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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPsec Working Group R. Housley 3 Internet Draft Vigil Security 4 expires in six months July 2003 6 Using AES CCM Mode With IPsec ESP 7 9 Status of this Memo 11 This document is an Internet-Draft and is in full conformance with all 12 provisions of Section 10 of RFC2026. 14 Internet-Drafts are working documents of the Internet Engineering Task 15 Force (IETF), its areas, and its working groups. Note that other 16 groups may also distribute working documents as Internet-Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six months 19 and may be updated, replaced, or obsoleted by other documents at any 20 time. It is inappropriate to use Internet-Drafts as reference 21 material or to cite them other than as "work in progress." 23 The list of current Internet-Drafts can be accessed at 24 http://www.ietf.org/ietf/1id-abstracts.txt 26 The list of Internet-Draft Shadow Directories can be accessed at 27 http://www.ietf.org/shadow.html. 29 This document is a submission to the IETF Internet Protocol Security 30 (IPsec) Working Group. Please send comments on this document to the 31 working group mailing list (ipsec@lists.tislabs.com). 33 Distribution of this memo is unlimited. 35 Abstract 37 This document describes the use of AES CCM Mode, with an explicit 38 initialization vector, as an IPsec Encapsulating Security Payload 39 (ESP) mechanism to provide confidentiality, data origin 40 authentication, connectionless integrity. 42 Table of Contents 44 1 Introduction .............................................. 3 45 1.1 Conventions Used In This Document ......................... 3 46 2 AES-CCM Mode .............................................. 3 47 3 ESP Payload ............................................... 5 48 3.1 Initialization Vector ..................................... 5 49 3.2 Encrypted Payload ......................................... 5 50 3.3 Authentication Data ....................................... 6 51 4 Nonce Format .............................................. 6 52 5 AAD Construction .......................................... 7 53 6 Packet Expansion .......................................... 7 54 7 IKE Conventions ........................................... 7 55 7.1 Keying Material and Salt Values ........................... 8 56 7.2 Phase 1 Identifier ........................................ 8 57 7.3 Phase 2 Identifier ........................................ 9 58 7.4 Key Length Attribute ...................................... 9 59 8 Test Vectors .............................................. 9 60 9 Security Considerations ................................... 9 61 10 Design Rationale .......................................... 10 62 11 IANA Considerations ....................................... 11 63 12 Acknowledgments ........................................... 12 64 13 References ................................................ 12 65 13.1 Normative References ...................................... 12 66 13.2 Informative References .................................... 12 67 14 Author's Address .......................................... 14 68 13 Full Copyright Statement .................................. 14 70 1. Introduction 72 The Advanced Encryption Standard (AES) [AES] is a block cipher, and 73 it can be used in many different modes. This document describes the 74 use of AES in CCM (Counter with CBC-MAC) mode (AES-CCM), with an 75 explicit initialization vector (IV), as an IPsec Encapsulating 76 Security Payload (ESP) [ESP] mechanism to provide confidentiality, 77 data origin authentication, connectionless integrity. 79 This document does not provide an overview of IPsec. However, 80 information about how the various components of IPsec and the way in 81 which they collectively provide security services is available in 82 [ARCH] and [ROAD]. 84 1.1. Conventions Used In This Document 86 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 87 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 88 document are to be interpreted as described in [STDWORDS]. 90 2. AES-CCM Mode 92 CCM is a generic authenticate-and-encrypt block cipher mode [CCM]. 93 In this specification, CCM is used with the AES [AES] block cipher. 95 AES-CCM has two parameters: 97 M M indicates the size of the integrity check value (ICV). 98 CCM defines values of 4, 6, 8, 10, 12, 14, and 16 octets; 99 However, to maintain alignment and provide adequate 100 security, only the values that are a multiple of four and 101 are at least eight are permitted. Implementations MUST 102 support M values of 8 octets and 16 octets, and 103 implementations MAY support an M value of 12 octets. 