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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. 'F3411-19' Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DRIP Working Group A. Wiethuechter 3 Internet-Draft S. Card 4 Intended status: Standards Track AX Enterprize, LLC 5 Expires: 20 December 2021 R. Moskowitz 6 HTT Consulting 7 18 June 2021 9 DRIP Authentication Formats 10 draft-ietf-drip-auth-01 12 Abstract 14 This document describes how to include trust into the ASTM Remote ID 15 specification defined in ASTM F3411-19 under a Broadcast Remote ID 16 (RID) scenario. It defines a few different message schemes (based on 17 the Authentication Message) that can be used to assure past messages 18 sent by a UA and also act as an assurance for UA trustworthiness in 19 the absence of Internet connectivity at the receiving node. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on 20 December 2021. 38 Copyright Notice 40 Copyright (c) 2021 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 45 license-info) in effect on the date of publication of this document. 46 Please review these documents carefully, as they describe your rights 47 and restrictions with respect to this document. Code Components 48 extracted from this document must include Simplified BSD License text 49 as described in Section 4.e of the Trust Legal Provisions and are 50 provided without warranty as described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 55 1.1. DRIP Requirements Addressed . . . . . . . . . . . . . . . 3 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 57 2.1. Required Terminology . . . . . . . . . . . . . . . . . . 4 58 2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4 59 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3.1. Problem Space and Focus . . . . . . . . . . . . . . . . . 4 61 3.2. Reasoning for IETF DRIP Authentication . . . . . . . . . 4 62 3.3. ASTM Authentication Message . . . . . . . . . . . . . . . 5 63 4. DRIP Authentication Formats . . . . . . . . . . . . . . . . . 6 64 4.1. UAS ID Signature . . . . . . . . . . . . . . . . . . . . 6 65 4.2. Operator ID Signature . . . . . . . . . . . . . . . . . . 7 66 4.3. Message Set Signature . . . . . . . . . . . . . . . . . . 8 67 4.4. Specific Method . . . . . . . . . . . . . . . . . . . . . 9 68 4.4.1. DRIP Frame Format . . . . . . . . . . . . . . . . . . 9 69 4.4.2. DRIP Wrapper Format . . . . . . . . . . . . . . . . . 11 70 4.4.3. DRIP Manifest Format . . . . . . . . . . . . . . . . 11 71 4.4.4. DRIP Link Format . . . . . . . . . . . . . . . . . . 13 72 5. Transport Methods & Recommendations . . . . . . . . . . . . . 13 73 5.1. Legacy Advertisements (Bluetooth 4.X) . . . . . . . . . . 13 74 5.2. Extended Advertisements (Bluetooth 5.X, WiFi NaN, WiFi 75 Beacon) . . . . . . . . . . . . . . . . . . . . . . . . . 14 76 5.3. DRIP Recommendations . . . . . . . . . . . . . . . . . . 14 77 6. ICAO Considerations . . . . . . . . . . . . . . . . . . . . . 14 78 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 79 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 80 8.1. Manifest Hash Length . . . . . . . . . . . . . . . . . . 15 81 8.2. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 15 82 8.3. Trust Timestamp Offsets . . . . . . . . . . . . . . . . . 16 83 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 84 10. Appendix A: Thoughts on ASTM Authentication Message . . . . . 16 85 11. Appendix B: DRIP Attestations . . . . . . . . . . . . . . . . 17 86 11.1. Self-Attestation (Axx) . . . . . . . . . . . . . . . . . 17 87 11.2. Attestation (Axy) . . . . . . . . . . . . . . . . . . . 18 88 11.3. Concise Attestation (C-Axy) . . . . . . . . . . . . . . 19 89 11.4. Mutual Attestation (M-Axy) . . . . . . . . . . . . . . . 20 90 11.5. Link Attestation (L-Axy) . . . . . . . . . . . . . . . . 21 91 11.6. Broadcast Attestation (B-Axy) . . . . . . . . . . . . . 22 92 11.7. Link Certificate (L-Cxy) . . . . . . . . . . . . . . . . 24 93 11.8. Mutual Certificate (M-Cxy) . . . . . . . . . . . . . . . 24 94 11.9. Example Registration with Attestation . . . . . . . . . 25 95 12. Appendix C: DRIP Broadcast Attestation Structure . . . . . . 26 96 12.1. Attestor Hierarchical Host Identity Tag . . . . . . . . 27 97 12.2. Attestation Data . . . . . . . . . . . . . . . . . . . . 27 98 12.3. Trust Timestamp . . . . . . . . . . . . . . . . . . . . 27 99 12.4. Signing Timestamp . . . . . . . . . . . . . . . . . . . 27 100 12.5. Attestor Signature . . . . . . . . . . . . . . . . . . . 28 101 13. Appendix D: Forward Error Correction . . . . . . . . . . . . 28 102 13.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 28 103 13.1.1. Single Page FEC . . . . . . . . . . . . . . . . . . 28 104 13.1.2. Multi Page FEC . . . . . . . . . . . . . . . . . . . 29 105 13.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . . 29 106 13.2.1. Single Page FEC . . . . . . . . . . . . . . . . . . 29 107 13.2.2. Multi Page FEC . . . . . . . . . . . . . . . . . . . 29 108 13.3. FEC Limitations . . . . . . . . . . . . . . . . . . . . 29 109 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 110 14.1. Normative References . . . . . . . . . . . . . . . . . . 29 111 14.2. Informative References . . . . . . . . . . . . . . . . . 30 112 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30 114 1. Introduction 116 UA Systems (UAS) are usually in a volatile environment when it comes 117 to communication. UA are generally small with little computational 118 (or flying) horsepower to carry standard communication equipment. 119 This limits the mediums of communication to few viable options. 