Internet-Draft DRIP Auth Formats December 2023
Wiethuechter, et al. Expires 16 June 2024 [Page]
Workgroup:
DRIP Working Group
Internet-Draft:
draft-ietf-drip-auth-42
Published:
Intended Status:
Standards Track
Expires:
Authors:
A. Wiethuechter, Ed.
AX Enterprize, LLC
S. Card
AX Enterprize, LLC
R. Moskowitz
HTT Consulting

DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote ID

Abstract

The Drone Remote Identification Protocol (DRIP), plus trust policies and periodic access to registries, augments Unmanned Aircraft System (UAS) Remote Identification (RID), enabling local real time assessment of trustworthiness of received RID messages and observed UAS, even by Observers then lacking Internet access. This document defines DRIP message types and formats to be sent in Broadcast RID Authentication Messages to verify that attached and recent detached messages were signed by the registered owner of the DRIP Entity Tag (DET) claimed.

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 https://datatracker.ietf.org/drafts/current/.

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 16 June 2024.

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Table of Contents

1. Introduction

The initial regulations (e.g., [FAA-14CFR]) and standards (e.g., [F3411]) for Unmanned Aircraft (UA) Systems (UAS) Remote Identification and tracking (RID) do not address trust. However, this is a requirement that needs to be addressed for various different parties that have a stake in the safe operation of National Airspace Systems (NAS). DRIP's goal is to specify how RID can be made trustworthy and available in both Internet and local-only connected scenarios, especially in emergency situations.

UAS often operate in a volatile environment. Small UA offer little capacity for computation and communication. UAS RID must also be accessible with ubiquitous and inexpensive devices without modification. This limits options.

Generally, two communication schemes for UAS RID are considered: Broadcast and Network. This document focuses on adding trust to Broadcast RID (Section 3.2 of [RFC9153]).

Senders can make any claims the RID message formats allow. Observers have no standardized means to assess the trustworthiness of message content, nor verify whether the messages were sent by the UA identified therein, nor confirm that the UA identified therein is the one they are visually observing. Indeed, Observers have no way to detect whether the messages were sent by a UA, or spoofed by some other transmitter (e.g., a laptop or smartphone) anywhere in direct wireless broadcast range. Authentication is the primary strategy for mitigating this issue.

1.1. DRIP Entity Tag (DET) Authentication Goals for Broadcast RID

ASTM [F3411] Authentication Messages (Message Type 0x2), when used with DET-based formats, enable a high level of trust that the content of other ASTM Messages was generated by their claimed registered source. These messages are designed to provide the Observers with trustworthy and immediately actionable information. Appendix A provides a high-level overview of the various states of trustworthiness that may be used along with these formats.

This authentication approach also provides some error correction (Section 5) as mandated by the United States (US) Federal Aviation Administration (FAA) [FAA-14CFR], which is missing from [F3411] over Legacy Transports (Bluetooth 4.x).

These DRIP enhancements to ASTM's [F3411] further support the important use case of Observers who may be offline at the time of observation.

A summary of DRIP requirements [RFC9153] addressed herein is provided in Section 7.

  • Note: The Endorsement (used in Section 4.2) that proves that a DET is registered MUST come from its immediate parent in the registration hierarchy, e.g., a DRIP Identity Management Entity (DIME) [drip-registries]. In the definitive hierarchy, the parent of the UA is its HHIT Domain Authority (HDA), the parent of an HDA is its Registered Assigning Authority (RAA), etc. It is also assumed that all DRIP-aware entities use a DET as their identifier during interactions with other DRIP-aware entities.

2. Terminology

2.1. Required 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. Definitions

This document makes use of the terms (CAA, Observer, USS, UTM, etc.) defined in [RFC9153]. Other terms (such as DIME) are from [RFC9434], while others (DET, RAA, HDA, etc.) are from [RFC9374].

In addition, the following terms are defined for this document:

Extended Transports:

  • Use of extended advertisements (Bluetooth 5.x), service info (Wi-Fi Neighbor Awareness Networking), or vendor specific information element (IEEE 802.11 Beacon) in broadcast frames as specified in [F3411]. Must use ASTM Message Pack (Message Type 0xF).

Legacy Transports:

  • Use of broadcast frames (Bluetooth 4.x) as specified in [F3411].

Manifest:

  • an immutable list of items being transported (in this specific case over wireless communication).

3. Background

3.1. Reasoning for IETF DRIP Authentication

[F3411] defines Authentication Message framing only. It does not define authentication formats or methods. It explicitly anticipates several signature options but does not fully define those. Annex A1 of [F3411] defines a Broadcast Authentication Verifier Service, which has a heavy reliance on Observer real-time connectivity to the Internet. Fortunately, [F3411] also allows third party standard Authentication Types using Type 5 Specific Authentication Method (SAM), several of which DRIP defines herein.

The standardization of specific formats to support the DRIP requirements in UAS RID for trustworthy communications over Broadcast RID is an important part of the chain of trust for a UAS ID. Per Section 5 of [RFC9434], Authentication formats are needed to relay information for Observers to determine trust. No existing formats (defined in [F3411] or other organizations leveraging this feature) provide the functionality to satisfy this goal resulting in the work reflected in this document.

3.1.1. UA Signed Evidence

When an Observer receives a DRIP-based Authentication Message (Section 4.3, Section 4.4, or Section 4.5) containing UA-signed Evidence (in an Endorsement structure Section 4.1) it MUST validate the signature using the Host Identity (HI) corresponding to the UA's DRIP Entity Tag (DET).

The UA's HI SHOULD be retrieved from DNS. If not available it may have been revoked. Note that accurate revocation status is a DIME inquiry; DNS non-response is a hint that a DET is expired or revoked. It MAY be retrieved from a local cache, if present. The local cache is typically populated by DNS lookups and/or by received Broadcast Endorsements (Section 3.1.2).

Once the Observer has the registered UA's DET and HI, all further (or cached previous) DRIP-based Authentication Messages using the UA DET can be validated. Signed content, tied to the DET, can now be trusted to have been signed by the holder of the private key corresponding to the DET.

Whether the content is true is a separate question which DRIP cannot address, but sanity checks (Section 6) are possible.

3.1.2. DIME Endorsements of Subordinate DET

When an Observer receives a DRIP Link Authentication Message (Section 4.2) containing an Endorsement by the DIME of a child DET registration, it MUST validate the signature using the HI corresponding to the DIME's DET.

The DIME's HI, SHOULD be retrieved from DNS (e.g., Section 5 of [drip-registries]), when available. It MAY be cached from a prior DNS lookup or be stored in a distinct local store.

3.1.3. DIME Hierarchy Endorsements

An Observer can receive a series of DRIP Link Authentication Messages (Section 4.2), each one pertaining to a DIME's registration in the DIME above it in the hierarchy. Similar to Section 3.1.2, each link in this chain MUST be validated.

3.1.4. UAS RID Trust

Section 3.1.1, Section 3.1.2, and Section 3.1.3 complete the trust chain for the claimed DET and associated HI (public key), but the chain cannot yet be trusted as having any relevance to the observed UA because replay attacks are trivial. At this point, the key nominally possessed by the UA is trusted but the UA has not yet been proven to possess that private key.

It is necessary for the UA to prove possession by dynamically signing data that is unique and unpredictable but easily verified by the Observer. This can be a DRIP Wrapper or Manifest (Section 4.3, Section 4.4) containing an ASTM Message that fulfills the requirements. Verification of this signed data MUST be performed by the Observer as part of the received UAS RID information trust assessment (Section 6.4.2).

