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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) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DRIP R. Moskowitz 3 Internet-Draft HTT Consulting 4 Intended status: Standards Track S. Card 5 Expires: 9 April 2022 A. Wiethuechter 6 AX Enterprize 7 6 October 2021 9 UAS Operator Privacy for RemoteID Messages 10 draft-moskowitz-drip-operator-privacy-08 12 Abstract 14 This document describes a method of providing privacy for UAS 15 Operator/Pilot information specified in the ASTM UAS Remote ID and 16 Tracking messages. 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 9 April 2022. 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 . . . . . . . 6 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 . . . . 7 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 . . . . . . . . . . . . . . . . . . . . . 8 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 . . . . . . . . . . . . . . . . . . . . . 9 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 . . . . . . . . . . . . . 10 75 10.4. KMAC Security as a KDF . . . . . . . . . . . . . . . . . 10 76 11. Normative References . . . . . . . . . . . . . . . . . . . . 10 77 12. Informative References . . . . . . . . . . . . . . . . . . . 11 78 Appendix A. Feistel Scheme . . . . . . . . . . . . . . . . . . . 12 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 its UAS ID (null padded to 20 characters per [F3411-19]) and 189 a 256 bit random nonce to the USS. These are inputs, along with the 190 ECDH keys to produce the shared secret as follows. 192 A 64 bit UNIX timestamp for the operation time is also included in 193 the Operation Security Context. This will be used in the IV 194 construction. 196 Per [NIST.SP.800-56Cr1], Section 4.1, Option 3: 198 Shared Secret = KMAC128(salt, IKM, L, S) 200 L is the derived key bit length. Since only a single key is needed, 201 L=128. 203 S is the byte string 01001011 || 01000100 || 01000110, which 204 represents the sequence of characters "K", "D", and "F" in 8-bit 205 ASCII. 207 salt = Nonce-USS | Nonce-UAS 209 There are special security considerations for IKM per [RFC7748]. The 210 IKM as follows: 212 IKM = Diffie-Hellman secret | USS-ID | RID 214 4. System Message Privacy 216 The System Message contains 8 bytes of Operator specific information: 217 Longitude and Latitude of the Remote Operator (Pilot in the field 218 description) of the UA. The GCS MAY encrypt these as follows. 220 Editors Note: The next version of [F3411-19], currently in ballot, is 221 adding a 4 byte Operator Altitude field, thus increasing the Operator 222 specific information to 12 bytes. This change will be delineated via 223 Protocol Version field. 225 The 8 bytes of Operator information are encrypted, using the ECIES 226 derived 128 bit shared secret, with one of the cipher's specified 227 below. The choice of cipher is based on USS policy and is agreed to 228 as part of the operation registration. AES-CFB32 is the recommended 229 default cipher. 231 ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to 232 allow Bit 2 of the Flags byte in the System Message set to "1" to 233 indicate the Operator information is encrypted. 235 The USS similarly decrypts these 8 bytes and provides the information 236 to authorized entities. 238 4.1. Rules for encrypting System Message content 240 If the Operator location is encrypted the encrypted bit flag MUST be 241 set to 1. 243 The Operator MAY be notified by the USS that the operation has 244 entered a location or time where privacy of Operator location is not 245 allowed. In this case the Operator MUST disable this privacy feature 246 and send the location unencrypted or land the UA or route around the 247 restricted area. 249 If the UAS looses connectivity to the USS, the privacy feature SHOULD 250 be disabled or land the UA. 252 If the operation is in an area or time with no Internet Connectivity, 253 the privacy feature MUST NOT be used. 255 4.2. Rules for decrypting System Message content 257 An Observer receives a System Message with the encrypt bit set to 1. 258 The Observer sends a query to its USS Display Provider containing the 259 UA's ID and the encrypted fields. 261 The USS Display Provider MAY deny the request if the Observer does 262 not have the proper authorization. 264 The USS Display Provider MAY reply to the request with the decrypted 265 fields if the Observer has the proper authorization. 267 The USS Display Provider MAY reply to the request with the decrypting 268 key if the Observer has the proper authorization. 270 The Observer MAY notify the USS through its USS Display Provider that 271 content privacy for a UAS in this location/time is not allowed. If 272 the Observer has the proper authorization for this action, the USS 273 notifies the Operator to disable this privacy feature. 275 5. Operator ID Message Privacy 277 The Operator ID Message contains the 20 byte Operator ID. The GCS 278 MAY encrypt these as follows. 280 The 20 bytes Operator ID is encrypted, using the ECIES derived 128 281 bit shared secret, with one of the cipher's specified below. The 282 choice of cipher is based on USS policy and is agreed to as part of 283 the operation registration. AES-CFB32 is the recommended default 284 cipher. 