121 Observer systems (e.g. smartphones and tablets) place further 122 constraints on the communication options. The Remote ID Broadcast 123 messages MUST be available to applications on these platforms without 124 modifying the devices. 126 The ASTM standard [F3411-19] focuses on two ways of communicating to 127 a UAS for RID: Broadcast and Network. 129 This document will focus on adding trust to Broadcast RID in the 130 current (and an expanded) Authentication Message format. 132 1.1. DRIP Requirements Addressed 134 The following [drip-requirements] will be addressed: 136 GEN 1: Provable Ownership This will be addressed using the DRIP 137 Link. 139 GEN 2: Provable Binding This requirement is addressed using the DRIP 140 Link, Manifest. 142 GEN 3: Provable Registration This requirement is addressed using the 143 DRIP Link. 145 2. Terminology 147 2.1. Required Terminology 149 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 150 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 151 "OPTIONAL" in this document are to be interpreted as described in BCP 152 14 [RFC2119] [RFC8174] when, and only when, they appear in all 153 capitals, as shown here. 155 2.2. Definitions 157 See [drip-requirements] for common DRIP terms. 159 Aircraft: In this document whenever the word Aircraft is used it is 160 referring to an Unmanned Aircraft (UA) not a Manned Aircraft. 162 3. Background 164 3.1. Problem Space and Focus 166 The current standard for Remote ID (RID) does not, in any meaningful 167 capacity, address the concerns of trust in the UA space with 168 communication in the Broadcast RID environment. This is a 169 requirement that will need to be addressed eventually for various 170 different parties that have a stake in the UA industry. 172 The following subsections will provide a high level reference to the 173 ASTM standard for Authentication Messages and how their current 174 limitations effect trust in the Broadcast RID environment. 176 3.2. Reasoning for IETF DRIP Authentication 178 The ASTM Authentication Message has provisions in [F3411-19] to allow 179 for other organizations to define (and standardize) Authentication 180 formats. The standardization of special formats to support the DRIP 181 requirements in UAS RID for trustworthy communications over Broadcast 182 RID is an important part of the chain of trust for a UAS ID. No 183 existing formats (defined by ASTM or others) was flexible enough to 184 satisfy this goal resulting in the work reflected in this document. 186 3.3. ASTM Authentication Message 188 Page 0: 189 0 1 2 3 190 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 1 191 +---------------+-----------------------------------------------+ 192 | Auth Header | | 193 +---------------+ ASTM Authentication Headers +---------------+ 194 | | | 195 +-----------------------------------------------+ | 196 | | 197 | | 198 | | 199 | Authentication Data / Signature | 200 | | 201 | | 202 | | 203 +---------------------------------------------------------------+ 205 Page 1 - 15: 206 0 1 2 3 207 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 1 208 +---------------+-----------------------------------------------+ 209 | Auth Header | | 210 +---------------+ | 211 | | 212 | | 213 | | 214 | | 215 | Authentication Data / Signature | 216 | | 217 | | 218 | | 219 | | 220 +---------------------------------------------------------------+ 222 Auth Header (1 byte): 223 Contains Authentication Type (AuthType) and Page Number. 225 ASTM Authentication Headers: (6 bytes) 226 Contains other header information for the Authentication 227 Message from ASTM UAS RID Standard. 229 Authentication Data / Signature: (0 to 255 bytes) 230 Opaque authentication data. 232 Figure 1: Standard ASTM Authentication Message format 234 The above diagram is the format defined by ASTM [F3411-19] that is 235 the frame which everything this document fits into. The specific 236 details of the ASTM headers are abstracted away as they are not 237 necessarily required for this document. 239 There is a 25th byte exclude in the diagrams that comes before the 240 Auth Header. This is the ASTM Header and consists of the Protocol 241 Version and Message Type of the given message frame/page. 243 4. DRIP Authentication Formats 245 To keep consistent formatting across the different mediums (Bluetooth 246 4, Bluetooth 5 and WiFi NaN) and their independent restrictions the 247 authentication data being sent is REQUIRED to fit within the first 9 248 pages (Page 0 through Page 8) of the Authentication Message (giving a 249 max of 201 bytes). The rest of the pages of the message is reserved 250 exclusively for Forward Error Correction bytes and is only present on 251 Bluetooth 4. 253 4.1. UAS ID Signature 255 The existing ASTM [F3411-19] Authentication Type 0x1 can be used to 256 send a fresh Self-Attestation of the UA over 7 pages. 258 0 1 2 3 259 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 1 260 +---------------+---------------+---------------+---------------+ 261 | | 262 | UA Hierarchical | 263 | Host Identity Tag | 264 | | 265 +---------------+---------------+---------------+---------------+ 266 | | 267 | | 268 | | 269 | UA Host Identity | 270 | | 271 | | 272 | | 273 | | 274 +---------------+---------------+---------------+---------------+ 275 | Trust Timestamp | 276 +---------------+---------------+---------------+---------------+ 277 | Signing Timestamp | 278 +---------------+---------------+---------------+---------------+ 279 | | 280 | | 281 | | 282 | | 283 | | 284 | | 285 | | 286 | UA Signature | 287 | | 288 | | 289 | | 290 | | 291 | | 292 | | 293 | | 294 | | 295 +---------------+---------------+---------------+---------------+ 297 Figure 2: DRIP UAS ID Signature 299 4.2. Operator ID Signature 301 The existing ASTM [F3411-19] Authentication Type 0x2 can be used to 302 send a static Self-Attestation of the Operator over 7 pages. 