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Internet DRIP Working Group Internet-Draft This document describes how to add trust into the Broadcast Remote ID (RID) specification discussed in the DRIP Architecture. It defines message types and associated formats (sent within the Authentication Message) that can be used to authenticate past messages sent by an unmanned aircraft (UA) and provide proof of UA trustworthiness even in the absence of Internet connectivity at the receiving node.

Introduction Unmanned Aircraft Systems (UAS) operate usually in a volatile environment when it comes to communication. Unmanned Aircraft (UA) are generally small with little computational (or flying) horsepower to carry standard communication equipment. This limits the media of communication to few viable options. Observer systems (e.g., smartphones and tablets) place further constraints on the communication options. The Broadcast Remote ID (RID) messages must be available to applications on these platforms without modifying the devices. As discussed in two communication schemes to a UAS for Remote ID (RID) are considered: Broadcast and Network RID. This document focuses on adding trust to Broadcast RID (Section 3.2 of ) via the Authentication Message by combining dynamically signed data with an endorsement of the UA's identity from a DRIP Identity Management Entity (DIME). This authentication approach also provides the missing, but United States (US) Federal Aviation Administration (FAA) mandated, error correction for the Bluetooth 4.x transmissions (see ). This is error correction not only for the authentication message itself, but indirectly, to other messages authenticated via the Manifest method (see ). A summary of addressed DRIP requirements is provided in .

UAS Observers and DRIP Authentication Without authentication, a UA Observer has no basis for trust. As the messages are sent via wireless broadcast, they may be sourced anywhere within wireless range and making any claims desired by the sender. The DRIP Specific Authentication Methods, as defined herein, when properly used enables a high level of trust on other ASTM message content and source. These messages are designed to provide the Observers with actionable information.

UA Endorsement When an Observer receives a DRIP-based Authentication Message (, , ) that only contains the UA DET, timestamps, and signature; it SHOULD use the DRIP Entity Tag (DET) to retrieve the Host Identity (HI) from DNS (Section 5, ) or a local cache to validate the signature. Once the Observer has the DET/HI pair, all further (or cached previous) DRIP Authentication Messages can be validated. The content signed over can now be trusted to have been sent by the holder of the private key corresponding to the HI and DET but not the context of it.

DIME Endorsement When an Observer receives a DRIP Link Authentication Message (), that contains an endorsement of the UA DET DIME registration (Appendix B); it SHOULD use the DET of the DIME to retrieve the DIME HI from DNS (Section 5, ) or a local cache to validate the signature. The UA DET/HI pair is now known to be registered in a given DIME. As the HI is encapsulated in the data being endorsed all further (or previously received and cached) DRIP Authentication Messages using the UA DET can be validated.

Chain of DIMEs to Trust Root An Observer can receive a series of DRIP Link Authentication Messages () each one pertaining to a DIMEs registration on the chain. Similar to , each link can be validated. A chain of DIME Endorsements () can also be obtained via DNS. This is done by decomposing the received DET and altering the HID values and performing CERT lookups containing a copy of DIME Endorsements.

Authentication Content Correlation While the content of DRIP Authentication Messages can be validated via their signature this does not resolves issues due to context of that information (as noted in ). After signature validation the Observer MUST use other sources of information, for example a visual confirmation of UA position, to correlate against and provided context. When a correlation does not make sense it SHOULD be rejected as if the signature failed to validate.

Terminology

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 when, and only when, they appear in all capitals, as shown here.

Definitions This document makes use of the terms defined in . In addition, the following terms are defined: DRIP Entity Tag (DET):

• An HHIT that is used as an identifier in DRIP as specified in .

DRIP Identity Management Entity:

• Registry service for DETs and other information in DRIP as specified in .

Legacy Transports:

• use of broadcast frames (Bluetooth 4.x) as specified in .

Extended Transports:

• use of extended advertisements (Bluetooth 5.x), service info (Wi-Fi NaN) or vendor specific element information (Wi-Fi BEACON) in broadcast frames as specified in . Must use ASTM Message Pack (Message Type 0xF).

Hierarchial Host Identity Tag (HHIT):

• A special-use, non-routable, IPv6 address constructed as specified in .

