| Internet-Draft | auth-formats | November 2021 |
| Wiethuechter, et al. | Expires 12 May 2022 | [Page] |
- Workgroup:
- DRIP Working Group
- Internet-Draft:
- draft-ietf-drip-auth-03
- Published:
- Intended Status:
- Standards Track
- Expires:
DRIP Authentication Formats for Broadcast Remote ID
Abstract
This document describes how to include trust into the ASTM Remote ID specification defined in ASTM F3411 under Broadcast Remote ID (RID). It defines a few message schemes (sent within the Authentication Message) that can be used to authenticate past messages sent by a unmanned aircraft (UA) and provide proof of UA trustworthiness even in the absence of Internet connectivity at the receiving node.¶
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/.¶
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This Internet-Draft will expire on 12 May 2022.¶
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
1. Introduction
Unmanned Aircraft Systems (UAS) are usually in a volatile environment when it comes to communication. UA are generally small with little computational (or flying) horsepower to carry standard communication equipment. This limits the mediums of communication to few viable options.¶
Observer systems (e.g. smartphones and tablets) place further constraints on the communication options. The Remote ID Broadcast messages MUST be available to applications on these platforms without modifying the devices.¶
The ASTM [F3411] standard focuses on two ways of communicating to a UAS for Remote ID (RID): Broadcast and Network.¶
This document will focus on adding trust to Broadcast RID via the Authentication Message by combining dynamically signed data with an Attestation of the UA's identity from a Registry.¶
1.1. DRIP Requirements Addressed
The following [drip-requirements] will be addressed:¶
- GEN 1: Provable Ownership
- This will be addressed using the DRIP Link and DRIP Wrapper or DRIP Manifest.¶
- GEN 2: Provable Binding
- This requirement is addressed using the DRIP Wrapper or DRIP Manifest.¶
- GEN 3: Provable Registration
- This requirement is addressed using the DRIP Link.¶
See Section 7.3 for further clarification.¶
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
See [drip-requirements] for common DRIP terms.¶
- Aircraft:
- In this document whenever the word Aircraft is used it is referring to an Unmanned Aircraft (UA) not a Manned Aircraft.¶
- Legacy Transports:
- uses broadcast frames (Bluetooth 4.x).¶
- Extended Transports:
- uses the extended advertisements (Bluetooth 5.X), service info (Wi-Fi NaN) or vendor specific element information (Wi-Fi BEACON). Must use ASTM [F3411] Message Pack (Message Type 0xF).¶
3. Background
3.1. Problem Space and Focus
The current standard for Remote ID does not, in any meaningful capacity, address the concerns of trust in the UA space with communication in the Broadcast RID environment. This is a requirement that will need to be addressed eventually for various different parties that have a stake in the UA industry.¶
3.2. Reasoning for IETF DRIP Authentication
The ASTM Authentication Message has provisions in [F3411] to allow for other organizations to standardize additional Authentication formats beyond those explicitly in [F3411]. 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. 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.¶
3.3. ASTM Authentication Message
The ASTM Authentication Message (Message Type 0x2) is a unique message in the Broadcast [F3411] standard as it is the only one that is paged.¶
3.3.1. Authentication Page
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 |
| |
| |
+---------------+---------------+---------------+---------------+
Page Header: (1 byte)
Authentication Type (4 bits)
Page Number (4 bits)
Authentication Payload: (23 bytes per page)
Authentication Payload, including headers. Null padded.
3.3.1.1. Authentication Type
[F3411] has the following subset of Authentication Type's defined and that can be used in the Page Header:¶
| Authentication Type | Description |
|---|---|
| 0x2 | Operator ID Signature |
| 0x3 | Message Set Signature |
| 0x5 | Specific Authentication Method |
3.3.1.1.1. Specific Authentication Method (SAM)
This document leverages Authentication Type 0x5, Specific Authentication Method (SAM), defining a set of SAM Types in Section 6.3. Other Authentication Types are also used in DRIP and their use is defined in Section 6.¶
3.3.1.2. Page Number
There is a technical maximum of 16-pages (indexed 0 to 15 in the Page Header) that can be sent for a single Authentication Message, with each page carrying a max 23-byte Authentication Payload. See Section 3.3.2 for more details.¶
3.3.1.3. Authentication Payload Field
The following is shown in its complete format.¶
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 .