304 0 1 2 3 305 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 1 306 +---------------+---------------+---------------+---------------+ 307 | | 308 | Operator Hierarchical | 309 | Host Identity Tag | 310 | | 311 +---------------+---------------+---------------+---------------+ 312 | | 313 | | 314 | | 315 | Operator Host Identity | 316 | | 317 | | 318 | | 319 | | 320 +---------------+---------------+---------------+---------------+ 321 | Trust Timestamp | 322 +---------------+---------------+---------------+---------------+ 323 | Signing Timestamp | 324 +---------------+---------------+---------------+---------------+ 325 | | 326 | | 327 | | 328 | | 329 | | 330 | | 331 | | 332 | Operator Signature | 333 | | 334 | | 335 | | 336 | | 337 | | 338 | | 339 | | 340 | | 341 +---------------+---------------+---------------+---------------+ 343 Figure 3: DRIP Operator ID Signature 345 4.3. Message Set Signature 347 When running under Extended Advertisements, the existing ASTM 348 [F3411-19] Authentication Type 0x3 can be used to sign over the 349 adjacent ASTM Messages in the Message Pack (0xF). 351 The concatenation of all messages in the Message Pack (excluding 352 Authentication) before signing MUST be in Message Type order and be 353 placed between the UA HHIT and Signing Timestamp field. 355 0 1 2 3 356 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 1 357 +---------------+---------------+---------------+---------------+ 358 | | 359 | UA Hierarchical | 360 | Host Identity Tag | 361 | | 362 +---------------+---------------+---------------+---------------+ 363 | Trust Timestamp | 364 +---------------+---------------+---------------+---------------+ 365 | Signing Timestamp | 366 +---------------+---------------+---------------+---------------+ 367 | | 368 | | 369 | | 370 | | 371 | | 372 | | 373 | | 374 | UA Signature | 375 | | 376 | | 377 | | 378 | | 379 | | 380 | | 381 | | 382 | | 383 +---------------+---------------+---------------+---------------+ 385 Figure 4: DRIP Message Set Signature 387 4.4. Specific Method 389 Under Specific Method (Authentication Type 0x5) is where the main set 390 of DRIP Authentication Formats are defined. These formats unlike the 391 previous ones are more well defined and can include Forward Error 392 Correction data. 394 4.4.1. DRIP Frame Format 396 This is specified when the SAM ID is DRIP Frame. It is encapsulated 397 by the ASTM Authentication Message (Section 3.3) and fills the 398 Authentication Data / Signature field in Figure 1. 400 0 1 2 3 401 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 1 402 +---------------+---------------+---------------+---------------+ 403 | SAM ID | | 404 +---------------+ | 405 . . 406 . DRIP Authentication Data . 407 . . 408 | | 409 +---------------+---------------+---------------+---------------+ 410 | | 411 . . 412 . Forward Error Correction . 413 . . 414 | | 415 +---------------+---------------+---------------+---------------+ 417 SAM ID (1 byte): 418 The SAM ID (Specific Authentication Method ID) is 419 defined by ASTM under AuthType 0x5 and values allocated 420 by ICAO. 422 For DRIP there are four SAM IDs allocated: 424 SAM ID | Value 425 -----------------------------+------- 426 DRIP Frame | 0 427 DRIP Wrapper | 1 428 DRIP Manifest | 2 429 DRIP Link | 3 431 DRIP Authentication Data (0 to 200 bytes): 432 DRIP Authentication data. 434 Forward Error Correction (0 to 161 bytes): 435 Forward Error Correction data. 437 Figure 5: DRIP Frame Format 439 4.4.1.1. Specific Authentication Method ID (SAM ID) 441 Defined by ASTM (only under AuthType 0x5), values are allocated by 442 ICAO. For DRIP there are four SAM IDs: DRIP Frame, DRIP Wrapper, 443 DRIP Manifest and DRIP Link. 445 4.4.1.2. DRIP Authentication Data 447 This field has a maximum size of 200 bytes. If the data is less than 448 the max and a page is only partially filled then the rest of the 449 partially filled page must be null padded. Note that the Length 450 field in the Authentication Message is set to the length of the DRIP 451 Authentication Data and MUST NOT include the Forward Error 452 Correction. 454 When possible the DRIP Broadcast Attestation Structure (Section 12) 455 should be used in this space. 457 4.4.1.3. Forward Error Correction 459 This field has a maximum size of 161 bytes and SHOULD be page aligned 460 at start. The number of pages present after the data indicate the 461 FEC scheme. When a single page of FEC is present an XOR operation 462 MUST be used. When there are multiple pages of FEC (2 or more) a 463 Reed Solomon method MUST be used. 465 See Section 13 for more. 467 4.4.2. DRIP Wrapper Format 469 This is specified when the SAM ID is DRIP Wrapper. It is 470 encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast 471 Attestation Structure (Section 12); filling the Attestation Data 472 (Section 12.2) field with full (25-byte) ASTM Messages. The minimum 473 number of ASTM Messages being 1 (Editors Note: Is this minimum 1 or 474 0?) and the max being 4. The encapsulated ASTM Messages MUST be in 475 Message Type order as defined by ASTM. All message types except 476 Authentication (0x2) and Message Pack (0xF) are allowed. 