HHIT Domain Authority (HDA):

• A class of DIME usually associated with a USS in UTM.

Hierarchial ID (HID):

• Encoding of the RAA and HDA into the HHIT structure as defined in .

Host Identity (HI):

• Public key have of an asymmetric keypair used in generating a HHIT as specified in .

Registered Assigning Authority (RAA):

• A class of DIME usually associated with a CAA such as the US FAA.

Background

Problem Space and Focus The initial standards for RID (, ) do not 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 for various different parties that have a stake in the UA industry. DRIP's goal as stated in the charter is:

• to specify how RID can be made trustworthy and available in both Internet and local-only connected scenarios, especially in emergency situations.

This document focuses on providing the first observable "link" of this trust chain over Broadcast RID; with an importance of the observer being offline. This first link is the primary stepping stone for an observer to gain access and use "enhanced related services".

Broadcast RID Radio Frequency Options A UA has the option of broadcasting using Bluetooth (4 and 5) or Wi-Fi (BEACON or NAN), see . With Bluetooth, FAA and other Civil Aviation Authorities (CAA) mandate transmitting simultaneously over both 4 and 5. With Wi-Fi, use of BEACON is recommended. Wi-Fi NAN is another option, depending on the CAA. Bluetooth 4.x presents a payload size challenge in that it can only transmit 25 bytes of payload where the others all can support larger payloads.

Reasoning for IETF DRIP Authentication defines Authentication Message framing only. It does not define authentication formats or methods. It explicitly anticipates several signature options, but does not fully define even those. Annex A1 defines a Broadcast Authentication Verifier Service, which has a heavy reliance on real-time connectivity to the Internet (specifically into UTM) that is not always guaranteed. Fortunately, also allows third party standard Authentication Types, 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 in Section 5, there is a need to have Authentication formats to relay information for observers to determine trust. No existing formats (defined in or other organizations leveraging this feature) provide the functionality to satisfy this goal resulting in the work reflected in this document.

ASTM Authentication Message The ASTM Authentication Message (Message Type 0x2) is a unique message in the Broadcast standard as it is the only one that is larger than the Bluetooth 4.x frame size. To address this, it is defined as a set of "pages" that each fits into a single Bluetooth 4.x broadcast frame. For other media these pages are still used but all in a single frame.

Authentication Page

Standard ASTM Authentication Message Page

This document leverages Authentication Type 0x5, Specific Authentication Method (SAM), as the principal authentication container, defining a set of SAM Types in . This is denoted in every Authentication Page in the Page Header. The SAM Type is denoted as a field in the Authentication Payload (see ). The Authentication Message is structured as a set of pages. 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 maximum 23-byte Authentication Payload. See for more details. Over Bluetooth 4.x, these pages are "fragmented" into separate Bluetooth 4.x broadcast frames. 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 (Bluetooth or Wi-Fi).

Authentication Payload Field is the source data view of the data fields found in the Authentication Message as defined by . This data is placed into 's Authentication Payload, spanning multiple pages.

ASTM Authentication Message Fields

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 .

ASTM 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 transport that can hold the least amount of authentication data is Bluetooth 5.x and Wi-Fi BEACON 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 denotes the length in bytes of Authentication Data / Signature only) MUST NOT exceed the value of 201.

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). The Bluetooth 4.x Legacy Transport does not have supporting FEC so with DRIP Authentication the following application level FEC scheme is used to add FEC. This section is only used for Bluetooth 4.x transmission/reception. 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 supports 2 different FEC encodings. Single Page FEC is the simpler scheme, but can correct for only a single erased page. Multiple Page FEC is more complex, but can correct for multiple erased pages. The data added during FEC is not included in the Authentication Data / Signature but instead in the Additional Data field of . This may cause the Authentication Message to exceed 9-pages, up to a maximum of 16-pages.

General Encoding Rules For DRIP the FEC data MUST start on a new ASTM Authentication Page. To do this once the results of parity encoding is placed in the Additional Data field of with null padding before it to line up with the next page. The Additional Data Length field is set to number of padding bytes + number of parity bytes.

