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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) W. Feghali 5 Intended status: Standards Track Intel Corp. 6 Expires: October 2, 2014 P. Hoffman 7 VPN Consortium 8 March 31, 2014 10 Cryptographic Algorithm Implementation Requirements and Usage Guidance 11 for Encapsulating Security Payload (ESP) and Authentication Header (AH) 12 draft-ietf-ipsecme-esp-ah-reqts-03 14 Abstract 16 This Internet Draft is standards track proposal to update to the 17 Cryptographic Algorithm Implementation Requirements for ESP and AH; 18 it also adds usage guidance to help in the selection of these 19 algorithms. 21 The Encapsulating Security Payload (ESP) and Authentication Header 22 (AH) protocols makes use of various cryptographic algorithms to 23 provide confidentiality and/or data origin authentication to 24 protected data communications in the IP Security (IPsec) 25 architecture. To ensure interoperability between disparate 26 implementations, the IPsec standard specifies a set of mandatory-to- 27 implement algorithms. This document specifies the current set of 28 mandatory-to-implement algorithms for ESP and AH, specifies 29 algorithms that should be implemented because they may be promoted to 30 mandatory at some future time, and also recommends against the 31 implementation of some obsolete algorithms. Usage guidance is also 32 provided to help the user of ESP and AH best achieve their security 33 goals through appropriate choices of cryptographic algorithms. 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 October 2, 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 . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . 7 82 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 83 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 84 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 85 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 86 9.1. Normative References . . . . . . . . . . . . . . . . . . 9 87 9.2. Informative References . . . . . . . . . . . . . . . . . 9 88 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 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", "MAY", and "OPTIONAL" in this 134 document are to be interpreted as described in [RFC2119]. 136 Following [RFC4835], we define some additional key words: 138 MUST- This term means the same as MUST. However, we expect that at 139 some point in the future this algorithm will no longer be a MUST. 141 SHOULD+ This term means the same as SHOULD. However, it is likely 142 that an algorithm marked as SHOULD+ will be promoted at some 143 future time to be a MUST. 145 SHOULD- This term means the same as SHOULD. However, it is likely 146 that an algorithm marked as SHOULD- will be deprecated to a MAY or 147 worse in a future version of this document. 149 2. Implementation Requirements 151 This section specifies the cryptographic algorithms that MUST be 152 implemented, and provides guidance about ones that SHOULD or SHOULD 153 NOT be implemented. 155 In the following sections, all AES modes are for 128-bit AES. 192-bit 156 and 256-bit AES MAY be supported for those modes, but the 157 requirements here are for 128-bit AES. 159 2.1. ESP Authenticated Encryption (Combined Mode Algorithms) 161 ESP combined mode algorithms provide both confidentiality and 162 authentication services; in cryptographic terms, these are 163 authenticated encryption algorithms [RFC5116]. Authenticated 164 encryption transforms are listed in the ESP encryption transforms 165 IANA registry. 167 Requirement Authenticated Encryption Algorithm 168 ----------- ---------------------------------- 169 SHOULD+ AES-GCM with a 16 octet ICV [RFC4106] 170 MAY AES-CCM [RFC4309] 172 2.2. ESP Encryption Algorithms 174 Requirement Encryption Algorithm 175 ----------- -------------------------- 176 MUST NULL [RFC2410] 177 MUST AES-CBC [RFC3602] 178 MAY AES-CTR [RFC3686] 179 MAY TripleDES-CBC [RFC2451] 180 MUST NOT DES-CBC [RFC2405] 182 2.3. ESP Authentication Algorithms 184 Requirement Authentication Algorithm (notes) 185 ----------- ----------------------------- 186 MUST HMAC-SHA1-96 [RFC2404] 187 SHOULD+ AES-GMAC with AES-128 [RFC4543] 188 SHOULD AES-XCBC-MAC-96 [RFC3566] 189 MAY NULL [RFC4303] 191 Note that the requirement level for NULL authentication depends on 192 the type of encryption used. When using authenticated encryption 193 from Section 2.1, the requirement for NULL encryption is the same as 194 the requirement for the authenticated encryption itself. When using 195 the encryption from Section 2.2, the requirement for NULL encryption 196 is truly "MAY"; see Section 3 for more detail. 