478 To determine the number of messages wrapped the receiver can check 479 that the length of the Attestation Data (Section 12.2) field of the 480 DRIP Broadcast Attestation (Section 12) is a multiple of 25-bytes. 482 4.4.2.1. Wrapper Limitations 484 TODO 486 4.4.3. DRIP Manifest Format 488 This format is specified when SAM ID is set to DRIP Manifest. It is 489 encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast 490 Attestation Structure (Section 12); filling the Attestation Data 491 (Section 12.2) field with 8-byte hashes of previous ASTM Messages. 493 By hashing previously sent messages and signing them we gain trust in 494 UAs previous reports. An observer who has been listening for any 495 considerable length of time can hash received messages and cross 496 check against listed hashes. This is a way to evade the limitation 497 of a maximum of 4 messages in the Wrapper Format and reduce overhead. 499 (Editors Note: Manifests MUST NOT be of a length multiple of 25-bytes 500 or 48-bytes.) 502 4.4.3.1. Hash Algorithms and Operation 504 The hash algorithm used for the Manifest Message is the same hash 505 algorithm used in creation of the HHIT that is signing the Manifest. 507 A standard HHIT would be using cSHAKE128 from [NIST.SP.800-185]. 508 With cSHAKE128, the hash is computed as follows: 510 cSHAKE128(Message, 128, "", "Remote ID Auth Hash") 512 4.4.3.2. Pseudo-Blockchain Hashes 514 Two special hashes are included in all Manifest messages; a previous 515 manifest hash, which links to the previous manifest message, as well 516 as a current manifest hash. This gives a pseudo-blockchain 517 provenance to the manifest message that could be traced back if the 518 observer was present for extended periods of time. 520 Creation: During creation and signing of this message format this 521 field MUST be set to 0. So the signature will be based on this 522 field being 0, as well as its own hash. It is an open question of 523 if we compute the hash, then sign or sign then compute. 525 Cycling: There a few different ways to cycle this message. We can 526 "roll up" the hash of 'current' to 'previous' when needed or to 527 completely recompute the hash. This mostly depends on the 528 previous note. 530 4.4.3.3. Manifest Limitations 532 A potential limitation to this format is dwell time of the UA. If 533 the UA is not sticking to a general area then most likely the 534 Observer will not obtain many (if not all) of the messages in the 535 manifest. Without the original messages received no verification can 536 be done. Examples of such scenarios include delivery or survey UA. 538 Another limitation is the length of hash, which is discussed in 539 Section 8. 541 4.4.4. DRIP Link Format 543 This format is specified when SAM ID is set to DRIP Link. It is 544 encapsulated by the DRIP Frame (Section 4.4.1) and Broadcast 545 Attestation Structure (Section 12) but the attestation has already 546 taken place, thus the UA need not dynamically sign the structure. 548 See Broadcast Attestation as defined in [drip-rid] and Section 11.6. 550 4.4.4.1. Link Limitations 552 TODO 554 5. Transport Methods & Recommendations 556 5.1. Legacy Advertisements (Bluetooth 4.X) 558 With Legacy Advertisements the goal is to attempt to bring reliable 559 receipt of the paged Authentication Message. Forward Error 560 Correction (Section 4.4.1.3) MUST be enabled when using Legacy 561 Advertising methods (such as Bluetooth 4.X). 563 Under ASTM Bluetooth 4.X rules, transmission of dynamic messages are 564 at least every 1 second while static messages (which is what 565 Authentication is classified under) are sent at least every 3 566 seconds. 568 Under DRIP the Certificate Message MUST be transmitted to properly 569 meet the GEN 1 and GEN 3 requirement. 571 The ASTM Message Wrapper and Manifest both satisfy the GEN 2 572 requirement. At least one MUST be implemented to comply with the GEN 573 2 requirement. 575 A single Manifest can carry at most (using the full 10 page limit and 576 8 byte hashes) 12 unique hashes of previously sent messages (of any 577 type). This results in a total of 22 (12 + 10) frames of Bluetooth 578 data being transmitted over Bluetooth. 580 In comparison the Message Wrapper sends 6 pages (each a single frame) 581 for each wrapped message. For backwards compatibility the 582 implementation should also send the standard ASTM message that was 583 wrapped for non-DRIP compliant receivers to obtain. This method 584 results in 84 total Bluetooth frames (12 + (12 * 6)) sent. 586 The question of which is better suited is up to the implementation. 588 5.2. Extended Advertisements (Bluetooth 5.X, WiFi NaN, WiFi Beacon) 590 Under the ASTM specification, Bluetooth 5 or WiFi NaN transport of 591 Remote ID is to use the Message Pack (Type 0xF) format for all 592 transmissions. Under Message Pack all messages are sent together (in 593 Message Type order) in a single Bluetooth frame (up to 9 single frame 594 equivalent messages). Message Packs are required by ASTM to be sent 595 at a rate of 1 per second (like dynamic messages). 597 Without any fragmentation or loss of pages with transmission Forward 598 Error Correction (Section 4.4.1.3) MUST NOT be used as it is 599 impractical. 601 5.3. DRIP Recommendations 603 For DRIP it is RECOMMENDED the following Authentication Formats are 604 sent: 606 1. DRIP Link using the Broadcast Attestation of HID Root and the CAA 608 2. DRIP Link using the Broadcast Attestation of CAA and the USS 610 3. DRIP Link using the Broadcast Attestation of USS and the UA 612 4. Any other DRIP Authentication Format where the UA is dynamically 613 signing data 615 6. ICAO Considerations 617 DRIP requests the following SAM IDs to be allocated: 619 1. DRIP Frame 621 2. DRIP Wrapper 623 3. DRIP Manifest 625 4. DRIP Link 627 7. IANA Considerations 629 This document does not require any actions by IANA. 631 8. Security Considerations 632 8.1. Manifest Hash Length 634 For DRIP Manifest an 8-byte hash length has been selected by the 635 authors for a number of reasons. 