105 L L indicates the size of the length field in octets. CCM 106 defines values of L between 2 octets and 8 octets. 107 Implementations MUST support an L value of 4 octets, which 108 accommodates a full Jumbogram [JUMBO]; however, the length 109 includes all of the encrypted data, which also includes 110 the ESP Padding, Pad Length, and Next Header fields. 112 There are four inputs to CCM originator processing: 114 key 115 A single key is used to calculate the ICV using CBC-MAC and to 116 perform payload encryption using counter mode. AES supports 117 key sizes of 128 bits, 192 bits, and 256 bits. The default key 118 size is 128 bits, and implementations MUST support this key 119 size. Implementations MAY also support key sizes of 192 bits 120 and 256 bits. 122 nonce 123 The size of the nonce depends on the value selected for the 124 parameter L. It is 15-L octets. Implementations MUST support 125 a nonce of 11 octets. The construction of the nonce is 126 described in section 4. 128 payload 129 The payload of the ESP packet. The payload MUST NOT be longer 130 than 4,294,967,295 octets, which is the maximum size of a 131 Jumbogram [JUMBO]; however, the ESP Padding, Pad Length, and 132 Next Header fields are also part of the payload. 134 AAD 135 CCM provides data integrity and data origin authentication for 136 some data outside the payload. CCM does not allow additional 137 authenticated data (AAD) to be longer than 138 18,446,744,073,709,551,615 octets. The ICV is computed from 139 the ESP header, Payload, and ESP trailer fields, which is 140 significantly smaller than the CCM imposed limit. The 141 construction of the AAD described in section 5. 143 AES-CCM requires the encryptor to generate a unique per-packet value, 144 and communicate this value to the decryptor. This per-packet value 145 is one of the component parts of the nonce, and it is referred to as 146 the initialization vector (IV). The same IV and key combination MUST 147 NOT be used more than once. The encryptor can generate the IV in any 148 manner that ensures uniqueness. Common approaches to IV generation 149 include incrementing a counter for each packet and linear feedback 150 shift registers (LFSRs). 152 AES-CCM employs counter mode for encryption. As with any stream 153 cipher, reuse of the IV same value with the same key is catastrophic. 154 An IV collision immediately leaks information about the plaintext in 155 both packets. For this reason, it is inappropriate to use this CCM 156 with statically configured keys. Extraordinary measures would be 157 needed to prevent reuse of an IV value with the static key across 158 power cycles. To be safe, implementations MUST use fresh keys with 159 AES-CCM. The Internet Key Exchange (IKE) [IKE] protocol can be used 160 to establish fresh keys. 162 3. ESP Payload 164 The ESP payload is comprised of the IV followed by the ciphertext. 165 The payload field, as defined in [ESP], is structured as shown in 166 Figure 1. 168 0 1 2 3 169 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 1 170 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 171 | Initialization Vector | 172 | (8 octets) | 173 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 174 | | 175 ~ Encrypted Payload (variable) ~ 176 | | 177 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 178 | | 179 ~ Authentication Data (variable) ~ 180 | | 181 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 183 Figure 1. ESP Payload Encrypted with AES-CCM 185 3.1. Initialization Vector (IV) 187 The AES-CCM IV field MUST be eight octets. The IV MUST be chosen by 188 the encryptor in a manner that ensures that the same IV value is used 189 only once for a given key. The encryptor can generate the IV in any 190 manner that ensures uniqueness. Common approaches to IV generation 191 include incrementing a counter for each packet and linear feedback 192 shift registers (LFSRs). 194 Including the IV in each packet ensures that the decryptor can 195 generate the key stream needed for decryption, even when some 196 datagrams are lost or reordered. 198 3.2. Encrypted Payload 200 The encrypted payload contains the ciphertext. 202 AES-CCM mode does not require plaintext padding. However, ESP does 203 require padding to 32-bit word-align the authentication data. The 204 Padding, Pad Length, and Next Header fields MUST be concatenated with 205 the plaintext before performing encryption, as described in [ESP]. 207 3.3. Authentication Data 209 AES-CCM provides an encrypted ICV. The ICV provided by CCM is 210 carried in the Authentication Data fields without further encryption. 