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Etype encr...' -- The document date (April 19, 2013) is 4025 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- No issues found here. Summary: 0 errors (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group K. Burgin 3 Internet Draft National Security Agency 4 Intended Status: Informational M. Peck 5 Expires: October 21, 2013 The MITRE Corporation 6 April 19, 2013 8 AES Encryption with HMAC-SHA2 for Kerberos 5 9 draft-ietf-kitten-aes-cts-hmac-sha2-00 11 Abstract 13 This document specifies two encryption types and two corresponding 14 checksum types for Kerberos 5. The new types use AES in CTS mode 15 (CBC mode with ciphertext stealing) for confidentiality and HMAC with 16 a SHA-2 hash for integrity. 18 Status of this Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on October 21, 2013. 35 Copyright and License Notice 37 Copyright (c) 2013 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 2. Conventions used in this Document . . . . . . . . . . . . . . 3 54 3. Protocol Key Representation . . . . . . . . . . . . . . . . . 3 55 4. Key Generation from Pass Phrases . . . . . . . . . . . . . . . 3 56 5. Key Derivation Function . . . . . . . . . . . . . . . . . . . 4 57 6. Kerberos Algorithm Protocol Parameters . . . . . . . . . . . . 5 58 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 59 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 60 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 61 9.1. Normative References . . . . . . . . . . . . . . . . . . . 9 62 9.2. Informative References . . . . . . . . . . . . . . . . . . 9 63 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 10 64 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 66 1. Introduction 68 This document defines two encryption types and two corresponding 69 checksum types for Kerberos 5 using AES with 128-bit or 256-bit keys. 70 The new types conform to the framework specified in [RFC3961], but do 71 not use the simplified profile. 73 The new encryption types use AES in CTS mode (CBC mode with 74 ciphertext stealing) similar to [RFC3962] but with several 75 variations. 77 The new types use the PBKDF2 algorithm for key generation from 78 strings, with a modification to the use in [RFC3962] that the 79 pseudorandom function used by PBKDF2 is HMAC-SHA-256 or HMAC-SHA-384 80 instead of HMAC-SHA-1. 82 The new types use key derivation to produce keys for encryption, 83 integrity protection, and checksum operations as in [RFC3962]. 84 However, a key derivation function from [SP800-108] which uses the 85 SHA-256 or SHA-384 hash algorithm is used in place of the DK key 86 derivation function used in [RFC3961]. 88 The new types use the HMAC algorithm with a hash from the SHA-2 89 family for integrity protection and checksum operations. 91 2. Conventions used in this Document 93 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 94 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 95 document are to be interpreted as described in RFC 2119 [RFC2119]. 97 3. Protocol Key Representation 99 The AES key space is dense, so we can use random or pseudorandom 100 octet strings directly as keys. The byte representation for the key 101 is described in [FIPS197], where the first bit of the bit string is 102 the high bit of the first byte of the byte string (octet string). 104 4. Key Generation from Pass Phrases 106 We use a variation on the key generation algorithm specified in 107 Section 4 of [RFC3962] with the following changes: 109 * The pseudorandom function used by PBKDF2 will be the SHA-256 or 110 SHA-384 HMAC of the passphrase and salt, instead of the SHA-1 HMAC 111 of the passphrase and salt. If the enctype is "aes128-cts-hmac- 112 sha256-128", then HMAC-SHA-256 is used as the PRF. If the enctype 113 is "aes256-cts-hmac-sha384-192", then HMAC-SHA-384 is used as the 114 PRF. 116 * The salt MUST contain at least 128 random bits as required in 117 Section 5.1 of [SP800-132]. It MAY also contain other information 118 such as the principal's realm and name components. 