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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) -- Obsolete informational reference (is this intentional?): RFC 4835 (Obsoleted by RFC 7321) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. McGrew 3 Internet-Draft Cisco Systems 4 Obsoletes: 4835 (if approved) P. Hoffman 5 Intended status: Standards Track VPN Consortium 6 Expires: November 17, 2014 May 16, 2014 8 Cryptographic Algorithm Implementation Requirements and Usage Guidance 9 for Encapsulating Security Payload (ESP) and Authentication Header (AH) 10 draft-ietf-ipsecme-esp-ah-reqts-10 12 Abstract 14 This Internet Draft is a standards track proposal to update the 15 Cryptographic Algorithm Implementation Requirements for ESP and AH; 16 it also adds usage guidance to help in the selection of these 17 algorithms. 19 The Encapsulating Security Payload (ESP) and Authentication Header 20 (AH) protocols make use of various cryptographic algorithms to 21 provide confidentiality and/or data origin authentication to 22 protected data communications in the IP Security (IPsec) 23 architecture. To ensure interoperability between disparate 24 implementations, the IPsec standard specifies a set of mandatory-to- 25 implement algorithms. This document specifies the current set of 26 mandatory-to-implement algorithms for ESP and AH, specifies 27 algorithms that should be implemented because they may be promoted to 28 mandatory at some future time, and also recommends against the 29 implementation of some obsolete algorithms. Usage guidance is also 30 provided to help the user of ESP and AH best achieve their security 31 goals through appropriate choices of cryptographic algorithms. 33 This document obsoletes RFC 4835. 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at http://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on November 17, 2014. 51 Copyright Notice 53 Copyright (c) 2014 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents 58 (http://trustee.ietf.org/license-info) in effect on the date of 59 publication of this document. Please review these documents 60 carefully, as they describe your rights and restrictions with respect 61 to this document. Code Components extracted from this document must 62 include Simplified BSD License text as described in Section 4.e of 63 the Trust Legal Provisions and are provided without warranty as 64 described in the Simplified BSD License. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 69 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 70 2. Implementation Requirements . . . . . . . . . . . . . . . . . 4 71 2.1. ESP Authenticated Encryption (Combined Mode Algorithms) . 4 72 2.2. ESP Encryption Algorithms . . . . . . . . . . . . . . . . 4 73 2.3. ESP Authentication Algorithms . . . . . . . . . . . . . . 4 74 2.4. AH Authentication Algorithms . . . . . . . . . . . . . . 5 75 2.5. Summary of Changes from RFC 4835 . . . . . . . . . . . . 5 76 3. Usage Guidance . . . . . . . . . . . . . . . . . . . . . . . 5 77 4. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 6 78 4.1. Authenticated Encryption . . . . . . . . . . . . . . . . 6 79 4.2. Encryption Transforms . . . . . . . . . . . . . . . . . . 6 80 4.3. Authentication Transforms . . . . . . . . . . . . . . . . 7 81 5. Algorithm Diversity . . . . . . . . . . . . . . . . . . . . . 8 82 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 83 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 84 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 85 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 86 9.1. Normative References . . . . . . . . . . . . . . . . . . 9 87 9.2. Informative References . . . . . . . . . . . . . . . . . 9 88 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 90 1. Introduction 92 The Encapsulating Security Payload (ESP) [RFC4303] and the 93 Authentication Header (AH) [RFC4302] are the mechanisms for applying 94 cryptographic protection to data being sent over an IPsec Security 95 Association (SA) [RFC4301]. 97 To ensure interoperability between disparate implementations, it is 98 necessary to specify a set of mandatory-to-implement algorithms. 99 This ensures that there is at least one algorithm that all 100 implementations will have in common. This document specifies the 101 current set of mandatory-to-implement algorithms for ESP and AH, 102 specifies algorithms that should be implemented because they may be 103 promoted to mandatory at some future time, and also recommends 104 against the implementation of some obsolete algorithms. Usage 105 guidance is also provided to help the user of ESP and AH best achieve 106 their security goals through appropriate choices of mechanisms. 