3.2. ASTM Authentication Message

The Authentication Message (Message Type 0x2) is unique in the ASTM [F3411] Broadcast standard as it is the only message that can be larger than the Legacy Transport size. To address this limitation around transport size, it is defined as a set of "pages", each of which fits into a single Legacy Transport frame. For Extended Transports, pages are still used but all are in a single frame.

  • Informational Note: Message Pack (Message Type 0xF) is also larger than the Legacy Transport size but is limited for use only on Extended Transports where is can be supported.

The following sub-sections are a brief overview of the Authentication Message format defined in [F3411] for better context on how DRIP Authentication fills and uses various fields already defined by ASTM [F3411].

3.2.1. Authentication Page

This document leverages Authentication Type 0x5, Specific Authentication Method (SAM), as the principal authentication container, defining a set of SAM Types in Section 4. Authentication Type is encoded in every Authentication Page in the Page Header. The SAM Type is defined as a field in the Authentication Payload (see Section 3.2.3.1).

 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
+---------------+---------------+---------------+---------------+
|  Page Header  |                                               |
+---------------+                                               |
|                                                               |
|                                                               |
|                     Authentication Payload                    |
|                                                               |
|                                                               |
+---------------+---------------+---------------+---------------+
Figure 1: Standard ASTM Authentication Message Page

Page Header: (1 octet)

  • Authentication Type (4 bits) and Page Number (4 bits)

Authentication Payload: (23 octets per page)

  • Authentication Payload, including headers. Null padded. See Section 3.2.2.

The Authentication Message is structured as a set of pages per Figure 1. There is a technical maximum of 16 pages (indexed 0 to 15) that can be sent for a single Authentication Message, with each page carrying a maximum 23 octet Authentication Payload. See Section 3.2.4 for more details. Over Legacy Transports, these messages are "fragmented", with each page sent in a separate Legacy Transport frame.

Either as a single Authentication Message or a set of fragmented Authentication Message Pages, the structure is further wrapped by outer ASTM framing and the specific link framing.

3.2.2. Authentication Payload Field

Figure 2 is the source data view of the data fields found in the Authentication Message as defined by [F3411]. This data is placed into Figure 1's Authentication Payload, spanning multiple Authentication Pages.

 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
+---------------+---------------+---------------+---------------+
|                     Authentication Headers                    |
|                               +---------------+---------------+
|                               |                               |
+---------------+---------------+                               |
.                                                               .
.                Authentication Data / Signature                .
.                                                               .
|                                                               |
+---------------+---------------+---------------+---------------+
|      ADL      |                                               |
+---------------+                                               |
.                                                               .
.                       Additional Data                         .
.                                                               .
|                                                               |
+---------------+---------------+---------------+---------------+
Figure 2: ASTM Authentication Message Fields

Authentication Headers: (6 octets)

Authentication Data / Signature: (0 to 255 octets)

  • Opaque authentication data. The length of this payload is known through a field in the Authentication Headers (not shown here).

Additional Data Length (ADL): (1 octet - unsigned)

  • Length in octets of Additional Data.

Additional Data: (255 octets max)

  • Data that follows the Authentication Data / Signature but is not considered part of the Authentication Data thus is not covered by a signature. When Additional Data is being sent, a single unsigned octet (Additional Data Length) directly follows the Authentication Data / Signature and has the length, in octets, of the following Additional Data. For DRIP, this field is used to carry Forward Error Correction (FEC) data as defined in Section 5.

3.2.3. Specific Authentication Method (SAM)

3.2.3.1. SAM Data Format

Figure 3 is the general format to hold authentication data when using SAM and is placed inside the Authentication Data/Signature field in Figure 2.

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
+---------------+---------------+---------------+---------------+
|   SAM Type    |                                               |
+---------------+                                               |
.                                                               .
.                     SAM Authentication Data                   .
.                                                               .
|                                                               |
+---------------+---------------+---------------+---------------+
Figure 3: SAM Data Format

SAM Type: (1 octet)

  • The following SAM Types are allocated to DRIP:
Table 1: DRIP SAM Types
SAM Type Description
0x01 DRIP Link (Section 4.2)
0x02 DRIP Wrapper (Section 4.3)
0x03 DRIP Manifest (Section 4.4)
0x04 DRIP Frame (Section 4.5)
  • Note: ASTM International is the owner of these code points as they are defined in [F3411]. In accordance with Annex 5 of the ASTM's [F3411], the International Civil Aviation Organization (ICAO) has been selected by ASTM as the registrar to manage allocations of these code points. The list of which can be found at [ASTM-Remote-ID].

SAM Authentication Data: (0 to 200 octets)

  • Contains opaque authentication data formatted as defined by the preceding SAM Type.

3.2.4. ASTM Broadcast RID Constraints

3.2.4.1. Wireless Frame Constraints

A UA has the option of broadcasting using Bluetooth (4.x and 5.x), Wi-Fi NAN, or IEEE 802.11 Beacon, see Section 6. With Bluetooth, FAA and other Civil Aviation Authorities (CAA) mandate transmitting simultaneously over both 4.x and 5.x. The same application layer information defined in [F3411] MUST be transmitted over all the physical layer interfaces performing the function of RID.

Bluetooth 4.x presents a payload size challenge in that it can only transmit 25 octets of payload per frame while other transports can support larger payloads per frame. However, the [F3411] messaging framing dictated by Bluetooth 4.x constraints is inherited by [F3411] over other media.

3.2.4.2. Paged Authentication Message Constraints

To keep consistent formatting across the different transports (Legacy and Extended) and their independent restrictions, the authentication data being sent is REQUIRED to fit within the page limit that the most constrained existing transport can support. Under Broadcast RID, the Extended Transport that can hold the least amount of authentication data is Bluetooth 5.x at 9 pages.

As such DRIP transmitters are REQUIRED to adhere to the following when using the Authentication Message:

  1. Authentication Data / Signature data MUST fit in the first 9 pages (Page Numbers 0 through 8).
  2. The Length field in the Authentication Headers (which encodes the length in octets of Authentication Data / Signature only) MUST NOT exceed the value of 201. This includes the SAM Type but excludes Additional Data.
3.2.4.3. Timestamps

In ASTM [F3411] timestamps are a Unix-style timestamp with an epoch of 2019-01-01 00:00:00 UTC. For DRIP this format is adopted for Authentication to keep a common time format in Broadcast payloads.

Under DRIP there are two timestamps defined Valid Not Before (VNB) and Valid Not After (VNA).

Valid Not Before (VNB) Timestamp: (4 octets)

  • Timestamp denoting recommended time to start trusting data in. MUST follow the format defined in [F3411] as described above. MUST be set no earlier than the time the signature (across a given structure) is generated.

Valid Not After (VNA) Timestamp: (4 octets)

  • Timestamp denoting recommended time to stop trusting data. MUST follow the format defined in [F3411] as described above. Has an additional offset to push a short time into the future (relative to VNB) to avoid replay attacks. The exact offset is not defined in this document. Best practice identifying an acceptable offset should be used taking into consideration the UA environment, and propagation characteristics of the messages being sent, and clock differences between the UA and Observers. A reasonable time would be to set VNA 2 minutes after VNB.

4. DRIP Authentication Formats

All formats defined in this section are the content of the Authentication Data/Signature field in Figure 2 and use the Specific Authentication Method (SAM, Authentication Type 0x5). The first octet of the Authentication Data / Signature of Figure 2 is used to multiplex among these various formats.

When sending data over a medium that does not have underlying FEC, for example Legacy Transports, then Section 5 MUST be used.

