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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 April 26, 2021. 36 Copyright Notice 38 Copyright (c) 2020 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 . . . . . . . . . . . . . . . . . . . . . 8 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 . . . . . . . . . . . . . . . . . 9 76 11. Normative References . . . . . . . . . . . . . . . . . . . . 10 77 12. Informative References . . . . . . . . . . . . . . . . . . . 10 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 2. Terms and Definitions 129 2.1. Requirements Terminology 131 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 132 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 133 "OPTIONAL" in this document are to be interpreted as described in BCP 134 14 [RFC2119] [RFC8174] when, and only when, they appear in all 135 capitals, as shown here. 137 2.2. Definitions 139 See Drip Requirements [drip-requirements] for common DRIP terms. 141 ECIES 142 Elliptic Curve Integrated Encryption Scheme. A hybrid encryption 143 scheme which provides semantic security against an adversary who 144 is allowed to use chosen-plaintext and chosen-ciphertext attacks. 146 Keccak (KECCAK Message Authentication Code): 147 The family of all sponge functions with a KECCAK-f permutation as 148 the underlying function and multi-rate padding as the padding 149 rule. 151 KMAC (KECCAK Message Authentication Code): 152 A PRF and keyed hash function based on KECCAK. 154 3. The Operator - USS Security Relationship 156 All CAAs have rules defining which UAS must be registered to operate 157 in their National Airspace. This includes UAS and Operator 158 registration in a USS. Further, operator's are expected to report 159 flight operations to their USS. This operation reporting provides a 160 mechanism for the USS and operator to establish an operation security 161 context. Here it will be used to exchange public keys for use in 162 ECIES. 164 The operator's ECIES public key SHOULD be unique for each operation. 165 The USS ECIES public key may be unique for each operator and 166 operation, but not required. For best post-compromise security 167 (PCS), the USS ECIES public key should be changed over some 168 operational window. 170 The public key algorithm should be Curve25519 [RFC7748]. 171 Correspondingly, the ECIES 128 bit shared secret should be generated 172 using KMAC. 174 3.1. ECIES Shared Secret Generation 176 The KMAC function provides a new, more efficient, key derivation 177 function over HKDF [RFC5869]. This will be referred to as KKDF. 179 HKDF needs a minimum of 4 hash functions (e.g. SHA256). KKDF does 180 an equivalent shared secret generation in a single Keccak Sponge 181 operation. 183 When the USS - UAS Operation Security Context is established, the UAS 184 provides a 20 Character USS ID and a 256 bit random nonce to the USS. 185 These are inputs, along with the ECDH keys to produce the shared 186 secret as follows. 188 Per [NIST.SP.800-56Cr1], Section 4.1, Option 3: 190 Shared Secret = KMAC128(salt, IKM, L, S) 192 L is the derived key bit length. Since only a single key is needed, 193 L=128. 195 S is the byte string 01001011 || 01000100 || 01000110, which 196 represents the sequence of characters "K", "D", and "F" in 8-bit 197 ASCII. 199 salt = Nonce-USS | Nonce-UAS 201 There are special security considerations for IKM per [RFC7748]. The 202 IKM as follows: 204 IKM = Diffie-Hellman secret | USS-ID | RID 206 4. System Message Privacy 208 The System Message contains 8 bytes of Operator specific information: 209 Longitude and Latitude of the Remote Operator (Pilot in the field 210 description) of the UA. The GCS MAY encrypt these as follows. 212 The 8 bytes of Operator information are encrypted, using the ECIES 213 derived 128 bit shared secret, with one of the cipher's specified 214 below. The choice of cipher is based on USS policy and is agreed to 215 as part of the operation registration. AES-CFB32 is the recommended 216 default cipher. 218 ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to 219 allow Bit 2 of the Flags byte in the System Message set to "1" to 220 indicate the Operator information is encrypted. 