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This is achieved by encrypting, in place, those 17 fields containing Operator sensitive data using a hybrid ECIES. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at https://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on 21 October 2021. 36 Copyright Notice 38 Copyright (c) 2021 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 43 license-info) in effect on the date of publication of this document. 44 Please review these documents carefully, as they describe your rights 45 and restrictions with respect to this document. Code Components 46 extracted from this document must include Simplified BSD License text 47 as described in Section 4.e of the Trust Legal Provisions and are 48 provided without warranty as described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 3 54 2.1. Requirements Terminology . . . . . . . . . . . . . . . . 3 55 2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3 56 3. The Operator - USS Security Relationship . . . . . . . . . . 4 57 3.1. ECIES Shared Secret Generation . . . . . . . . . . . . . 4 58 4. System Message Privacy . . . . . . . . . . . . . . . . . . . 5 59 4.1. Rules for encrypting System Message content . . . . . . . 5 60 4.2. Rules for decrypting System Message content . . . . . . . 6 61 5. Operator ID Message Privacy . . . . . . . . . . . . . . . . . 6 62 5.1. Rules for encrypting Operator ID Message content . . . . 6 63 5.2. Rules for decrypting Operator ID Message content . . . . 7 64 6. Cipher choices for Operator PII encryption . . . . . . . . . 7 65 6.1. Using AES-CFB32 . . . . . . . . . . . . . . . . . . . . . 7 66 6.2. Using a Feistel scheme . . . . . . . . . . . . . . . . . 8 67 6.3. Using AES-CTR . . . . . . . . . . . . . . . . . . . . . . 8 68 7. DRIP Requirements addressed . . . . . . . . . . . . . . . . . 8 69 8. ASTM Considerations . . . . . . . . . . . . . . . . . . . . . 8 70 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 71 10. Security Considerations . . . . . . . . . . . . . . . . . . . 9 72 10.1. CFB32 Risks . . . . . . . . . . . . . . . . . . . . . . 9 73 10.2. Crypto Agility . . . . . . . . . . . . . . . . . . . . . 9 74 10.3. Key Derivation vulnerabilities . . . . . . . . . . . . . 9 75 10.4. KMAC Security as a KDF . . . . . . . . . . . . . . . . . 10 76 11. Normative References . . . . . . . . . . . . . . . . . . . . 10 77 12. Informative References . . . . . . . . . . . . . . . . . . . 11 78 Appendix A. Feistel Scheme . . . . . . . . . . . . . . . . . . . 11 79 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12 80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 82 1. Introduction 84 This document defines a mechanism to provide privacy in the ASTM 85 Remote ID and Tracking messages [F3411-19] by encrypting, in place, 86 those fields that contain sensitive UAS Operator/Pilot information. 87 Encrypting in place means that the ciphertext is exactly the same 88 length as the cleartext, and directly replaces it. 90 An example of and an initial application of this mechanism is the 8 91 bytes of UAS Operator/Pilot (hereafter called simply Operator) 92 longitude and latitude location in the ASTM System Message (Msg Type 93 0x4). This meets the Drip Requirements [drip-requirements], Priv-01. 95 It is assumed that the Operator, via the UAS, registers an operation 96 with its USS. During this operation registration, the UAS and USS 97 exchange public keys to use in the hybrid ECIES. The USS key may be 98 long lived, but the UAS key SHOULD be unique to a specific operation. 99 This provides protection if the ECIES secret is exposed from prior 100 operations. 102 The actual Tracking message field encryption MUST be an "encrypt in 103 place" cipher. There is rarely any room in the tracking messages for 104 a cipher IV or encryption MAC (AEAD tag). There is rarely any data 105 in the messages that can be used as an IV. The AES-CFB32 mode of 106 operation proposed here can encrypt a multiple of 4 bytes. 108 The System Message is not a simple, one-time, encrypt the PII with 109 the ECIES derived key. The Operator may move during a operation and 110 these fields change, correspondingly. Further, not all messages will 111 be received by the USS, so each message's encryption must stand on 112 its own and not be at risk of attack by the content of other 113 messages. 