4.1. Endorsement Structure for UA Signed Evidence

The Endorsement Structure for UA Signed Evidence (Figure 4) is used by the UA during flight to sign over information elements using the private key associated with the current UA DET. It is encapsulated by the SAM Authentication Data field of Figure 3.

This structure is used by the DRIP Wrapper (Section 4.3), Manifest Section 4.4, and Frame (Section 4.5). DRIP Link (Section 4.2) MUST NOT use it as it will not fit in the ASTM Authentication Message with its intended content (i.e., a Broadcast Endorsement).

 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
+---------------+---------------+---------------+---------------+
|                      VNB Timestamp by UA                      |
+---------------+---------------+---------------+---------------+
|                      VNA Timestamp by UA                      |
+---------------+---------------+---------------+---------------+
|                                                               |
.                                                               .
.                            Evidence                           .
.                                                               .
|                                                               |
+---------------+---------------+---------------+---------------+
|                                                               |
|                              UA                               |
|                        DRIP Entity Tag                        |
|                                                               |
+---------------+---------------+---------------+---------------+
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                          UA Signature                         |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
+---------------+---------------+---------------+---------------+
Figure 4: Endorsement Structure for UA Signed Evidence

Valid Not Before (VNB) Timestamp by UA: (4 octets)

Valid Not After (VNA) Timestamp by UA: (4 octets)

Evidence: (0 to 112 octets)

  • The evidence section MUST be filled in with data in the form of an opaque object specified in the DRIP Wrapper (Section 4.3), Manifest (Section 4.4), or Frame (Section 4.5).

UA DRIP Entity Tag: (16 octets)

  • This is the current DET [RFC9374] being used by the UA assumed to be a Specific Session ID (a type of UAS ID).

UA Signature: (64 octets)

  • Signature over concatenation of preceding fields (VNB, VNA, Evidence, and UA DET) using the keypair of the UA DET.

When using this structure, the UA is minimally self-endorsing its DET. The HI of the UA DET can be looked up by mechanisms described in [drip-registries] or by extracting it from a Broadcast Endorsement (see Section 4.2 and Section 6.3).

4.3. DRIP Wrapper

This SAM Type is used to wrap and sign over a list of other [F3411] Broadcast RID messages.

The evidence section of the Endorsement Structure for UA Signed Evidence (Section 4.1) is populated with up to four ASTM [F3411] Messages in a contiguous octet sequence. Only ASTM Message Types 0x0, 0x1, 0x3, 0x4, and 0x5 are allowed and must be in Message Type order as defined by [F3411]. These messages MUST include the Message Type and Protocol Version octet and MUST NOT include the Message Counter octet (thus are fixed at 25 octets in length).

4.3.1. Wrapped Count & Sanity Check

When decoding a DRIP Wrapper on a receiver, a calculation of the number of messages wrapped and a sanity check MUST be performed by using the number of octets (defined as wrapperLength) between the VNA Timestamp by UA and the UA DET as shown in Figure 6.

<CODE BEGINS>

if (wrapperLength MOD 25) != 0 {
  return DECODE_FAILURE;
}
wrappedCount = wrapperLength / 25;
if (wrappedCount == 0) {
  // DRIP Wrapper over extended transport
}
else if (wrappedCount > 4) {
  return DECODE_FAILURE;
} else {
  // standard DRIP Wrapper
}


<CODE ENDS>
Figure 6: Pseudo-code for Wrapper sanity check and number of messages calculation

4.3.2. Wrapper over Extended Transports

When using Extended Transports an optimization can be made to DRIP Wrapper to sign over co-located data in an ASTM Message Pack (Message Type 0xF).

To perform this optimization the Endorsement Structure for UA Signed Evidence is filled with the ASTM Messages to be in the ASTM Message Pack, the signature is generated, then the evidence field is cleared leaving the encoded form shown in Figure 7.

 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
+---------------+---------------+---------------+---------------+
|                      VNB Timestamp by UA                      |
+---------------+---------------+---------------+---------------+
|                      VNA Timestamp by UA                      |
+---------------+---------------+---------------+---------------+
|                                                               |
|                              UA                               |
|                        DRIP Entity Tag                        |
|                                                               |
+---------------+---------------+---------------+---------------+
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                          UA Signature                         |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
|                                                               |
+---------------+---------------+---------------+---------------+
Figure 7: DRIP Wrapper over Extended Transports

To verify the signature, the receiver MUST concatenate all the messages in the Message Pack (excluding Authentication Message found in the same Message Pack) in ASTM Message Type order and set the evidence section of the Endorsement Structure for UA Signed Evidence before performing signature verification.

The functionality of a Wrapper in this form is equivalent to Message Set Signature (Authentication Type 0x3) when running over Extended Transports. What the Wrapper provides is the same format but over both Extended and Legacy Transports allowing the transports to be similar. Message Set Signature also implies using the ASTM validator system architecture which depends on Internet connectivity for verification which the receiver may not have at the time of receipt of an Authentication Message. This is something the Wrapper, and all DRIP Authentication Formats, avoid when the UA key is obtained via a DRIP Link Authentication Message.

4.3.3. Wrapper Limitations

The primary limitation of the Wrapper is the bounding of up to 4 ASTM Messages that can be sent within it. Another limitation is that the format cannot be used as a surrogate for messages it is wrapping due to the potential that a receiver on the ground does not support DRIP. Thus, when a Wrapper is being used, the wrapped data must effectively be sent twice, once as a single framed message (as specified in [F3411]) and then again within the Wrapper.

4.4. DRIP Manifest

This SAM Type is used to create message manifests that contain hashes of previously sent ASTM Messages.

By hashing previously sent messages and signing them, we gain trust in a UA's previous reports without re-transmitting them. This is a way to evade the limitation of a maximum of 4 messages in the Wrapper (Section 4.3.3) and greatly reduce overhead.

An Observer who has been listening for any length of time MUST hash received messages and cross-check them against the Manifest hashes.

Judicious use of a Manifest enables an entire Broadcast RID message stream to be strongly authenticated with less than 100% overhead relative to a completely unauthenticated message stream (see Appendix B).

The evidence section of the Endorsement Structure for UA Signed Evidence (Section 4.1) is populated with 8-octet hashes of [F3411] Broadcast RID messages (up to 11) and three special hashes (Section 4.4.2). All these hashes MUST be concatenated to form a contiguous octet sequence in the evidence section.

The Previous Manifest Hash, Current Manifest Hash, and Broadcast Endorsement: HDA on UA Hash MUST always come before the ASTM Message Hashes as seen in Figure 8.

A receiver SHOULD use the Manifest to verify each ASTM Message hashed therein that it has previously received. It can do this without having received them all. A Manifest SHOULD typically encompass a single transmission cycle of messages being sent, see Section 6.4 and Appendix B.

 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
+---------------+---------------+---------------+---------------+
|                       Previous Manifest                       |
|                              Hash                             |
+---------------+---------------+---------------+---------------+
|                       Current Manifest                        |
|                              Hash                             |
+---------------+---------------+---------------+---------------+
|                Broadcast Endorsement: HDA on UA               |
|                              Hash                             |
+---------------+---------------+---------------+---------------+
|                                                               |
.                                                               .
.                      ASTM Message Hashes                      .
.                                                               .
|                                                               |
+---------------+---------------+---------------+---------------+
Figure 8: DRIP Manifest Evidence Structure

Previous Manifest Hash: (8 octets)

  • Hash of the previously sent Manifest Message.

Current Manifest Hash: (8 octets)

  • Hash of the current Manifest Message.