637 1. Hash lengths smaller than 8-bytes (for example 4-bytes) were 638 originally contemplated but ruled out by comments by various 639 cryptographers. The main concern raised in this forum was that 640 the length of hash would not provide strong resistance against 641 collision rate. The authors also after further review agreed 642 with this and also realized operationally it was not necessarily 643 viable. While 4-byte hashes would allow more messages to be 644 filled into a single DRIP Manifest payload (up to 22 individual 645 hashes) the length of time for the UA to stay in a single place 646 where the Observer would receive all the originally messages to 647 rehash to verify such a message was impractical. 649 2. Hash lengths larger than 8-bytes (for example 16-bytes) were also 650 considered by the authors. These got the approval of the 651 cryptographers but the number of hashes to send became much lower 652 (only 5 individual hashes). While this lower number is a more 653 reasonable number of original messages the Observer would have to 654 capture it would also mean that potentially more DRIP Manifests 655 would need to be sent. Overall the increase length of the hash 656 did not operationally justify the cost. 658 3. Simplifying the current design and locking it into using the same 659 hash as the HHIT instead of allowing for agility in either hash 660 algorithm or length seemed more realistic to the authors today. 662 8.2. Replay Attacks 664 The astute reader may note that the DRIP Link messages, which are 665 recommended to be sent under DRIP, are static in nature and contain 666 various timestamps. These Attestation Link message can easily be 667 replayed by an attacker who has copied them from previous broadcasts. 668 There are two things to mitigate this in DRIP: 670 1. If an attacker (who is smart and spoofs more than just the UAS 671 ID/data payloads) willing replays an Attestation Link message 672 they have in principle actually helped by ensuring the message is 673 sent more frequently and be received by potential Observers. 675 2. Under DRIP it is RECOMMENDED to send more than just DRIP Link 676 messages, specifically those that sign over changing data using 677 the current session keypair, and those messages are sent more 678 frequently. An aircraft beaconing these messages then actually 679 signing other messages using the keypair validates the data 680 receiver by an Observer. An UA who does not either run DRIP 681 themselves or does not have possession of the same private key, 682 would be clearly exposed upon signature verification. 684 8.3. Trust Timestamp Offsets 686 Note the discussion of Trust Timestamp Offsets here is in context of 687 the DRIP Wrapper (Section 4.4.2) and DRIP Manifest (Section 4.4.3) 688 messages. For DRIP Link (Section 4.4.4) messages these offsets are 689 set by the Attestor (typically a registry) and have their own set of 690 considerations as seen in (TODO: link to registry draft security 691 considerations here). 693 The offset of the Trust Timestamp (defined as a very short Expiration 694 Timestamp) is one that needs careful consideration for any 695 implementation. The offset should be shorter than any given flight 696 duration (typically less than an hour) but be long enough to be 697 received and processed by Observers (larger than a few seconds). It 698 recommended that 3-5 minutes should be sufficient to serve this 699 purpose in any scenario, but is not limited by design. 701 9. Acknowledgments 703 Ryan Quigley and James Mussi of AX Enterprize, LLC for early 704 prototyping to find holes in the draft specifications. 706 Soren Friis for pointing out that WiFi protocols would not give 707 access to the MAC Address, originally used in calculation of the 708 hashes for DRIP Manifest. Also for confirming that Message Packs 709 (0xF) can only carry up to 9 ASTM frames worth of data (9 710 Authentication pages) - this drove the requirement for max page 711 length of Authentication Data itself. 713 10. Appendix A: Thoughts on ASTM Authentication Message 715 (Editor Note: is this valid anymore to keep?) 717 The format standardized by the ASTM is designed with a few major 718 considerations in mind, which the authors of this document feel put 719 significant limitations on the expansion of the standard. 721 The primary consideration (in this context) is the use of the 722 Bluetooth 5.X Extended Frame format. This method allows for a 255 723 byte payload to be sent in what the ASTM refers to as a "Message 724 Pack". 726 The idea is to include up to five standard ASTM Broadcast RID 727 messages (each of which are 25 bytes) plus a single authentication 728 message (5 pages of 25 bytes each) in the Message Pack. The 729 reasoning is then the Authentication Message is for the entire 730 Message Pack. 732 The authors have no issues with this proposed approach; this is a 733 valid format to use for the Authentication Message provided by the 734 ASTM. However, by limiting the Authentication Message to ONLY five 735 pages in the standard it ignores the possibility of other formatting 736 options to be created and used. 738 Another issue with this format, not fully addressed in this document 739 is fragmentation. Under Bluetooth 4.X, each page is sent separately 740 which can result in lose of pages on the receiver. This is 741 disastrous as the loss of even a single page means any signature is 742 incomplete. 