. .
| |
+---------------+---------------+---------------+---------------+
Authentication Headers: (6 bytes)
Contains other header information for the Authentication
Message as defined in F3411.
Authentication Data / Signature: (0 to 255 bytes)
Opaque authentication data.
Additional Data Length (ADL): (1 byte - unsigned)
Length in bytes of Additional Data.
Additional Data: (0 to 255 bytes):
Data that follows the Authentication Data / Signature but
is not considered part of the Authentication Data.
Figure 2 is the abstract 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 pages.¶
When Additional Data is being sent, a single unsigned byte (Additional Data Length) directly follows the Authentication Data / Signature and has the length, in bytes, of the following Additional Data. For DRIP, this field is used to carry Forward Error Correction as defined in Section 4.¶
Full examples of Authentication Messages (fully paginated; both with and without Additional Data) can be found in Appendix B.¶
3.3.2. DRIP 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 of the most constrained existing transport can support. Under Broadcast RID the transport that can hold the least amount of authentication data is Bluetooth 5 and Wi-Fi BEACON at 9-pages.¶
As such DRIP transmitters are REQUIRED to adhere to the following:¶
4. Forward Error Correction
For Broadcast RID, Forward Error Correction (FEC) is provided by the lower layers in Extended Transports (Bluetooth 5.X, Wi-Fi NaN, and Wi-Fi BEACON). Legacy Transports do not have supporting FEC so with DRIP Authentication the following application level FEC scheme is used.¶
(Editors Note: add in self-protecting and more-than-self-protecting options, with their justifications)¶
(Editors Note: Bob M. mentions that the FEC should be page aligned and the ADL includes null padding at the start to page align the data)¶
4.1. Encoding
4.1.1. Single Page FEC
To generate the parity a simple XOR operation using the previous and current page is used. For Page 0, a 25-byte null pad is used for the previous page. The resulting parity fills the Additional Data field of [F3411] with the Additional Data Length field being set to 25.¶
4.1.2. Multi Page FEC
TODO (Reed Solomon)¶
(Editors Note: probably need a table to check against ADLs to which parameters of Reed Solomon are being used?) (Editors Note: this is the place to define if we are self-protecting or global-protecting with FEC...another multiplex byte here directly after the ADL?)¶
4.2. Decoding
Due to the nature of Bluetooth 4 and the existing ASTM paging structure an optimization can be used. If a Bluetooth frame fails its CRC check, then the frame is dropped without notification to the upper protocol layers. From the Remote ID perspective this means the loss of a complete frame/message/page. In Authentication Messages, each page is already numbered so the loss of a page allows the receiving application to build a "dummy" page filling the entire pages with nulls.¶
If Page 0 is being reconstructed an additional check of the Last Page Index to check against how many pages are actually present, MUST be performed for sanity. An additional check on the Data Length field SHOULD also be performed.¶
To determine if Single Page FEC or Multi-Page FEC has been used a simple check of the Additional Data Length (ADL) field can be used. If the ADL is equal to 25, then Single Page FEC is present, anything larger signals Multi-Page FEC.¶
4.2.1. Single Page FEC
Using the same methods as encoding, an XOR operation is used between the previous and current page (a 25-byte null pad is used as the start). The resulting 25-bytes should be the missing page.¶
4.2.2. Multi Page FEC
TODO (Reed Solomon)¶
(Editors Note: probably need a table to check against ADLs to which parameters of Reed Solomon are being used?) (Editors Note: this is the place to define if we are self-protecting or global-protecting with FEC...another multiplex byte here directly after the ADL?)¶
4.3. FEC Limitations
If more than one page is lost (>1/5 for 5-page messages, >1/10 for 10-page messages) than the error rate of the link is already beyond saving and the application has more issues to deal with.¶
(Editors Note: Is this valid anymore, for XOR yes but for multi-page FEC?)¶
5. Broadcast Attestation Structure
To directly support Broadcast RID a variation of the Attestation Structure format of [drip-registries] SHOULD be used when running DRIP under the various Authentication Types (filling the Authentication Data / Signature field of Figure 2) and SAM Types (filling the SAM Authentication Data field (Section 6.3.1.2)).¶
When using this structure the UA is always self-attesting its DRIP Entity Tag (DET). The Host Identity of the UA DET can be looked up by mechanisms described in [drip-registries] or by extracting it from Broadcast Attestation (see Section 6.3.2 and Section 7.3).¶
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
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
. .