Broadcast Endorsement: HDA on UA Hash: (8 octets)

  • Hash of the 136-octet contents of a DRIP Link (Section 4.2). Specifically the BE: HDA, UA.

ASTM Message Hash: (8 octets)

  • Hash of a single full ASTM Message using hash operations described in Section 4.4.3.

4.4.1. Hash Count & Sanity Check

When decoding a DRIP Manifest on a receiver, a calculation of the number of hashes and a sanity check can be performed by using the number of octets (defined as manifestLength) between the UA DET and the VNB Timestamp by UA such as shown in Figure 9.

<CODE BEGINS>

if (manifestLength MOD 8) != 0 {
  return DECODE_FAILURE
}
hashCount = (manifestLength / 8) - 3;


<CODE ENDS>
Figure 9: Pseudo-code for Manifest Sanity Check and Number of Hashes Calculation

4.4.2. Manifest Ledger Hashes

Three special hashes are included in all Manifests. The Previous Manifest Hash, links to the previous Manifest, and the Current Manifest Hash which is the currently filled Manifest. These two hashes act as a ledger of provenance to the Manifest that could be traced back if the Observer was present for extended periods of time.

The Broadcast Endorsement: HDA on UA Hash is included so there is a direct signature by the UA over the Broadcast Endorsement (see Section 4.2).

4.4.3. Hash Algorithms and Operation

The hash algorithm used for the Manifest is the same hash algorithm used in creation of the DET [RFC9374] that is signing the Manifest.

DET's using cSHAKE128 [NIST.SP.800-185] compute the hash as follows:

cSHAKE128(ASTM Message, 64, "", "Remote ID Auth Hash")

For OGAs other than "5" [RFC9374], use the construct appropriate for the associated hash. For example, for "2" which is ECDSA/SHA-384:

Ltrunc( SHA-384( ASTM Message | "Remote ID Auth Hash" ), 8 )

When building the list of hashes, the Previous Manifest Hash is known from the previous Manifest. For the first built Manifest this value is filled with a random nonce. The Current Manifest Hash is null filled while ASTM Messages are hashed and fill the ASTM Messages Hashes section. When all messages are hashed, the Current Manifest Hash is computed over the Previous Manifest Hash, Current Manifest Hash (null filled) and ASTM Messages Hashes. This hash value replaces the null filled Current Manifest Hash and becomes the Previous Manifest Hash for the next Manifest.

4.4.3.1. Legacy Transport Hashing

Under this transport DRIP hashes the full ASTM Message being sent over the Bluetooth Advertising frame. For paged ASTM Messages (currently only Authentication Messages) all the pages are concatenated together and hashed as one object. For all other Message Types each individual 25-octet message is hashed.

4.4.3.2. Extended Transport Hashing

Under this transport DRIP hashes the full ASTM Message Pack (Message Type 0xF) - regardless of its content.

4.5. DRIP Frame

This SAM Type is used when the authentication data does not fit in other defined formats under DRIP and is reserved for future expansion under DRIP if required.

The contents of Frame Evidence Data are not defined in this document. Other specifications MUST define the contents and register for a Frame Type.

 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
+---------------+---------------+---------------+---------------+
|  Frame Type   |                                               |
+---------------+                                               .
.                      Frame Evidence Data                      .
.                                                               .
|                                                               |
+---------------+---------------+---------------+---------------+
Figure 10: DRIP Frame

Frame Type: (1 octet)

  • Byte to sub-type for future different DRIP Frame formats. It takes the first octet in Figure 10, leaving 111 octets available for Frame Evidence Data. See Section 8.1 for Frame Type allocations.

5. Forward Error Correction

For Broadcast RID, FEC is provided by the lower layers in Extended Transports. The Bluetooth 4.x Legacy Transport does not have supporting FEC, so with DRIP Authentication the following application level scheme is used to add some FEC. When sending data over a medium that does not have underlying FEC, for example Bluetooth 4.x, then this section MUST be used.

The Bluetooth 4.x lower layers have error detection but not correction. Any frame in which Bluetooth detects an error is dropped and not delivered to higher layers (in our case, DRIP). Thus it can be treated as an erasure.

DRIP standardizes a single page FEC scheme using XOR parity across all page data of an Authentication Message. This allows the correction of single erased page in an Authentication Message. If more than a single page is missing then handling of an incomplete Authentication Message is determined by higher layers.

Other FEC schemes, to protect more than a single page of an Authentication Message or multiple [F3411] Messages, is left for future standardization if operational experience proves it necessary and/or practical.

The data added during FEC is not included in the Authentication Data / Signature, but instead in the Additional Data field of Figure 2. This may cause the Authentication Message to exceed 9-pages, up to a maximum of 16-pages.

5.1. Encoding

When encoding two things are REQUIRED:

  1. The FEC data MUST start on a new Authentication Page. To do this, the results of parity encoding MUST be placed in the Additional Data field of Figure 2 with null padding before it to line up with the next page. The Additional Data Length field MUST be set to number of padding octets + number of parity octets.
  2. The Last Page Index field (in Page 0) MUST be incremented from what it would have been without FEC by the number of pages required for the Additional Data Length field, null padding and FEC.

To generate the parity, a simple XOR operation using the previous parity page and current page is used. Only the 23-octet Authentication Payload field of Figure 1 is used in the XOR operations. For Page 0, a 23-octet null pad is used for the previous parity page.

Figure 11 shows an example of the last two pages (out of N) of an Authentication Message using DRIP Single Page FEC. The Additional Data Length is set to 33 as there are always 23 octets of FEC data and in this example 10 octets of padding to line it up into Page N.

Page N-1:
 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
+---------------+---------------+---------------+---------------+
|  Page Header  |                                               |
+---------------+                                               |
|                Authentication Data / Signature                |
|                                                               |
|               +---------------+---------------+---------------+
|               |    ADL=33     |                               |
+---------------+---------------+                               |
|                          Null Padding                         |
|                                                               |
+---------------+---------------+---------------+---------------+

Page N:
 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
+---------------+---------------+---------------+---------------+
|  Page Header  |                                               |
+---------------+                                               |
|                                                               |
|                     Forward Error Correction                  |
|                                                               |
|                                                               |
|                                                               |
+---------------+---------------+---------------+---------------+
Figure 11: Example Single Page FEC Encoding

5.2. Decoding

To determine if FEC has been used, a check of the Last Page Index is performed. In general if the Last Page Index field is one greater than that necessary to hold Length octets of Authentication Data then FEC has been used. Note however that if Length octets was exhausted exactly at the end of an Authentication Page then the Additional Data Length field will occupy the first octet of the following page the remainder of which under DRIP will be null padded. In this case the Last Page Index will have been incremented once for initializing the Additional Data Length field and once for FEC page, for a total of two additional pages.

To decode FEC in DRIP, a rolling XOR is used on each Authentication Page received in the current Authentication Message. A Message Counter, outside of the ASTM Message but specified in [F3411], is used to signal a different Authentication Message and to correlate pages to messages. This Message Counter is only single octet in length, so it will roll over (to 0x00) after reaching its maximum value (0xFF). If only a single page is missing in the Authentication Message the resulting parity octets should be the data of the erased page.

Authentication Page 0 contains various important fields, only located on that page, that help decode the full ASTM Authentication Message. If Page 0 has been reconstructed, the Last Page Index and Length fields MUST be sanity checked by DRIP. The pseudo-code in Figure 12 can be used for both checks.