108 The nature of cryptography is that new algorithms surface 109 continuously and existing algorithms are continuously attacked. An 110 algorithm believed to be strong today may be demonstrated to be weak 111 tomorrow. Given this, the choice of mandatory-to-implement algorithm 112 should be conservative so as to minimize the likelihood of it being 113 compromised quickly. Thought should also be given to performance 114 considerations as many uses of IPsec will be in environments where 115 performance is a concern. 117 The ESP and AH mandatory-to-implement algorithm(s) may need to change 118 over time to adapt to new developments in cryptography. For this 119 reason, the specification of the mandatory-to-implement algorithms is 120 not included in the main IPsec, ESP, or AH specifications, but is 121 instead placed in this document. Ideally, the mandatory-to-implement 122 algorithm of tomorrow should already be available in most 123 implementations of IPsec by the time it is made mandatory. To 124 facilitate this, this document identifies such algorithms, as they 125 are known today. There is no guarantee that the algorithms that we 126 believe today may be mandatory in the future will in fact become so. 127 All algorithms known today are subject to cryptographic attack and 128 may be broken in the future. 130 1.1. Requirements Language 132 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 133 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 134 "OPTIONAL" in this document are to be interpreted as described in 135 [RFC2119]. 137 Following [RFC4835], we define some additional key words: 139 MUST- This term means the same as MUST. However, we expect that at 140 some point in the future this algorithm will no longer be a MUST. 142 SHOULD+ This term means the same as SHOULD. However, it is likely 143 that an algorithm marked as SHOULD+ will be promoted at some 144 future time to be a MUST. 146 2. Implementation Requirements 148 This section specifies the cryptographic algorithms that MUST be 149 implemented, and provides guidance about ones that SHOULD or SHOULD 150 NOT be implemented. 152 In the following sections, all AES modes are for 128-bit AES. 192-bit 153 and 256-bit AES MAY be supported for those modes, but the 154 requirements here are for 128-bit AES. 156 2.1. ESP Authenticated Encryption (Combined Mode Algorithms) 158 ESP combined mode algorithms provide both confidentiality and 159 authentication services; in cryptographic terms, these are 160 authenticated encryption algorithms [RFC5116]. Authenticated 161 encryption transforms are listed in the ESP encryption transforms 162 IANA registry. 164 Requirement Authenticated Encryption Algorithm 165 ----------- ---------------------------------- 166 SHOULD+ AES-GCM with a 16 octet ICV [RFC4106] 167 MAY AES-CCM [RFC4309] 169 2.2. ESP Encryption Algorithms 171 Requirement Encryption Algorithm 172 ----------- -------------------------- 173 MUST NULL [RFC2410] 174 MUST AES-CBC [RFC3602] 175 MAY AES-CTR [RFC3686] 176 MAY TripleDES-CBC [RFC2451] 177 MUST NOT DES-CBC [RFC2405] 179 2.3. ESP Authentication Algorithms 181 Requirement Authentication Algorithm (notes) 182 ----------- ----------------------------- 183 MUST HMAC-SHA1-96 [RFC2404] 184 SHOULD+ AES-GMAC with AES-128 [RFC4543] 185 SHOULD AES-XCBC-MAC-96 [RFC3566] 186 MAY NULL [RFC4303] 188 Note that the requirement level for NULL authentication depends on 189 the type of encryption used. When using authenticated encryption 190 from Section 2.1, the requirement for NULL encryption is the same as 191 the requirement for the authenticated encryption itself. When using 192 the encryption from Section 2.2, the requirement for NULL encryption 193 is truly "MAY"; see Section 3 for more detail. 195 2.4. AH Authentication Algorithms 197 The requirements for AH are the same as for ESP Authentication 198 Algorithms, except that NULL authentication is inapplicable. 200 2.5. Summary of Changes from RFC 4835 202 The following is a summary of the changes from RFC 4835. 