1.1. Overview of UAS RID and Its Standardization

UAS RID is an application that enables UAS to be identified by UAS Traffic Management (UTM), UAS Service Suppliers (USS) (Appendix A), and third-party entities, such as law enforcement. Many considerations (e.g., safety and security) dictate that UAS be remotely identifiable.¶

Civil Aviation Authorities (CAAs) worldwide are mandating UAS RID. CAAs currently promulgate performance-based regulations that do not specify techniques but rather cite industry consensus technical standards as acceptable means of compliance.¶

USA Federal Aviation Administration (FAA)

The FAA published a Notice of Proposed Rule Making [NPRM] in 2019 and thereafter published a "Final Rule" in 2021 [FAA_RID], imposing requirements on UAS manufacturers and operators, both commercial and recreational. The rule states that Automatic Dependent Surveillance Broadcast (ADS-B) Out and transponders cannot be used to satisfy the UAS RID requirements on UAS to which the rule applies (see Appendix B).¶

European Union Aviation Safety Agency (EASA)

In pursuit of the "U-space" concept of a single European airspace safely shared by manned and unmanned aircraft, the EASA published a [Delegated] regulation in 2019, imposing requirements on UAS manufacturers and third-country operators, including but not limited to UAS RID requirements. The same year, the EASA also published a regulation [Implementing], laying down detailed rules and procedures for UAS operations and operating personnel, which then was updated in 2021 [Implementing_update]. A Notice of Proposed Amendment [NPA] was published in 2021 to provide more information about the development of acceptable means of compliance and guidance material to support U-space regulations.¶

American Society for Testing and Materials (ASTM)

ASTM International, Technical Committee F38 (UAS), Subcommittee F38.02 (Aircraft Operations), Work Item WK65041 developed an ASTM standard [F3411-22a], titled "Standard Specification for Remote ID and Tracking".¶

ASTM defines one set of UAS RID information and two means, Media Access Control (MAC) layer broadcast and IP layer network, of communicating it. If a UAS uses both communication methods, the same information must be provided via both means. [F3411-22a] is the technical standard basis of the Means Of Compliance (MOC) specified in [F3586-22]. The FAA has accepted [F3586-22] as a MOC to the FAA's UAS RID Final Rule [FAA_RID], with some caveats, as per [MOC-NOA]. Other CAAs are expected to accept the same or other MOCs likewise based on [F3411-22a].¶

3rd Generation Partnership Project (3GPP)

With Release 16, the 3GPP completed the UAS RID requirement study [TR-22.825] and proposed a set of use cases in the mobile network and services that can be offered based on UAS RID. The Release 17 study [TR-23.755] and specification [TS-23.255] focus on enhanced UAS service requirements and provide the protocol and application architecture support that will be applicable for both 4G and 5G networks. The study of Further Architecture Enhancement for Uncrewed Aerial Vehicles (UAV) and Urban Air Mobility (UAM) in Release 18 [FS_AEUA] further enhances the communication mechanism between UAS and USS/UTM. The DET in Section 3 may be used as the 3GPP CAA-level UAS ID for RID purposes.¶

1.2. Overview of Types of UAS Remote ID

This specification introduces two types of UAS Remote IDs as defined in ASTM [F3411-22a].¶

                +------------------------+
                | Unmanned Aircraft (UA) |
                +-----------o------------+
                            |
                            | app messages directly over
                            | one-way RF data link (no IP)
                            |
                            v
          +------------------o-------------------+
          | Observer's device (e.g., smartphone) |
          +--------------------------------------+

 +-------------+     ******************
 |     UA      |     *    Internet    *
 +--o-------o--+     *                *
    |       |        *                *     +------------+
    |       '--------*--(+)-----------*-----o            |
    |                *   |            *     |            |
    |       .--------*--(+)-----------*-----o Net-RID SP |
    |       |        *                *     |            |
    |       |        *         .------*-----o            |
    |       |        *         |      *     +------------+
    |       |        *         |      *
    |       |        *         |      *     +------------+
    |       |        *         '------*-----o            |
    |       |        *                *     | Net-RID DP |
    |       |        *         .------*-----o            |
    |       |        *         |      *     +------------+
    |       |        *         |      *
    |       |        *         |      *     +------------+
 +--o-------o--+     *         '------*-----o Observer's |
 |     GCS     |     *                *     | Device     |
 +-------------+     ******************     +------------+