General Decoding Rules 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 Length field SHOULD also be performed. To determine if Single Page FEC or Multiple Page FEC has been used a simple check of the Last Page Index can be used. If the number of pages left after the Length of Authentication Data is not exhausted then the remaining pages should be all FEC. The Additional Data Length byte can further confirm this; taking into account any null padding needed for page alignment.

Single Page

Encoding To generate the parity a simple XOR operation using the previous parity page and current page is used. Only the 23-byte Authentication Payload field of is used in the XOR operations. For Page 0, a 23-byte null pad is used for the previous parity page. shows 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 is 23-bytes of FEC data and 10-bytes of padding to line it up into Page N.

Example Single Page FEC Encoding

Decoding To decode Single Page 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 is used to signal a different Authentication Message and to correlate pages to them. This Message Counter is only 1-byte in length, so it will rollover (to 0x00) after reaching its maximum value (0xFF). If only 1-page is missing in the Authentication Message the resulting parity bytes should be the data of the erased page.

Multiple Page

Encoding For Multiple Page FEC there are two variations: Frame Recovery and Page Recovery. Both follow a similar process, but are offset at what data is protected. For DRIP the polynomial to use for Reed Solomon is: 1 + x^2 + x^3 + x^4 + x^8. This polynomial was selected as it commonly used in Reed Solomon implementations. A form of it was deployed by the National Aeronautics and Space Administration (NASA) for the Voyager probes ; a problem space with far tighter constraints than RID. is a generic example of Multiple Page FEC Authentication Message where 3-pages out of N are used for Reed Solomon FEC. The Additional Data Length is set to 72 as there are 10-bytes of padding and 62-bytes of parity from Reed Solomon.

Example Multiple Page FEC Encoding

• Informative Note: the last page in the example is padded to fill the full page as specified by .

Page Recovery Page Recovery in Multiple Page FEC protects the same content, just the Authentication Message, as Single Page FEC. The benefit is increased protection from a maximum of one page, up to the page maximum (minus pages being used for authentication). In Page Recovery, the Authentication Payload field of for each page is used. Reed Solomon is performed in a byte-wise fashion across each Authentication Page to generate a number of parity bytes. The number of these parity bytes directly corresponds to the number of pages of FEC to be append to the Authentication Message. The resulting parity bytes are placed to the corresponding bytes in the FEC pages. This can be considered a form of interleaving that takes advantage of the fixed page length. See for a detailed example of encoding for Page Recovery.

Frame Recovery Frame Recovery in Multiple Page FEC protects not just the Authentication Message it is carried in but also other ASTM Messages being sent. This is at the cost of much longer Authentication Messages. Up to 240 messages (255 minus 15 pages maximum for FEC) can be protected using Frame Recovery. For Frame Recovery both transmitter and receiver need to agree on what messages are being Reed Solomon'd over. It is RECOMMENDED that the data is limited to a full transmission set of ASTM Messages. For example in the European Union (EU) a full set of ASTM Messages would include: 1x Basic ID, 1x Location/Vector, 1x System and 1x Operator ID. With DRIP this would also included 1x Authentication that would be carrying the FEC (along with DRIP Authentication). Similar to , Reed Solomon is performed in a bytes-wise fashion across messages to generate the desired number of parity bytes. These messages MUST be in Message Type order when performing the Reed Solomon operation. All 25-bytes of the ASTM Message are used during this operation for Frame Recovery. After the computation the new pseudo-frames formed by the parity are concatenated together. The length of this data is used in the calculation of the Additional Data Length with the amount of padding needed to align to a new Authentication Page. The padding and parity data are then placed in the Additional Data field. See for a detailed example of encoding for Frame Recovery.

Decoding To determine if Page Recovery or Frame Recovery is used two modulo checks with the ADL after the length of the null-pad is removed are needed. One against the value of 23, and the other against the value of 25. If 23 comes back with a value of 0 then Page Recovery is being used. If 25 comes back with 0 then Frame Recovery is used. Any other combination indicates an error. As it is known which pages were not received in an Authentication Message (or were erased by Bluetooth due to detected errors), the Reed Solomon capacity can be dedicated exclusively to correction of erasures, rather than to detection and correction of errors, thereby doubling its effective capacity. This is accomplished by marking the erasures, i.e., filling the dummy page(s) or frames with nulls. For either Page Recovery or Frame recovery the first step on the receiver is to create empty (or dummy) Authentication Pages for any pages missing in the Authentication Message. Then the Additional Data can be extracted from the Authentication Message, have its null-padding removed and further processed.