744 With the current limitation of 5 pages, Forward Error Correction 745 (FEC) is nearly impossible without sacrificing the amount of data 746 sent. More pages would allow FEC to be performed on the 747 Authentication Message pages so loss of pages can be mitigated. 749 All these problems are further amplified by the speed at which UA fly 750 and the Observer's position to receive transmissions. There is no 751 guarantee that the Observer will receive all the pages of even a 5 752 page Authentication Message in the time it takes a UA to traverse 753 across their line of sight. Worse still is that is not including 754 other UA in the area, which congests the spectrum and could cause 755 further confusion attempting to collate messages from various UA. 756 This specific problem is out of scope for this document and our 757 solutions in general, but should be noted as a design consideration. 759 11. Appendix B: DRIP Attestations 761 11.1. Self-Attestation (Axx) 762 0 1 2 3 763 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 1 764 +---------------+---------------+---------------+---------------+ 765 | | 766 | Hierarchical | 767 | Host Identity Tag | 768 | | 769 +---------------+---------------+---------------+---------------+ 770 | | 771 | | 772 | | 773 | Host Identity | 774 | | 775 | | 776 | | 777 | | 778 +---------------+---------------+---------------+---------------+ 779 | Trust Timestamp | 780 +---------------+---------------+---------------+---------------+ 781 | Signing Timestamp | 782 +---------------+---------------+---------------+---------------+ 783 | | 784 | | 785 | | 786 | | 787 | | 788 | | 789 | | 790 | Signature | 791 | | 792 | | 793 | | 794 | | 795 | | 796 | | 797 | | 798 | | 799 +---------------+---------------+---------------+---------------+ 801 Length = 120-bytes 803 Figure 6: DRIP Self-Attestation 805 11.2. Attestation (Axy) 806 0 1 2 3 807 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 1 808 +---------------+---------------+---------------+---------------+ 809 | | 810 . . 811 . Axx . 812 . . 813 | | 814 +---------------+---------------+---------------+---------------+ 815 | | 816 . . 817 . Ayy . 818 . . 819 | | 820 +---------------+---------------+---------------+---------------+ 821 | Trust Timestamp by X | 822 +---------------+---------------+---------------+---------------+ 823 | Signing Timestamp by X | 824 +---------------+---------------+---------------+---------------+ 825 | | 826 | | 827 | | 828 | | 829 | | 830 | | 831 | | 832 | Signature by X | 833 | | 834 | | 835 | | 836 | | 837 | | 838 | | 839 | | 840 | | 841 +---------------+---------------+---------------+---------------+ 843 Length = 312-bytes 845 Figure 7: DRIP Attestation 847 11.3. Concise Attestation (C-Axy) 848 0 1 2 3 849 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 1 850 +---------------+---------------+---------------+---------------+ 851 | | 852 | Hierarchical | 853 | Host Identity Tag of X | 854 | | 855 +---------------+---------------+---------------+---------------+ 856 | | 857 | Hierarchical | 858 | Host Identity Tag of Y | 859 | | 860 +---------------+---------------+---------------+---------------+ 861 | Trust Timestamp by X | 862 +---------------+---------------+---------------+---------------+ 863 | Signing Timestamp by X | 864 +---------------+---------------+---------------+---------------+ 865 | | 866 | | 867 | | 868 | | 869 | | 870 | | 871 | | 872 | Signature by X | 873 | | 874 | | 875 | | 876 | | 877 | | 878 | | 879 | | 880 | | 881 +---------------+---------------+---------------+---------------+ 883 Length = 104-bytes 885 Figure 8: DRIP Concise Attestation 887 11.4. Mutual Attestation (M-Axy) 888 0 1 2 3 889 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 1 890 +---------------+---------------+---------------+---------------+ 891 | | 892 . . 893 . Axy . 894 . . 895 | | 896 +---------------+---------------+---------------+---------------+ 897 | Trust Timestamp by Y | 898 +---------------+---------------+---------------+---------------+ 899 | Signing Timestamp by Y | 900 +---------------+---------------+---------------+---------------+ 901 | | 902 | | 903 | | 904 | | 905 | | 906 | | 907 | | 908 | Signature by Y | 909 | | 910 | | 911 | | 912 | | 913 | | 914 | | 915 | | 916 | | 917 +---------------+---------------+---------------+---------------+ 919 Length = 384-bytes 921 Figure 9: DRIP Mutual Attestation 923 11.5. Link Attestation (L-Axy) 924 0 1 2 3 925 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 1 926 +---------------+---------------+---------------+---------------+ 927 | | 928 . . 929 . C-Axy . 930 . . 