. Attestation Data .
. .
| |
+---------------+---------------+---------------+---------------+
| Expiration Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
Attestation Data (0 to 116 bytes):
Opaque attestation data.
Expiration Timestamp by UA (4 bytes):
Timestamp denoting recommended time to trust data to.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
Attestation Data is a field with a maximum of 116-bytes, containing data that the UA is attesting during its flight.¶
The Expiration Timestamp MUST follow the format defined in [F3411]. That is a UNIX timestamp offset by 01/01/2019 00:00:00. An additional offset is then added to push the timestamp a short time into the future to avoid replay attacks. The offset used against the UNIX timestamp 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.¶
6. DRIP Authentication Formats
All formats defined in this section fill the Authentication Data / Signature field in Figure 2.¶
When sending data over a medium that does not have underlying Forward Error Correction (FEC), for example Bluetooth 4, then Section 4 MUST be used.¶
6.1. Operator ID Signature
The existing ASTM [F3411] Authentication Type 0x2 can be used to send a static Self-Attestation of the Operator.¶
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
+---------------+---------------+---------------+---------------+
| |
| Operator |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| Operator Host Identity |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| Expiration Timestamp |
+---------------+---------------+---------------+---------------+
| Signing Timestamp |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| Operator Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The Operator DET in byte form (network byte order).
Operator Host Identity (32-bytes):
HI of the Operator.
Expiration Timestamp by Operator (4 bytes):
Timestamp denoting recommended time to trust data to.
Signing Timestamp by Operator (4 bytes):
Current time at signing.
Operator Signature (64 bytes):
Signature over preceding fields using the keypair of
the Operator.
6.2. Message Set Signature
When running under Extended Transports, the existing ASTM [F3411] Authentication Type 0x3 can be used to sign over the adjacent ASTM Messages in the Message Pack (Message Type 0xF).¶
The concatenation of all messages in the Message Pack (excluding Authentication) before signing MUST be in Message Type order and be placed between the UA DRIP Entity Tag and Expiration Timestamp field.¶
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
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| Expiration Timestamp |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
Expiration Timestamp by UA (4 bytes):
Timestamp denoting recommended time to trust data to.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
6.3. Specific Authentication Method
For ASTM Specific Authentication Method (Authentication Type 0x5) a special SAM Type field, specified as the first byte of the Authentication Data / Signature by [F3411], is used to multiplex between various formats.¶
6.3.1. SAM Data Format
Figure 6 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 .
. .
| |
+---------------+---------------+---------------+---------------+
SAM Type (1 byte):
Byte defined by F3411 to multiplex SAMs
SAM Authentication Data (0 to 200 bytes):
Opaque SAM authentication data.
6.3.1.1. SAM Type
The SAM Type field is maintained by the International Civil Aviation Organization (ICAO) and for DRIP four are allocated:¶
| SAM Type | Description |
|---|---|
| 0x01 | DRIP Link (Section 6.3.2) |
| 0x02 | DRIP Wrapper (Section 6.3.3) |
| 0x03 | DRIP Manifest (Section 6.3.4) |
| 0x04 | DRIP Frame (Section 6.3.5) |
6.3.1.2. SAM Authentication Data
This field has a maximum size of 200-bytes, as defined by Section 3.3.2. When possible the Broadcast Attestation Structure (Section 5) should be used in this space.¶
6.3.2. DRIP Link
This SAM Type is used to transmit the Broadcast Attestation of the Registry (HDA) over the UA. Its structure is defined in [drip-registries] and is as follows:¶
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
+---------------+
| Link Type |
+---------------+---------------+---------------+---------------+
| |
| DRIP |
| Entity Tag of HDA |
| |
+---------------+---------------+---------------+---------------+
| |
| DRIP |
| Entity Tag of UA |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| Host Identity of UA |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| Expiration Timestamp by HDA |
+---------------+---------------+---------------+---------------+
| Signing Timestamp by HDA |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| Signature by HDA |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Link Type: (1-byte)
Multiplexing byte to define different Links being sent.
DRIP Entity Tag of HDA: (16-bytes)
DET of HDA.
DRIP Entity Tag of UA: (16-bytes)
DET of UA.
Host Identity of UA: (32-bytes)
HI of UA
Expiration Timestamp by HDA (4 bytes):
Timestamp denoting recommended time to trust data to.