1.3. Overview of USS Interoperability

With Net-RID, there is direct communication between each UAS and its USS. Multiple USS exchange information with the assistance of a DSS so all USS collectively have knowledge about all activities in a 4-D airspace. The interactions among an Observer, multiple UAS, and their USS are shown in Figure 3.¶

                +------+    +----------+    +------+
                | UAS1 |    | Observer |    | UAS2 |
                +---o--+    +-----o----+    +--o---+
                    |             |            |
              ******|*************|************|******
              *     |             |            |     *
              *     |         +---o--+         |     *
              *     |  .------o USS3 o------.  |     *
              *     |  |      +--o---+      |  |     *
              *     |  |         |          |  |     *
              *   +-o--o-+    +--o--+     +-o--o-+   *
              *   |      o----o DSS o-----o      |   *
              *   | USS1 |    +-----+     | USS2 |   *
              *   |      o----------------o      |   *
              *   +------+                +------+   *
              *                                      *
              *               Internet               *
              ****************************************

1.4. Overview of DRIP Architecture

Figure 4 illustrates a global UAS RID usage scenario. Broadcast RID links are not shown, as they reach from any UA to any listening receiver in range and thus would obscure the intent of the figure. Figure 4 shows, as context, some entities and interfaces beyond the scope of DRIP (as currently (2023) chartered). Multiple UAS are shown, each with its own UA controlled by its own GCS, potentially using the same or different USS, with the UA potentially communicating directly with each other (V2V), especially for low-latency, safety-related purposes (DAA).¶

***************                                        ***************
*    UAS1     *                                        *     UAS2    *
*             *                                        *             *
* +--------+  *                 DAA/V2V                *  +--------+ *
* |   UA   o--*----------------------------------------*--o   UA   | *
* +--o--o--+  *                                        *  +--o--o--+ *
*    |  |     *   +------+      Lookups     +------+   *     |  |    *
*    |  |     *   | GPOD o------.    .------o PSOD |   *     |  |    *
*    |  |     *   +------+      |    |      +------+   *     |  |    *
*    |  |     *                 |    |                 *     |  |    *
* C2 |  |     *     V2I      ************     V2I      *     |  | C2 *
*    |  '-----*--------------*          *--------------*-----'  |    *
*    |        *              *          *              *        |    *
*    |        o====Net-RID===*          *====Net-RID===o        |    *
* +--o--+     *              * Internet *              *     +--o--+ *
* | GCS o-----*--------------*          *--------------*-----o GCS | *
* +-----+     * Registration *          * Registration *     +-----+ *
*             * (and UTM)    *          * (and UTM)    *             *
***************              ************              ***************
                               |  |  |
                +----------+   |  |  |   +----------+
                | Public   o---'  |  '---o Private  |
                | Registry |      |      | Registry |
                +----------+      |      +----------+
                               +--o--+
                               | DNS |
                               +-----+

DAA:  Detect And Avoid
GPOD: General Public Observer Device
PSOD: Public Safety Observer Device
V2I:  Vehicle-to-Infrastructure
V2V:  Vehicle-to-Vehicle

DRIP is meant to leverage existing Internet resources (standard protocols, services, infrastructures, and business models) to meet UAS RID and closely related needs. DRIP will specify how to apply IETF standards, complementing [F3411-22a] and other external standards, to satisfy UAS RID requirements.¶

This document outlines the DRIP architecture in the context of the UAS RID architecture. This includes closing the gaps between the CAAs' concepts of operations and [F3411-22a] as it relates to the use of Internet technologies and UA-direct RF communications. Issues include, but are not limited to:¶