115 Another candidate message is the optional ASTM Operator ID Message 116 (Msg Type 0x5) with its 20 character Operator ID field. The Operator 117 ID does not change during an operation, so this is a one-time 118 encryption operation for the operation. The same cipher SHOULD be 119 used for all messages from the UAS and this will influence the cipher 120 selection. 122 Future applications of this mechanism may be provided. The content 123 of the System Message may change to meet CAA requirements, requiring 124 encrypting a different amount of data. At that time, they will be 125 added to this document. 127 Editor note: The Rules for allowing encryption need to be updated to 128 handle the UA operating in Broadcast Remote ID only mode. That is 129 conditions where the USS cannot notify the UAS to stop encrypting. 131 2. Terms and Definitions 133 2.1. Requirements Terminology 135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 137 "OPTIONAL" in this document are to be interpreted as described in BCP 138 14 [RFC2119] [RFC8174] when, and only when, they appear in all 139 capitals, as shown here. 141 2.2. Definitions 143 See Drip Requirements [drip-requirements] for common DRIP terms. 145 ECIES 146 Elliptic Curve Integrated Encryption Scheme. A hybrid encryption 147 scheme which provides semantic security against an adversary who 148 is allowed to use chosen-plaintext and chosen-ciphertext attacks. 150 Keccak (KECCAK Message Authentication Code): 151 The family of all sponge functions with a KECCAK-f permutation as 152 the underlying function and multi-rate padding as the padding 153 rule. 155 KMAC (KECCAK Message Authentication Code): 156 A PRF and keyed hash function based on KECCAK. 158 3. The Operator - USS Security Relationship 160 All CAAs have rules defining which UAS must be registered to operate 161 in their National Airspace. This includes UAS and Operator 162 registration in a USS. Further, operator's are expected to report 163 flight operations to their USS. This operation reporting provides a 164 mechanism for the USS and operator to establish an operation security 165 context. Here it will be used to exchange public keys for use in 166 ECIES. 168 The operator's ECIES public key SHOULD be unique for each operation. 169 The USS ECIES public key may be unique for each operator and 170 operation, but not required. For best post-compromise security 171 (PCS), the USS ECIES public key should be changed over some 172 operational window. 174 The public key algorithm should be Curve25519 [RFC7748]. 175 Correspondingly, the ECIES 128 bit shared secret should be generated 176 using KMAC [NIST.SP.800-185]. 178 3.1. ECIES Shared Secret Generation 180 The KMAC function provides a new, more efficient, key derivation 181 function over HKDF [RFC5869]. This will be referred to as KKDF. 183 HKDF needs a minimum of 4 hash functions (e.g. SHA256). KKDF does 184 an equivalent shared secret generation in a single Keccak Sponge 185 operation. 187 When the USS - UAS Operation Security Context is established, the UAS 188 provides a 20 Character USS ID and a 256 bit random nonce to the USS. 189 These are inputs, along with the ECDH keys to produce the shared 190 secret as follows. 192 Per [NIST.SP.800-56Cr1], Section 4.1, Option 3: 194 Shared Secret = KMAC128(salt, IKM, L, S) 196 L is the derived key bit length. Since only a single key is needed, 197 L=128. 199 S is the byte string 01001011 || 01000100 || 01000110, which 200 represents the sequence of characters "K", "D", and "F" in 8-bit 201 ASCII. 203 salt = Nonce-USS | Nonce-UAS 205 There are special security considerations for IKM per [RFC7748]. The 206 IKM as follows: 208 IKM = Diffie-Hellman secret | USS-ID | RID 210 4. System Message Privacy 212 The System Message contains 8 bytes of Operator specific information: 213 Longitude and Latitude of the Remote Operator (Pilot in the field 214 description) of the UA. The GCS MAY encrypt these as follows. 216 The 8 bytes of Operator information are encrypted, using the ECIES 217 derived 128 bit shared secret, with one of the cipher's specified 218 below. The choice of cipher is based on USS policy and is agreed to 219 as part of the operation registration. AES-CFB32 is the recommended 220 default cipher. 