120 * The final key derivation step uses the algorithm KDF-HMAC-SHA2 121 defined below in Section 5 instead of the DK function. 123 * If no string-to-key parameters are specified, the default number 124 of iterations is raised to 32,768. 126 To ensure that different long-term keys are used with different 127 enctypes, we prepend the enctype name to the salt string, separated 128 by a null byte. The enctype name is "aes128-cts-hmac-sha256-128" or 129 "aes256-cts-hmac-sha384-192" (without the quotes). The user's long- 130 term key is derived as follows 132 saltp = enctype-name | 0x00 | salt 133 tkey = random-to-key(PBKDF2(passphrase, saltp, 134 iter_count, keylength)) 135 key = KDF-HMAC-SHA2(tkey, "kerberos") where "kerberos" is the 136 byte string {0x6b65726265726f73}. 138 where the pseudorandom function used by PBKDF2 is HMAC-SHA-256 when 139 the enctype is "aes128-cts-hmac-sha256-128" and HMAC-SHA-384 when the 140 enctype is "aes256-cts-hmac-sha384-192", the value for keylength is 141 the AES key length, and the algorithm KDF-HMAC-SHA2 is defined in 142 Section 5. 144 5. Key Derivation Function 146 We use a key derivation function from Section 5.1 of [SP800-108] 147 which uses the HMAC algorithm as the PRF. The counter i is expressed 148 as four octets in big-endian order. The length of the output key in 149 bits (denoted as k) is also represented as four octets in big-endian 150 order. The "Label" input to the KDF is the usage constant supplied 151 to the key derivation function, and the "Context" input is null. In 152 the following summary, | indicates concatenation. The random-to-key 153 function is the identity function, as defined in Section 6. The k- 154 truncate function is defined in [RFC3961], Section 5.1. 156 When the encryption type is aes128-cts-hmac-sha256-128, the output 157 key length k is 128 bits for all applications of KDF-HMAC-SHA2(key, 158 constant) which is computed as follows: 160 n = 1 161 K1 = HMAC-SHA-256(key, 00 00 00 01 | constant | 0x00 | 00 00 00 80) 162 DR(key, constant) = k-truncate(K1) 163 KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant)) 165 When the encryption type is aes256-cts-hmac-sha384-192, the output 166 key length k is 256 bits when computing the base-key and Ke, and the 167 output key length k is 192 bits when deriving Kc and Ki. KDF-HMAC- 168 SHA2(key, constant) is computed as follows: 170 If deriving Kc or Ki (the constant ends with 0x99 or 0x55): 171 k = 192 172 n = 1 173 K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 00 C0) 174 DR(key, constant) = k-truncate(K1) 175 KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant)) 177 Otherwise (if deriving Ke or deriving the base-key from a 178 passphrase as described in Section 4): 179 k = 256 180 n = 1 181 K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 01 00) 182 DR(key, constant) = k-truncate(K1) 183 KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant)) 185 The constants used for key derivation are the same as those used in 186 the simplified profile. 188 6. Kerberos Algorithm Protocol Parameters 190 The following parameters apply to the encryption types aes128-cts- 191 hmac-sha256-128 and aes256-cts-hmac-sha384-192. 193 The key-derivation function described in the previous section is used 194 to produce the three intermediate keys. Typically, CBC mode [SP800- 195 38A] requires the input be padded to a multiple of the encryption 196 algorithm block size, which is 128 bits for AES. However, to avoid 197 ciphertext expansion, we use the CBC-CS3 variant to CBC mode defined 198 in [SP800-38A+] (this mode is also referred to as CTS). Note that 199 [SP800-38A+] requires the plaintext length to be greater than or 200 equal to the block size. 202 Each encryption will use a freshly generated 16-octet nonce generated 203 at random by the message originator. The initialization vector (IV) 204 used by AES is obtained by xoring the random nonce with the 205 cipherstate. 