• the design of trustworthy remote identifiers required by GEN-1 (Section 3), especially but not exclusively for use as single-use session IDs,¶
• mechanisms to leverage the Domain Name System (DNS) [RFC1034] for registering and publishing public and private information (see Sections 4.1 and 4.2), as required by REG-1 and REG-2,¶
• specific authentication methods and message payload formats to enable verification that Broadcast RID messages were sent by the claimed sender (Section 5) and that the sender is in the claimed DRIP Identity Management Entity (DIME) (see Sections 4 and 5), as required by GEN-2,¶
• harvesting Broadcast RID messages for UTM inclusion, with the optional DRIP extension of Crowdsourced Remote ID (CS-RID) (Section 6), using the DRIP support for gateways required by GEN-5 [RFC9153],¶
• methods for instantly establishing secure communications between an Observer and the pilot of an observed UAS (Section 7), using the DRIP support for dynamic contact required by GEN-4 [RFC9153], and¶
• privacy in UAS RID messages (personal data protection) (Section 10).¶

This document should serve as a main point of entry into the set of IETF documents addressing the basic DRIP requirements.¶

2.1. Additional Abbreviations

DET:

DRIP Entity Tag¶

EdDSA:

Edwards-curve Digital Signature Algorithm¶

HHIT:

Hierarchical HIT¶

HI:

Host Identity¶

HIP:

Host Identity Protocol¶

HIT:

Host Identity Tag¶

2.2. Additional Definitions

This section introduces the terms "Claim", "Evidence", "Endorsement", and "Certificate", as used in DRIP. A DRIP certificate has a different context compared with security certificates and Public Key Infrastructure used in X.509.¶

Claim:

A claim shares the same definition as a claim in Remote ATtestation procedureS (RATS) [RFC9334]; it is a piece of asserted information, sometimes in the form of a name/value pair. It could also been seen as a predicate (e.g., "X is Y", "X has property Y", and most importantly "X owns Y" or "X is owned by Y").¶

Evidence:

Evidence in DRIP borrows the same definition as in RATS [RFC9334], that is, a set of claims.¶

Endorsement:

An Endorsement is inspired from RATS [RFC9334]; it is a secure (i.e., signed) statement vouching the integrity and veracity of evidence.¶

Certificate:

A certificate in DRIP is an endorsement, strictly over identity information, signed by a third party. This third party should be one with no stake in the endorsement over which it is signing.¶

DRIP Identity Management Entity (DIME):

A DIME is an entity that performs functions similar to a domain registrar/registry. A DIME vets Claims and/or Evidence from a registrant and delivers back Endorsements and/or Certificates in response. It is a high-level entity in the DRIP registration/provisioning process that can hold the role of HHIT Domain Authority (HDA), Registered Assigning Authority (RAA), or root of trust (typically the HHIT prefix owner or DNS apex owner) for DETs.¶

3.1. UAS Remote Identifiers Problem Space

A DRIP entity identifier needs to be "Trustworthy" (see DRIP requirements GEN-1, ID-4, and ID-5 in [RFC9153]). This means that given a sufficient collection of UAS RID messages, an Observer can establish that the identifier claimed therein uniquely belongs to the claimant. To satisfy DRIP requirements and maintain important security properties, the DRIP identifier should be self-generated by the entity it names (e.g., a UAS) and registered (e.g., with a USS; see DRIP requirements GEN-3 and ID-2).¶

However, Broadcast RID, especially its support for Bluetooth 4, imposes severe constraints. A previous revision of the ASTM UAS RID, [F3411-19], allowed a UAS ID of types (1, 2, and 3), each of 20 bytes. [F3411-22a] adds type 4, Specific Session ID, for other Standards Development Organizations (SDOs) to extend ASTM UAS RID. Type 4 uses one byte to index the Specific Session ID subtype, leaving 19 bytes (see ID-1 of DRIP Requirements [RFC9153]). As described in Section 3 of [RFC9153], ASTM has allocated Specific Session ID subtype 1 to IETF DRIP.¶

The maximum ASTM UAS RID Authentication Message payload is 201 bytes each for Authentication Types 1, 2, 3, and 4. [F3411-22a] adds Authentication Type 5 for other SDOs (including the IETF) to extend ASTM UAS RID with Specific Authentication Methods (SAMs). With Type 5, one of the 201 bytes is consumed to index the SAM Type, leaving only 200 bytes for DRIP authentication payloads, including one or more DRIP entity identifiers and associated authentication data.¶