Page Recovery To decode Page Recovery, the received FEC data along with the Authentication Payload of each Authentication Page has Reed Solomon performed using erasures byte-wise across the pages. The results should have all pages of the Authentication Message recovered. The receiver SHOULD validate the rebuilt message before decoding the actual authentication.

Frame Recovery To decode Frame Recovery, the receiver breaks the Additional Data into 25-byte chunks. This will produce the pseudo-frames of parity bytes from the Authentication Message. To build the rest of the message set, static messages such as Basic ID and Operator ID are constant and can be filled in using any received copy that is cache by the receiver for that UA. Dynamic messages in the set, such as Location/Vector or System can be nulled out to have Reed Solomon alway recover them and the results checked against cached copies from recent transmissions of the UA. With all the frames in the set, Reed Solomon can be used in an erasure mode to decode byte-wise across the frames to fill in the erasures and rebuild the entire set of messages. Validation of the Authentication Message SHOULD be performed before further processing of authentication data.

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-bytes 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. This should be avoided where possible in an effort to maintain efficiency.

DRIP Authentication Formats All formats defined in this section are the content for the Authentication Data / Signature field in and use the Specific Authentication Method (SAM, Authentication Type 0x5). The first byte of the Authentication Data / Signature of , is used to multiplex between these various formats. When sending data over a medium that does not have underlying Forward Error Correction (FEC), for example Bluetooth 4.x, then MUST be used. gives a high-level overview of a state machine for decoding and determining a trustworthiness state. shows an example of using the formats defined in this section.

DRIP Authentication Field Definitions ASTM Message (25-bytes):

• Full ASTM Message as defined in ; specifically Message Types 0x0, 0x1, 0x3, 0x4, and 0x5

ASTM Message Hash (12-bytes):

• Hash of a single full ASTM Message using hash operations described in (). Multiple hashes MUST be in Message Type order.

Attestation Data (0 to 112 bytes):

• Opaque attestation data that the UA is attesting during its flight in .

Broadcast Endorsement (136-bytes):

• DIME HI over UA DET/HI. Generated by a DIME during a UA DET, being used as a Session ID, registration. Used in .

Current Manifest Hash (12-bytes):

• Hash of the current Manifest Message (). See .

Frame Type (1-byte):

• Sub-type for future different DRIP Frame formats. See .

Not Before Timestamp by UA (4-bytes):

• Timestamp denoting recommended time to start trusting data in . MUST follow the format defined in . That is a Unix-style timestamp but with an epoch of 01/01/2019 00:00:00. MUST be set no earlier than the time the signature is generated.

Not After Timestamp by UA (4-bytes):

• Timestamp denoting recommended time to stop trusting data in . MUST follow the format defined in . That is a Unix-style timestamp but with an epoch of 01/01/2019 00:00:00 with an additional offset is then added to push a short time into the future (relative to Not Before Timestamp) to avoid replay attacks. The offset used against the Unix-style 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. A reasonable time would be to set Not After Timestamp 2 minutes after Not Before Timestamp.

Previous Manifest Hash (12-bytes):

• Hash of the previously sent Manifest Message (). See .

UA DRIP Entity Tag (16-bytes):

• The UA DET in byte form (network byte order) and is part of .

UA Signature (64-bytes):

• Signature over all 4 preceding fields of using the HI of the UA.

Broadcast Attestation Structure Variations of the Attestation Structure format of MUST be used when running DRIP Authentication under the DRIP SAM Types (filling the SAM Authentication Data field ()). The only differences are that the timestamps are set by the UA and the Attestor Identity Information is set to the DET of the UA. When using this structure, the UA is minimally self-attesting its DET. It may be attesting the DET registration in a specific HID (see ). The HI of the UA DET can be looked up by mechanisms described in or by extracting it from a Broadcast Endorsement (see and ).

