by the registrar identified via the embedded hierarchical information or its registration validated by registration attestations messages [drip-authentication], then the HHIT is either fraudulent or revoked/expired. In-depth discussion of these processes are out of scope for this document.

This contrasts with using general identifiers (e.g., a Universally Unique IDentifiers (UUID) [RFC4122] or device serial numbers as the subject in an X.509 [RFC5280] certificate. In either case, there can be no unique proof of ownership/registration.

For example, in a multi-Certificate Authority (multi-CA) 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.

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.

2.3. Definitions

This document uses the terms defined in Section 2.2 of [RFC9153]. 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 (HHIT Domain Authority):
The 14-bit field that identifies the HHIT Domain Authority under a Registered Assigning Authority (RAA). See Figure 1.
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 as defined in [RFC9063].
HID (Hierarchy ID):
The 28-bit field providing the HIT Hierarchy ID. See Figure 1.
HIP (Host Identity Protocol)
The origin [RFC7401] of HI, HIT, and HHIT.
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. It refers in particular to all the functions referenced from [NIST.FIPS.202] and [NIST.SP.800-185].
KMAC (KECCAK Message Authentication Code [NIST.SP.800-185]):
A Pseudo Random Function (PRF) and keyed hash function based on KECCAK.
RAA (Registered Assigning Authority):
The 14-bit field identifying the business or organization that manages a registry of HDAs. See Figure 1.
RVS (Rendezvous Server):
A Rendezvous Server such as 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 Host Identity Tag (HIT) space, constructed as an Overlay Routable Cryptographic Hash IDentifier (ORCHID) [RFC7343]. By adding two levels of hierarchical administration control, the HHIT provides for device registration/ownership, thereby enhancing the trust framework for HITs.

The 128-bit HHITs represent the HI in only a 64-bit hash, rather than the 96 bits in HITs. 4 of these 32 freed up bits expand the Suite ID to 8 bits, and the other 28 bits are used 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 described in Section 3.5.1.

A HHIT is built from the following fields (Figure 1):

               14 bits| 14 bits              8 bits
              +-------+-------+         +--------------+
              |  RAA  | HDA   |         |HHIT Suite ID |
              +-------+-------+         +--------------+
               \              |    ____/   ___________/
                \             \  _/    ___/
                 \             \/     /
   |    p bits    |  28 bits   |8bits|      o=92-p bits       |
   | IPV6 Prefix  |    HID     |HHSI |      ORCHID hash       |

Figure 1: HHIT Format

The Context ID (generated with openssl rand) for the ORCHID hash is:

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

Context IDs are allocated out of the namespace introduced for Cryptographically Generated Addresses (CGA) Type Tags [RFC3972].

3.1. HHIT Prefix for RID Purposes

The IPv6 HHIT prefix MUST be distinct from that used in the flat-space HIT as allocated in [RFC7343]. Without this distinct prefix, the first 4 bits of the RAA would be interpreted as the HIT Suite ID per HIPv2 [RFC7401].

Initially, for DET use, one 28-bit prefix should be assigned out of the IANA IPv6 Special Purpose Address Block ([RFC6890]).

     HHIT Use     Bits  Value
     DET          28    TBD6 (suggested value 2001:30::/28)

Other prefixes may be added in the future either for DET use or other applications of HHITs. For a prefix to be added to the registry in Section 8.2, its usage and HID allocation process have to be publicly available.

3.2. HHIT Suite IDs

The HHIT Suite IDs specify the HI and hash algorithms. These are a superset of the 4/8-bit HIT Suite ID as defined in Section 5.2.10 of [RFC7401].

The HHIT values of 1 - 15 map to the basic 4-bit HIT Suite IDs. HHIT values of 17 - 31 map to the extended 8-bit HIT Suite IDs. HHIT values unique to HHIT will start with value 32.