198 2.4. AH Authentication Algorithms 200 The requirements for AH are the same as for ESP Authentication 201 Algorithms, except that NULL authentication is inapplicable. 203 2.5. Summary of Changes 205 Old New 206 Requirement Requirement Algorithm (notes) 207 ---- ----------- ----------------- 208 MAY SHOULD+ AES-GCM with a 16 octet ICV [RFC4106] 209 MAY SHOULD+ AES-GMAC with AES-128 [RFC4543] 210 MUST- MAY TripleDES-CBC [RFC2451] 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 Both confidentiality and authentication SHOULD be provided. If 226 confidentiality is not needed, then authentication MAY be provided. 227 Confidentiality without authentication is not effective [DP07] and 228 SHOULD NOT be used. We describe each of these cases in more detail 229 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 perfers ESP wth NULL encryption over AH, but AH is still 242 required in some protocols; further, AH is more appropriate when 243 there are security-sensitive options in the IP header. It is not 244 possible to provide effective confidentiality without authentication, 245 because the lack of authentication undermines the efficacy of 246 encryption [B96][V02]. Therefore, an encryption transform MUST NOT 247 be used with a NULL authentication transform (unless the encryption 248 transform is an authenticated encryption transform from Section 2.1). 250 Triple-DES SHOULD NOT be used in any scenario in which multiple 251 gigabytes of data will be encrypted with a single key. As a 64-bit 252 block cipher, it leaks information about plaintexts above that 253 "birthday bound" [M13]. Triple-DES CBC is listed as a MAY implement 254 for the sake of backwards compatibility, but its use is discouraged. 256 4. Rationale 258 This section explains the principles behind the implementation 259 requirements described above. 261 The algorithms listed as MAY-implement are not meant to be endorsed 262 over other non-standard alternatives. All of the algorithms that 263 appeared in [RFC4835] are included in this document, for the sake of 264 continuity. In some cases, these algorithms have moved from being 265 SHOULD-implement to MAY-implement algorithms. 267 4.1. Authenticated Encryption 269 This document encourages the use of authenticated encryption 270 algorithms because they can provide significant efficiency and 271 throughput advantages, and the tight binding between authentication 272 and encryption can be a security advantage [RFC5116]. 274 AES-GCM [RFC4106] brings significant performance benefits [KKGEGD], 275 has been incorporated into IPsec recommendations [RFC6379] and has 276 emerged as the preferred authenticated encryption method in IPsec and 277 other standards. 279 4.2. Encryption Transforms 281 Since ESP encryption is optional, support for the "NULL" algorithm is 282 required to maintain consistency with the way services are 283 negotiated. Note that while authentication and encryption can each 284 be "NULL", they MUST NOT both be "NULL" [RFC4301] [H10]. 286 AES Counter Mode (AES-CTR) is an efficient encryption method, but it 287 provides no authentication capability. The AES-GCM authenticated 288 encryption method has all of the advantages of AES-CTR, while also 289 providing authentication. Thus this document moves AES-CTR from a 290 SHOULD to a MAY. 292 The Triple Data Encryption Standard (TDES) is obsolete because of its 293 small block size; as with all 64-bit block ciphers, it SHOULD NOT be 294 used to encrypt more than one gigabyte of data with a single key 295 [M13]. Its key size is smaller than that of the Advanced Encryption 296 Standard (AES), while at the same time its performance and efficiency 297 is worse. Thus, its use in new implementations is discouraged. 299 The Data Encryption Standard (DES) is obsolete because of its small 300 key size and small block size. There have been publicly demonstrated 301 and open-design special-purpose cracking hardware. Therefore, its 302 use is has been changed to MUST NOT in this document. 304 4.3. Authentication Transforms 306 AES-GMAC provides good security along with performance advantages, 307 even over HMAC-MD5. In addition, it uses the same internal 308 components as AES-GCM and is easy to implement in a way that shares 309 components with that authenticated encryption algorithm. 311 The MD5 hash function has been found to not meet its goal of 312 collision resistance; it is so weak that its use in digital 313 signatures is highly discouraged [RFC6151]. There have been 314 theoretical results against HMAC-MD5, but that message authentication 315 code does not seem to have a practical vulnerability. Thus, it may 316 not be urgent to remove HMAC-MD5 from the existing protocols. 