Broadcast Attestation Structure

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

SAM Data Format

SAM Type The SAM Type field is maintained by the International Civil Aviation Organization (ICAO) and for DRIP four are planned to be allocated:

SAM Type	Description
0x01	DRIP Link ()
0x02	DRIP Wrapper ()
0x03	DRIP Manifest ()
0x04	DRIP Frame ()

SAM Authentication Data This field has a maximum size of 200-bytes, as defined by . The Broadcast Attestation Structure () MUST be used in this space.

DRIP Link The DRIP Link SAM Type is used to transmit Broadcast Endorsements. For example, the Broadcast Endorsement: DIME, UA is sent (see ) as a DRIP Link message. The structure is defined in and an example of it can be found in . DRIP Link is important as its contents are used to provide trust in the DET/HI pair that the UA is currently broadcasting. This message does not require Internet connectivity to perform signature validations of the contents when the DIME DET/HI is in the receiver's cache. It also provides the UA HI so that connectivity is not required when performing validation of other DRIP Authentication Messages.

DRIP Link

This DRIP Authentication Message is used in conjunction with other DRIP SAM Types (such as Manifest or Wrapper) that contain data that is guaranteed to be unique, unpredictable and easily cross checked by the receiving device. The only message type satisfying these requirements is the Location/Vector Message (Message Type 0x2). The hash of such a message MAY merely be included in a DRIP Manifest, but an entire such message SHOULD be encapsulated in a DRIP Wrapper periodically for stronger security.

DRIP Wrapper This SAM Type is used to wrap and sign over a list of other Broadcast RID messages. It MUST use the Broadcast Attestation Structure (). The Attestation Data field is filled with full (25-byte) 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 . 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 () is a multiple of 25-bytes.

DRIP Wrapper over Legacy Transports

Wrapper over Extended Transports To send the DRIP Wrapper over Extended Transports the messages being wrapped are co-located with the Authentication Message in a ATM Message Pack (Message Type 0xF). The ASTM Messages are removed from the DRIP Wrapper after signing (as they are redundant) leaving the following structure that is placed into the SAM Authentication Data of and sent in the same Message Pack.

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 Message Type order and place them between the UA DRIP Entity Tag and Not Before Timestamp before performing signature verification. The functionality of Wrapper in this form is identical to Authentication Type 0x3 (Message Set Signature) when running over Extended Transports. What 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 relies on Internet connectivity for verification which the receiver may not have at the time of receipt of an Authentication Message. This is something Wrapper, and all DRIP Authentication Formats, avoid when the UA key is obtained via a DRIP Link Authentication Message.

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

DRIP Manifest This SAM Type is used to create message manifests. It MUST use the Broadcast Attestation Structure (). By hashing previously sent messages and signing them we gain trust in UAs previous reports. An observer who has been listening for any 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 Broadcast messages. A receiver does not need to have received every message in the manifest to verify it. A manifest SHOULD typically encompass a single transmission cycle of messages being sent, see .

DRIP Manifest

Hash Count The number of hashes in the Manifest can be variable (3-9). An easy way to determine the number of hashes is to take the length of the data between the end of the UA DRIP Entity Tag and Not Before Timestamp by UA and divide it by the hash length (12). If this value is not rational, the message is invalid.

Message Hash Algorithms and Operation The hash algorithm used for the Manifest Message is the same hash algorithm used in creation of the DET that is signing the Manifest. An DET using cSHAKE128 computes the hash as follows:

• • Informative Note: specifies cSHAKE128 but is open for the expansion of other OGAs.

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.

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

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.

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.

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 MUST use the Broadcast Attestation Structure ().

DRIP Frame

Frame Type Byte to sub-type for future different DRIP Frame formats. It takes the first byte of Attestation Data in leaving 111-bytes for Frame Attestation Data.