As HHIT introduces a new Suite ID, EdDSA/cSHAKE128, and since this is of value to HIPv2, it will be allocated out of the 4-bit HIT space and result in an update to HIT Suite IDs. Future HHIT Suite IDs may be allocated similarly, or may come out of the additional space made available by going to 8 bits.

The following HHIT Suite IDs are defined:

     HHIT Suite          Value
     RESERVED            0
     RSA,DSA/SHA-256     1    [RFC7401]
     ECDSA/SHA-384       2    [RFC7401]
     ECDSA_LOW/SHA-1     3    [RFC7401]
     EdDSA/cSHAKE128     TBD3 (suggested value 5)   (RECOMMENDED)

3.2.1. HDA custom HIT Suite IDs

Support for 8-bit HHIT Suite IDs allows for HDA custom HIT Suite IDs. These will be assigned values greater than 15 as follows:

     HHIT Suite             Value
     HDA Private Use 1      TBD4 (suggested value 254)
     HDA Private Use 2      TBD5 (suggested value 255)

These custom HIT Suite IDs, for example, 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.

They should not be used to create a "de facto standardization". Section 8.2 states that additional Suite IDs can be made through IETF Review.

3.3. The Hierarchy ID (HID)

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

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

The rationale for the 14/14 HID split is described in Appendix B.

The two levels of hierarchy allows for CAAs to have it least one RAA for their National Air Space (NAS). Within its RAA(s), the CAAs can delegate HDAs as needed. There may be other RAAs allowed to operate within a given NAS; this is a policy decision of each CAA.

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) or Japan Civil Aviation Bureau (JCAB) could be an RAA.

The RAA is a 14-bit field (16,384 RAAs). The management of this space is further elaborated in [drip-registries]. An RAA MUST provide a set of services to allocate HDAs to organizations. It SHOULD 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 HIP RVS servers for the HID. The zone delegation is also covered in [drip-registries].

As DETs under an administrative control may be used in many different domains (e.g., commercial, recreation, military), RAAs should be allocated in blocks (e.g. 16-19) with consideration on the likely size of a particular usage. Alternatively, different prefixes can be used to separate different domains of use of HHITs.

The RAA DNS zone within the UAS DNS tree may be a PTR for its RAA. It may be a zone in an HHIT specific DNS zone. Assume that the RAA is decimal 100. The PTR record could be constructed as follows:   IN PTR

Note that if the zone is ultimately used, some registrar will need to manage this for all HHIT applications. Thus further thought will be needed in the actual zone tree and registration process [drip-registries].

3.3.2. The Hierarchical HIT Domain Authority (HDA)

An HDA may be an Internet Service Provider (ISP), UAS Service Supplier (USS), or any third party that takes on the business to provide UAS services management, HIP RVSs or other needed services such as those required for HHIT and/or HIP-enabled devices.

The HDA is a 14-bit field (16,384 HDAs per RAA) assigned by an RAA is further elaborated in [drip-registries]. An HDA must maintain public and private UAS registration information and should maintain a set of RVS servers for UAS clients that may use HIP. How this is done and scales to the potentially millions of customers are outside the scope of this document, though covered in [drip-registries]. 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. Such policy is out of scope.

3.4. Edward-Curve Digital Signature Algorithm for HHITs

The Edwards-Curve Digital Signature Algorithm (EdDSA) [RFC8032] is specified here for use as HIs per HIPv2 [RFC7401].

The intent in this document is to add EdDSA as a HI algorithm for DETs, but doing so impacts the HIP parameters used in a HIP exchange. The subsections of this section document the required updates of HIP parameters. Other than the HIP DNS RR (Resource Record), these should not be needed in a DRIP implementation that does not use HIP.

See Section 3.2 for use of the HIT Suite in the context of DRIP.

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 5.2.9 of [RFC7401].

     profiles    Values

     EdDSA       TBD1 (suggested value 13) [RFC8032]    (RECOMMENDED) HIP Parameter support for EdDSA

The addition of EdDSA as a HI algorithm requires a subfield in the HIP HOST_ID parameter (Section 5.2.9 of [RFC7401]) as was done for ECDSA when used in a HIP exchange.