<CODE BEGINS>

function decode_check(auth_pages[], decoded_lpi, decoded_length) {
  // check decoded_lpi does not exceed maximum value
  if (decoded_lpi >= 16) {
    return DECODE_FAILURE
  }

  // check that decoded length does not exceed DRIP maximum value
  if (decoded_length > 201) {
    return DECODE_FAILURE
  }

  // grab the page at index where length ends and extract its data
  auth_data = auth_pages[(decoded_length - 17) / 23].data
  // find the index of last auth byte
  last_auth_byte = (17 + (23 * last_auth_page)) - decoded_length

  // look for non-nulls after the last auth byte
  if (auth_data[(last_auth_byte + 2):] has non-nulls) {
    return DECODE_FAILURE
  }

  // check that byte directly after last auth byte is null
  if (auth_data[last_auth_byte + 1] equals null) {
    return DECODE_FAILURE
  }

  // we set our presumed Additional Data Length (ADL)
  presumed_adl = auth_data[last_auth_byte + 1]
  // use the presumed ADL to calculate a presumed LPI
  presumed_lpi = (presumed_adl + decoded_length - 17) / 23

  // check that presumed LPI and decoded LPI match
  if (presumed_lpi not equal decoded_lpi) {
    return DECODE_FAILURE
  }
  return DECODE_SUCCESS
}


<CODE ENDS>
Figure 12: Pseudo-code for Decode Checks

5.3. FEC Limitations

The worst-case scenario is when the Authentication Data / Signature ends perfectly on a page (Page N-1). This means the Additional Data Length would start the next page (Page N) and have 22 octets worth of null padding to align the FEC to begin at the start of the next page (Page N+1). In this scenario, an entire page (Page N) is being wasted just to carry the Additional Data Length.

6. Requirements & Recommendations

6.1. Legacy Transports

Under DRIP, the goal is to attempt to bring reliable receipt of the paged Authentication Message using Legacy Transports. FEC (Section 5) MUST be used, per mandated RID rules (for example the US FAA RID Rule [FAA-14CFR]), when using Legacy Transports (such as Bluetooth 4.x).

Under [F3411], Authentication Messages are transmitted at the static rate (at least every 3 seconds). Any DRIP Authentication Messages containing dynamic data (such as the DRIP Wrapper) MAY be sent at the dynamic rate (at least every 1 second).

6.2. Extended Transports

Under the ASTM specification, Extended Transports of RID must use the Message Pack (Message Type 0xF) format for all transmissions. Under Message Pack, ASTM Messages are sent together (in Message Type order) in a single frame (up to 9 single frame equivalent messages under Legacy Transports). Message Packs are required by [F3411] to be sent at a rate of 1 per second (like dynamic messages).

Without any fragmentation or loss of pages with transmission FEC (Section 5) MUST NOT be used as it is impractical.

6.3. Authentication

It is REQUIRED that a UA send the following DRIP Authentication Formats to fulfill the requirements in [RFC9153]:

  1. SHOULD: send DRIP Link (Section 4.2) using the BE: Apex, RAA (satisfying GEN-3); at least once per 5 minutes. Apex in this context is the DET prefix owner
  2. MUST: send DRIP Link (Section 4.2) using the BE: RAA, HDA (satisfying GEN-3); at least once per 5 minutes
  3. MUST: send DRIP Link (Section 4.2) using the BE: HDA, UA (satisfying ID-5, GEN-1 and GEN-3); at least once per minute
  4. MUST: send any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest (Section 4.4) or DRIP Wrapper (Section 4.3)) where the UA is dynamically signing data that is guaranteed to be unique, unpredictable and easily cross checked by the receiving device (satisfying ID-5, GEN-1 and GEN-2); at least once per 5 seconds

6.4. Operational

UAS operation may impact the frequency of sending DRIP Authentication messages. When a UA dwells at an approximate location, and the channel is heavily used by other devices, less frequent message authentication may be sufficient for an Observer. Contrast this with a UA traversing an area, where every message should be authenticated as soon as possible for greatest success as viewed by the receiver.

Thus how/when these DRIP Authentication Messages are sent is up to each implementation. Further complication comes in comparing Legacy and Extended Transports. In Legacy, each message is a separate hash within the Manifest. So, when the UA remains within a small volume of airspace, it may lean toward occasional message authentication. In Extended Transports, the hash is over the Message Pack so only few hashes need to be in a Manifest. A single Manifest can handle a potential two Message Packs (for a full set of messages) and a DRIP Link Authentication Message for the BE: HDA, UA.

A separate issue is the frequency of transmitting the DRIP Link Authentication Message for the BE: HDA, UA when using the Manifest. This message content is static; its hash never changes radically. The only change is the 4-octet timestamp in the Authentication Message headers. Thus, potentially, in a dwell style operation it can be sent once per minute, where its hash is in every Manifest. A receiver can cache all DRIP Link Authentication Message for the BE: HDA, UA to mitigate potential packet loss.

The following operational configuration is RECOMMENDED (in alignment with Section 6.3):

  1. Per CAA requirements, generate and transmit a set of ASTM Messages (example; Basic ID, Location and System).
  2. Under Extended Transports, generate and include in the same Message Pack as the CAA required ASTM Messages a DRIP Wrapper as specified in Section 4.3.2 (implicitly wrapping and signing messages in the pack).
  3. Under Legacy Transports, generate and transmit every 5 seconds a DRIP Manifest (Section 4.4) hashing as many sets as desired of recent CAA required ASTM Messages. The system MAY periodically replace the DRIP Manifest with a DRIP Wrapper (Section 4.3) containing at least a Location Message (Message Type 0x2).
  4. Under both Legacy or Extended Transports, generate and transmit a DRIP Link's (Section 4.2) containing; BE: HDA, UA every minute, BE: RAA, HDA every 5 minutes, BE: Apex, RAA every 5 minutes.

The reasoning and math behind these recommendations can be found in Appendix B.

6.4.1. DRIP Wrapper

The DRIP Wrapper MUST NOT be used in place of sending the ASTM messages as is. All receivers MUST be able to process all the messages specified in [F3411]. Sending them within the DRIP Wrapper makes them opaque to receivers lacking support for DRIP Authentication Messages. Thus, messages within a Wrapper are sent twice: in the clear and authenticated within the Wrapper. The DRIP Manifest would seem to be a more efficient use of the transport channel.

The DRIP Wrapper has a specific use case for DRIP aware receivers. For receiver plotting Location Messages (Message Type 0x2) on a map display an embedded Location Message in a DRIP Wrapper can be marked differently (e.g., via color) to signify trust in the Location data.

6.4.2. UAS RID Trust Assessment

As described in Section 3.1.4, the receiver MUST perform validation of the data being received in Broadcast RID. This is because trust in a key is different from trust that an observed UA possesses that key.

A chain of DRIP Links provides trust in a key. A message containing rapidly changing, not predictable far in advance (relative to typical operational flight times) but sanity-checkable data, signed by that key, provides trust that some agent with access to that data also possesses that key. If the sanity check involves correlating physical world observations of the UA with claims in that data, then the probability is high that the observed UA is (or is collaborating with or observed in real time by) the agent with the key.

After signature verification of any DRIP Authentication Message containing UAS RID information elements (e.g., DRIP Wrapper Section 4.3) the Observer MUST use other sources of information to correlate against and perform validation. An example of another source of information is a visual confirmation of the UA position.

When correlation of these different data streams does not match in acceptable thresholds, the data SHOULD be rejected as if the signature failed to validate. Acceptable thresholds limits and what happens after such a rejection are out of scope for this document.

7. Summary of Addressed DRIP Requirements

The following [RFC9153] requirements are addressed in this document:

ID-5: Non-spoofability

GEN-1: Provable Ownership

GEN-2: Provable Binding

GEN-3: Provable Registration

8. IANA Considerations

8.1. IANA DRIP Registry

This document requests two new registries, for DRIP SAM Type and DRIP Frame Type, under the DRIP registry group.