Signing Timestamp by HDA (4 bytes):
Current time at signing.
HDA Signature (64 bytes):
Signature over preceding fields using the keypair of
the HDA.
This DRIP format MUST be used in conjunction with the DRIP Manifest with the hash of the DRIP Link message and other dynamic data (such as the Location Message (Message Type 0x2)).¶
6.3.2.1. Link Type
| Link Type | Description |
|---|---|
| 0x00 | Reserved |
| 0x01 | HDA to UA |
| 0x02 | RAA to HDA |
| 0x03 | Root to RAA |
| 0x05 | HDA to Operator |
| 0x06 | Operator to UA |
6.3.2.2. Link Limitations
See Section 10.2 for details on why this structure is not dynamically signed.¶
6.3.3. DRIP Wrapper
This SAM Type is used to wrap and sign over a list of other [F3411] Broadcast RID messages. It MUST use the Broadcast Attestation Structure (Section 5).¶
The Attestation Data field is filled with full (25-byte) [F3411] Broadcast RID messages. The minimum number being 1 and the maximum being 4. The encapsulated messages MUST be in Message Type order as defined by [F3411]. All message types except Authentication (Message Type 0x2) and Message Pack (Message Type 0xF) are allowed.¶
To determine the number of messages wrapped the receiver can check that the length of the Attestation Data field of the DRIP Broadcast Attestation (Section 5) is a multiple of 25-bytes.¶
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
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| ASTM Message |
| |
| |
| |
+ +---------------+---------------+---------------+
| | |
+---------------+ |
| |
| ASTM Message |
| |
| |
| |
+ +---------------+---------------+
| | |
+---------------+---------------+ |
| |
| |
| ASTM Message |
| |
| |
+ +---------------+
| | |
+---------------+---------------+---------------+ |
| |
| |
| ASTM Message |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| Expiration Timestamp |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
ASTM Message (25 bytes):
Full ASTM Message.
Expiration Timestamp by UA (4 bytes):
Timestamp denoting recommended time to trust data to.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
6.3.4. DRIP Manifest
This SAM Type is used to create message manifests. It MUST use the Broadcast Attestation Structure (Section 5).¶
By hashing previously sent messages and signing them we gain trust in UAs previous reports. An observer who has been listening for any considerable length of time can hash received messages and cross-check against listed hashes. This is a way to evade the limitation of a maximum of 4 messages in the Wrapper Format and reduce overhead.¶
The Attestation Data field is filled with 12-byte hashes of previous [F3411] Broadcast messages.¶
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
+---------------+
| Msg Window |
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
+---------------+---------------+---------------+---------------+
| |
| Previous Manifest Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| Current Manifest Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| |
| ASTM Message Hash |
| |
+---------------+---------------+---------------+---------------+
| Expiration Timestamp |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Msg Window (1 byte):
Variable window size - TODO
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
Previous Manifest Hash (12 bytes):
See Section 6.3.4.3.
Current Manifest Hash (12 bytes):
See Section 6.3.4.3.
ASTM Message Hash (12 bytes):
Hash of a single full ASTM Message. Multiple hashes should
be in Message Type order.
Expiration Timestamp by UA (4 bytes):
Timestamp denoting recommended time to trust data to.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
6.3.4.1. Hash Algorithms and Operation
The hash algorithm used for the Manifest Message is the same hash algorithm used in creation of the HHIT that is signing the Manifest.¶
An HHIT using cSHAKE128 [NIST.SP.800-185] computes the hash as follows:¶
cSHAKE128(ASTM Message, 96, "", "Remote ID Auth Hash")¶
6.3.4.1.1. Legacy Transport Hashing
Under this transport DRIP hashes the full ASTM Message being sent over the Bluetooth Advertising frame. For Authentication Messages all the Authentication Message Pages are concatenated together and hashed as one object. For all other Message Types the 25-byte message is hashed.¶
6.3.4.1.2. Extended Transport Hashing
Under this transport DRIP hashes the full ASTM Message Pack (Message Type 0xF) - regardless of its content.¶
6.3.4.2. Variable Message Window
Windows of number of ASTM Messages the manifest is applicable over.¶
(Editors Note: needs better text here and justification of inclusion)¶
6.3.4.3. Pseudo-Blockchain Hashes
Two special hashes are included in all Manifest messages; a previous manifest hash, which links to the previous manifest message, as well as a current manifest hash. This gives a pseudo-blockchain provenance to the manifest message that could be traced back if the observer was present for extended periods of time.¶
- Creation:
- During creation and signing of this message format this field MUST be set to 0. So the signature will be based on this field being 0, as well as its own hash. It is an open question of if we compute the hash, then sign or sign then compute.¶
- Cycling:
- There a few different ways to cycle this message. We can "roll up" the hash of 'current' to 'previous' when needed or to completely recompute the hash. This mostly depends on the previous note.¶
6.3.4.4. Manifest Limitations
A potential limitation to this format is dwell time of the UA. If the UA is not sticking to a general area then most likely the Observer will not obtain many (if not all) of the messages in the manifest. Examples of such scenarios include delivery or survey UA.¶
Another limitation is the length of hash, which is discussed in Section 10.1.¶
6.3.5. DRIP Frame
This SAM Type is for when the authentication data does not fit in other defined formats under DRIP and is reserved for future expansion under DRIP if required. This SAM Type SHOULD use the Broadcast Attestation Structure (Section 5).¶
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 |
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
. .
