Drone Remote Identification Protocol (DRIP) Architecture

1.1. Overview of Unmanned Aircraft System (UAS) Remote ID (RID) and Standardization

UAS Remote Identification (RID) is an application that enables a UAS to be identified by Unmanned Aircraft Systems Traffic Management (UTM) and UAS Service Supplier (USS) (Appendix A) or third party entities such as law enforcement. Many considerations (e.g., safety) 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 clearly 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)¶

• 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, EASA also published an [Implementing] regulation laying down detailed rules and procedures for UAS operations and operating personnel 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 the U-space regulation.¶

American Society for Testing and Materials (ASTM)¶

• ASTM International, Technical Committee F38 (UAS), Subcommittee F38.02 (Aircraft Operations), Work Item WK65041, developed the ASTM [F3411] Standard Specification for Remote ID and Tracking.¶

• ASTM defines one set of UAS RID information and two means, MAC-layer broadcast and IP-layer network, of communicating it. If an UAS uses both communication methods, the same information must be provided via both means. [F3411] is cited by the FAA in its UAS RID final rule [FAA_RID] as "a potential means of compliance" to a Remote ID rule.¶

The 3rd Generation Partnership Project (3GPP)¶

• With release 16, the 3GPP completed the UAS RID requirement study [TS-22.825] and proposed a set of use cases in the mobile network and services that can be offered based on UAS RID. Release 17 specification focuses on enhanced UAS service requirements and provides 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) [FS_AEUA] in release 18 further enhances the communication mechanism between UAS and USS/UTM. The DRIP Entity Tag in Section 3 may be used as the 3GPP CAA-level UAS ID for Remote Identification purposes.¶

1.2. Overview of Types of UAS Remote ID

This specification introduces two types UAS Remote ID defined in ASTM [F3411].¶

            +------------------------+
            | 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 4D 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 (2022) chartered).¶

***************                                        ***************
*    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] 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 presenting the gaps between the CAAs' Concepts of Operations and [F3411] as it relates to the use of Internet technologies and UA direct RF communications. Issues include, but are not limited to:¶

• • Design of trustworthy remote identifiers (Section 3).¶

• • Mechanisms to leverage Domain Name System (DNS [RFC1034]), Extensible Provisioning Protocol (EPP [RFC5731]) and Registration Data Access Protocol (RDAP) ([RFC9082]) for publishing public and private information (see Section 4.1 and Section 4.2).¶

• • Specific authentication methods and message payload formats to enable verification that Broadcast RID messages were sent by the claimed sender (Section 5) and that sender is in the claimed registry (Section 4 and Section 5).¶

• • Harvesting Broadcast RID messages for UTM inclusion, with the optional DRIP extension of Crowd Sourced 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].¶

• • Privacy in UAS RID messages (PII protection) (Section 9).¶

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 "Claims", "Assertions", "Attestations", and "Certificates" as used in DRIP. DRIP certificate has a different context compared with security certificates and Public Key Infrastructure used in X.509.¶

Claims:¶

• A claim in DRIP is a predicate (e.g., "X is Y", "X has property Y", and most importantly "X owns Y" or "X is owned by Y").¶

Assertions:¶

• An assertion in DRIP is a set of claims. This definition is borrowed from JWT [RFC7519] and CWT [RFC8392].¶

Attestations:¶

• An attestation in DRIP is a signed assertion. The signer may be the claimant or a related party with stake in the assertion(s). Under DRIP this is normally used when an entity asserts a relationship with another entity, along with other information, and the asserting entity signs the assertion, thereby making it an attestation.¶

Certificates:¶

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

3.1. UAS Remote Identifiers Problem Space

A DRIP entity identifier needs to be "Trustworthy" (See DRIP Requirement 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 Requirements GEN-3 and ID-2).¶

Broadcast RID, especially its support for Bluetooth 4.x, imposes severe constraints. ASTM UAS RID [F3411] allows a UAS ID of types 1, 2 and 3 of 20 bytes; a revision to [F3411], currently in balloting (as of Oct 2021), adds type 4, Specific Session ID, to be standardized by IETF and other standards development organizations (SDOs) as extensions to ASTM UAS RID, consumes one of those bytes to index the sub-type, leaving only 19 for the identifier (see DRIP Requirement ID-1).¶