211 Implementations MUST support ICV sizes of 8 octets and 16 octets. 212 Implementations MAY also support ICV 12 octets. 214 4. Nonce Format 216 Each packet conveys the IV that is necessary to construct the 217 sequence of counter blocks used by counter mode to generate the key 218 stream. The AES counter block 16 octets. One octet is used for the 219 CCM Flags, and 4 octets are used for the block counter, as specified 220 by the CCM L parameter. The remaining octets are the nonce. These 221 octets occupy the second through the twelfth octets in the counter 222 block. Figure 2 shows the format of the nonce. 224 0 1 2 3 225 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 1 226 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 227 | Salt | 228 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 229 | Initialization Vector | 230 | | 231 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 233 Figure 2. Nonce Format 235 The components of the nonce are as follows: 237 Salt 238 The salt field is 24 bits. As the name implies, it contains an 239 unpredictable value. It MUST be assigned at the beginning of 240 the security association. The salt value need not be secret, 241 but it MUST NOT be predictable prior to the beginning of the 242 security association. 244 Initialization Vector 245 The IV field is 64 bits. As described in section 3.1, the IV 246 MUST be chosen by the encryptor in a manner that ensures that 247 the same IV value is used only once for a given key. 249 This construction permits each packet to consist of up to: 251 2^32 blocks = 4,294,967,296 blocks 252 = 68,719,476,736 octets 254 This construction provides more key stream for each packet than is 255 needed to handle any IPv6 Jumbogram [JUMBO]. 257 5. AAD Construction 259 The data integrity and data origin authentication for the SPI and 260 (Extended) Sequence Number fields is provided without encrypting 261 them. Two formats are defined: one for 32-bit sequence numbers and 262 one for 64-bit extended sequence numbers. The format with 32-bit 263 sequence numbers is shown in Figure 3, and the format with 64-bit 264 extended sequence numbers is shown in Figure 4. 266 0 1 2 3 267 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 1 268 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 269 | SPI | 270 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 271 | 32-bit Sequence Number | 272 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 274 Figure 3. AAD Format with 32-bit Sequence Number 276 0 1 2 3 277 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 1 278 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 279 | SPI | 280 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 281 | 64-bit Extended Sequence Number | 282 | | 283 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 285 Figure 4. AAD Format with 64-bit Extended Sequence Number 287 6. Packet Expansion 289 The initialization vector (IV) and the integrity check value (ICV) is 290 the only sources of packet expansion. The IV always adds 8 octets to 291 the front of the payload. The ICV is added at the end of the 292 payload, and the CCM parameter M determines the size of the ICV. 293 Implementations MUST support M values of 8 octets and 16 octets, and 294 implementations MAY also support an M value of 12 octets. 296 7. IKE Conventions 298 This section describes the conventions used to generate keying 299 material and salt values for use with AES-CCM using the Internet Key 300 Exchange (IKE) [IKE] protocol. The identifiers and attributes needed 301 to negotiate a security association which uses AES-CCM are also 302 defined. 304 7.1. Keying Material and Salt Values 306 As previously described, implementations MUST use fresh keys with 307 AES-CCM. IKE can be used to establish fresh keys. This section 308 describes the conventions for obtaining the unpredictable salt value 309 for use in the nonce from IKE. Note that this convention provides a 310 salt value that is secret as well as unpredictable. 312 IKE makes use of a pseudo-random function (PRF) to derive keying 313 material. The PRF is used iteratively to derive keying material of 314 arbitrary size, called KEYMAT. Keying material is extracted from the 315 output string without regard to boundaries. 317 The size of KEYMAT MUST be three octets longer than is needed for the 318 associated AES key. The keying material is used as follows: 320 AES-CCM with a 128 bit key 321 The KEYMAT requested for each AES-CCM key is 19 octets. The 322 first 16 octets are the 128-bit AES key, and the remaining 323 three octets are used as the salt value in the counter block. 