. Attestation Data .
. .
| |
+---------------+---------------+---------------+---------------+
| Expiration Timestamp |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Frame Type (1 byte):
Multiplexing frame type.
UA DRIP Entity Tag (16 bytes):
The UA DET in byte form (network byte order).
Attestation Data (0 to 115 bytes):
Opaque attestation data.
Expiration Timestamp by UA (4 bytes):
Timestamp denoting recommended time to trust data to.
UA Signature (64 bytes):
Signature over preceding fields using the keypair of
the UA.
6.3.5.1. Frame Limitations
With the Broadcast Attestation Structure only 115-bytes of Attestation Data are free for use.¶
7. Requirements & Recommendations
7.1. Legacy Transports
With Legacy Advertisements the goal is to attempt to bring reliable receipt of the paged Authentication Message. Forward Error Correction (Section 4) MUST be used when using Legacy Advertising methods (such as Bluetooth 4.X).¶
Under ASTM Bluetooth 4.X rules, transmission of dynamic messages are at least every 1 second. DRIP Authentication Messages typically contain dynamic data (such as the DRIP Manifest or DRIP Wrapper) and must be sent at the dynamic rate of 1 per second.¶
7.2. Extended Transports
Under the ASTM specification, Bluetooth 5.X Wi-Fi NaN, and Wi-Fi BEACON transport of Remote ID is to use the Message Pack (Message Type 0xF) format for all transmissions. Under Message Pack messages are sent together (in Message Type order) in a single Bluetooth 5 extended frame (up to 9 single frame equivalent messages under Bluetooth 4.X). Message Packs are required by ASTM to be sent at a rate of 1 per second (like dynamic messages).¶
Without any fragmentation or loss of pages with transmission Forward Error Correction (Section 4) MUST NOT be used as it is impractical.¶
7.3. Authentication
It is REQUIRED that an aircraft send the following Authentication Formats to fulfill the [drip-requirements]:¶
- DRIP Link using the Broadcast Attestation of USS and the UA (satisfying GEN-1 and GEN-3)¶
- Any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest or DRIP Wrapper) where the UA is dynamically signing data (satisfying GEN-1 and GEN-2)¶
It is RECOMMENDED the following set of Authentication Formats are sent for support of offline Observers:¶
- DRIP Link using the Broadcast Attestation of HID Root and the CAA (satisfies GEN-3)¶
- DRIP Link using the Broadcast Attestation of CAA and the USS (satisfies GEN-3)¶
- DRIP Link using the Broadcast Attestation of USS and the UA (satisfies GEN-1 and GEN-3)¶
- Any other DRIP Authentication Format (RECOMMENDED: DRIP Manifest or DRIP Wrapper) where the UA is dynamically signing data (satisfies GEN-1 and GEN-2)¶
7.4. Operational
UAS operation may impact the frequency of sending DRIP Authentication messages. Where a UA is dwelling in one location, and the channel is heavily used by other devices, "occasional" message authentication may be sufficient for an observer. Contrast this with a UA traversing an area, and then 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 contrasting Legacy and Extended Transports. In Legacy, each message is a separate hash within the Manifest. So, again in dwelling, 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 HDA UA assertion.¶
A separate issue is the frequency of transmitting the DRIP Link Authentication Message for the HDA UA assertion. This message content is static; its hash never changes radically. The only change is the 4-byte timestamp in the Authentication Message headers. Thus, potentially, in a dwelling 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 HDA UA assertion to mitigate potential packet loss.¶
The preferred mode of operation is to send the HDA UA assertion every 3 seconds and Manifest messages immediately after a set of UA operation messages (e.g. Basic, Location, and System messages).¶
7.4.1. 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]. Only sending them within the DRIP Wrapper will make 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 format 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 received Location Messages (Message Type 0x2) on a map display an embedded Location Message in a DRIP Wrapper can be colored differently to signify trust in the Location data - be it current or previous Location reports that are wrapped.¶