207 The ciphertext is the concatenation of the random nonce, the output 208 of AES in CBC-CS3 mode, and the HMAC of the nonce concatenated with 209 the AES output. The HMAC is computed using either SHA-256 or SHA- 210 384. The output of SHA-256 is truncated to 128 bits and the output 211 of SHA-384 is truncated to 192 bits. Sample test vectors are given in 212 Appendix A. 214 Decryption is performed by removing the HMAC, verifying the HMAC 215 against the remainder, and then decrypting the remainder if the HMAC 216 is correct. 218 The encryption and checksum mechanisms below use the following 219 notation from [RFC3961]. 221 HMAC output size, h 222 message block size, m 223 encryption/decryption functions, E and D 224 cipher block size, c 226 Encryption Mechanism for AES-CTS-HMAC-SHA2 227 ------------------------------------------------------------------------ 229 protocol key format 128- or 256-bit string 231 specific key structure Three protocol-format keys: { Kc, Ke, Ki }. 233 required checksum As defined below. 234 mechanism 236 key-generation seed key size (128 or 256 bits) 237 length 239 cipher state Random nonce of length c (128 bits) 241 initial cipher state All bits zero 243 encryption function N = random nonce of length c (128 bits) 244 IV = N + cipherState (+ denotes XOR) 245 C = E(Ke, plaintext, IV) 246 using CBC-CS3-Encrypt defined 247 in [SP800-38A+] 248 H = HMAC(Ki, N | C) 249 ciphertext = N | C | H[1..h] 250 cipherState = N 252 decryption function (N, C, H) = ciphertext 253 if (H != HMAC(Ki, N | C)[1..h]) 254 stop, report error 255 IV = N + cipherState (+ denotes XOR) 256 P = D(Ke, C, IV) 257 using CBC-CS3-Decrypt defined 258 in [SP800-38A+] 259 cipherState = N 261 pseudo-random function Kp = KDF-HMAC-SHA2(protocol-key, "prf") 262 PRF = HMAC(Kp, octet-string) 264 key generation functions: 266 string-to-key function tkey = random-to-key(PBKDF2(passphrase, saltp, 267 iter_count, 268 keylength)) 269 base-key = KDF-HMAC-SHA2(tkey, "kerberos") 271 where the pseudorandom function used by PBKDF2 272 is HMAC-SHA-256 or HMAC-SHA-384 as described 273 in Section 4. 275 default string-to-key 00 00 80 00 276 parameters 278 random-to-key function identity function 280 key-derivation function KDF-HMAC-SHA2 as defined in Section 5. The 281 key usage number is expressed as four octets 282 in big-endian order. 284 Kc = KDF-HMAC-SHA2(base-key, usage | 0x99) 285 Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA) 286 Ki = KDF-HMAC-SHA2(base-key, usage | 0x55); 288 Checksum Mechanism for AES-CTS-HMAC-SHA2 289 ------------------------------------------------------------------------ 290 associated cryptosystem AES-128-CTS or AES-256-CTS as appropriate 292 get_mic HMAC(Kc, message)[1..h] 294 verify_mic get_mic and compare 296 Using this profile with each key size gives us two each of encryption 297 and checksum algorithm definitions. 299 +--------------------------------------------------------------------+ 300 | encryption types | 301 +--------------------------------------------------------------------+ 302 | type name etype value key size | 303 +--------------------------------------------------------------------+ 304 | aes128-cts-hmac-sha256-128 TBD1 128 | 305 | aes256-cts-hmac-sha384-192 TBD2 256 | 306 +--------------------------------------------------------------------+ 308 +--------------------------------------------------------------------+ 309 | checksum types | 310 +--------------------------------------------------------------------+ 311 | type name sumtype value length | 312 +--------------------------------------------------------------------+ 313 | hmac-sha256-128-aes128 TBD3 128 | 314 | hmac-sha384-192-aes256 TBD4 192 | 315 +--------------------------------------------------------------------+ 317 These checksum types will be used with the corresponding encryption 318 types defined above. 