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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 May 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. Keying material is extracted from the output string 315 without regard to boundaries. 317 IKE uses the PRF to generate an output stream that parsed into five 318 keys: SK_d, SK_ai, SK_ar, SK_ei, and SK_er. SK_d is used to derive 319 new keys for the child security associations. SK_ai and SK_ar are 320 the authentication keys for the initiator and the responder, 321 respectively. SK_ei and SK_er are the encryption keys for the 322 initiator and the responder, respectively. 324 SK_ai and SK_ei are used to protect traffic from the initiator to the 325 responder. SK_ar and SK_er are used to protect traffic from the 326 responder to the initiator. 328 The size of SK_ei and SK_er are each three octets longer than is 329 needed by the associated AES key. The keying material is used as 330 follows: 332 AES-CCM with a 128 bit key 333 SK_ei and SK_er are each 19 octets. The first 16 octets are 334 the 128-bit AES key, and the remaining three octets are used as 335 the salt value in the counter block. 337 AES-CCM with a 192 bit key 338 SK_ei and SK_er are each 27 octets. The first 24 octets are 339 the 192-bit AES key, and the remaining three octets are used as 340 the salt value in the counter block. 342 AES-CCM with a 256 bit key 343 SK_ei and SK_er are each 35 octets. The first 32 octets are 344 the 256-bit AES key, and the remaining three octets are used as 345 the salt value in the counter block. 347 7.2. Phase 1 Identifier 349 This document does not specify the conventions for using AES-CCM for 350 IKE Phase 1 negotiations. For AES-CCM to be used in this manner, a 351 separate specification is needed, and an Encryption Algorithm 352 Identifier needs to be assigned. 354 7.3. Phase 2 Identifier 356 For IKE Phase 2 negotiations, IANA has assigned three ESP Transform 357 Identifiers for AES-CCM with an explicit IV: 359 for AES-CCM with an 8 octet ICV; 360 for AES-CCM with a 12 octet ICV; and 361 for AES-CCM with a 16 octet ICV. 363 7.4. Key Length Attribute 365 Since the AES supports three key lengths, the Key Length attribute 366 MUST be specified in the IKE Phase 2 exchange [DOI]. The Key Length 367 attribute MUST have a value of 128, 192, or 256. 369 8. Test Vectors 371 To be supplied. 373 9. Security Considerations 375 AES-CCM employs counter (CTR) mode for confidentiality. If a counter 376 value is ever used for more that one packet with the same key, then 377 the same key stream will be used to encrypt both packets, and the 378 confidentiality guarantees are voided. 380 What happens if the encryptor XORs the same key stream with two 381 different packet plaintexts? Suppose two packets are defined by two 382 plaintext byte sequences P1, P2, P3 and Q1, Q2, Q3, then both are 383 encrypted with key stream K1, K2, K3. The two corresponding 384 ciphertexts are: 386 (P1 XOR K1), (P2 XOR K2), (P3 XOR K3) 388 (Q1 XOR K1), (Q2 XOR K2), (Q3 XOR K3) 390 If both of these two ciphertext streams are exposed to an attacker, 391 then a catastrophic failure of confidentiality results, since: 393 (P1 XOR K1) XOR (Q1 XOR K1) = P1 XOR Q1 394 (P2 XOR K2) XOR (Q2 XOR K2) = P2 XOR Q2 395 (P3 XOR K3) XOR (Q3 XOR K3) = P3 XOR Q3 397 Once the attacker obtains the two plaintexts XORed together, it is 398 relatively straightforward to separate them. Thus, using any stream 399 cipher, including AES-CTR, to encrypt two plaintexts under the same 400 key stream leaks the plaintext. 402 Therefore, AES-CCM should not be used with statically configured 403 keys. Extraordinary measures would be needed to prevent the reuse of 404 a counter block value with the static key across power cycles. To be 405 safe, implementations MUST use fresh keys with AES-CCM. The Internet 406 Key Exchange (IKE) [IKE] protocol can be used to establish fresh 407 keys. 409 When IKE is used to establish fresh keys between two peer entities, 410 separate keys are established for the two traffic flows. If a 411 different mechanism is used to establish fresh keys, one that 412 establishes only a single key to encrypt packets, then there is a 413 high probability that the peers will select the same IV values for 414 some packets. Thus, to avoid counter block collisions, ESP 415 implementations that permit use of the same key for encrypting and 416 decrypting packets with the same peer MUST ensure that the two peers 417 assign different salt values to the security association (SA). 419 AES with a 128-bit key is vulnerable to the birthday attack after 420 2^64 blocks are encrypted with a single key, regardless of the mode 421 used. Since ESP with Extended Sequence Numbers allows for up to 2^64 422 packets in a single security association (SA), there is real 423 potential for more than 2^64 blocks to be encrypted with one key. 424 Implementations SHOULD generate a fresh key before 2^64 blocks are 425 encrypted with the same key, or implementations SHOULD make use of 426 the longer AES key sizes. Note that ESP with 32-bit Sequence Numbers 427 will not exceed 2^64 blocks even if all of the packets are maximum- 428 length Jumbograms. 