For HIP hosts that implement EdDSA as the algorithm, the following EdDSA curves are represented by the following fields:

   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
  |         EdDSA Curve           |             NULL              /
  /                         Public Key                            |

  EdDSA Curve   Curve label
  Public Key    Represented in Octet-string format      [RFC8032]

Figure 2

For hosts that implement EdDSA as a HIP algorithm the following EdDSA curves are required:

     Algorithm    Curve            Values

     EdDSA        RESERVED         0
     EdDSA        EdDSA25519       1 [RFC8032]          (RECOMMENDED)
     EdDSA        EdDSA25519ph     2 [RFC8032]
     EdDSA        EdDSA448         3 [RFC8032]          (RECOMMENDED)
     EdDSA        EdDSA448ph       4 [RFC8032] HIP DNS RR support for EdDSA

The HIP DNS RR is defined in [RFC8005]. It uses the values defined for the 'Algorithm Type' of the IPSECKEY RR [RFC4025] for its PK Algorithm field.

The new EdDSA HI uses [RFC8080] for the IPSECKEY RR encoding:

   Value  Description

   TBD2 (suggested value 4)
          An EdDSA key is present, in the format defined in [RFC8080]


The HIT_SUITE_LIST parameter contains a list of the supported HIT suite IDs of the HIP Responder. Based on the HIT_SUITE_LIST, the HIP 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:

     HIT Suite        Value
     EdDSA/cSHAKE128  TBD3 (suggested value 5)   (RECOMMENDED)

Table 1 provides more detail on the above HIT Suite combination.

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 update to ORCHIDv2 [RFC7343], as it can produce ORCHIDv2 output.

3.5.1. Adding Additional Information to the ORCHID

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

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


Prefix          : A constant 28-bit-long bitstring value
                  (IPV6 prefix)

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.

The new ORCHID function is as follows:

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


Prefix (p)      : An IPv6 prefix of length p (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.
                  When used for HHIT generation this is the
                  HHIT Suite ID.

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

p + n + o + m = 128 bits

The ORCHID length MUST be 128 bits. With a 28-bit IPv6 prefix, the remaining 100 bits can be divided in any manner between the additional information ("Info"), OGA ID, and the hash output. Care must be considering the size of the hash portion, taking into account risks like pre-image attacks. 64 bits, as used in Hierarchical HITs may be as small as is acceptable. The size of 'n' is determined as what is left; in the case of the 8-bit OGA used for HHIT, this is 28 bits.

3.5.2. ORCHID Encoding

This update 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 ("Info"), 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 28 bits for the HID and 8 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 cSHAKE function call for this update 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 a /28 prefix 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 HITs use the Context ID defined in Section 3. Encoding ORCHIDs for HIPv2

This section discusses how to provide backwards compatibility for ORCHIDv2 [RFC7343] as used in HIPv2 [RFC7401].

For HIPv2, the Prefix is 2001:20::/28 (Section 6 of [RFC7343]). 'Info' is zero-length (i.e., not included), and OGA ID is 4-bit. Thus, the HI Hash is 96-bit length. 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 update, the decoding of an ORCHID is determined by the Prefix and OGA 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 Section 8.2.

3.5.4. Decoding ORCHIDs for HIPv2

This section is included to provide backwards compatibility for ORCHIDv2 [RFC7343] as used for HIPv2 [RFC7401].

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

4. Hierarchical HITs as DRIP Entity Tags

HHITs for UAS ID (called, DETs) use the new EdDSA/SHAKE128 HIT suite defined in Section 3.4 (GEN-2 in [RFC9153]). This hierarchy, cryptographically bound within the HHIT, provides the information for finding the UA's HHIT registry (ID-3 in [RFC9153]).

The 2022 forthcoming updated release of ASTM Standard Specification for Remote ID and Tracking [F3411] adds support for DETs. This is within the UAS ID type 4, "Specific Session ID (SSI)".