320 7. IANA Considerations 322 IANA is requested to assign: 324 1. Encryption type numbers for aes128-cts-hmac-sha256-128 and 325 aes256-cts-hmac-sha384-192 in the Kerberos Encryption Type 326 Numbers registry. 328 Etype encryption type Reference 329 ----- --------------- --------- 330 TBD1 aes128-cts-hmac-sha256-128 [this document] 331 TBD2 aes256-cts-hmac-sha384-192 [this document] 333 2. Checksum type numbers for hmac-sha256-128-aes128 and 334 hmac-sha384-192-aes256 in the Kerberos Checksum Type Numbers 335 registry. 337 Sumtype Checksum type Size Reference 338 ------- ------------- ---- --------- 339 TBD3 hmac-sha256-128-aes128 16 [this document] 340 TBD4 hmac-sha384-192-aes256 24 [this document] 342 8. Security Considerations 344 This specification requires implementations to generate random 345 values. The use of inadequate pseudo-random number generators 346 (PRNGs) can result in little or no security. The generation of 347 quality random numbers is difficult. NIST Special Publication 800-90 348 [SP800-90] and [RFC4086] offer random number generation guidance. 350 This document specifies a mechanism for generating keys from pass 351 phrases or passwords. The salt and iteration count resist brute 352 force and dictionary attacks, however, it is still important to 353 choose or generate strong passphrases. 355 9. References 357 9.1. Normative References 359 [SP800-38A+] National Institute of Standards and Technology, 360 "Recommendation for Block Cipher Modes of Operation: 361 Three Variants of Ciphertext Stealing for CBC Mode", 362 Addendum to NIST Special Publication 800-38A, October 363 2010. 365 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 366 Requirement Levels", BCP 14, RFC 2119, March 1997. 368 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 369 Kerberos 5", RFC 3961, February 2005. 371 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) 372 Encryption for Kerberos 5", RFC 3962, February 2005. 374 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 375 "Randomness Requirements for Security", BCP 106, 376 RFC 4086, June 2005. 378 [FIPS197] National Institute of Standards and Technology, 379 "Advanced Encryption Standard (AES)", FIPS PUB 197, 380 November 2001. 382 9.2. Informative References 384 [SP800-38A] National Institute of Standards and Technology, 385 "Recommendation for Block Cipher Modes of Operation - 386 Methods and Techniques", NIST Special Publication 800- 387 38A, February 2001. 389 [SP800-90] National Institute of Standards and Technology, 390 Recommendation for Random Number Generation Using 391 Deterministic Random Bit Generators (Revised), NIST 392 Special Publication 800-90, March 2007. 394 [SP800-108] National Institute of Standards and Technology, 395 "Recommendation for Key Derivation Using Pseudorandom 396 Functions", NIST Special Publication 800-108, October 397 2009. 399 [SP800-132] National Institute of Standards and Technology, 400 "Recommendation for Password-Based Key Derivation, Part 401 1: Storage Applications", NIST Special Publication 800- 402 132, June 2010. 404 Appendix A. Test Vectors 406 Sample results for string-to-key conversion: 408 Iteration count = 32768 409 Pass phrase = "password" 410 Saltp for creating 128-bit master key: 411 61 65 73 31 32 38 2D 63 74 73 2D 68 6D 61 63 2D 412 73 68 61 32 35 36 2D 31 32 38 00 F3 60 61 DC E2 413 E1 B3 59 00 83 87 46 B8 78 2F 1D 41 54 48 45 4E 414 41 2E 4D 49 54 2E 45 44 55 72 61 65 62 75 72 6E 415 (The saltp is "aes128-cts-hmac-sha256-128" | 0x00 | 416 16 random bytes | "ATHENA.MIT.EDUraeburn") 417 128-bit master key: 418 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 420 Saltp for creating 256-bit master key: 421 61 65 73 32 35 36 2D 63 74 73 2D 68 6D 61 63 2D 422 73 68 61 33 38 34 2D 31 39 32 00 F3 60 61 DC E2 423 E1 B3 59 00 83 87 46 B8 78 2F 1D 41 54 