Likewise, the maximum ASTM UAS RID [F3411] Authentication Message payload is 201 bytes for most authentication types. A type 5 is also added in this revision for IETF and other SDOs to develop Specific Authentication Methods as extensions to ASTM UAS RID. One byte out of 201 bytes is consumed to index the sub-type which leaves only 200 for DRIP authentication payloads, including one or more DRIP entity identifiers and associated authentication data.¶

3.2. 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 keypair. In this method, the UAS ID is cryptographically derived directly from the public key. The proof of UAS ID ownership (verifiable attestation, 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 cryptographic hash feature of second-preimage resistance. The cryptographically-bound addition of the Hierarchy and an 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 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 Network RID SHOULD be provisioned likewise; the private key resides only in the ultimate source of Network RID messages (i.e., on the UA itself if the GCS is merely relaying rather than sourcing Network RID messages). Each Observer device SHOULD be provisioned either with public keys of the DRIP identifier root registries or certificates for subordinate registries.¶

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 used with HIP to strongly mutually authenticate and encrypt communications.¶

A self-attestation of a HHIT used as a UAS ID can be done in as little as 84 bytes when Ed25519 [RFC8032] is used, by avoiding an explicit encoding technology like ASN.1 or Concise Binary Object Representation (CBOR [RFC8949]). This attestation consists of only the HHIT, a timestamp, and the EdDSA signature on them.¶

Ed25519 [RFC8032] is used as the HHIT signing algorithm as [RFC9153] GEN-1 and ID-5 can best be met by restricting the HI to 32 bytes. A larger public key would rule out the offline attestation 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 8).¶

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

3.3. 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. This is the defense against a cryptographic hash second pre-image attack on the HHIT (e.g., multiple HIs yielding the same HHIT, see Requirement ID-3). A lookup of the HHIT into this registration data provides the registered HI for HHIT proof of ownership. A first-come-first-served registration for a HHIT provides deterministic access to any other needed actionable information based on inquiry access authority (more details in Section 4.2).¶

3.4. HHIT as a Cryptographic Identifier

The only (known to the authors at the time of this 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 and a cryptographic hash of that information concatenated with the entity's public key, etc. Although hash collisions may occur, the registrar can detect them and reject registration requests rather than issue credentials, e.g., by enforcing a first-claimed, first-attested policy. Pre-image hash attacks are also mitigated through this registration process, locking the HHIT to a specific HI¶

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 Network RID client. It would present a new interface to a CS-RID SDSP, similar to but readily distinguishable from that between a GCS and a Net-RID SP.¶

6.2. The CS-RID SDSP

A CS-RID SDSP aggregates and processes (e.g., estimates UA location using multilateration when possible) information collected by CS-RID Finders. A CS-RID SDSP should appear (i.e., present the same interface) to a Net-RID SP as a Net-RID DP.¶

8.1. 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 either from the UA or 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 wtih secure store for the private key.¶

8.2. Post Quantum Computing Out Of Scope

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 HHIT for each operation, the attack window could be limited to the duration of the operation.¶

Finally, as the HHIT contains 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.¶

8.3. Denial Of Service (DOS) Protection Out Of Scope

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. Subtle DOS attacks of message content altering are not practical with the basic message error correction provided. Finally, this whole architecture is put forth to make DOS spoofing/replay attacks very hard.¶

A.1. Operation Concept

The National Aeronautics and Space Administration (NASA) and FAA's effort 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 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) submission and application for airspace authorization in real-time by checking against multiple aeronautical databases such as airspace classification and operating rules associated with it, 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 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), pre-flight authorization must be obtained from a USS when operating beyond-visual-of-sight (BVLOS). The USS either accepts or rejects the received operational intent (flight plan) from the UAS. Accepted UAS operation may share its current flight data such as GPS position and altitude to USS. The USS may keep the UAS operation status near real-time and may keep it as a record for overall airspace air traffic monitoring.¶