Note to RFC Editor: This, and all references to F3411 need to be updated to this new version which is in final ASTM editing. A new link and replacement text will be provided when it is published.

The original UAS ID Types 1 - 3 allow for an UAS ID with a maximum length of 20 bytes, this new SSI (Type 4) uses the first byte of the ID for the SSI Type, thus restricting the UAS ID of this type to a maximum of 19 bytes. The SSI Types initially assigned are:

ID 1
IETF - DRIP Drone Remote ID Protocol (DRIP) entity ID.
ID 2
3GPP - IEEE 1609.2-2016 HashedID8

4.1. Nontransferablity of DETs

A HI and its DET 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 an HHIT within the CTA 2063-A format is not simple. The CTA 2063-A format is defined as follows:

Serial Number   :=  MFR Code | Length Code | MFR SN


MFR Code     : 4 character code assigned by ICAO
                 (International Civil Aviation Organization,
                  a UN Agency).

Length Code  : 1 character Hex encoding of MFR SN length (1-F).

MFR SN       : Alphanumeric code (0-9, A-Z except O and I).
                Maximum length of 15 characters.

There is no place for the HID; there will need to be a mapping service from Manufacturer Code to HID. The HHIT Suite ID and ORCHID hash will take the full 15 characters (as described below) of the MFR SN field.

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 HHIT Suite ID and ORCHID hash in 15 characters. Although, programmatically, 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 72 bits (HHIT Suite ID | ORCHID hash) of the HHIT SHALL be left-padded with 3 bits of zeros. This 75-bit number will be encoded into the 15-character MFR SN field using the digit/letters above. The manufacturer MUST use a Length Code of F (15).

Using the sample DET from Section 5 that is for HDA=20 under RAA=10 and having the ICAO CTA MFR Code of 8653, the 20-character CTA 2063-A Serial Number would be:


A mapping service (e.g., DNS) MUST provide a trusted (e.g., via DNSSEC [RFC4034]) conversion of the 4-character Manufacturer Code to high-order 58 bits (Prefix | HID) of the HHIT. Definition of this mapping service is currently out of scope of this document.

It should be noted that this encoding would only be used in the Basic ID Message (Section 2.2 of [RFC9153]). The DET is used in the Authentication Messages (i.e., the messages that provide framing for authentication data only).

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 8).

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 rationale at the time of developing ORCHID was attacks against these fields are Denial-of-Service (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 (Section 4.5).

4.5. DRIP Entity Tag (DET) Registry

DETs are registered to HDAs. A registration process, [drip-registries], ensures DET global uniqueness (ID-4 in [RFC9153]). It also provides the mechanism to create UAS public/private data that are associated with the DET (REG-1 and REG-2 in [RFC9153]).

4.6. Remote ID Authentication using DETs

The EdDSA25519 HI (Section 3.4) underlying the DET can be used in an 84-byte self-proof attestation (timestamp, HHIT, and signature of these) to provide proof of Remote ID ownership (GEN-1 in [RFC9153]). In practice, the Wrapper and Manifest authentication formats (Sections 6.3.3 and 6.3.4 of [drip-authentication]) implicitly provide this self-attestation. A lookup service like DNS can provide the HI and registration proof (GEN-3 in [RFC9153]).

Similarly, for Observers without Internet access, a 200-byte offline self-attestation could provide the same Remote ID ownership proof. This attestation would contain the HDA's signing of the UA's HHIT, itself signed by the UA's HI. Only a small cache that contains the HDA's HI/HHIT and HDA meta-data is needed by the Observer. However, such an object would just fit in the ASTM Authentication Message (Section 2.2 of [RFC9153]) with no room for growth. In practice, [drip-authentication] provides this offline self-attestation in two authentication messages: the HDA's certification of the UA's HHIT registration in a Link authentication message whose hash is sent in a Manifest authentication message.