222 The USS similarly decrypts these 8 bytes and provides the information 223 to authorized entities. 225 4.1. Rules for encrypting System Message content 227 If the Operator location is encrypted the encrypted bit flag MUST be 228 set to 1. 230 The Operator MAY be notified by the USS that the operation has 231 entered a location or time where privacy of Operator location is not 232 allowed. In this case the Operator MUST disable this privacy feature 233 and send the location unencrypted or land the UA or route around the 234 restricted area. 236 If the UAS looses connectivity to the USS, the privacy feature SHOULD 237 be disabled or land the UA. 239 If the operation is in an area or time with no Internet Connectivity, 240 the privacy feature MUST NOT be used. 242 4.2. Rules for decrypting System Message content 244 An Observer receives a System Message with the encrypt bit set to 1. 245 The Observer sends a query to its USS Display Provider containing the 246 UA's ID and the encrypted fields. 248 The USS Display Provider MAY deny the request if the Observer does 249 not have the proper authorization. 251 The USS Display Provider MAY reply to the request with the decrypted 252 fields if the Observer has the proper authorization. 254 The USS Display Provider MAY reply to the request with the decrypting 255 key if the Observer has the proper authorization. 257 The Observer MAY notify the USS through its USS Display Provider that 258 content privacy for a UAS in this location/time is not allowed. If 259 the Observer has the proper authorization for this action, the USS 260 notifies the Operator to disable this privacy feature. 262 5. Operator ID Message Privacy 264 The Operator ID Message contains 20 bytes for Operator the ID. The 265 GCS MAY encrypt these as follows. 267 The 20 bytes Operator ID is encrypted, using the ECIES derived 128 268 bit shared secret, with one of the cipher's specified below. The 269 choice of cipher is based on USS policy and is agreed to as part of 270 the operation registration. AES-CFB32 is the recommended default 271 cipher. 273 ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to 274 allow Operator ID Type in the Operator ID Message set to "1" to 275 indicate the Operator ID is encrypted. 277 The USS similarly decrypts these 20 bytes and provides the 278 information to authorized entities. 280 5.1. Rules for encrypting Operator ID Message content 282 If the Operator ID is encrypted the Operator ID Type field MUST be 283 set to 1. 285 The Operator MAY be notified by the USS that the operation has 286 entered a location or time where privacy of Operator ID is not 287 allowed. In this case the Operator MUST disable this privacy feature 288 and send the ID unencrypted or land the UA or route around the 289 restricted area. 291 If the UAS looses connectivity to the USS, the privacy feature SHOULD 292 be disabled or land the UA. 294 If the operation is in an area or time with no Internet Connectivity, 295 the privacy feature MUST NOT be used. 297 5.2. Rules for decrypting Operator ID Message content 299 An Observer receives a Operator ID Message with the Operator ID Type 300 field set to 1. The Observer sends a query to its USS Display 301 Provider containing the UA's ID and the encrypted fields. 303 The USS Display Provider MAY deny the request if the Observer does 304 not have the proper authorization. 306 The USS Display Provider MAY reply to the request with the decrypted 307 fields if the Observer has the proper authorization. 309 The USS Display Provider MAY reply to the request with the decrypting 310 key if the Observer has the proper authorization. 312 The Observer MAY notify the USS through its USS Display Provider that 313 content privacy for a UAS in this location/time is not allowed. If 314 the Observer has the proper authorization for this action, the USS 315 notifies the Operator to disable this privacy feature. 317 6. Cipher choices for Operator PII encryption 319 6.1. Using AES-CFB32 321 CFB32 is defined in [NIST.SP.800-38A], Section 6.3. This is the 322 Cipher Feedback (CFB) mode operating on 32 bits at a time. This 323 variant of CFB can be used to encrypt any multiple of 4 bytes of 324 cleartext. 