Frame Type	Name	Description
0x00	Reserved	Reserved
0xC0-0xFF	Experimental	Experimental Use

Requirements & Recommendations

Legacy Transports With Legacy Advertisements the goal is to attempt to bring reliable receipt of the paged Authentication Message. FEC () MUST be used, per mandated RID rules (for example the US FAA RID Rule ), when using Legacy Advertising methods (such as Bluetooth 4.x). Under ASTM Bluetooth 4.x rules, transmission of dynamic messages is at least every 1 second. DRIP Authentication Messages typically contain dynamic data (such as the DRIP Manifest or DRIP Wrapper) and should be sent at the dynamic rate of 1 per second.

Extended Transports Under the ASTM specification, Bluetooth 5.x, Wi-Fi NaN, and Wi-Fi BEACON transport of RID 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.x extended frame (up to 9 single frame equivalent messages under Bluetooth 4). 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 FEC () MUST NOT be used as it is impractical.

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

1. SHOULD: send DRIP Link using the Broadcast Endorsement: DIME:Apex, DIME:RAA (satisfying GEN-3); at last once per 5 minutes
2. MUST: send DRIP Link using the Broadcast Endorsement: DIME:RAA, DIME:HDA(satisfying GEN-3); at least once per 5 minutes
3. MUST: send DRIP Link using the Broadcast Endorsement: DIME: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 or DRIP Wrapper) 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

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 Broadcast Endorsement: DIME, UA. A separate issue is the frequency of transmitting the DRIP Link Authentication Message for the Broadcast Endorsement: DIME, UA when using a Manifest Message. 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 Broadcast Endorsement: DIME, UA to mitigate potential packet loss. The preferred mode of operation is to send the Broadcast Endorsement: DIME, UA every 10 seconds and Manifest messages immediately after a set of UA operation messages (e.g. Basic, Location, and System messages).

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 . 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 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.

Summary of Addressed DRIP Requirements The following are addressed in this document: ID-5: Non-spoofability

• Addressed using the DRIP Wrapper (), DRIP Manifest () or DRIP Frame ().

GEN-1: Provable Ownership

• Addressed using the DRIP Link () and DRIP Wrapper (), DRIP Manifest () or DRIP Frame ().

GEN-2: Provable Binding

• Addressed using the DRIP Wrapper (), DRIP Manifest () or DRIP Frame ().

GEN-3: Provable Registration

• Addressed using the DRIP Link ().

ICAO Considerations DRIP requests the following SAM Types to be allocated:

1. DRIP Link
2. DRIP Wrapper
3. DRIP Manifest
4. DRIP Frame

IANA Considerations

IANA DRIP Registry This document requests a new subregistry for Frame Type under the DRIP registry.

DRIP Frame Type:

This 8-bit valued subregistry is for Frame Types in DRIP Frame Authentication Messages. Future additions to this subregistry are to be made through Expert Review (Section 4.5 of ). The following values are defined:

Security Considerations

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 DRIP 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:

1. If an attacker (who is smart and spoofs more than just the UAS ID/data payloads) willing replays an DRIP Link message they have in principle actually helped by ensuring the message is sent more frequently and be received by potential Observers.
2. It is REQUIRED to send more than DRIP Link messages, specifically those that sign over changing data using the current session keypair, and those messages are sent more frequently. An UA 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.

Trust Timestamp Offsets Note the discussion of Trust Timestamp Offsets here is in context of the DRIP Wrapper () and DRIP Manifest () messages. For DRIP Link () messages these offsets are set by the Attestor (typically a DIME) and have their own set of considerations as seen in . 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.

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 maximum page length of Authentication Data itself.
• Many thanks to Rick Salz for the secdir review.

References Normative References This document defines terminology and requirements for solutions produced by the Drone Remote Identification Protocol (DRIP) Working Group. These solutions will support Unmanned Aircraft System Remote Identification and tracking (UAS RID) for security, safety, and other purposes (e.g., initiation of identity-based network sessions supporting UAS applications). DRIP will facilitate use of existing Internet resources to support RID and to enable enhanced related services, and it will enable online and offline verification that RID information is trustworthy. AX Enterprize AX Enterprize HTT Consulting Intel Linköping University This document describes an architecture for protocols and services to support Unmanned Aircraft System (UAS) Remote Identification (RID) and tracking, plus UAS RID-related communications. This architecture adheres to the requirements listed in the DRIP Requirements document (RFC9153). In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Information Technology Laboratory Information Technology Laboratory Information Technology Laboratory National Institute of Standards and Technology