204 Old New 205 Requirement Requirement Algorithm (notes) 206 ---- ----------- ----------------- 207 MAY SHOULD+ AES-GCM with a 16 octet ICV [RFC4106] 208 MAY SHOULD+ AES-GMAC with AES-128 [RFC4543] 209 MUST- MAY TripleDES-CBC [RFC2451] 210 SHOULD NOT MUST NOT DES-CBC [RFC2405] 211 SHOULD+ SHOULD AES-XCBC-MAC-96 [RFC3566] 212 SHOULD MAY AES-CTR [RFC3686] 214 3. Usage Guidance 216 Since ESP and AH can be used in several different ways, this document 217 provides guidance on the best way to utilize these mechanisms. 219 ESP can provide confidentiality, data origin authentication, or the 220 combination of both of those security services. AH provides only 221 data origin authentication. Background information on those security 222 services is available [RFC4949]. In the following, we shorten "data 223 origin authentication" to "authentication". 225 Providing both confidentiality and authentication offers the best 226 security. If confidentiality is not needed, providing authentication 227 can still be useful. Confidentiality without authentication is not 228 effective [DP07] and therefore SHOULD NOT be used. We describe each 229 of these cases in more detail below. 231 To provide both confidentiality and authentication, an authenticated 232 encryption transform from Section 2.1 SHOULD be used in ESP, in 233 conjunction with NULL authentication. Alternatively, an ESP 234 encryption transform and ESP authentication transform MAY be used 235 together. It is NOT RECOMMENDED to use ESP with NULL authentication 236 in conjunction with AH; some configurations of this combination of 237 services have been shown to be insecure [PD10]. 239 To provide authentication without confidentiality, an authentication 240 transform MUST be used in either ESP or AH. The IPsec community 241 generally prefers ESP with NULL encryption over AH. AH is still 242 required in some protocols and operational environments when there 243 are security-sensitive options in the IP header, such as source 244 routing headers; ESP inherently cannot protect those IP options. It 245 is not possible to provide effective confidentiality without 246 authentication, because the lack of authentication undermines the 247 trustworthiness of encryption [B96][V02]. Therefore, an encryption 248 transform MUST NOT be used with a NULL authentication transform 249 (unless the encryption transform is an authenticated encryption 250 transform from Section 2.1). 252 Triple-DES SHOULD NOT be used in any scenario in which multiple 253 gigabytes of data will be encrypted with a single key. As a 64-bit 254 block cipher, it leaks information about plaintexts above that 255 "birthday bound" [M13]. Triple-DES CBC is listed as a MAY implement 256 for the sake of backwards compatibility, but its use is discouraged. 258 4. Rationale 260 This section explains the principles behind the implementation 261 requirements described above. 263 The algorithms listed as "MAY implement" are not meant to be endorsed 264 over other non-standard alternatives. All of the algorithms that 265 appeared in [RFC4835] are included in this document, for the sake of 266 continuity. In some cases, these algorithms have moved from being 267 "SHOULD implement" to "MAY implement" algorithms. 269 4.1. Authenticated Encryption 271 This document encourages the use of authenticated encryption 272 algorithms because they can provide significant efficiency and 273 throughput advantages, and the tight binding between authentication 274 and encryption can be a security advantage [RFC5116]. 276 AES-GCM [RFC4106] brings significant performance benefits [KKGEGD], 277 has been incorporated into IPsec recommendations [RFC6379] and has 278 emerged as the preferred authenticated encryption method in IPsec and 279 other standards. 281 4.2. Encryption Transforms 283 Since ESP encryption is optional, support for the "NULL" algorithm is 284 required to maintain consistency with the way services are 285 negotiated. Note that while authentication and encryption can each 286 be "NULL", they MUST NOT both be "NULL" [RFC4301] [H10]. 288 AES Counter Mode (AES-CTR) is an efficient encryption method, but it 289 provides no authentication capability. The AES-GCM authenticated 290 encryption method has all of the advantages of AES-CTR, while also 291 providing authentication. Thus this document moves AES-CTR from a 292 SHOULD to a MAY. 294 The Triple Data Encryption Standard (TDES) is obsolete because of its 295 small block size; as with all 64-bit block ciphers, it SHOULD NOT be 296 used to encrypt more than one gigabyte of data with a single key 297 [M13]. Its key size is smaller than that of the Advanced Encryption 298 Standard (AES), while at the same time its performance and efficiency 299 is worse. Thus, its use in new implementations is discouraged. 