3.2. HHIT as a Cryptographic Identifier

The only (known to the authors at the time of writing) existing types of IP-address-compatible identifiers cryptographically derived from the public keys of the identified entities are Cryptographically Generated Addresses (CGAs) [RFC3972] and Host Identity Tags (HITs) [RFC7401]. CGAs and HITs lack registration/retrieval capability. To provide this, each HHIT embeds plaintext information designating the hierarchy within which it is registered, a cryptographic hash of that information concatenated with the entity's public key, etc. Although hash collisions may occur, the DIME can detect them and reject registration requests rather than issue credentials, e.g., by enforcing a First Come First Served policy [RFC8126]. Preimage hash attacks are also mitigated through this registration process, locking the HHIT to a specific HI.¶

3.3. HHIT as a Trustworthy DRIP Entity Identifier

A Remote UAS ID that can be trustworthy for use in Broadcast RID can be built from an asymmetric key pair. In this method, the UAS ID is cryptographically derived directly from the public key. The proof of UAS ID ownership (verifiable endorsement versus mere claim) is guaranteed by signing this cryptographic UAS ID with the associated private key. The association between the UAS ID and the private key is ensured by cryptographically binding the public key with the UAS ID; more specifically, the UAS ID results from the hash of the public key. The public key is designated as the HI, while the UAS ID is designated as the HIT.¶

By construction, the HIT is statistically unique through the mandatory use of cryptographic hash functions with second-preimage resistance. The cryptographically bound addition of the hierarchy and a HHIT registration process provide complete, global HHIT uniqueness. This registration forces the attacker to generate the same public key rather than a public key that generates the same HHIT. This is in contrast to general IDs (e.g., a Universally Unique Identifier (UUID) or device serial number) as the subject in an X.509 certificate.¶

A UA equipped for Broadcast RID MUST be provisioned not only with its HHIT but also with the HI public key from which the HHIT was derived and the corresponding private key to enable message signature.¶

A UAS equipped for DRIP-enhanced Network RID MUST be provisioned likewise; the private key resides only in the ultimate source of Network RID messages. If the GCS is the source of the Network RID messages, the GCS MUST hold the private key. If the UA is the source of the Network RID messages and they are being relayed by the GCS, the UA MUST hold the private key, just as a UA that directly connects to the network rather than through its GCS.¶

Each Observer device functioning with Internet connectivity MAY be provisioned either with public keys of the DRIP identifier root registries or certificates for subordinate registries; each Observer device that needs to operate without Internet connectivity at any time MUST be so provisioned.¶

HHITs can also be used throughout the USS/UTM system. Operators and Private Information Registries, as well as other UTM entities, can use HHITs for their IDs. Such HHITs can facilitate DRIP security functions, such as those used with HIP, to strongly mutually authenticate and encrypt communications.¶

A self-endorsement of a HHIT used as a UAS ID can be done in as little as 88 bytes when Ed25519 [RFC8032] is used by only including the 16-byte HHIT, two 4-byte timestamps, and the 64-byte Ed25519 signature.¶

Ed25519 [RFC8032] is used as the HHIT mandatory-to-implement signing algorithm, as GEN-1 and ID-5 [RFC9153] can best be met by restricting the HI to 32 bytes. A larger public key would rule out the offline endorsement feature that fits within the 200-byte Authentication Message maximum length. Other algorithms that meet this 32-byte constraint can be added as deemed needed.¶

A DRIP identifier can be assigned to a UAS as a static HHIT by its manufacturer, such as a single HI and derived HHIT encoded as a hardware serial number, per [CTA2063A]. Such a static HHIT SHOULD only be used to bind one-time-use DRIP identifiers to the unique UA. Depending upon implementation, this may leave a HI private key in the possession of the manufacturer (see also Section 9).¶

In general, Internet access may be needed to validate Endorsements or Certificates. This may be obviated in the most common cases (e.g., endorsement of the UAS ID), even in disconnected environments, by prepopulating small caches on Observer devices with DIME public keys and a chain of Endorsements or Certificates (tracing a path through the DIME tree). This is assuming all parties on the trust path also use HHITs for their identities.¶

3.4. HHIT for DRIP Identifier Registration and Lookup

UAS RID needs a deterministic lookup mechanism that rapidly provides actionable information about the identified UA. Given the size constraints imposed by the Bluetooth 4 broadcast media, the UAS ID itself needs to be a non-spoofable inquiry input into the lookup.¶