326 The Operator includes a 64 bit UNIX timestamp for the operation time, 327 along with its operation pubic key. The Operator also includes the 328 UA MAC address (or multiple addresses if flying multiple UA). 330 The 128 bit IV for AES-CFB32 is constructed by the Operator and USS 331 as: SHAKE128(MAC|UTCTime|Message_Type, 128). Inclusion of the ASTM 332 Message_Type ensures a unique IV for each Message type that contains 333 PII to encrypt. 335 AES-CFB32 would then be used to encrypt the Operator information. 337 6.2. Using a Feistel scheme 339 If the encryption speed doesn't matter, we can use the following 340 approach based on the Feistel scheme. This approach is already being 341 used in format-preserving encryption (e.g. credit card numbers). The 342 Feistal scheme is explained in Appendix A. 344 6.3. Using AES-CTR 346 If 2 bytes of the Message can be set aside to contain a counter that 347 is incremented each time the Operator information changes, AES-CTR 348 can be used as follows. 350 The Operator includes a 64 bit UNIX timestamp for the operation time, 351 along with its operation pubic key. The Operator also includes the 352 UA MAC address (or multiple addresses if flying multiple UA). 354 The high order bits of an AES-CTR counter is constructed by the 355 Operator and USS as: SHAKE128(MAC|UTCTime|Message_Type, 112). 356 Inclusion of the ASTM Message_Type ensures a unique IV for each 357 Message type that contains PII to encrypt. 359 AES-CTR would then be used to encrypt the Operator information. 361 7. DRIP Requirements addressed 363 This document provides solution to PRIV-1 for PII in the ASTM System 364 Message. 366 8. ASTM Considerations 368 ASTM will need to make the following changes to the "Flags" in the 369 System Message (Msg Type 0x4): 371 Bit 2: 372 Value 1 for encrypted; 0 for cleartext (see Section 4). 374 ASTM will need to make the following changes to the "Operator ID 375 Type" in the Operator ID Message (Msg Type 0x5): 377 Operator ID Type 378 Value 1 for encrypted Operator ID (see Section 5). 380 9. IANA Considerations 382 TBD 384 10. Security Considerations 386 An attacker has no known text after decrypting to determine a 387 successful attack. An attacker can make assumptions about the high 388 order byte values for Operator Longitude and Latitude that may 389 substitute for known cleartext. There is no knowledge of where the 390 operator is in relation to the UA. Only if changing location values 391 "make sense" might an attacker assume to have revealed the operator's 392 location. 394 10.1. CFB32 Risks 396 Using the same IV for different Operator information values with 397 CFB32 presents a cyptoanalysis risk. Typically only the low order 398 bits would change as the Operators position changes. The risk is 399 mitigated due to the short-term value of the data. Further analysis 400 is need to properly place risk. 402 10.2. Crypto Agility 404 The ASTM Remote ID Messages do not provide any space for a crypto 405 suite indicator or any other method to manage crypto agility. 407 All crypto agility is left to the USS policy and the relation between 408 the USS and operator/UAS. The selection of the ECIES public key 409 algorithm, the shared secret key derivation function, and the actual 410 symmetric cipher used for on the System Message are set by the USS 411 which informs the operator what to do. 413 10.3. Key Derivation vulnerabilities 415 [RFC7748] warns about using Curve25519 and Curve448 in Diffie-Hellman 416 for key derivation: 418 Designers using these curves should be aware that for each public 419 key, there are several publicly computable public keys that are 420 equivalent to it, i.e., they produce the same shared secrets. Thus 421 using a public key as an identifier and knowledge of a shared secret 422 as proof of ownership (without including the public keys in the key 423 derivation) might lead to subtle vulnerabilities. 425 This applies here, but may have broader consequences. Thus two 426 endpoint IDs are included with the Diffie-Hellman secret. 