286 ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to 287 allow Operator ID Type in the Operator ID Message set to "1" to 288 indicate the Operator ID is encrypted. 290 The USS similarly decrypts these 20 bytes and provides the 291 information to authorized entities. 293 5.1. Rules for encrypting Operator ID Message content 295 If the Operator ID is encrypted the Operator ID Type field MUST be 296 set to 1. 298 The Operator MAY be notified by the USS that the operation has 299 entered a location or time where privacy of Operator ID is not 300 allowed. In this case the Operator MUST disable this privacy feature 301 and send the ID unencrypted or land the UA or route around the 302 restricted area. 304 If the UAS looses connectivity to the USS, the privacy feature SHOULD 305 be disabled or land the UA. 307 If the operation is in an area or time with no Internet Connectivity, 308 the privacy feature MUST NOT be used. 310 5.2. Rules for decrypting Operator ID Message content 312 An Observer receives a Operator ID Message with the Operator ID Type 313 field set to 1. The Observer sends a query to its USS Display 314 Provider containing the UA's ID and the encrypted fields. 316 The USS Display Provider MAY deny the request if the Observer does 317 not have the proper authorization. 319 The USS Display Provider MAY reply to the request with the decrypted 320 fields if the Observer has the proper authorization. 322 The USS Display Provider MAY reply to the request with the decrypting 323 key if the Observer has the proper authorization. 325 The Observer MAY notify the USS through its USS Display Provider that 326 content privacy for a UAS in this location/time is not allowed. If 327 the Observer has the proper authorization for this action, the USS 328 notifies the Operator to disable this privacy feature. 330 6. Cipher choices for Operator PII encryption 331 6.1. Using AES-CFB32 333 CFB32 is defined in [NIST.SP.800-38A], Section 6.3. This is the 334 Cipher Feedback (CFB) mode operating on 32 bits at a time. This 335 variant of CFB can be used to encrypt any multiple of 4 bytes of 336 cleartext. 338 The Operator includes a 64 bit UNIX timestamp for the operation time, 339 along with its operation pubic key. The Operator also includes the 340 UA MAC address (or multiple addresses if flying multiple UA). 342 The 128 bit IV for AES-CFB32 is constructed by the Operator and USS 343 as: SHAKE128(MAC|UTCTime|Message_Type, 128). Inclusion of the ASTM 344 Message_Type ensures a unique IV for each Message type that contains 345 PII to encrypt. 347 AES-CFB32 would then be used to encrypt the Operator information. 349 6.2. Using a Feistel scheme 351 If the encryption speed doesn't matter, we can use the following 352 approach based on the Feistel scheme. This approach is already being 353 used in format-preserving encryption (e.g. credit card numbers). The 354 Feistal scheme is explained in Appendix A. 356 6.3. Using AES-CTR 358 If 2 bytes of the Message can be set aside to contain a counter that 359 is incremented each time the Operator information changes, AES-CTR 360 can be used as follows. 362 The Operator includes a 64 bit UNIX timestamp for the operation time, 363 along with its operation pubic key. The Operator also includes the 364 UA MAC address (or multiple addresses if flying multiple UA). 366 The high order bits of an AES-CTR counter is constructed by the 367 Operator and USS as: SHAKE128(MAC|UTCTime|Message_Type, 112). 368 Inclusion of the ASTM Message_Type ensures a unique IV for each 369 Message type that contains PII to encrypt. 371 AES-CTR would then be used to encrypt the Operator information. 373 7. DRIP Requirements addressed 375 This document provides solution to PRIV-1 for PII in the ASTM System 376 Message. 378 8. ASTM Considerations 380 ASTM will need to make the following changes to the "Flags" in the 381 System Message (Msg Type 0x4): 383 Bit 2: 384 Value 1 for encrypted; 0 for cleartext (see Section 4). 386 ASTM will need to make the following changes to the "Operator ID 387 Type" in the Operator ID Message (Msg Type 0x5): 389 Operator ID Type 390 Value 1 for encrypted Operator ID (see Section 5). 392 9. IANA Considerations 394 TBD 396 10. Security Considerations 398 An attacker has no known text after decrypting to determine a 399 successful attack. An attacker can make assumptions about the high 400 order byte values for Operator Longitude and Latitude that may 401 substitute for known cleartext. There is no knowledge of where the 402 operator is in relation to the UA. Only if changing location values 403 "make sense" might an attacker assume to have revealed the operator's 404 location. 406 10.1. CFB32 Risks 408 Using the same IV for different Operator information values with 409 CFB32 presents a cyptoanalysis risk. Typically only the low order 410 bits would change as the Operators position changes. The risk is 411 mitigated due to the short-term value of the data. Further analysis 412 is need to properly place risk. 414 10.2. Crypto Agility 416 The ASTM Remote ID Messages do not provide any space for a crypto 417 suite indicator or any other method to manage crypto agility. 