A DRIP registration process based on the explicit hierarchy within a HHIT provides manageable uniqueness of the HI for the HHIT. The hierarchy is defined in [RFC9374] and consists of 2 levels: an RAA and then an HDA. The registration within this hierarchy is the defense against a cryptographic hash second-preimage attack on the HHIT (e.g., multiple HIs yielding the same HHIT; see Requirement ID-3 in [RFC9153]). The First Come First Served registration policy is adequate.¶

A lookup of the HHIT into the DIME provides the registered HI for HHIT proof of ownership and deterministic access to any other needed actionable information based on inquiry access authority (more details in Section 4.2).¶

4.1. Public Information Registry

4.2. Private Information Registry

6.1. The CS-RID Finder

A CS-RID Finder is the gateway for Broadcast Remote ID Messages into UTM. It performs this gateway function via a CS-RID SDSP. A CS-RID Finder could implement, integrate, or accept outputs from a Broadcast RID receiver. However, it should not depend upon a direct interface with a GCS, Net-RID SP, Net-RID DP, or Net-RID client. It would present a new interface to a CS-RID SDSP, similar to but readily distinguishable from that which a UAS (UA or GCS) presents to a Net-RID SP.¶

6.2. The CS-RID SDSP

A CS-RID SDSP aggregates and processes (e.g., estimates UA locations using multilateration when possible) information collected by CS-RID Finders. A CS-RID SDSP should present the same interface to a Net-RID SP as it does to a Net-RID DP and to a Net-RID DP as it does to a Net-RID SP, but its data source must be readily distinguishable via Finders rather than direct from the UAS itself.¶

9.1. Private Key Physical Security

The security provided by asymmetric cryptographic techniques depends upon protection of the private keys. It may be necessary for the GCS to have the key pair to register the HHIT to the USS. Thus, it may be the GCS that generates the key pair and delivers it to the UA, making the GCS a part of the key security boundary. Leakage of the private key, from either the UA or the GCS, to the component manufacturer is a valid concern, and steps need to be in place to ensure safe keeping of the private key. Since it is possible for the UAS RID sender of a small harmless UA (or the entire UA) to be carried by a larger dangerous UA as a "false flag", it is out of scope to deal with secure storage of the private key.¶

9.2. Quantum Resistant Cryptography

There has been no effort as of yet in DRIP to address post quantum computing cryptography. Small UAS and Broadcast Remote ID communications are so constrained that current post quantum computing cryptography is not applicable. Fortunately, since a UA may use a unique HHIT for each operation, the attack window can be limited to the duration of the operation. One potential future DRIP use for post quantum cryptography is for key pairs that have long usage lives but that rarely, if ever, need to be transmitted over bandwidth constrained links, such as for serial numbers or operators. As the HHIT contains the ID for the cryptographic suite used in its creation, a future post quantum computing safe algorithm that fits Remote ID constraints may be readily added. This is left for future work.¶

9.3. Denial-of-Service (DoS) Protection

Remote ID services from the UA use a wireless link in a public space. As such, they are open to many forms of RF jamming. It is trivial for an attacker to stop any UA messages from reaching a wireless receiver. Thus, it is pointless to attempt to provide relief from DoS attacks, as there is always the ultimate RF jamming attack. Also, DoS may be attempted with spoofing/replay attacks; for which, see Section 9.4.¶

9.4. Spoofing and Replay Protection

As noted in Section 5, spoofing is combatted by the intrinsic self-attesting properties of HHITs, plus their registration. Also, as noted in Section 5, to combat replay attacks, a receiver MUST NOT trust any claims nominally received from an observed UA (not even the Basic ID message purportedly identifying that UA) until the receiver verifies that the private key used to sign those claims is trusted, that the sender actually possesses that key, and that the sender appears indeed to be that observed UA. This requires receiving a complete chain of endorsement links from a root of trust to the UA's leaf DET, plus a message containing suitable nonce-like data signed with the private key corresponding to that DET, and verifying all the foregoing. The term "nonce-like" describes data that is readily available to the prover and the verifier, changes frequently, is not predictable by the prover, and can be checked quickly at low computational cost by the verifier; a Location/Vector message is an obvious choice.¶