US Gaithersburg

Informative References Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. HTT Consulting AX Enterprize AX Enterprize Linköping University This document describes the use of Hierarchical Host Identity Tags (HHITs) as a self-asserting and thereby trustable Identifier for use as the UAS Remote ID. HHITs include explicit hierarchy to provide Registrar discovery for 3rd-party ID attestation. AX Enterprize, LLC AX Enterprize, LLC HTT Consulting RTFM llp This document details the required mechanisms for the registration and discovery of DRIP Entity Tags (DETs). The registration process relies upon the Extensible Provisioning Protocol (EPP). The discovery process leverages DNS, DNSSEC, and related technology. The lookup process relies upon the Registration Data Access Protocol (RDAP). The DETs can be registered with as their "raw public keys" or in X.509 certificates.

Authentication State Diagrams & Color Scheme ASTM Authentication has only 3 states: None, Invalid or Valid. This is because under ASTM the idea is that Authentication is done by an external service hosted somewhere on the Internet so it is assumed you will always get some sort of answer back. With DRIP this classification becomes more complex with the support of "offline" scenarios where the receiver does not have Internet connectivity. With the use of asymmetric keys this means the public key (PK) must somehow be obtained - gets more into detail how these keys are stored on DNS and one reason for DRIP Authentication is to send PK's over Broadcast RID. There are two keys of interest: the PK of the UA and the PK of the DIME. This document gives a clear way to send the PK of the UA over the Broadcast RID messages. The key of the DIME can be sent over Broadcast RID using the same mechanisms (see and ) but is not required due to potential operational constraints of sending multiple DRIP Link messages. As such there are scenarios where you may have part of the key-chain but not all of it. The intent of this appendix is to give some kind of recommended way to classify these various states and convey it to the user through colors and state names/text.

State Colors The table below lays out the RECOMMENDED colors to associate with state.

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 DIME is marked as trusted
Questionable	Orange	Inconsistent verification results
Unverified	Red	Invalid verification results
Conflicting	Purple	Inconsistent verification results and DIME is marked as trusted

State Diagrams This section gives some RECOMMENDED state flows that DRIP should follow. Note that the state diagrams do not have all error conditions mapped.

Notations

Diagram Notations Transition Option False/No -----> Transition Option True/Yes ]]>

General

Standard Authentication Colors/State o 1 o+----->| None | o---------------------o ooooo +------+ | v ooooo +-------------+ o 2 o+----->| Unsupported | ooooo +-------------+ | ^ v | +---------+ ooooo | | Partial |<-----+o 3 o | +---------+ ooooo | | | v + ooooo ooooo o-------------o o 4 o------>o 5 o------>| SAM Decoder | ooooo ooooo o-------------o + | v o------------------o | AuthType Decoder | o------------------o ]]>

Transition	Transition Query	Next State/Process/Transition (Yes, No)
1	Receiving Authentication Pages?	2, None
2	Authentication Type Supported?	3, Unsupported
3	All Pages of Authentication Message Received?	4, Partial
4	Is Authentication Type received 5?	5, AuthType Decoder
5	Is SAM Type Supported?	SAM Decoder, Unsupported

DRIP SAM

DRIP SAM Decoder o 6 o------>| DRIP Wrapper/Manifest/Frame | o-------------o ooooo o-----------------------------o + | ^ | | | v v | o-----------o o--------------------o | | DRIP Link |--->| Update State Cache | | o-----------o o--------------------o | | | v | o--------------o ooooo o----------------------o | NOP / Return |<------+o 7 o----->| Extract Message from | o--------------o ooooo | Verification Queue | o----------------------o ]]>

Transition	Transition Query	Next State/Process/Transition (Yes, No)
6	Is SAM Type DRIP Link?	DRIP Link, DRIP Wrapper/Manifest/Frame
7	Messages in Verification Queue?	Extract Message from Verification Queue, NOP / Return