Hashes of any previously sent ASTM messages can be placed in a Manifest authentication message (GEN-2 in [RFC9153]). When a Location/Vector Message (i.e., a message that provides UA location, altitude, heading, speed, and status) hash along with the hash of the HDA's UA HHIT attestation are sent in a Manifest authentication message and the Observer can visually see a UA at the claimed location, the Observer has a very strong proof of the UA's Remote ID.

All this behavior and how to mix these authentication messages into the flow of UA operation messages are detailed in [drip-authentication].

5. DRIP Entity Tags (DETs) in DNS

There are two approaches for storing and retrieving DETs using DNS. The following are examples of how this may be done. This will serve as guidance to the actual deployment of DETs in DNS. However, this document does not intend to provide a recommendation. Further DNS-related considerations are covered in [drip-registries].

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 [RFC9153]). For example, the USS for the HHIT could be found via the following: assume the RAA is decimal 100 and the HDA is decimal 50. The PTR record is constructed as follows:   IN PTR

The individual DETs may be potentially too numerous (e.g., 60 - 600M) and dynamic (e.g., new DETs every minute for some HDAs) to store in a signed, DNS zone. The HDA SHOULD provide DNS service for its zone and provide the HHIT detail response.

The DET reverse lookup can be a standard IPv6 reverse look up, or it can leverage off the HHIT structure. Using the allocated prefix for HHITs TBD6 [suggested value 2001:30::/28] (See Section 3.1), the RAA is 10 and the HDA is 20, 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

This DNS entry for the DET can also provide a revocation service. For example, instead of returning the HI RR it may return some record showing that the HI (and thus DET) has been revoked. Guidance on revocation service will be provided in [drip-registries].

6. Other UAS Traffic Management (UTM) Uses of HHITs Beyond DET

HHITs will be used within the UTM architecture beyond DET (and USS in UA ID registration and authentication), for example, as a Ground Control Station (GCS) HHIT ID. Some GCS will use its HHIT for securing its Network Remote ID (to USS HHIT) and Command and Control (C2, Section 2.2.2 of [RFC9153]) transports.

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. Details about such issues are out of the scope of this document).

7. Summary of Addressed DRIP Requirements

This document provides the details to solutions for GEN 1 - 3, ID 1 - 5, and REG 1 - 2 requirements that are described in [RFC9153].

8. IANA Considerations

8.1. New Well-Known IPv6 prefix for DETs

Since the DET format is not compatible with [RFC7343], IANA is requested to allocate a new prefix following this template for the IPv6 Special-Purpose Address Registry.

Address Block:
IANA is requested to allocate a new 28-bit prefix out of the IANA IPv6 Special Purpose Address Block, namely 2001::/23, as per [RFC6890] (TBD6, suggested: 2001:30::/28).
This block should be named "DRIP Entity Tags (DETs) Prefix".
This document.
Allocation Date:
Date this document published.
Termination Date:
Globally Reachable:

8.2. New IANA DRIP Registry

This document requests IANA to create a new registry titled "Drone Remote ID Protocol" registry. The following two subregistries should be created under that registry.

Hierarchical HIT (HHIT) Prefixes:
Initially, for DET use, one 28-bit prefix should be assigned out of the IANA IPv6 Special Purpose Address Block, namely 2001::/23, as per [RFC6890]. Future additions to this subregistry are to be made through Expert Review (Section 4.5 of [RFC8126]). Entries with network-specific prefixes may be present in the registry.

     HHIT Use     Bits  Value
     DET          28    TBD6 (suggested value 2001:30::/28)

Hierarchical HIT (HHIT) Suite ID:
This 8-bit valued subregistry is a superset of the 4/8-bit "HIT Suite ID" subregistry of the "Host Identity Protocol (HIP) Parameters" registry in [IANA-HIP]. Future additions to this subregistry are to be made through IETF Review (Section 4.8 of [RFC8126]). The following HHIT Suite IDs are defined:

     HHIT Suite          Value
     RESERVED            0
     RSA,DSA/SHA-256     1    [RFC7401]
     ECDSA/SHA-384       2    [RFC7401]
     ECDSA_LOW/SHA-1     3    [RFC7401]
     EdDSA/cSHAKE128     TBD3 (suggested value 5)   (RECOMMENDED)
     HDA Private Use 1   TBD4 (suggested value 254)
     HDA Private Use 2   TBD5 (suggested value 255)

  • The HHIT Suite ID values 1 - 31 are reserved for IDs that MUST be replicated as HIT Suite IDs (Section 8.4) as is TBD3 here. Higher values (32 - 255) are for those Suite IDs that need not or cannot be accommodated as a HIT Suite ID.

8.3. IANA CGA Registry Update

This document requests that this document be added to the reference field for the "CGA Extension Type Tags" registry [IANA-CGA], where IANA registers the following Context ID:

Context ID:
The Context ID (Section 3) shares the namespace introduced for CGA Type Tags. Defining new Context IDs follow the rules in Section 8 of [RFC3972]:

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

8.4. IANA HIP Registry Updates

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

Host ID:
This document defines the new EdDSA Host ID with value TBD1 (suggested: 13) (Section 3.4.1) in the "HI Algorithm" subregistry of the "Host Identity Protocol (HIP) Parameters" registry.

     profiles    Values

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

EdDSA Curve Label:
This document specifies a new algorithm-specific subregistry named "EdDSA Curve Label". The values for this subregistry are defined in Section Future additions to this subregistry are to be made through IETF Review (Section 4.8 of [RFC8126]).

     Algorithm    Curve            Values

     EdDSA        RESERVED         0
     EdDSA        EdDSA25519       1 [RFC8032]          (RECOMMENDED)
     EdDSA        EdDSA25519ph     2 [RFC8032]
     EdDSA        EdDSA448         3 [RFC8032]          (RECOMMENDED)
     EdDSA        EdDSA448ph       4 [RFC8032]
                                   5-65535               Unassigned

HIT Suite ID:
This document defines the new HIT Suite of EdDSA/cSHAKE with value TBD3 (suggested: 5) (Section 3.4.2) in the "HIT Suite ID" subregistry of the "Host Identity Protocol (HIP) Parameters" registry.

     HIT Suite        Value
     EdDSA/cSHAKE128  TBD3 (suggested value 5)   (RECOMMENDED)

  • The HIT Suite ID 4-bit values 1 - 15 and 8-bit values 0x00 - 0x0F MUST be replicated as HHIT Suite IDs (Section 8.2) as is TBD3 here.

8.5. IANA IPSECKEY Registry Update

This document requests IANA to make the following change to the "IPSECKEY Resource Record Parameters" [IANA-IPSECKEY] registry:

This document defines the new IPSECKEY value TBD2 (suggested: 4) (Section in the "Algorithm Type Field" subregistry of the "IPSECKEY Resource Record Parameters" registry.

   Value  Description

   TBD2 (suggested value 4)
          An EdDSA key is present, in the format defined in [RFC8080]

9. Security Considerations

The 64-bit hash in HHITs presents a real risk of second pre-image cryptographic hash attack Section 9.5. 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.

However, with today's computing power, producing 2^64 EdDSA keypairs and then generating the corresponding HHIT is economically feasible. Consider that a *single* bitcoin mining ASIC can do on the order of 2^46 sha256 hashes a second or about 2^62 hashes in a single day. The point being, 2^64 is not prohibitive, especially as this can be done in parallel.

Now it should be noted that the 2^64 attempts is for stealing a specific HHIT. Consider a scenario of a street photography company with 1,024 UAs (each with its own HHIT); you'd be happy stealing any one of them. Then rather than needing to satisfy a 64-bit condition on the cSHAKE128 output, you need only satisfy what is equivalent to a 54-bit condition (since there are 2^10 more opportunities for success).

Thus, although the probability of a collision or pre-image attack is low in a collection of 1,024 HHITs out of a total population of 2^64, per Section 9.5, it is computationally and economically feasible. Therefore, the HHIT registration and HHIT/HI registration validation is strongly recommended.