9.5. Timestamps and Time Sources

Section 6 and, more fundamentally, Section 3.3 both require timestamps. In Broadcast RID messages, [F3411-22a] specifies both 32-bit Unix-style UTC timestamps (seconds since midnight going into the 1st day of 2019, rather than 1970) and 16-bit relative timestamps (tenths of seconds since the start of the most recent hour or other specified event). [F3411-22a] requires that 16-bit timestamp accuracy, relative to the time of applicability of the data being timestamped, also be reported, with a worst allowable case of 1.5 seconds. [F3411-22a] does not specify the time source, but GNSS is generally assumed, as latitude, longitude, and geodetic altitude must be reported and most small UAS use GNSS for positioning and navigation.¶

GCS often have access to cellular LTE or other time sources better than the foregoing, and such better time sources would be required to support multilateration in Section 6, but such better time sources cannot be assumed generally for purposes of security analysis.¶

A.1. Operation Concept

The efforts of the National Aeronautics and Space Administration (NASA) and FAA to integrate UAS operations into the national airspace system (NAS) led to the development of the concept of UTM and the ecosystem around it. The UTM concept was initially presented in 2013, and version 2.0 was published in 2020 [FAA_UAS_Concept_Of_Ops].¶

The eventual concept refinement, initial prototype implementation, and testing were conducted by the joint FAA and NASA UTM research transition team. World efforts took place afterward. The Single European Sky ATM Research (SESAR) started the Concept of Operation for EuRopean UTM Systems (CORUS) project to research its UTM counterpart concept, namely [U-Space]. This effort is led by the European Organization for the Safety of Air Navigation (EUROCONTROL).¶

Both NASA and SESAR have published their UTM concepts of operations to guide the development of their future air traffic management (ATM) system and ensure safe and efficient integration of manned and unmanned aircraft into the national airspace.¶

UTM comprises UAS operations infrastructure, procedures, and local regulation compliance policies to guarantee safe UAS integration and operation. The main functionality of UTM includes, but is not limited to, providing means of communication between UAS operators and service providers and a platform to facilitate communication among UAS service providers.¶

A.2. UAS Service Supplier (USS)

A USS plays an important role to fulfill the key performance indicators (KPIs) that UTM has to offer. Such an entity acts as a proxy between UAS operators and UTM service providers. It provides services like real-time UAS traffic monitoring and planning, aeronautical data archiving, airspace and violation control, interacting with other third-party control entities, etc. A USS can coexist with other USS to build a large service coverage map that can load-balance, relay, and share UAS traffic information.¶

The FAA works with UAS industry shareholders and promotes the Low Altitude Authorization and Notification Capability [LAANC] program, which is the first system to realize some of the envisioned functionality of UTM. The LAANC program can automate UAS operational intent (flight plan) submissions and applications for airspace authorization in real time by checking against multiple aeronautical databases, such as airspace classification and operating rules associated with it, the FAA UAS facility map, special use airspace, Notice to Airmen (NOTAM), and Temporary Flight Restriction (TFR).¶

A.3. UTM Use Cases for UAS Operations

This section illustrates a couple of use case scenarios where UAS participation in UTM has significant safety improvement.¶

1. For a UAS participating in UTM and taking off or landing in controlled airspace (e.g., Class Bravo, Charlie, Delta, and Echo in the United States), the USS under which the UAS is operating is responsible for verifying UA registration, authenticating the UAS operational intent (flight plan) by checking against a designated UAS facility map database, obtaining the air traffic control (ATC) authorization, and monitoring the UAS flight path in order to maintain safe margins and follow the pre-authorized sequence of authorized 4-D volumes (route).¶
2. For a UAS participating in UTM and taking off or landing in uncontrolled airspace (e.g., Class Golf in the United States), preflight authorization must be obtained from a USS when operating BVLOS. The USS either accepts or rejects the received operational intent (flight plan) from the UAS. An accepted UAS operation may, and in some cases must, share its current flight data, such as GPS position and altitude, to the USS. The USS may maintain (and provide to authorized requestors) the UAS operation status near real time in the short term and may retain at least some of it in the longer term, e.g., for overall airspace air traffic monitoring.¶