8. ICAO Considerations
DRIP requests the following SAM Type's to be allocated:¶
9. IANA Considerations
This document does not require any actions by IANA.¶
(Editors Note: needed for Link Types?)¶
10. Security Considerations
10.1. Manifest Hash Length
For DRIP Manifest an 12-byte hash length has been selected by the authors for a number of reasons.¶
- Hash lengths smaller than 8-bytes (for example 4-bytes) were originally contemplated but ruled out by comments by various cryptographers. The main concern raised in this forum was that the length of hash would not provide strong resistance against collision rate. The authors also after further review agreed with this and also realized operationally it was not necessarily viable. While 4-byte hashes would allow more messages to be filled into a single DRIP Manifest payload (up to 22 individual hashes) the length of time for the UA to stay in a single place where the Observer would receive all the originally messages to rehash to verify such a message was impractical.¶
- Hash lengths larger than 8-bytes (for example 12 or 16-bytes) were also considered by the authors. These got the approval of the cryptographers but the number of hashes to send became much lower (only 5 individual hashes). While this lower number is a more reasonable number of original messages the Observer would have to capture it would also mean that potentially more DRIP Manifests would need to be sent. Overall the increase length of the hash did not operationally justify the cost.¶
- Simplifying the current design and locking it into using the same hash as the HHIT instead of allowing for agility in either hash algorithm or length seemed more realistic to the authors today.¶
10.2. Replay Attacks
The astute reader may note that the DRIP Link messages, which are recommended to be sent, are static in nature and contain various timestamps. These Attestation Link messages can easily be replayed by an attacker who has copied them from previous broadcasts. There are two things to mitigate this in DRIP:¶
- If an attacker (who is smart and spoofs more than just the UAS ID/data payloads) willing replays an Attestation Link message they have in principle actually helped by ensuring the message is sent more frequently and be received by potential Observers.¶
- It is RECOMMENDED to send more than just DRIP Link messages, specifically those that sign over changing data using the current session keypair, and those messages are sent more frequently. An aircraft beaconing these messages then actually signing other messages using the keypair validates the data receiver by an Observer. An UA who does not either run DRIP themselves or does not have possession of the same private key, would be clearly exposed upon signature verification.¶
10.3. Trust Timestamp Offsets
Note the discussion of Trust Timestamp Offsets here is in context of the DRIP Wrapper (Section 6.3.3) and DRIP Manifest (Section 6.3.4) messages. For DRIP Link (Section 6.3.2) messages these offsets are set by the Attestor (typically a registry) and have their own set of considerations as seen in (Editors Note: link to registry draft security considerations here).¶
The offset of the Trust Timestamp (defined as a very short Expiration Timestamp) 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 recommended that 3-5 minutes should be sufficient to serve this purpose in any scenario, but is not limited by design.¶
11. 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) - this drove the requirement for max page length of Authentication Data itself.¶
12. References
12.1. Normative References
- [F3411]
- "Standard Specification for Remote ID and Tracking", .
- [NIST.SP.800-185]
- Kelsey, J., Change, S., and R. Perlner, "SHA-3 Derived Functions: cSHAKE, KMAC, TupleHash and ParallelHash", NIST Special Publication SP 800-185, DOI 10.6028/nist.sp.800-185, , <http://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>.
12.2. Informative References
- [drip-registries]
- Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP Registries", Work in Progress, Internet-Draft, draft-wiethuechter-drip-registries-01, , <https://www.ietf.org/archive/id/draft-wiethuechter-drip-registries-01.txt>.