222 ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to 223 allow Bit 2 of the Flags byte in the System Message set to "1" to 224 indicate the Operator information is encrypted. 226 The USS similarly decrypts these 8 bytes and provides the information 227 to authorized entities. 229 4.1. Rules for encrypting System Message content 231 If the Operator location is encrypted the encrypted bit flag MUST be 232 set to 1. 234 The Operator MAY be notified by the USS that the operation has 235 entered a location or time where privacy of Operator location is not 236 allowed. In this case the Operator MUST disable this privacy feature 237 and send the location unencrypted or land the UA or route around the 238 restricted area. 240 If the UAS looses connectivity to the USS, the privacy feature SHOULD 241 be disabled or land the UA. 243 If the operation is in an area or time with no Internet Connectivity, 244 the privacy feature MUST NOT be used. 246 4.2. Rules for decrypting System Message content 248 An Observer receives a System Message with the encrypt bit set to 1. 249 The Observer sends a query to its USS Display Provider containing the 250 UA's ID and the encrypted fields. 252 The USS Display Provider MAY deny the request if the Observer does 253 not have the proper authorization. 255 The USS Display Provider MAY reply to the request with the decrypted 256 fields if the Observer has the proper authorization. 258 The USS Display Provider MAY reply to the request with the decrypting 259 key if the Observer has the proper authorization. 261 The Observer MAY notify the USS through its USS Display Provider that 262 content privacy for a UAS in this location/time is not allowed. If 263 the Observer has the proper authorization for this action, the USS 264 notifies the Operator to disable this privacy feature. 266 5. Operator ID Message Privacy 268 The Operator ID Message contains 20 bytes for Operator the ID. The 269 GCS MAY encrypt these as follows. 271 The 20 bytes Operator ID is encrypted, using the ECIES derived 128 272 bit shared secret, with one of the cipher's specified below. The 273 choice of cipher is based on USS policy and is agreed to as part of 274 the operation registration. AES-CFB32 is the recommended default 275 cipher. 277 ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to 278 allow Operator ID Type in the Operator ID Message set to "1" to 279 indicate the Operator ID is encrypted. 281 The USS similarly decrypts these 20 bytes and provides the 282 information to authorized entities. 284 5.1. Rules for encrypting Operator ID Message content 286 If the Operator ID is encrypted the Operator ID Type field MUST be 287 set to 1. 289 The Operator MAY be notified by the USS that the operation has 290 entered a location or time where privacy of Operator ID is not 291 allowed. In this case the Operator MUST disable this privacy feature 292 and send the ID unencrypted or land the UA or route around the 293 restricted area. 295 If the UAS looses connectivity to the USS, the privacy feature SHOULD 296 be disabled or land the UA. 298 If the operation is in an area or time with no Internet Connectivity, 299 the privacy feature MUST NOT be used. 301 5.2. Rules for decrypting Operator ID Message content 303 An Observer receives a Operator ID Message with the Operator ID Type 304 field set to 1. The Observer sends a query to its USS Display 305 Provider containing the UA's ID and the encrypted fields. 307 The USS Display Provider MAY deny the request if the Observer does 308 not have the proper authorization. 310 The USS Display Provider MAY reply to the request with the decrypted 311 fields if the Observer has the proper authorization. 313 The USS Display Provider MAY reply to the request with the decrypting 314 key if the Observer has the proper authorization. 316 The Observer MAY notify the USS through its USS Display Provider that 317 content privacy for a UAS in this location/time is not allowed. If 318 the Observer has the proper authorization for this action, the USS 319 notifies the Operator to disable this privacy feature. 