325 AES-CCM with a 192 bit key 326 The KEYMAT requested for each AES-CCM key is 27 octets. The 327 first 24 octets are the 192-bit AES key, and the remaining 328 three octets are used as the salt value in the counter block. 330 AES-CCM with a 256 bit key 331 The KEYMAT requested for each AES-CCM key is 35 octets. The 332 first 32 octets are the 256-bit AES key, and the remaining 333 three octets are used as the salt value in the counter block. 335 7.2. Phase 1 Identifier 337 This document does not specify the conventions for using AES-CCM for 338 IKE Phase 1 negotiations. For AES-CCM to be used in this manner, a 339 separate specification is needed, and an Encryption Algorithm 340 Identifier needs to be assigned. 342 7.3. Phase 2 Identifier 344 For IKE Phase 2 negotiations, IANA has assigned three ESP Transform 345 Identifiers for AES-CCM with an explicit IV: 347 for AES-CCM with an 8 octet ICV; 348 for AES-CCM with a 12 octet ICV; and 349 for AES-CCM with a 16 octet ICV. 351 7.4. Key Length Attribute 352 Since the AES supports three key lengths, the Key Length attribute 353 MUST be specified in the IKE Phase 2 exchange [DOI]. The Key Length 354 attribute MUST have a value of 128, 192, or 256. 356 8. Test Vectors 358 Section 8 of [CCM] provides test vectors that will assist 359 implementers with AES-CCM mode. 361 9. Security Considerations 363 AES-CCM employs counter (CTR) mode for confidentiality. If a counter 364 value is ever used for more that one packet with the same key, then 365 the same key stream will be used to encrypt both packets, and the 366 confidentiality guarantees are voided. 368 What happens if the encryptor XORs the same key stream with two 369 different packet plaintexts? Suppose two packets are defined by two 370 plaintext byte sequences P1, P2, P3 and Q1, Q2, Q3, then both are 371 encrypted with key stream K1, K2, K3. The two corresponding 372 ciphertexts are: 374 (P1 XOR K1), (P2 XOR K2), (P3 XOR K3) 376 (Q1 XOR K1), (Q2 XOR K2), (Q3 XOR K3) 378 If both of these two ciphertext streams are exposed to an attacker, 379 then a catastrophic failure of confidentiality results, since: 381 (P1 XOR K1) XOR (Q1 XOR K1) = P1 XOR Q1 382 (P2 XOR K2) XOR (Q2 XOR K2) = P2 XOR Q2 383 (P3 XOR K3) XOR (Q3 XOR K3) = P3 XOR Q3 385 Once the attacker obtains the two plaintexts XORed together, it is 386 relatively straightforward to separate them. Thus, using any stream 387 cipher, including AES-CTR, to encrypt two plaintexts under the same 388 key stream leaks the plaintext. 390 Therefore, AES-CCM should not be used with statically configured 391 keys. Extraordinary measures would be needed to prevent the reuse of 392 a counter block value with the static key across power cycles. To be 393 safe, implementations MUST use fresh keys with AES-CCM. The Internet 394 Key Exchange (IKE) [IKE] protocol can be used to establish fresh 395 keys. 397 When IKE is used to establish fresh keys between two peer entities, 398 separate keys are established for the two traffic flows. If a 399 different mechanism is used to establish fresh keys, one that 400 establishes only a single key to encrypt packets, then there is a 401 high probability that the peers will select the same IV values for 402 some packets. Thus, to avoid counter block collisions, ESP 403 implementations that permit use of the same key for encrypting and 404 decrypting packets with the same peer MUST ensure that the two peers 405 assign different salt values to the security association (SA). 407 Regardless of the mode used, AES with a 128-bit key is vulnerable to 408 the birthday attack after 2^64 blocks are encrypted with a single 409 key. Since ESP with Extended Sequence Numbers allows for up to 2^64 410 packets in a single security association (SA), there is real 411 potential for more than 2^64 blocks to be encrypted with one key. 412 Implementations SHOULD generate a fresh key before 2^64 blocks are 413 encrypted with the same key, or implementations SHOULD make use of 414 the longer AES key sizes. Note that ESP with 32-bit Sequence Numbers 415 will not exceed 2^64 blocks even if all of the packets are maximum- 416 length Jumbograms. 418 10. Design Rationale 420 In the development of this specification, the use of the ESP sequence 421 number field instead of an explicit IV field was considered. This 422 section documents the rationale for the selection of an explicit IV. 423 This selection is not a cryptographic security issue, as either 424 approach will prevent counter block collisions. 426 The use of the explicit IV does not dictate the manner that the 427 encryptor uses to assign the per-packet value in the counter block. 428 This is desirable for several reasons. 