931 | | 932 +---------------+---------------+---------------+---------------+ 933 | Trust Timestamp by Y | 934 +---------------+---------------+---------------+---------------+ 935 | Signing Timestamp by Y | 936 +---------------+---------------+---------------+---------------+ 937 | | 938 | | 939 | | 940 | | 941 | | 942 | | 943 | | 944 | Signature by Y | 945 | | 946 | | 947 | | 948 | | 949 | | 950 | | 951 | | 952 | | 953 +---------------+---------------+---------------+---------------+ 955 Length = 176-bytes 957 Figure 10: DRIP Link Attestation 959 11.6. Broadcast Attestation (B-Axy) 960 0 1 2 3 961 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 1 962 +---------------+---------------+---------------+---------------+ 963 | | 964 | Hierarchical | 965 | Host Identity Tag of X | 966 | | 967 +---------------+---------------+---------------+---------------+ 968 | | 969 | Hierarchical | 970 | Host Identity Tag of Y | 971 | | 972 +---------------+---------------+---------------+---------------+ 973 | | 974 | | 975 | | 976 | Host Identity of Y | 977 | | 978 | | 979 | | 980 | | 981 +---------------+---------------+---------------+---------------+ 982 | Trust Timestamp by X | 983 +---------------+---------------+---------------+---------------+ 984 | Signing Timestamp by X | 985 +---------------+---------------+---------------+---------------+ 986 | | 987 | | 988 | | 989 | | 990 | | 991 | | 992 | | 993 | Signature by X | 994 | | 995 | | 996 | | 997 | | 998 | | 999 | | 1000 | | 1001 | | 1002 +---------------+---------------+---------------+---------------+ 1004 Length = 136-bytes 1006 Figure 11: DRIP Broadcast Attestation 1008 11.7. Link Certificate (L-Cxy) 1010 0 1 2 3 1011 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 1012 +---------------+---------------+---------------+---------------+ 1013 | | 1014 | Hierarchical | 1015 | Host Identity Tag of Z | 1016 | | 1017 +---------------+---------------+---------------+---------------+ 1018 | | 1019 . . 1020 . L-Axy . 1021 . . 1022 | | 1023 +---------------+---------------+---------------+---------------+ 1024 | Trust Timestamp by Z | 1025 +---------------+---------------+---------------+---------------+ 1026 | Signing Timestamp by Z | 1027 +---------------+---------------+---------------+---------------+ 1028 | | 1029 | | 1030 | | 1031 | | 1032 | | 1033 | | 1034 | | 1035 | Signature by Z | 1036 | | 1037 | | 1038 | | 1039 | | 1040 | | 1041 | | 1042 | | 1043 | | 1044 +---------------+---------------+---------------+---------------+ 1046 Length = 264-bytes 1048 Figure 12: DRIP Link Certificate 1050 11.8. Mutual Certificate (M-Cxy) 1051 0 1 2 3 1052 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 1053 +---------------+---------------+---------------+---------------+ 1054 | | 1055 . . 1056 . Azz . 1057 . . 1058 | | 1059 +---------------+---------------+---------------+---------------+ 1060 | | 1061 . . 1062 . M-Axy . 1063 . . 1064 | | 1065 +---------------+---------------+---------------+---------------+ 1066 | Trust Timestamp by Z | 1067 +---------------+---------------+---------------+---------------+ 1068 | Signing Timestamp by Z | 1069 +---------------+---------------+---------------+---------------+ 1070 | | 1071 | | 1072 | | 1073 | | 1074 | | 1075 | | 1076 | | 1077 | Signature by Z | 1078 | | 1079 | | 1080 | | 1081 | | 1082 | | 1083 | | 1084 | | 1085 | | 1086 +---------------+---------------+---------------+---------------+ 1088 Length = 264-bytes 1090 Figure 13: DRIP Mutual Certificate 1092 11.9. Example Registration with Attestation 1094 1. X generates Axx and Y generates Ayy 1096 2. Y sends Ayy to X 1097 3. X verified Ayy; composes Axy, C-Axy, B-Axy; sends Axy, C-Axy, 1098 B-Axy and B-Axy's from parents 1100 4. Y composes M-Axy and L-Axy 1102 5. Y broadcasts B-Axy's 1104 12. Appendix C: DRIP Broadcast Attestation Structure 1106 When possible the following format should be used in the DRIP 1107 Authentication Data (Section 4.4.1.2) field. 1109 0 1 2 3 1110 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 1111 +---------------+---------------+---------------+---------------+ 1112 | | 1113 | Attestor Hierarchical | 1114 | Host Identity Tag | 1115 | | 1116 +---------------+---------------+---------------+---------------+ 1117 | | 1118 . . 1119 . Attestation Data . 1120 . . 1121 | | 1122 +---------------+---------------+---------------+---------------+ 1123 | Trust Timestamp | 1124 +---------------+---------------+---------------+---------------+ 1125 | Signing Timestamp | 1126 +---------------+---------------+---------------+---------------+ 1127 | | 1128 | | 1129 | | 1130 | | 1131 | | 1132 | | 1133 | | 1134 | Attestor Signature | 1135 | | 1136 | | 1137 | | 1138 | | 1139 | | 1140 | | 1141 | | 1142 | | 1143 +---------------+---------------+---------------+---------------+ 1144 Attestor Hierarchial Host Identity Tag (16 bytes): 1145 The Attestors HHIT in byte form (network byte order). 1147 Attestation Data (0 to 112 bytes): 1148 Opaque attestation data. 1150 Trust Timestamp (4 bytes): 1151 Timestamp denoting recommended time to trust data to. 1153 Signing Timestamp (4 bytes): 1154 Current time at signing. 1156 Attestor Signature (64 bytes): 1157 Signature over preceding fields using the keypair of 1158 the Attestor. 1160 Figure 14: DRIP Broadcast Attestation Data Structure 1162 12.1. Attestor Hierarchical Host Identity Tag 1164 The HHIT is an enhancement of the Host Identity Tag (HIT) [RFC7401] 1165 introducing hierarchy and how they are used in UAS RID as defined in 1166 [drip-rid]. 1168 12.2. Attestation Data 1170 This field has a maximum of 112 bytes in length. It is nominally 1171 filled with data as defined by the SAM ID being set or other sub- 1172 multiplexer in the authentication payload. 1174 12.3. Trust Timestamp 1176 The Trust Timestamp is of the format defined in [F3411-19]. That is 1177 a UNIX timestamp offset by 01/01/2019 00:00:00. An additional offset 1178 is then added to push the timestamp a short time into the future to 1179 avoid replay attacks. 1181 The offset used against the UNIX timestamp is not defined in this 1182 document. Best practices to identify a acceptable offset should be 1183 used taking into consideration the UA environment, and propagation 1184 characteristics of the messages being sent. 1186 12.4. Signing Timestamp 1188 Follows the format defined in [F3411-19]. That is a UNIX timestamp 1189 offset by 01/01/2019 00:00:00. 