321 6. Cipher choices for Operator PII encryption 323 6.1. Using AES-CFB32 325 CFB32 is defined in [NIST.SP.800-38A], Section 6.3. This is the 326 Cipher Feedback (CFB) mode operating on 32 bits at a time. This 327 variant of CFB can be used to encrypt any multiple of 4 bytes of 328 cleartext. 330 The Operator includes a 64 bit UNIX timestamp for the operation time, 331 along with its operation pubic key. The Operator also includes the 332 UA MAC address (or multiple addresses if flying multiple UA). 334 The 128 bit IV for AES-CFB32 is constructed by the Operator and USS 335 as: SHAKE128(MAC|UTCTime|Message_Type, 128). Inclusion of the ASTM 336 Message_Type ensures a unique IV for each Message type that contains 337 PII to encrypt. 339 AES-CFB32 would then be used to encrypt the Operator information. 341 6.2. Using a Feistel scheme 343 If the encryption speed doesn't matter, we can use the following 344 approach based on the Feistel scheme. This approach is already being 345 used in format-preserving encryption (e.g. credit card numbers). The 346 Feistal scheme is explained in Appendix A. 348 6.3. Using AES-CTR 350 If 2 bytes of the Message can be set aside to contain a counter that 351 is incremented each time the Operator information changes, AES-CTR 352 can be used as follows. 354 The Operator includes a 64 bit UNIX timestamp for the operation time, 355 along with its operation pubic key. The Operator also includes the 356 UA MAC address (or multiple addresses if flying multiple UA). 358 The high order bits of an AES-CTR counter is constructed by the 359 Operator and USS as: SHAKE128(MAC|UTCTime|Message_Type, 112). 360 Inclusion of the ASTM Message_Type ensures a unique IV for each 361 Message type that contains PII to encrypt. 363 AES-CTR would then be used to encrypt the Operator information. 365 7. DRIP Requirements addressed 367 This document provides solution to PRIV-1 for PII in the ASTM System 368 Message. 370 8. ASTM Considerations 372 ASTM will need to make the following changes to the "Flags" in the 373 System Message (Msg Type 0x4): 375 Bit 2: 376 Value 1 for encrypted; 0 for cleartext (see Section 4). 378 ASTM will need to make the following changes to the "Operator ID 379 Type" in the Operator ID Message (Msg Type 0x5): 381 Operator ID Type 382 Value 1 for encrypted Operator ID (see Section 5). 384 9. IANA Considerations 386 TBD 388 10. Security Considerations 390 An attacker has no known text after decrypting to determine a 391 successful attack. An attacker can make assumptions about the high 392 order byte values for Operator Longitude and Latitude that may 393 substitute for known cleartext. There is no knowledge of where the 394 operator is in relation to the UA. Only if changing location values 395 "make sense" might an attacker assume to have revealed the operator's 396 location. 398 10.1. CFB32 Risks 400 Using the same IV for different Operator information values with 401 CFB32 presents a cyptoanalysis risk. Typically only the low order 402 bits would change as the Operators position changes. The risk is 403 mitigated due to the short-term value of the data. Further analysis 404 is need to properly place risk. 406 10.2. Crypto Agility 408 The ASTM Remote ID Messages do not provide any space for a crypto 409 suite indicator or any other method to manage crypto agility. 411 All crypto agility is left to the USS policy and the relation between 412 the USS and operator/UAS. The selection of the ECIES public key 413 algorithm, the shared secret key derivation function, and the actual 414 symmetric cipher used for on the System Message are set by the USS 415 which informs the operator what to do. 417 10.3. Key Derivation vulnerabilities 419 [RFC7748] warns about using Curve25519 and Curve448 in Diffie-Hellman 420 for key derivation: 422 Designers using these curves should be aware that for each public 423 key, there are several publicly computable public keys that are 424 equivalent to it, i.e., they produce the same shared secrets. Thus 425 using a public key as an identifier and knowledge of a shared secret 426 as proof of ownership (without including the public keys in the key 427 derivation) might lead to subtle vulnerabilities. 429 This applies here, but may have broader consequences. Thus two 430 endpoint IDs are included with the Diffie-Hellman secret. 