DRIP SAM Type:
This registry is a mirror for SAM Types containing the subset of allocations used by DRIP Authentication Messages. Future additions MUST be done through ICAO using their process. The following values have been allocated by ICAO to the IETF and are defined here:
Table 2: DRIP SAM Types
SAM Type Name Description
0x01 DRIP Link Format to hold Broadcast Endorsements
0x02 DRIP Wrapper Authenticate full ASTM Messages
0x03 DRIP Manifest Authenticate hashes of ASTM Messages
0x04 DRIP Frame Format for future DRIP authentication
DRIP Frame Type:
This 8-bit valued registry is for Frame Types in DRIP Frame Authentication Messages. Future additions to this registry are to be made through Expert Review (Section 4.5 of [RFC8126]) for the values of 0x01 to 0x9F and First Come, First Served (Section 4.4 of [RFC8126]) for values 0xA0 to 0xEF. The following values are defined:
Table 3: DRIP Frame Types
Frame Type Name Description
0x00 Reserved Reserved
0x01 - 0x9F Reserved Reserved: Expert Review
0xA0 - 0xEF Reserved Reserved: First Come, First Served
0xF0 - 0xFF Experimental Experimental Use

Criteria that should be applied by the designated experts includes determining whether the proposed registration duplicates existing functionality and whether the registration description is clear and fits the purpose of this registry.

Registration requests MUST be sent to drip-reg-review@ietf.org and be evaluated within a three-week review period on the advice of one or more designated experts. Within that review period, the designated experts will either approve or deny the registration request, and communicate their decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions to successfully register the prefix.

Registration requests that are undetermined for a period longer than 28 days can be brought to the IESG's attention for resolution.

9. Security Considerations

9.1. Replay Attacks

DRIP Link messages are static in nature. These DRIP Link messages can easily be replayed by an attacker who has copied them from previous broadcasts.

If an attacker (who is smart and spoofs more than just the UAS ID/data payloads) willingly replays a DRIP Link message, they have in principle actually helped by ensuring the DRIP Link is sent more frequently and be received by potential Observers.

The primary mitigation is that the UA is REQUIRED to send more than DRIP Link messages, specifically the Manifest and/or Wrapper messages that sign over changing data ASTM Messages (e.g., Location/Vector Messages) using the DET private key. A UA sending these messages then actually signing these and other messages using the DET key provides the Observer with data that proves real-time signing. A UA that does not either run DRIP themselves or does not have possession of the same private key, would be clearly exposed upon signature verification.

9.2. VNA Timestamp Offsets for DRIP Authentication Formats

Note the discussion of VNA Timestamp offsets here is in the context of the DRIP Wrapper (Section 4.3), DRIP Manifest (Section 4.4), and DRIP Frame (Section 4.5). For DRIP Link (Section 4.2) these offsets are set by the DIME and have their own set of considerations in [drip-registries].

The offset of the VNA Timestamp by UA is one that needs careful consideration for any implementation. The offset should be shorter than any given flight duration (typically less than an hour) but be long enough to be received and processed by Observers (larger than a few seconds). It is recommended that 3-5 minutes should be sufficient to serve this purpose in any scenario, but is not limited by design.

10. Acknowledgments

  • Ryan Quigley and James Mussi of AX Enterprize, LLC for early prototyping to find holes in the draft specifications.
  • Soren Friis for pointing out that Wi-Fi implementations would not always give access to the MAC Address, originally used in calculation of the hashes for DRIP Manifest. Also, for confirming that Message Packs (0xF) can only carry up to 9 ASTM frames worth of data (9 Authentication pages).

  • Thanks to the following reviewers:

    • Rick Salz (secdir)
    • Matt Joras (genart)

11. References

11.1. Normative References

[F3411]
"F3411-22a: Standard Specification for Remote ID and Tracking", .
[NIST.SP.800-185]
Kelsey, J., Change, S., Perlner, R., and NIST, "SHA-3 derived functions: cSHAKE, KMAC, TupleHash and ParallelHash", NIST Special Publications (General) 800-185, DOI 10.6028/NIST.SP.800-185, , <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-185.pdf>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC9153]
Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A. Gurtov, "Drone Remote Identification Protocol (DRIP) Requirements and Terminology", RFC 9153, DOI 10.17487/RFC9153, , <https://www.rfc-editor.org/info/rfc9153>.
[RFC9374]
Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov, "DRIP Entity Tag (DET) for Unmanned Aircraft System Remote ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, , <https://www.rfc-editor.org/info/rfc9374>.
[RFC9434]
Card, S., Wiethuechter, A., Moskowitz, R., Zhao, S., Ed., and A. Gurtov, "Drone Remote Identification Protocol (DRIP) Architecture", RFC 9434, DOI 10.17487/RFC9434, , <https://www.rfc-editor.org/info/rfc9434>.

11.2. Informative References

[ASTM-Remote-ID]
"ICAO Remote ID Number Registration", , <https://www.icao.int/airnavigation/IATF/Pages/ASTM-Remote-ID.aspx>.
[drip-registries]
Wiethuechter, A. and J. Reid, "DRIP Entity Tag (DET) Identity Management Architecture", Work in Progress, Internet-Draft, draft-ietf-drip-registries-14, , <https://datatracker.ietf.org/doc/html/draft-ietf-drip-registries-14>.
[FAA-14CFR]
"Remote Identification of Unmanned Aircraft", , <https://www.govinfo.gov/content/pkg/FR-2021-01-15/pdf/2020-28948.pdf>.
[RFC8126]
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <https://www.rfc-editor.org/info/rfc8126>.

Appendix A. Authentication States

ASTM Authentication has only three states: None, Invalid, and Valid. This is because, under ASTM, the authentication is done by an external service hosted somewhere on the Internet so it is assumed an authoritative response will always be returned. This classification becomes more complex in DRIP with the support of "offline" scenarios where a receiver does not have Internet connectivity. With the use of asymmetric cryptography this means that the public key (PK) must somehow be obtained. [drip-registries] gets more into detail how these keys are stored on DNS and one use of DRIP Authentication messages is to send PK's over Broadcast RID.

There are a few keys of interest: the PK of the UA and the PK's of relevant DIMEs. This document describes how to send the PK of the UA over the Broadcast RID messages. The key of DIMEs are sent over Broadcast RID using the same mechanisms (see Section 4.2 and Section 6.3) but MAY be sent at a far lower rate due to potential operational constraints (such as saturation of limited bandwidth). As such, there are scenarios where part of the key-chain may be unavailable at the moment a full Authentication Message is received and processed.

The intent of this appendix is to give a recommended way to classify these various states and convey it to the user through colors and state names/text. These states can apply to either a single authentication message, a DET (and its associated public key), and/or a sender.

The table below lays out the RECOMMENDED colors to associate with state and a brief description of each.

Table 4: Authentication State Names, Colors & Descriptions
State Color Details
None Black No Authentication being received (as yet)
Partial Gray Authentication being received but missing pages
Unsupported Brown Authentication Type/SAM Type of received message not supported
Unverifiable Yellow Data needed for signature verification is missing
Verified Green Valid signature verification and content validation
Trusted Blue evidence of Verified and DIME is marked as only registering DETs for trusted entities
Unverified Red Invalid signature verification or content validation
Questionable Orange evidence of both Verified & Unverified for the same claimed sender
Conflicting Purple evidence of both Trusted & Unverified for the same claimed sender

A.1. None: Black

The default state where no authentication information has yet to be received.