430 10. Design Rationale 432 In the development of this specification, the use of the ESP sequence 433 number field instead of an explicit IV field was considered. This 434 section documents the rationale for the selection of an explicit IV. 435 This selection is not a cryptographic security issue, as either 436 approach will prevent counter block collisions. 438 The use of the explicit IV does not dictate the manner that the 439 encryptor uses to assign the per-packet value in the counter block. 440 This is desirable for several reasons. 442 1. Only the encryptor can ensure that the value is not used for 443 more than one packet, so there is no advantage to selecting a 444 mechanism that allows the decryptor to determine whether counter 445 block values collide. Damage from the collision is done, whether 446 the decryptor detects it or not. 448 2. The use of explicit IVs allows adders, LFSRs, and any other 449 technique that meets the time budget of the encryptor, so long as 450 the technique results in a unique value for each packet. Adders 451 are simple and straightforward to implement, but due to carries, 452 they do not execute in constant time. LSFRs offer an alternative 453 that executes in constant time. 455 3. Complexity is in control of the implementer. Further, the 456 decision made by the implementer of the encryptor does not make 457 the decryptor more (or less) complex. 459 4. The assignment of the per-packet counter block value needs to 460 be inside the assurance boundary. Some implementations assign the 461 sequence number inside the assurance boundary, but others do not. 462 A sequence number collision does not have the dire consequences, 463 but, as described in section 6, a collision in counter block 464 values has disastrous consequences. 466 5. Using the sequence number as the IV is possible in those 467 architectures where the sequence number assignment is performed 468 within the assurance boundary. In this situation, the sequence 469 number and the IV field will contain the same value. 471 6. By decoupling the IV and the sequence number, architectures 472 where the sequence number assignment is performed outside the 473 assurance boundary are accommodated. 475 The use of an explicit IV field directly follows from the decoupling 476 of the sequence number and the per-packet counter block value. The 477 additional overhead (64 bits for the IV field) is acceptable. This 478 overhead is significantly less overhead associated with Cipher Block 479 Chaining (CBC) mode. As normally employed, CBC requires a full block 480 for the IV and, on average, half of a block for padding. AES-CCM 481 confidentiality processing with an explicit IV has about one-third of 482 the overhead as AES-CBC, and the overhead is constant for each 483 packet. 485 11. IANA Considerations 487 IANA has assigned nine ESP transform numbers for use with AES-CCM 488 with an explicit IV: 490 for AES-CCM with an 8 octet ICV; 491 for AES-CCM with a 12 octet ICV; and 492 for AES-CCM with a 16 octet ICV. 494 12. Acknowledgements 496 Doug Whiting and Niels Ferguson worked with me to develop CCM mode. 497 We developed CCM mode as part of the IEEE 802.11i security effort. 498 One of the most attractive aspects of CCM mode is that it is 499 unencumbered by patents. I acknowledge the companies that supported 500 the development of an unencumbered authenticated encryption mode (in 501 alphabetical order): 503 Hifn 504 Intersil 505 MacFergus 506 RSA Security 508 13. References 510 This section provides normative and informative references. 512 13.1. Normative References 514 [AES] NIST, FIPS PUB 197, "Advanced Encryption Standard 515 (AES)," November 2001. 517 [DOI] Piper, D., "The Internet IP Security Domain of 518 Interpretation for ISAKMP," RFC 2407, November 1998. 520 [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)," 521 Work In Progress. . 523 [CCM] Whiting, D., Housley, R., and N. Ferguson, 524 "Counter with CBC-MAC (CCM)," Work In Progress. 525 . 527 [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate 528 Requirement Levels," RFC 2119, March 1997. 530 13.2. Informative References 532 [ARCH] Kent, S. and R. Atkinson, "Security Architecture for 533 the Internet Protocol," RFC 2401, November 1998. 535 [IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange 536 (IKE)," RFC 2409, November 1998. 538 [ROAD] Thayer, R., N. Doraswamy and R. Glenn, "IP Security 539 Document Roadmap," RFC 2411, November 1998. 541 14. Author's Address 543 Russell Housley 544 Vigil Security, LLC 545 918 Spring Knoll Drive 546 Herndon, VA 20170 547 USA 548 housley@vigilsec.com 550 Full Copyright Statement 552 Copyright (C) The Internet Society 2003. All Rights Reserved. 554 This document and translations of it may be copied and furnished to 555 others, and derivative works that comment on or otherwise explain it 556 or assist in its implementation may be prepared, copied, published 557 and distributed, in whole or in part, without restriction of any 558 kind, provided that the above copyright notice and this paragraph are 559 included on all such copies and derivative works. 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