432 10.4. KMAC Security as a KDF 434 Section 4.1 of NIST SP 800-185 [NIST.SP.800-185] states: 436 "The KECCAK Message Authentication Code (KMAC) algorithm is a PRF and 437 keyed hash function based on KECCAK . It provides variable-length 438 output" 440 That is, the output of KMAC is indistinguishable from a random 441 string, regardless of the length of the output. As such, the output 442 of KMAC can be divided into multiple substrings, each with the 443 strength of the function (KMAC128 or KMAC256) and provided that a 444 long enough key is used, as discussed in Sec. 8.4.1 of SP 800-185. 446 For example KMAC128(K, X, 512, S), where K is at least 128 bits, can 447 produce 4 128 bit keys each with a strength of 128 bits. That is a 448 single sponge operation is replacing perhaps 5 HMAC-SHA256 operations 449 (each 2 SHA256 operations) in HKDF. 451 11. Normative References 453 [NIST.SP.800-185] 454 Kelsey, J., Change, S., and R. Perlner, "SHA-3 derived 455 functions: cSHAKE, KMAC, TupleHash and ParallelHash", 456 National Institute of Standards and Technology report, 457 DOI 10.6028/nist.sp.800-185, December 2016, 458 . 460 [NIST.SP.800-38A] 461 Dworkin, M., "Recommendation for block cipher modes of 462 operation :: methods and techniques", National Institute 463 of Standards and Technology report, 464 DOI 10.6028/nist.sp.800-38a, 2001, 465 . 467 [NIST.SP.800-56Cr1] 468 Barker, E., Chen, L., and R. Davis, "Recommendation for 469 key-derivation methods in key-establishment schemes", 470 National Institute of Standards and Technology report, 471 DOI 10.6028/nist.sp.800-56cr1, April 2018, 472 . 474 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 475 Requirement Levels", BCP 14, RFC 2119, 476 DOI 10.17487/RFC2119, March 1997, 477 . 479 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 480 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 481 May 2017, . 483 12. Informative References 485 [drip-requirements] 486 Card, S. W., Wiethuechter, A., Moskowitz, R., and A. 487 Gurtov, "Drone Remote Identification Protocol (DRIP) 488 Requirements", Work in Progress, Internet-Draft, draft- 489 ietf-drip-reqs-09, 17 February 2021, 490 . 492 [F3411-19] ASTM International, "Standard Specification for Remote ID 493 and Tracking", February 2020, 494 . 496 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand 497 Key Derivation Function (HKDF)", RFC 5869, 498 DOI 10.17487/RFC5869, May 2010, 499 . 501 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 502 for Security", RFC 7748, DOI 10.17487/RFC7748, January 503 2016, . 505 Appendix A. Feistel Scheme 507 This approach is already being used in format-preserving encryption. 509 According to the theory, to provide CCA security guarantees (CCA = 510 Chosen Ciphertext Attacks) for m-bit encryption X |-> Y, we should 511 choose d >= 6. It seems very ineffective that when shortening the 512 block length, we have to use 6 times more block encryptions. On the 513 other hand, we preserve both the block cipher interface and security 514 guarantees in a simple way. 516 How to encrypt an m-bit plaintext X using an n-bit block cipher 517 E = {E_K} for n > m? 519 Enc(X, K): 520 1. Y <- X. 521 2. Split Y into 2 equal parts: Y = Y1 || Y2 522 (let us assume for simplicity that m is even). 523 3. For i = 1, 2, ..., d do: 524 Y <- Y2 || (Y1 ^ first_m/2_bits(E_K(Y2 || Ci)), 525 where Ci is a (n - m/2)-bit round constant. 526 4. Y <- Y2 || Y1. 527 5. Return Y. 529 Dec(Y, K): 530 1. X <- Y. 531 2. Split X into 2 equal parts: X = X1 || X2. 532 3. For i = d, ..., 2, 1 do: 533 X <- X2 || (X1 ^ first_m/2_bits(E_K(X2 || Ci)). 534 4. X <- X2 || X1. 535 5. Return X. 537 Acknowledgments 539 The recommended ciphers come from discussions on the IRTF CFRG 540 mailing list. 542 Authors' Addresses 544 Robert Moskowitz 545 HTT Consulting 546 Oak Park, MI 48237 547 United States of America 549 Email: rgm@labs.htt-consult.com 551 Stuart W. Card 552 AX Enterprize 553 4947 Commercial Drive 554 Yorkville, NY 13495 555 United States of America 557 Email: stu.card@axenterprize.com 559 Adam Wiethuechter 560 AX Enterprize 561 4947 Commercial Drive 562 Yorkville, NY 13495 563 United States of America 565 Email: adam.wiethuechter@axenterprize.com