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 messages containing DETs using DNS to find the HI for the DET. As such, use of DNSSEC by the DET registries is recommended to provide trust in HI retrieval.

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 detailed in [drip-authentication].

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.

9.1. Post Quantum Computing out of scope

As stated in Section 8.1 of [drip-architecture], there has been no effort, at this time, to address post quantum computing cryptography. UAs and Broadcast Remote ID communications are so constrained that current post quantum computing cryptography is not applicable. Plus since a UA may use a unique DET for each operation, the attack window could be limited to the duration of the operation.

HHITs contain the ID for the cryptographic suite used in its creation, a future post quantum computing safe algorithm that fits the Remote ID constraints may readily be added.

9.2. DET Trust in ASTM messaging

The DET in the ASTM Basic ID 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 ID 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 (Section 4.6) 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 ID 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 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. Only then does an observer gain full trust in the DET of the UA.

DETs obtained via the Network RID path provides a different approach to trust. Here the UAS SHOULD be securely communicating to the USS, thus asserting DET trust.

9.3. DET Revocation

The DNS entry for the DET can also provide a revocation service. For example, instead of returning the HI RR it may return some record showing that the HI (and thus DET) has been revoked. Guidance on revocation service will be provided in [drip-registries].

9.4. Privacy Considerations

There is no expectation of privacy for DETs; it is not part of the privacy normative requirements listed in, Section 4.3.1, of [RFC9153]. DETs are broadcast in the clear over the open air via Bluetooth and Wi-Fi. 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 MAC address randomization. For Bluetooth 4 Remote ID messaging, the MAC address is used by observers to link the Basic ID 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 Wi-Fi 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 mandatory 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 DET only starts the changes that need to be implemented.

These privacy realities may present challenges for the EU U-space (Appendix A) program.

9.5. 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.

Thus an adversary trying to generate a collision and 'steal' the DET would run afoul of this registration process and associated validation process mentioned in Section 1.1.

10. References

10.1. Normative References

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, , <>.
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, , <>.
Laganier, J., "Host Identity Protocol (HIP) Domain Name System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005, , <>.
Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, , <>.
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

10.2. Informative References

"A CFRG review of draft-ietf-drip-rid", , <>.
CORUS, "U-space Concept of Operations", , <>.
ANSI/CTA, "Small Unmanned Aerial Systems Serial Numbers", , <>.
Card, S. W., Wiethuechter, A., Moskowitz, R., Zhao, S., and A. Gurtov, "Drone Remote Identification Protocol (DRIP) Architecture", Work in Progress, Internet-Draft, draft-ietf-drip-arch-22, , <>.
Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote ID", Work in Progress, Internet-Draft, draft-ietf-drip-auth-10, , <>.
Wiethuechter, A., Card, S., Moskowitz, R., and J. Reid, "DRIP Entity Tag Registration & Lookup", Work in Progress, Internet-Draft, draft-ietf-drip-registries-03, , <>.
ASTM International, "Standard Specification for Remote ID and Tracking", <>.
United States Federal Aviation Administration (FAA), "Remote Identification of Unmanned Aircraft", , <>.
IANA, "Cryptographically Generated Addresses (CGA) Message Type Name Space", <>.
IANA, "Host Identity Protocol (HIP) Parameters", <>.
IANA, "IPSECKEY Resource Record Parameters", <>.
Bertoni, G., Daemen, J., Peeters, M., Van Assche, G., and R. Van Keer, "The Keccak Function", <>.
Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, DOI 10.17487/RFC3972, , <>.
Richardson, M., "A Method for Storing IPsec Keying Material in DNS", RFC 4025, DOI 10.17487/RFC4025, , <>.
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, , <>.
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, , <>.
Laganier, J. and L. Eggert, "Host Identity Protocol (HIP) Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004, , <>.
Sury, O. and R. Edmonds, "Edwards-Curve Digital Security Algorithm (EdDSA) for DNSSEC", RFC 8080, DOI 10.17487/RFC8080, , <>.
Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <>.
Moskowitz, R., Ed. and M. Komu, "Host Identity Protocol Architecture", RFC 9063, DOI 10.17487/RFC9063, , <>.
Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A. Gurtov, "Drone Remote Identification Protocol (DRIP) Requirements and Terminology", RFC 9153, DOI 10.17487/RFC9153, , <>.
Blanchet, M., "Finding the Authoritative Registration Data Access Protocol (RDAP) Service", STD 95, RFC 9224, DOI 10.17487/RFC9224, , <>.