301 The Data Encryption Standard (DES) is obsolete because of its small 302 key size and small block size. There have been publicly demonstrated 303 and open-design special-purpose cracking hardware. Therefore, its 304 use is has been changed to MUST NOT in this document. 306 4.3. Authentication Transforms 308 AES-GMAC provides good security along with performance advantages, 309 even over HMAC-MD5. In addition, it uses the same internal 310 components as AES-GCM and is easy to implement in a way that shares 311 components with that authenticated encryption algorithm. 313 The MD5 hash function has been found to not meet its goal of 314 collision resistance; it is so weak that its use in digital 315 signatures is highly discouraged [RFC6151]. There have been 316 theoretical results against HMAC-MD5, but that message authentication 317 code does not seem to have a practical vulnerability. Thus, it may 318 not be urgent to remove HMAC-MD5 from the existing protocols. 320 SHA-1 has been found to not meet its goal of collision resistance. 321 However, HMAC-SHA-1 does not rely on this property, and HMAC-SHA-1 is 322 believed to be secure. 324 The HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 are believed to 325 provide a good security margin, and they perform adequately on many 326 platforms. However, these algorithms are not recommended for 327 implementation in this document, because HMAC-SHA-1 support is 328 widespread and its security is good, AES-GMAC provides good security 329 with better performance, and Authenticated Encryption algorithms do 330 not need any authentication methods. 332 AES-XCBC has not seen widespread deployment, despite being previously 333 being recommended as a SHOULD+ in RFC 4835. Thus this draft lists it 334 only as a SHOULD. 336 5. Algorithm Diversity 338 When the AES cipher was first adopted, it was decided to continue 339 encouraging the implementation of Triple-DES, in order to provide 340 algorithm diversity. But the passage of time has eroded the 341 viability of Triple-DES as an alternative to AES. As it is a 64-bit 342 block cipher, its security is inadequate at high data rates (see 343 Section 4.2). Its performance in software and FPGAs is considerably 344 worse than that of AES. Since it would not be possible to use 345 Triple-DES as an alternative to AES in high data rate environments, 346 or in environments where its performance could not keep up the 347 requirements, the rationale of retaining Triple-DES to provide 348 algorithm diversity is disappearing. (Of course, this does not 349 change the rationale of retaining Triple-DES in IPsec implementations 350 for backwards compatibility.) 352 Recent discussions in the IETF have started considering how to make 353 the selection of a different cipher that could provide algorithm 354 diversity in IPsec and other IETF standards. That work is expected 355 to take a long time and involve discussions among many participants 356 and organizations. 358 It is important to bear in mind that it is very highly unlikely that 359 an exploitable flaw will be found in AES (e.g., a flaw that required 360 less than a terabyte of known plaintext, when AES is used in a 361 conventional mode of operation). The only reason that algorithm 362 diversity deserves any consideration is because the problems that 363 would be caused if such a flaw were found would be so large. 365 6. Acknowledgements 367 Some of the wording herein was adapted from [RFC4835], the document 368 that this one obsoletes. That RFC itself borrows from earlier RFCs, 369 notably RFC 4305 and 4307. RFC 4835, RFC 4305, and RFC 4307 were 370 authored by Vishwas Manral, Donald Eastlake, and Jeffrey Schiller 371 respectively. 373 Thanks are due to Wajdi Feghali, Brian Weis, Cheryl Madson, Dan 374 Harkins, Paul Wouters, Ran Atkinson, Scott Fluhrer, Tero Kivinen, and 375 Valery Smyslov for insightful feedback on this draft. 377 [[[ This paragraph exists so that the nits checker doesn't barf. It 378 is to be removed before this is published as an RFC. [RFC2404] 379 [RFC2405] [RFC2410] [RFC2451] [RFC3566] [RFC3602] [RFC3686] [RFC4309] 380 [RFC4543] ]]] 382 7. IANA Considerations 384 None. 386 8. Security Considerations 388 The security of a system that uses cryptography depends on both the 389 strength of the cryptographic algorithms chosen and the strength of 390 the keys used with those algorithms. The security also depends on 391 the engineering and administration of the protocol used by the system 392 to ensure that there are no non-cryptographic ways to bypass the 393 security of the overall system. 