A.2. Partial: Gray

A pending state where authentication pages are being received but a full authentication message has yet to be compiled.

A.3. Unsupported: Brown

A state wherein authentication data is being or has been received, but cannot be used, as the Authentication Type or SAM Type is not supported by the receiver.

A.4. Unverifiable: Yellow

A pending state where a full authentication message has been received but other information, such as public keys to verify signatures, is missing.

A.5. Verified: Green

A state where all authentication messages that have been received, up to that point from that claimed sender, pass signature verification and the requirement of Section 6.4.2 has been met.

A.6. Trusted: Blue

A state where all authentication messages that have been received, up to that point, from that claimed sender, have passed signature verification, the requirement of Section 6.4.2 has been met, and the public key of the sending UA is marked as trusted.

The sending UA key will have been marked as trusted if the relevant DIMEs only register DETs (of subordinate DIMEs, UAS operators, and UA) that have been vetted as per their published registration policies, and those DIMEs have been marked, by the owner (individual or organizational) of the receiver, as per that owner's policy, as trusted to register DETs only for trusted parties.

A.7. Questionable: Orange

A state where there is a mix of authentication messages received that are Verified (Appendix A.5) and Unverified (Appendix A.8).

Transition to this state is from Verified if a subsequent message fails verification so would have otherwise been marked Unverified, or from Unverified if a subsequent message passes verification or validation so would otherwise have been marked Verified, or from either of those state upon mixed results on the requirement of Section 6.4.2.

A.8. Unverified: Red

A state where all authentication messages that have been received, up to that point, from that claimed sender, failed signature verification or the requirement of Section 6.4.2.

A.9. Conflicting: Purple

A state where there is a mix of authentication messages received that are Trusted (Appendix A.6) and Unverified (Appendix A.8) and the public key of the aircraft is marked as trusted.

Transition to this state is from Trusted if a subsequent message fails verification so would have otherwise been marked Unverified, or from Unverified if a subsequent message passes verification or validation and policy checks so would otherwise have been marked Trusted, or from either of those state upon mixed results on the requirement of Section 6.4.2.

Appendix B. Operational Recommendation Analysis

The recommendations found in (Section 6.4) may seem heavy handed and specific. This appendix lays out the math and assumptions made to come to the recommendations listed there. This section is solely based on operations using Legacy Transports; as such, all calculations of frame counts for DRIP included FEC using Section 5.

B.1. Methodology

In the US, the required ASTM Messages to be transmitted every second are: Basic ID (0x1), Location (0x2), and System (0x4). Typical implementations will most likely send at a higher rate (2x sets per cycle) resulting in 6 frames sent per cycle. Transmitting this set of message more than once a second is not discouraged but awareness is needed to avoid congesting the RF spectrum, causing further issues.

  • Informational Note: In Europe, the Operator ID Message (0x5) is also included; pushing the frame count to 8 per cycle. In Japan, two Basic ID (0x0), Location (0x1), and Authentication (0x2) are required, likely pushing the frame count beyond 8.

To calculate the frame count of a given DRIP Authentication Message the following formula is used:

  • 1 + ceiling((((16 + 8 + 64) + (Item Size * Item Count) + 2) - 16) / 23) + 1

The leading 1 is counting for the Page 0 which is always present. The DET (16 octets), timestamps (8 octets) and signature (64 octets) all make up the required fields for DRIP. Item Size (in octets) is size of each item in a given format; for a Wrapper it is 25 (a full ASTM Message), while for a Manifest it is 8 (a single hash). 2 more is added to account for the SAM Type and the ADL octet. The value 16 is the number of octets not counted (as they are part of Page 0 which is already counted for). 23 is the number of octets per Authentication Page (pages 1 - 15). After dividing by 23 the value is raised to the nearest whole value as we can only send full frames, not partial. The final 1 is counting for a single page of FEC applied in DRIP under Bluetooth 4.x.

  • Informational Note: for DRIP Link the Item Size is 48 and Item Count is 1; resulting in a frame count of 8

Comparing DRIP Wrapper and Manifest Authentication Message frame counts we have the following:

Table 5: Frame Counts
Authenticated Frames Wrapper Frames Manifest Frames Total Wrapper Frames Total Manifest Frames
1 7 7 8 8
2 8 7 10 9
3 9 7 12 10
4 10 8 14 12
5 N/A 8 N/A 13
6 N/A 8 N/A 14
7 N/A 9 N/A 16
8 N/A 9 N/A 17
9 N/A 10 N/A 19
10 N/A 10 N/A 20
11 N/A 10 N/A 21
12 N/A 12 N/A 24

Note that for Manifest Frames the calculations use an Item Count that is 2 + Authentication Frames. This is to account for the two special hashes.

The values in Total Frames is calculated by adding in the Item Count (to either the Wrapper Frames or Manifest Frames column) to account for the ASTM Messages being sent outside the Authentication Message.

B.2. Examples

B.2.1. Legend

The following legend applies to all example schedules in this section.

A = Basic ID Message (0x0) ID Type 1
B = Basic ID Message (0x0) ID Type 2
C = Basic ID Message (0x0) ID Type 3
D = Basic ID Message (0x0) ID Type 4
V = Location/Vector Message (0x1)
I = Self ID Message (0x3)
S = System Message (0x4)
O = Operator ID Message (0x5)

L[y,z] = DRIP Link Authentication Message (0x2)
W[y,z] = DRIP Wrapper Authentication Message (0x2)
M[y,z] = DRIP Manifest Authentication Message (0x2)
  y = Start Page
  z = End Page

# = Empty Frame Slot
* = Message in DRIP Manifest Authentication Message

B.2.2. ASTM Maximum Schedule Example

For this example we will assume the following ASTM Messages are in play:

  • 1x Basic ID (0x0) set as ID Type for Serial Number (0x1)
  • 1x Basic ID (0x0) set as ID Type for CAA Assigned ID (0x2)
  • 1x Basic ID (0x0) set as ID Type for UTM Assigned ID (0x3)
  • 1x Basic ID (0x0) set as ID Type for Specific Session ID (0x4)
  • 2x Location (0x1)
  • 1x Self ID (0x3)
  • 2x System (0x4)
  • 2x Operator ID (0x5)

This message set uses all single frame ASTM Messages, sending a set of them (Location, System and Operator ID) at a rate of 2 per second. Two Basic IDs are sent in a single second and rotate between the 4 defined (1x per type). A single Self ID is sent every second. All messages in a given second, if appear more than once, are exact duplicates.

This example, as exactly presented here, would never make sense in practice, as a Single Use Session ID is pointless in conjunction with any other Basic ID. This is just to show that you could send everything, that doing so would have an overhead not much over 100%, and that you could create a reasonable practical schedule by simply "puncturing" this one (omitting those messages you don't need or want).