1191 12.5. Attestor Signature 1193 The signature is generated over all the preceding data. ASTM/DRIP 1194 Headers are exclude from this operation only information within the 1195 Broadcast Attestation Structure (Section 12) is signed. 1197 13. Appendix D: Forward Error Correction 1199 (Editors Note: move specifics of FEC (everything below) into its own 1200 draft for titled Integrity Protection) 1202 Remote ID data can be sent across many different broadcast link 1203 media, all with different characteristics. To enable robustness in 1204 Remote ID transmission media that has Forward Error Correction 1205 capability SHOULD be used. 1207 In cases where FEC is not available below the equivalent of the 1208 transport layer (known as Legacy Advertisements) DRIP Authentication 1209 REQUIRES that an application level FEC scheme is used. In cases 1210 where FEC is available below the equivalent of the transport layer 1211 (known as Extended Advertisements) DRIP MUST NOT use any application 1212 level FEC and instead SHALL rely on the lower layers FEC 1213 functionality. 1215 For current Remote ID the media options are the following: 1217 Legacy Advertisements: Bluetooth 4.X 1219 Extended Advertisements: WiFi NAN, WiFi Beacon, Bluetooth 5.X 1221 (Editors Note: add in self-protecting and more-than-self-protecting 1222 options, with their justifications) 1224 13.1. Encoding 1226 13.1.1. Single Page FEC 1228 When generating the parity the first byte of every Authentication 1229 Page MUST be exclude from the XOR operation. For pages 1 through N 1230 this leaves the data portion of the page while page 0 will include a 1231 number of headers along with 17 bytes of data. 1233 To generate the parity a simple XOR operation using the previous and 1234 current page is used. For page 0, a 23 byte null pad is used for the 1235 previous page. The resulting 23 bytes of parity is appended in one 1236 full page (always the last) allowing for recovery when any single 1237 page is lost in transmission. 1239 13.1.2. Multi Page FEC 1241 TODO (Reed Solomon) 1243 13.2. Decoding 1245 Due to the nature of Bluetooth 4 and the existing ASTM paging 1246 structure an optimization can be used. If a Bluetooth frame fails 1247 its CRC check, then the frame is dropped without notification to the 1248 upper protocol layers. From the Remote ID perspective this means the 1249 loss of a complete frame/message/page. In Authentication Messages, 1250 each page is already numbered so the loss of a page allows the 1251 receiving application to build a "dummy" page filling the 1252 Authentication Data field (and ASTM Authentication Headers fields if 1253 page 0) with nulls. 1255 If page 0 is being reconstructed an additional check of the Page 1256 Count, to check against how many pages are actually present, MUST be 1257 performed for sanity. An additional check on the Data Length field 1258 SHOULD also be performed. 1260 13.2.1. Single Page FEC 1262 Using the same methods as encoding, an XOR operation is used between 1263 is the current page). The resulting 23 bytes is the data of the 1264 missing page. 1266 13.2.2. Multi Page FEC 1268 TODO (Reed Solomon) 1270 13.3. FEC Limitations 1272 If more than one page is lost (>1/5 for 5 page messages, >1/10 for 10 1273 page messages) than the error rate of the link is already beyond 1274 saving and the application has more issues to deal with. 1276 (Editors Note: Is this valid anymore, for XOR yes but for multi-page 1277 FEC?) 1279 14. References 1281 14.1. Normative References 1283 [F3411-19] "Standard Specification for Remote ID and Tracking", 1284 February 2020. 1286 [NIST.SP.800-185] 1287 Kelsey, J., Change, S., and R. Perlner, "SHA-3 Derived 1288 Functions: cSHAKE, KMAC, TupleHash and ParallelHash", NIST 1289 Special Publication SP 800-185, 1290 DOI 10.6028/nist.sp.800-185, December 2016, 1291 . 1294 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1295 Requirement Levels", BCP 14, RFC 2119, 1296 DOI 10.17487/RFC2119, March 1997, 1297 . 1299 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1300 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1301 May 2017, . 1303 14.2. Informative References 1305 [drip-requirements] 1306 Card, S. W., Wiethuechter, A., Moskowitz, R., and A. 1307 Gurtov, "Drone Remote Identification Protocol (DRIP) 1308 Requirements", Work in Progress, Internet-Draft, draft- 1309 ietf-drip-reqs-13, 14 June 2021, 1310 . 1313 [drip-rid] Moskowitz, R., Card, S. W., Wiethuechter, A., and A. 1314 Gurtov, "UAS Remote ID", Work in Progress, Internet-Draft, 1315 draft-ietf-drip-uas-rid-01, 9 September 2020, 1316 . 1319 [identity-claims] 1320 Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP 1321 Identity Claims", Work in Progress, Internet-Draft, draft- 1322 wiethuechter-drip-identity-claims-03, 2 November 2020, 1323 . 1326 [RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T. 1327 Henderson, "Host Identity Protocol Version 2 (HIPv2)", 1328 RFC 7401, DOI 10.17487/RFC7401, April 2015, 1329 . 1331 Authors' Addresses 1332 Adam Wiethuechter 1333 AX Enterprize, LLC 1334 4947 Commercial Drive 1335 Yorkville, NY 13495 1336 United States of America 1338 Email: adam.wiethuechter@axenterprize.com 1340 Stuart Card 1341 AX Enterprize, LLC 1342 4947 Commercial Drive 1343 Yorkville, NY 13495 1344 United States of America 1346 Email: stu.card@axenterprize.com 1348 Robert Moskowitz 1349 HTT Consulting 1350 Oak Park, MI 48237 1351 United States of America 1353 Email: rgm@labs.htt-consult.com