395 This document concerns itself with the selection of cryptographic 396 algorithms for the use of ESP and AH, specifically with the selection 397 of mandatory-to-implement algorithms. The algorithms identified in 398 this document as "MUST implement" or "SHOULD implement" are not known 399 to be broken at the current time, and cryptographic research so far 400 leads us to believe that they will likely remain secure into the 401 foreseeable future. However, this is not necessarily forever. We 402 would therefore expect that new revisions of this document will be 403 issued from time to time that reflect the current best practice in 404 this area. 406 9. References 408 9.1. Normative References 410 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 411 Requirement Levels", BCP 14, RFC 2119, March 1997. 413 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 414 Internet Protocol", RFC 4301, December 2005. 416 [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December 417 2005. 419 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 420 4303, December 2005. 422 9.2. Informative References 424 [B96] Bellovin, S., "Problem areas for the IP security protocols 425 (Proceedings of the Sixth Usenix Unix Security 426 Symposium)", 1996. 428 [DP07] Degabriele, J. and K. Paterson, "Attacking the IPsec 429 Standards in Encryption-only Configurations (IEEE 430 Symposium on Privacy and Security)", 2007. 432 [H10] Hoban, A., "Using Intel AES New Instructions and PCLMULQDQ 433 to Significantly Improve IPSec Performance on Linux", 434 2010. 436 [KKGEGD] Kounavis, M., Kang, X., Grewal, K., Eszenyi, M., Gueron, 437 S., and D. Durham, "Encrypting the Internet (SIGCOMM)", 438 2010. 440 [M13] McGrew, D., "Impossible plaintext cryptanalysis and 441 probable-plaintext collision attacks of 64-bit block 442 cipher modes", 2012. 444 [PD10] Paterson, K. and J. Degabriele, "On the (in)security of 445 IPsec in MAC-then-encrypt configurations (ACM Conference 446 on Computer and Communications Security, ACM CCS)", 2010. 448 [RFC2404] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within 449 ESP and AH", RFC 2404, November 1998. 451 [RFC2405] Madson, C. and N. Doraswamy, "The ESP DES-CBC Cipher 452 Algorithm With Explicit IV", RFC 2405, November 1998. 454 [RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and 455 Its Use With IPsec", RFC 2410, November 1998. 457 [RFC2451] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher 458 Algorithms", RFC 2451, November 1998. 460 [RFC3566] Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96 Algorithm 461 and Its Use With IPsec", RFC 3566, September 2003. 463 [RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher 464 Algorithm and Its Use with IPsec", RFC 3602, September 465 2003. 467 [RFC3686] Housley, R., "Using Advanced Encryption Standard (AES) 468 Counter Mode With IPsec Encapsulating Security Payload 469 (ESP)", RFC 3686, January 2004. 471 [RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode 472 (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 473 4106, June 2005. 475 [RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM 476 Mode with IPsec Encapsulating Security Payload (ESP)", RFC 477 4309, December 2005. 479 [RFC4543] McGrew, D. and J. Viega, "The Use of Galois Message 480 Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543, 481 May 2006. 483 [RFC4835] Manral, V., "Cryptographic Algorithm Implementation 484 Requirements for Encapsulating Security Payload (ESP) and 485 Authentication Header (AH)", RFC 4835, April 2007. 487 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 488 4949, August 2007. 490 [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated 491 Encryption", RFC 5116, January 2008. 493 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 494 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 495 RFC 6151, March 2011. 497 [RFC6379] Law, L. and J. Solinas, "Suite B Cryptographic Suites for 498 IPsec", RFC 6379, October 2011. 500 [V02] Vaudenay, S., "Security Flaws Induced by CBC Padding - 501 Applications to SSL, IPSEC, WTLS ... (EUROCRYPT)", 2002. 503 Authors' Addresses 505 David McGrew 506 Cisco Systems 508 Email: mcgrew@cisco.com 510 Paul Hoffman 511 VPN Consortium 513 Email: paul.hoffman@vpnc.org