+-----------------------------------------------------------+
|                        Frame Slots                        |
| 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 |
+----+----+----+----+----+----+----+----+----+----+----+----+
| A* | V* | S  | O  | B  | V  | S* | O  | I  |   L/W[0,2]   |
+----+----+----+----+----+----+----+----+----+----+----+----+
| C* | V  | S* | O  | D* | V* | S  | O  | I* |   L/W[3,5]   |
+----+----+----+----+----+----+----+----+----+----+----+----+
| A  | V* | S  | O* | B* | V  | S  | O  | I  |L/W[6,7] | ## |
+----+----+----+----+----+----+----+----+----+----+----+----+
| C  | V  | S  | O  | D  | V  | S  | O  | I  |    M[0,2]    |
+----+----+----+----+----+----+----+----+----+----+----+----+
| A  | V  | S  | O  | B  | V  | S  | O  | I  |    M[3,5]    |
+----+----+----+----+----+----+----+----+----+----+----+----+
| C  | V  | S  | O  | D  | V  | S  | O  | I  |    M[6,8]    |
+----+----+----+----+----+----+----+----+----+----+----+----+
| A  | V  | S  | O  | B  | V  | S  | O  | I  |M[9]| ## | ## |
+----+----+----+----+----+----+----+----+----+----+----+----+
Figure 13: Example Legacy Transport Transmit Schedule

Manifest messages in the schedule are filled with unique messages from previously transmitted messages before the new Manifest is sent. In Figure 13, this is denoted by the * symbol as being part of the Manifest. In Figure 13, messages are eligible for the Manifest in the very first cycle of transmission. In future iterations, 56 messages are eligible across the 7 seconds it takes to send the previous Manifest and the next Link/Wrapper. Care should be given into the selection of messages for a Manifest as there is a limit of 10 hashes.

  • Informational Note: the term "unique message" above is used as in the example schedule the 2nd Location and System messages MAY be exact copies of the previous Location and System messages sent in the same second. Duplicates of this kind SHOULD NOT be included in a Manifest.

In the schedule the Wrapper and the Link messages switch back and forth the contents of them are changing in the following order:

Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Link: Apex on RAA
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Wrapper: Location (0x1), System (0x4)
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Link: Apex on RAA
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Wrapper: Location (0x1), System (0x4)
Link: IANA on UAS RID Apex

Any messages not required for a local jurisdiction can be removed from the schedule. It is RECOMMENDED this empty frame slot is left empty to help with timing due to RF constraints/concerns. For example, in the US the Self ID (0x3) and Operator ID (0x5) are not required and can be ignored in the above figures. Only one Basic ID (0x0) is selected in the US at any given time, opening up three (3) more slots.

B.2.3. US Example 

+-------------------------------------------------------------------+
|                          Frame Slots                              |
| 00      | 01 | 02 | 03 | 04-07  | 08      | 09 | 10 | 11 | 12-15  |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[0,3] | {A|C|D} | V  | I  | S  | M[4,7] |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[8,9] | {A|C|D} | V  | I  | S  | M[1,4] |
|         |    |    |    | L[0]   |         |    |    |    |        |
|         |    |    |    | M[0]   |         |    |    |    |        |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[5,8] | {A|C|D} | V  | I  | S  | M[9]   |
|         |    |    |    |        |         |    |    |    | L[1]   |
|         |    |    |    |        |         |    |    |    | M[0,1] |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[2,5] | {A|C|D} | V  | I  | S  | M[6,9] |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | L[2]   | {A|C|D} | V  | I  | S  | M[3,6] |
|         |    |    |    | M[0,2] |         |    |    |    |        |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[7,9] | {A|C|D} | V  | I  | S  | M[0,3] |
|         |    |    |    | L[3]   |         |    |    |    |        |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[4,7] | {A|C|D} | V  | I  | S  | M[8,9] |
|         |    |    |    |        |         |    |    |    | L[4]   |
|         |    |    |    |        |         |    |    |    | M[0]   |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[1,4] | {A|C|D} | V  | I  | S  | M[5,8] |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[9]   | {A|C|D} | V  | I  | S  | M[2,5] |
|         |    |    |    | L[5]   |         |    |    |    |        |
|         |    |    |    | M[0,1] |         |    |    |    |        |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[6,9] | {A|C|D} | V  | I  | S  | L[6]   |
|         |    |    |    |        |         |    |    |    | M[0,2] |
+---------+----+----+----+--------+---------+----+----+----+--------+
| {A|C|D} | V  | I  | S  | M[3,6] | {A|C|D} | V  | I  | S  | M[7,9] |
|         |    |    |    |        |         |    |    |    | L[7]   |
+---------+----+----+----+--------+---------+----+----+----+--------+
Figure 14: US Example Legacy Transport Transmit Schedule

B.2.4. EU Example 

+---------------------------------------------------------+
|                       Frame Slots                       |
| 00      | 01-04   | 05-09  | 10      | 11-14   | 15-19  |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[0,4] | {A|C|D} | V,I,S,O | M[5,9] |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | L[0]   | {A|C|D} | V,I,S,O | M[4,8] |
|         |         | M[0,3] |         |         |        |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[9]   | {A|C|D} | V,I,S,O | M[3,7] |
|         |         | L[1]   |         |         |        |
|         |         | M[0,2] |         |         |        |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[8,9] | {A|C|D} | V,I,S,O | M[2,6] |
|         |         | L[2]   |         |         |        |
|         |         | M[0,1] |         |         |        |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[7,9] | {A|C|D} | V,I,S,O | M[1,5] |
|         |         | L[3]   |         |         |        |
|         |         | M[0]   |         |         |        |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[6,9] | {A|C|D} | V,I,S,O | M[0,3] |
|         |         | L[4]   |         |         |        |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[5,9] | {A|C|D} | V,I,S,O | L[5]   |
|         |         |        |         |         | M[0,3] |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[4,8] | {A|C|D} | V,I,S,O | M[9]   |
|         |         |        |         |         | L[6]   |
|         |         |        |         |         | M[0,2] |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[3,7] | {A|C|D} | V,I,S,O | M[8,9] |
|         |         |        |         |         | L[7]   |
|         |         |        |         |         | M[0,1] |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[2,6] | {A|C|D} | V,I,S,O | M[7,9] |
|         |         |        |         |         | L[0]   |
|         |         |        |         |         | M[0]   |
+---------+---------+--------+---------+---------+--------+
| {A|C|D} | V,I,S,O | M[1,5] | {A|C|D} | V,I,S,O | M[6,9] |
|         |         |        |         |         | L[1]   |
+---------+---------+--------+---------+---------+--------+
Figure 15: EU Example Legacy Transport Transmit Schedule

B.2.5. JP Example 

+-----------------------------------------+
|               Frame Slots               |
| 00-04     | 05-09  | 10-14     | 15-19  |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[0,4] | A,B,V,I,S | M[5,9] |
+-----------+--------+-----------+--------+
| A,B,V,I,S | L[0]   | A,B,V,I,S | M[4,8] |
|           | M[0,3] |           |        |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[9]   | A,B,V,I,S | M[3,7] |
|           | L[1]   |           |        |
|           | M[0,2] |           |        |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[8,9] | A,B,V,I,S | M[2,6] |
|           | L[2]   |           |        |
|           | M[0,1] |           |        |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[7,9] | A,B,V,I,S | M[1,5] |
|           | L[3]   |           |        |
|           | M[0]   |           |        |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[6,9] | A,B,V,I,S | M[0,4] |
|           | L[4]   |           |        |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[5,9] | A,B,V,I,S | L[5]   |
|           |        |           | M[0,3] |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[4,8] | A,B,V,I,S | M[9]   |
|           |        |           | L[6]   |
|           |        |           | M[0,2] |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[3,7] | A,B,V,I,S | M[8,9] |
|           |        |           | L[7]   |
|           |        |           | M[0,1] |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[2,6] | A,B,V,I,S | M[7,9] |
|           |        |           | L[0]   |
|           |        |           | M[0]   |
+-----------+--------+-----------+--------+
| A,B,V,I,S | M[1,5] | A,B,V,I,S | M[6,9] |
|           |        |           | L[1]   |
+-----------+--------+-----------+--------+
Figure 16: JP Example Legacy Transport Transmit Schedule

Authors' Addresses

Adam Wiethuechter (editor)
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Stuart Card
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America