DRIP Link

DRIP Link State Decoder o 8 o+----->o 9 o+----->| Unverifiable | o-----------o ooooo ooooo +--------------+ | | |-------------' v ooooo +------------+ o 10 o+----->| Unverified | ooooo +------------+ | v o---------------------o | Add UA DET/PK | | to Key Cache | o---------------------o | v ooooo +----------+ o 11 o+------>| Verified | ooooo +----------+ | ^ v | o-------------------------o | Mark UA DET/PK | | as Trusted in Key Cache | o-------------------------o ]]>

Transition	Transition Query	Next State/Process/Transition (Yes, No)
8	DIME DET/PK in Key Cache?	10, 9
9	DIME PK found Online?	10, Unverifiable
10	DIME Signature Verified?	Add UA DET/PK to Key Cache, Unverified
11	DIME DET/PK marked as Trusted in Key Cache?	Mark UA DET/PK as Trusted in Key Cache, Verified

DRIP Wrapper/Manifest/Frame

DRIP Wrapper/Manifest/Frame State Decoder o 13 o+----->| Add Message to | ooooo ooooo | Verification Queue | | | o--------------------o | | |-------------' v ooooo ooooo ooooo +------------+ o 14 o+----->o 15 o+----->o 16 o+----->| Unverified | ooooo ooooo ooooo +------------+ | | | v v | ooooo +-------------+ | o 17 o+----->| Conflicting | | ooooo +-------------+ | | | v v ooooo +--------------+ o 18 o---------------->| Questionable | ooooo +--------------+ + | v ooooo +----------+ o 19 o+----->| Verified | ooooo +----------+ | v +---------+ | Trusted | +---------+ ]]>

Transition	Transition Query	Next State/Process/Transition (Yes, No)
12	UA DET/PK in Key Cache?	14, 13
13	UA PK found Online?	14, Add Message to Verification Queue
14	UA Signature Verified?	17, 15
15	Has past Messages of this type been marked as Trusted?	Conflicting, 16
16	Has past Messages of this type been marked as Questionable or Verified?	Questionable, Unverified
17	Has past Messages of this type been marked as Conflicting?	Conflicting, 18
18	Has past Messages of this type been marked as Questionable or Unverified?	Questionable, 19
19	Is UA DET/PK marked as Trusted in Key Cache?	Trusted, Verified

Broadcast Endorsement: DIME, UA

Example DRIP Broadcast Endorsement: DIME, UA

Example TX/RX Flow In this example the UA is sending all DRIP Authentication Message formats (DRIP Link, DRIP Wrapper and DRIP Manifest) during flight, along with standard ASTM Messages. The objective is to show the combinations of messages that must be received to properly validate a DRIP equipped UA and examples of their various states (as described in ). As the above example shows to properly authenticate both a DRIP Link and a DRIP Wrapper or DRIP Manifest are required.

FEC Examples

Multiple Page: Page Recovery The following example is an Authentication Message with 7 pages that 3 pages of parity are to be generated for. The first column is just the Page Header with a visual space here to show the boundary. For Page Recovery the first column is ignored and the last 23-bytes of each page are extracted to have Reed Solomon performed on them in a column wise fashion to produce parity bytes. This can be considered a form of interleaving that takes advantage of the fixed page length. For the example the following 3-bytes of parity are generated with the first byte of each page: Each set of parity is the placed into a pseudo-frame as follows (each byte in its own message in the same column). Below is an example of the full parity generated and each 23-bytes of parity added into the additional pages as Additional Data:

Multiple Page: Frame Recovery Below is an example of a number of messages. The first column is an additional ASTM Header that contain the Message Type; with a visual space for clarity. The last 24-bytes are the actual message contents; be it location information or an Authentication Page. A similar process is followed as in . Here every column of bytes has parity generated for it (even the ASTM Header). In the below example 5-bytes of parity are generated using the ASTM Header column: After doing this to all columns the following pseudo-frames would have been generated: These 25-byte chunks are now concatenated together and are placed in Authentication Pages, using the Additional Data, 23-bytes at a time. In the below figure the first column is the ASTM Header as before, the second column is the Page Header for each Authentication Page and then last column is the 23-bytes of data for each page.