- [drip-requirements]
- Card, S. W., Wiethuechter, A., Moskowitz, R., and A. Gurtov, "Drone Remote Identification Protocol (DRIP) Requirements", Work in Progress, Internet-Draft, draft-ietf-drip-reqs-18, , <https://www.ietf.org/archive/id/draft-ietf-drip-reqs-18.txt>.
- [drip-rid]
- Moskowitz, R., Card, S. W., Wiethuechter, A., and A. Gurtov, "UAS Remote ID", Work in Progress, Internet-Draft, draft-ietf-drip-uas-rid-01, , <https://www.ietf.org/archive/id/draft-ietf-drip-uas-rid-01.txt>.
Appendix A. Authentication Coloring Scheme
For DRIP there are various Authentication states. The below diagram is the recommended state diagram to determine Authentication status:¶
Each state has a specific color associated with it:¶
| State | Color | Details |
|---|---|---|
| None | Black | No Authentication being received |
| Partial | Gray | Authentication being received but missing pages |
| Unsupported | Brown | Authentication Type/SAM Type of received message not supported |
| Unverifiable | Yellow | Data needed for verification missing |
| Verified | Green | Valid verification results |
| Trusted | Blue | Valid verification results and HDA is marked as trusted |
| Questionable | Orange | Inconsistent verification results |
| Unverified | Red | Invalid verification results |
| Conflicting | Purple | Inconsistent verification results and HDA is marked as trusted |
Appendix B. Example Authentication Messages
B.1. Authentication Data Only
This is an example of an Authentication Message with 52-bytes of Authentication Data.¶
Page 0: 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 Headers +---------------+ | | | +---------------+---------------+---------------+ | | | | Authentication Data / Signature | | | | | +---------------+---------------+---------------+---------------+ Page 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 | | | | | | | +---------------+---------------+---------------+---------------+ Page 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 +---------------+---------------+---------------+---------------+ | Page Header | | +---------------+ | | Authentication Data / Signature | | | | +---------------+---------------+---------------+ | | | +---------------+ | | Null Padding | | | +---------------+---------------+---------------+---------------+¶
B.2. Authentication Data & Additional Data
This example has 52-bytes of Authentication Data and 20-bytes of Additional Data.¶
Page 0: 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 Headers +---------------+ | | | +---------------+---------------+---------------+ | | | | Authentication Data / Signature | | | | | +---------------+---------------+---------------+---------------+ Page 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 | | | | | +---------------+---------------+---------------+---------------+ Page 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 +---------------+---------------+---------------+---------------+ | Page Header | | +---------------+ | | Authentication Data / Signature | | | | +---------------+---------------+---------------+ | | ADL | | +---------------+---------------+ | | Additional Data | | | +---------------+---------------+---------------+---------------+ Page 3: 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 | | +---------------+ | | Additional Data | | +---------------+ | | | +---------------+---------------+---------------+ | | | | Null Padding | | | +---------------+---------------+---------------+---------------+¶
B.3. DRIP Link Example
This DRIP Link example includes FEC for a single page.¶
Page 0: 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 Headers +---------------+ | | SAM Type | +---------------+---------------+---------------+---------------+ | | | Broadcast Attestation | | | | | +---------------+---------------+---------------+---------------+ Page 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 | | +---------------+ | | | | | | Broadcast Attestation | | | | | +---------------+---------------+---------------+---------------+ Page 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 +---------------+---------------+---------------+---------------+ | Page Header | | +---------------+ | | | | | | Broadcast Attestation | | | | | +---------------+---------------+---------------+---------------+ Page 3: 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 | | +---------------+ | | | | | | Broadcast Attestation | | | | | +---------------+---------------+---------------+---------------+ Page 4: 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 | | +---------------+ | | | | | | Broadcast Attestation | | | | | +---------------+---------------+---------------+---------------+ Page 5: 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 | | +---------------+ | | | | | | Broadcast Attestation | | | | | +---------------+---------------+---------------+---------------+ Page 6: 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 | Broadcast Attestation / +---------------+---------------+---------------+---------------+ / | ADL | | +---------------+---------------+ | | | | Forward Error Correction | | | | | +---------------+---------------+---------------+---------------+ Page 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 +---------------+---------------+---------------+---------------+ | Page Header | | +---------------+ Forward Error Correction | | | +---------------+---------------+---------------+---------------+ | | | | | Null Padding | | | +---------------+---------------+---------------+---------------+¶