48 45 4E 424 41 2E 4D 49 54 2E 45 44 55 72 61 65 62 75 72 6E 425 (The saltp is "aes256-cts-hmac-sha384-192" | 0x00 | 426 16 random bytes | "ATHENA.MIT.EDUraeburn") 427 256-bit master key: 428 6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 429 00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52 431 Sample results for key derivation: 433 enctype aes128-cts-hmac-sha256-128: 434 128-bit master key: 435 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 436 Kc value for key usage 2 (constant = 0x0000000299): 437 B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3 438 Ke value for key usage 2 (constant = 0x00000002AA): 439 9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E 440 Ki value for key usage 2 (constant = 0x0000000255): 441 9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C 443 enctype aes256-cts-hmac-sha384-192: 444 256-bit master key: 445 6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 446 00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52 447 Kc value for key usage 2 (constant = 0x0000000299): 448 EF 57 18 BE 86 CC 84 96 3D 8B BB 50 31 E9 F5 C4 449 BA 41 F2 8F AF 69 E7 3D 450 Ke value for key usage 2 (constant = 0x00000002AA): 451 56 AB 22 BE E6 3D 82 D7 BC 52 27 F6 77 3F 8E A7 452 A5 EB 1C 82 51 60 C3 83 12 98 0C 44 2E 5C 7E 49 453 Ki value for key usage 2 (constant = 0x0000000255): 454 69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6 455 22 C4 D0 0F FC 23 ED 1F 457 Sample encryptions (using the default cipher state): 459 128-bit master key: 460 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 461 128-bit AES key (Ke, key usage 2): 462 9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E 463 128-bit HMAC key (Ki, key usage 2): 464 9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C 465 Plaintext: 466 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 467 10 11 12 13 14 468 IV | Ciphertext | Authentication Tag: 469 8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17 470 13 64 FB 39 DC C0 E3 D9 83 A7 DB 5B 4B 9F FB CA 471 42 F6 65 88 29 F2 1F C8 95 75 AE 93 C7 57 18 AB 472 3C 7C FB 28 E1 474 256-bit master key: 475 6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 476 00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52 477 256-bit AES key (Ke, key usage 2): 478 56 AB 22 BE E6 3D 82 D7 BC 52 27 F6 77 3F 8E A7 479 A5 EB 1C 82 51 60 C3 83 12 98 0C 44 2E 5C 7E 49 480 192-bit HMAC key (Ki, key usage 2): 481 69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6 482 22 C4 D0 0F FC 23 ED 1F 483 Plaintext: 484 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 485 10 11 12 13 14 486 IV | Ciphertext | Authentication Tag: 487 8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17 488 50 CB FF DC DF 38 69 D7 0B EA FF C3 2C 47 0B C6 489 5B 72 C3 37 2D 6E D7 B3 47 E9 0B BD 8F 31 F5 79 490 58 F9 69 50 BA A1 41 64 6E 65 6C F6 7C 492 Sample checksums: 494 Checksum type: hmac-sha256-128-aes128 495 128-bit master key: 496 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 497 128-bit HMAC key (Kc, key usage 2): 498 B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3 499 Plaintext: 500 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 501 10 11 12 13 14 502 Checksum: 503 D7 83 67 18 66 43 D6 7B 41 1C BA 91 39 FC 1D EE 505 Checksum type: hmac-sha384-192-aes256 506 256-bit master key: 507 6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 508 00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52 509 192-bit HMAC key (Kc, key usage 2): 510 EF 57 18 BE 86 CC 84 96 3D 8B BB 50 31 E9 F5 C4 511 BA 41 F2 8F AF 69 E7 3D 512 Plaintext: 513 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 514 10 11 12 13 14 515 Checksum: 516 45 EE 79 15 67 EE FC A3 7F 4A C1 E0 22 2D E8 0D 517 43 C3 BF A0 66 99 67 2A 519 Authors' Addresses 521 Kelley W. Burgin 522 National Security Agency 524 EMail: kwburgi@tycho.ncsc.mil 526 Michael A. Peck 527 The MITRE Corporation 529 EMail: mpeck@mitre.org