Appendix A. EU U-Space RID Privacy Considerations

The 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 the 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.

This is potentially a contrary expectation as that presented in Section 9.4. U-space will have to deal with this reality within the GDPR regulations. Still, DETs as defined here present a large step in the right direction for agnostic IDs.

The project lists "E-Identification" and "E-Registrations" services as to be developed. These services can use DETs and follow the privacy considerations outlined in this document for DETs.

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 DET (Hierarchical HIT) provides 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. The 14/14 HID split

The following explains the logic behind selecting to divide the 28 bits of the HID into 2 14-bit components.

At this writing ICAO has 273 member "States", each may want to control RID assignment within its National Air Space (NAS). Some members may want separate RAAs to use for Civil, general Government, and Military use. They may also want allowances for competing Civil RAA operations. It is reasonable to plan for 8 RAAs per ICAO member (plus regional aviation organizations like in the European Union). Thus at a start a 4,096 RAA space is advised.

There will be requests by commercial entities for their own, RAA allotments. Examples could include international organizations that will be using UAS and international delivery service associations. These may be smaller than the RAA space needed by ICAO member States and could be met with a 2,048 space allotment, but as will be seen, might as well be 4,096 as well.

This may well cover currently understood RAA entities. There will be future new applications, branching off into new areas. So yet another space allocation should be set aside. If this is equal to all that has been reserved, we should allow for 16,384 (2^14) RAAs.

The HDA allocation follows a different logic from that of RAAs. Per Appendix C, an HDA should be able to easily assign 63M RIDs and even manage 663M with a "first come, first assigned" registration process. For most HDAs this is more than enough, and a single HDA assignment within their RAA will suffice. Most RAAs will only delegate to a couple HDAs for their operational needs. But there are major exceptions that point to some RAAs needing large numbers of HDA assignments.

Delivery service operators like Amazon (est. 30K delivery vans) and UPS (est. 500K delivery vans) may choose, for anti-tracking reasons, to use unique RIDs per day or even per operation. 30K delivery UA could need 11M upwards to 44M RIDs. Anti-tracking would be hard to provide if the HID were the same for a delivery service fleet, so such a company may turn to an HDA that provides this service to multiple companies so that who's UA is who's is not evident in the HID. A USS providing this service could well use multiple HDA assignments per year, depending on strategy.

Perhaps a single RAA providing HDAs for delivery service (or similar behaving) UAS could 'get by' with a 2048 HDA space (11-bits). So the HDA space could well be served with only 12 bits allocated out of the 28-bit HID space. But as this is speculation, and it will take years of deployment experience, a 14-bit HDA space has been selected.

There may also be 'small' ICAO member States that opt for a single RAA and allocate their HDAs for all UA that are permitted in their NAS. The HDA space is large enough that some to use part for government needs as stated above and for small commercial needs. Or the State may use a separate, consecutive RAA for commercial users. Thus it would be 'easy' to recognize State-approved UA by HID high-order bits.

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. Nicholas Gajcowski [cfrg-comment] provided a concise hash pre-image security assessment via the CFRG list.

Many thanks to Michael Richardson and Brian Haberman for the iotdir review, Magnus Nystrom for the secdir review, Elwyn Davies for genart review and DRIP co-chair and draft shepherd, Mohamed Boucadair for his extensive comments and help on document clarity.

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