428 10.4. KMAC Security as a KDF 430 Section 4.1 of NIST SP 800-185 [NIST.SP.800-185] states: 432 "The KECCAK Message Authentication Code (KMAC) algorithm is a PRF and 433 keyed hash function based on KECCAK . It provides variable-length 434 output" 436 That is, the output of KMAC is indistinguishable from a random 437 string, regardless of the length of the output. As such, the output 438 of KMAC can be divided into multiple substrings, each with the 439 strength of the function (KMAC128 or KMAC256) and provided that a 440 long enough key is used, as discussed in Sec. 8.4.1 of SP 800-185. 442 For example KMAC128(K, X, 512, S), where K is at least 128 bits, can 443 produce 4 128 bit keys each with a strength of 128 bits. That is a 444 single sponge operation is replacing perhaps 5 HMAC-SHA256 operations 445 (each 2 SHA256 operations) in HKDF. 447 11. Normative References 449 [NIST.SP.800-185] 450 Kelsey, J., Change, S., and R. Perlner, "SHA-3 derived 451 functions: cSHAKE, KMAC, TupleHash and ParallelHash", 452 National Institute of Standards and Technology report, 453 DOI 10.6028/nist.sp.800-185, December 2016, 454 . 456 [NIST.SP.800-38A] 457 Dworkin, M., "Recommendation for block cipher modes of 458 operation :", National Institute of Standards and 459 Technology report, DOI 10.6028/nist.sp.800-38a, 2001, 460 . 462 [NIST.SP.800-56Cr1] 463 Barker, E., Chen, L., and R. Davis, "Recommendation for 464 key-derivation methods in key-establishment schemes", 465 National Institute of Standards and Technology report, 466 DOI 10.6028/nist.sp.800-56cr1, April 2018, 467 . 469 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 470 Requirement Levels", BCP 14, RFC 2119, 471 DOI 10.17487/RFC2119, March 1997, 472 . 474 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 475 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 476 May 2017, . 478 12. Informative References 480 [drip-requirements] 481 Card, S., Wiethuechter, A., Moskowitz, R., and A. Gurtov, 482 "Drone Remote Identification Protocol (DRIP) 483 Requirements", Work in Progress, Internet-Draft, draft- 484 ietf-drip-reqs-05, October 16, 2020, 485 . 487 [F3411-19] ASTM International, "Standard Specification for Remote ID 488 and Tracking", February 2020, 489 . 491 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand 492 Key Derivation Function (HKDF)", RFC 5869, 493 DOI 10.17487/RFC5869, May 2010, 494 . 496 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 497 for Security", RFC 7748, DOI 10.17487/RFC7748, January 498 2016, . 500 Appendix A. Feistel Scheme 502 This approach is already being used in format-preserving encryption. 504 According to the theory, to provide CCA security guarantees (CCA = 505 Chosen Ciphertext Attacks) for m-bit encryption X |-> Y, we should 506 choose d >= 6. It seems very ineffective that when shortening the 507 block length, we have to use 6 times more block encryptions. On the 508 other hand, we preserve both the block cipher interface and security 509 guarantees in a simple way. 511 How to encrypt an m-bit plaintext X using an n-bit block cipher 512 E = {E_K} for n > m? 514 Enc(X, K): 515 1. Y <- X. 516 2. Split Y into 2 equal parts: Y = Y1 || Y2 517 (let us assume for simplicity that m is even). 518 3. For i = 1, 2, ..., d do: 519 Y <- Y2 || (Y1 ^ first_m/2_bits(E_K(Y2 || Ci)), 520 where Ci is a (n - m/2)-bit round constant. 521 4. Y <- Y2 || Y1. 522 5. Return Y. 524 Dec(Y, K): 525 1. X <- Y. 526 2. Split X into 2 equal parts: X = X1 || X2. 527 3. For i = d, ..., 2, 1 do: 528 X <- X2 || (X1 ^ first_m/2_bits(E_K(X2 || Ci)). 529 4. X <- X2 || X1. 530 5. Return X. 532 Acknowledgments 534 The recommended ciphers come from discussions on the IRTF CFRG 535 mailing list. 537 Authors' Addresses 539 Robert Moskowitz 540 HTT Consulting 541 Oak Park, MI 48237 542 United States of America 544 Email: rgm@labs.htt-consult.com 546 Stuart W. Card 547 AX Enterprize 548 4947 Commercial Drive 549 Yorkville, NY 13495 550 United States of America 552 Email: stu.card@axenterprize.com 554 Adam Wiethuechter 555 AX Enterprize 556 4947 Commercial Drive 557 Yorkville, NY 13495 558 United States of America 560 Email: adam.wiethuechter@axenterprize.com