430 1. Only the encryptor can ensure that the value is not used for 431 more than one packet, so there is no advantage to selecting a 432 mechanism that allows the decryptor to determine whether counter 433 block values collide. Damage from the collision is done, whether 434 the decryptor detects it or not. 436 2. The use of explicit IVs allows adders, LFSRs, and any other 437 technique that meets the time budget of the encryptor, so long as 438 the technique results in a unique value for each packet. Adders 439 are simple and straightforward to implement, but due to carries, 440 they do not execute in constant time. LFSRs offer an alternative 441 that executes in constant time. 443 3. Complexity is in control of the implementer. Further, the 444 decision made by the implementer of the encryptor does not make 445 the decryptor more (or less) complex. 447 4. The assignment of the per-packet counter block value needs to 448 be inside the assurance boundary. Some implementations assign the 449 sequence number inside the assurance boundary, but others do not. 450 A sequence number collision does not have the dire consequences, 451 but, as described in section 6, a collision in counter block 452 values has disastrous consequences. 454 5. Using the sequence number as the IV is possible in those 455 architectures where the sequence number assignment is performed 456 within the assurance boundary. In this situation, the sequence 457 number and the IV field will contain the same value. 459 6. By decoupling the IV and the sequence number, architectures 460 where the sequence number assignment is performed outside the 461 assurance boundary are accommodated. 463 The use of an explicit IV field directly follows from the decoupling 464 of the sequence number and the per-packet counter block value. The 465 additional overhead (64 bits for the IV field) is acceptable. This 466 overhead is significantly less overhead associated with Cipher Block 467 Chaining (CBC) mode. As normally employed, CBC requires a full block 468 for the IV and, on average, half of a block for padding. AES-CCM 469 confidentiality processing with an explicit IV has about one-third of 470 the overhead as AES-CBC, and the overhead is constant for each 471 packet. 473 11. IANA Considerations 475 IANA has assigned nine ESP transform numbers for use with AES-CCM 476 with an explicit IV: 478 for AES-CCM with an 8 octet ICV; 479 for AES-CCM with a 12 octet ICV; and 480 for AES-CCM with a 16 octet ICV. 482 12. Acknowledgements 484 Doug Whiting and Niels Ferguson worked with me to develop CCM mode. 485 We developed CCM mode as part of the IEEE 802.11i security effort. 486 One of the most attractive aspects of CCM mode is that it is 487 unencumbered by patents. I acknowledge the companies that supported 488 the development of an unencumbered authenticated encryption mode (in 489 alphabetical order): 491 Hifn 492 Intersil 493 MacFergus 494 RSA Security 496 Also, I thank Tero Kivinen for his comprehensive review of this 497 document. 499 13. References 501 This section provides normative and informative references. 503 13.1. Normative References 505 [AES] NIST, FIPS PUB 197, "Advanced Encryption Standard 506 (AES)," November 2001. 508 [DOI] Piper, D., "The Internet IP Security Domain of 509 Interpretation for ISAKMP," RFC 2407, November 1998. 511 [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)," 512 Work In Progress. . 514 [CCM] Whiting, D., Housley, R., and N. Ferguson, 515 "Counter with CBC-MAC (CCM)," Work In Progress. 516 . 518 [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate 519 Requirement Levels," RFC 2119, March 1997. 521 13.2. Informative References 523 [ARCH] Kent, S. and R. Atkinson, "Security Architecture for 524 the Internet Protocol," RFC 2401, November 1998. 526 [IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange 527 (IKE)," RFC 2409, November 1998. 529 [ROAD] Thayer, R., N. Doraswamy and R. Glenn, "IP Security 530 Document Roadmap," RFC 2411, November 1998. 532 14. Author's Address 534 Russell Housley 535 Vigil Security, LLC 536 918 Spring Knoll Drive 537 Herndon, VA 20170 538 USA 539 housley@vigilsec.com 541 Full Copyright Statement 543 Copyright (C) The Internet Society 2003. All Rights Reserved. 545 This document and translations of it may be copied and furnished to 546 others, and derivative works that comment on or otherwise explain it 547 or assist in its implementation may be prepared, copied, published 548 and distributed, in whole or in part, without restriction of any 549 kind, provided that the above copyright notice and this paragraph are 550 included on all such copies and derivative works. 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