419 All crypto agility is left to the USS policy and the relation between 420 the USS and operator/UAS. The selection of the ECIES public key 421 algorithm, the shared secret key derivation function, and the actual 422 symmetric cipher used for on the System Message are set by the USS 423 which informs the operator what to do. 425 10.3. Key Derivation vulnerabilities 427 [RFC7748] warns about using Curve25519 and Curve448 in Diffie-Hellman 428 for key derivation: 430 Designers using these curves should be aware that for each public 431 key, there are several publicly computable public keys that are 432 equivalent to it, i.e., they produce the same shared secrets. Thus 433 using a public key as an identifier and knowledge of a shared secret 434 as proof of ownership (without including the public keys in the key 435 derivation) might lead to subtle vulnerabilities. 437 This applies here, but may have broader consequences. Thus two 438 endpoint IDs are included with the Diffie-Hellman secret. 440 10.4. KMAC Security as a KDF 442 Section 4.1 of NIST SP 800-185 [NIST.SP.800-185] states: 444 "The KECCAK Message Authentication Code (KMAC) algorithm is a PRF and 445 keyed hash function based on KECCAK . It provides variable-length 446 output" 448 That is, the output of KMAC is indistinguishable from a random 449 string, regardless of the length of the output. As such, the output 450 of KMAC can be divided into multiple substrings, each with the 451 strength of the function (KMAC128 or KMAC256) and provided that a 452 long enough key is used, as discussed in Sec. 8.4.1 of SP 800-185. 454 For example KMAC128(K, X, 512, S), where K is at least 128 bits, can 455 produce 4 128 bit keys each with a strength of 128 bits. That is a 456 single sponge operation is replacing perhaps 5 HMAC-SHA256 operations 457 (each 2 SHA256 operations) in HKDF. 459 11. Normative References 461 [NIST.SP.800-185] 462 Kelsey, J., Change, S., and R. Perlner, "SHA-3 derived 463 functions: cSHAKE, KMAC, TupleHash and ParallelHash", 464 National Institute of Standards and Technology report, 465 DOI 10.6028/nist.sp.800-185, December 2016, 466 . 468 [NIST.SP.800-38A] 469 Dworkin, M., "Recommendation for block cipher modes of 470 operation :: methods and techniques", National Institute 471 of Standards and Technology report, 472 DOI 10.6028/nist.sp.800-38a, 2001, 473 . 475 [NIST.SP.800-56Cr1] 476 Barker, E., Chen, L., and R. Davis, "Recommendation for 477 key-derivation methods in key-establishment schemes", 478 National Institute of Standards and Technology report, 479 DOI 10.6028/nist.sp.800-56cr1, April 2018, 480 . 482 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 483 Requirement Levels", BCP 14, RFC 2119, 484 DOI 10.17487/RFC2119, March 1997, 485 . 487 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 488 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 489 May 2017, . 491 12. Informative References 493 [drip-requirements] 494 Card, S. W., Wiethuechter, A., Moskowitz, R., and A. 495 Gurtov, "Drone Remote Identification Protocol (DRIP) 496 Requirements", Work in Progress, Internet-Draft, draft- 497 ietf-drip-reqs-18, 8 September 2021, 498 . 501 [F3411-19] ASTM International, "Standard Specification for Remote ID 502 and Tracking", February 2020, 503 . 505 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand 506 Key Derivation Function (HKDF)", RFC 5869, 507 DOI 10.17487/RFC5869, May 2010, 508 . 510 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 511 for Security", RFC 7748, DOI 10.17487/RFC7748, January 512 2016, . 514 Appendix A. Feistel Scheme 516 This approach is already being used in format-preserving encryption. 518 According to the theory, to provide CCA security guarantees (CCA = 519 Chosen Ciphertext Attacks) for m-bit encryption X |-> Y, we should 520 choose d >= 6. It seems very ineffective that when shortening the 521 block length, we have to use 6 times more block encryptions. On the 522 other hand, we preserve both the block cipher interface and security 523 guarantees in a simple way. 525 How to encrypt an m-bit plaintext X using an n-bit block cipher 526 E = {E_K} for n > m? 528 Enc(X, K): 529 1. Y <- X. 530 2. Split Y into 2 equal parts: Y = Y1 || Y2 531 (let us assume for simplicity that m is even). 532 3. For i = 1, 2, ..., d do: 533 Y <- Y2 || (Y1 ^ first_m/2_bits(E_K(Y2 || Ci)), 534 where Ci is a (n - m/2)-bit round constant. 535 4. Y <- Y2 || Y1. 536 5. Return Y. 538 Dec(Y, K): 539 1. X <- Y. 540 2. Split X into 2 equal parts: X = X1 || X2. 541 3. For i = d, ..., 2, 1 do: 542 X <- X2 || (X1 ^ first_m/2_bits(E_K(X2 || Ci)). 543 4. X <- X2 || X1. 544 5. Return X. 546 Acknowledgments 548 The recommended ciphers come from discussions on the IRTF CFRG 549 mailing list. 551 Authors' Addresses 553 Robert Moskowitz 554 HTT Consulting 555 Oak Park, MI 48237 556 United States of America 558 Email: rgm@labs.htt-consult.com 559 Stuart W. Card 560 AX Enterprize 561 4947 Commercial Drive 562 Yorkville, NY 13495 563 United States of America 565 Email: stu.card@axenterprize.com 567 Adam Wiethuechter 568 AX Enterprize 569 4947 Commercial Drive 570 Yorkville, NY 13495 571 United States of America 573 Email: adam.wiethuechter@axenterprize.com