318 SHA-1 has been found to not meet its goal of collision resistance. 319 However, HMAC-SHA-1 does not rely on this property, and HMAC-SHA-1 is 320 believed to be secure. 322 The HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 are believed to 323 provide a good security margin, and they perform adequately on many 324 platforms. However, these algorithms are not recommended for 325 implementation in this document, because HMAC-SHA-1 support is 326 widespread and its security is good, AES-GMAC provides good security 327 with better performance, and Authenticated Encryption algorithms do 328 not need any authentication methods. 330 AES-XCBC has not seen widespread deployment, despite being previously 331 being recommended as a SHOULD+ in RFC4305. Thus this draft lists it 332 only as a SHOULD. 334 5. Algorithm Diversity 335 When the AES cipher was first adopted, it was decided to continue 336 encouraging the implementation of Triple-DES, in order to provide 337 algorithm diversity. But the passage of time has eroded the 338 viability of Triple-DES as an alternative to AES. As it is a 64-bit 339 block cipher, its security is inadequate at high data rates (see 340 Section 4.2). Its performance in software and FPGAs is considerably 341 worse than that of AES. Since it would not be possible to use 342 Triple-DES as an alternative to AES in high data rate environments, 343 or in environments where its performance could not keep up the 344 requirements, the rationale of retaining Triple-DES to provide 345 algorithm diversity is disappearing. (Of course, this does not 346 change the rationale of retaining Triple-DES in IPsec implementations 347 for backwards compability.) 349 Recent discussions in the IETF have started considering how to make 350 the selection of a different cipher that could provide algorithm 351 diversity in IPsec and other IETF standards. That work is expected 352 to take a long time and involve discussions among many participants 353 and organizations. 355 It is important to bear in mind that it is very highly unlikely that 356 an exploitable flaw will be found in AES (e.g., a flaw that required 357 less than a terabyte of known plaintext, when AES is used in a 358 conventional mode of operation). The only reason that algorithm 359 diversity deserves any consideration is because the problems that 360 would be caused if such a flaw were found would be so large. 362 6. Acknowledgements 364 Much of the wording herein was adapted from [RFC4835], the parent 365 document of this document. That RFC itself borrows from [RFC4305], 366 which borrows in turn from [RFC4307]. RFC4835, RFC4305, and RFC4307 367 were authored by Vishwas Manral, Donald Eastlake, and Jeffrey 368 Schiller respectively. 370 Thanks are due to Brian Weis, Cheryl Madson, Dan Harkins, Paul 371 Wouters, Ran Atkinson, Scott Fluhrer, Tero Kivinen, and Valery 372 Smyslov for insightful feedback on this draft. 374 7. IANA Considerations 376 None. 378 8. Security Considerations 380 The security of a system that uses cryptography depends on both the 381 strength of the cryptographic algorithms chosen and the strength of 382 the keys used with those algorithms. The security also depends on 383 the engineering and administration of the protocol used by the system 384 to ensure that there are no non-cryptographic ways to bypass the 385 security of the overall system. 387 This document concerns itself with the selection of cryptographic 388 algorithms for the use of ESP and AH, specifically with the selection 389 of mandatory-to-implement algorithms. The algorithms identified in 390 this document as "MUST implement" or "SHOULD implement" are not known 391 to be broken at the current time, and cryptographic research so far 392 leads us to believe that they will likely remain secure into the 393 foreseeable future. However, this is not necessarily forever. We 394 would therefore expect that new revisions of this document will be 395 issued from time to time that reflect the current best practice in 396 this area. 398 9. References 400 9.1. Normative References 402 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 403 Requirement Levels", BCP 14, RFC 2119, March 1997. 405 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 406 Internet Protocol", RFC 4301, December 2005. 408 [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December 409 2005. 411 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 412 4303, December 2005. 414 9.2. Informative References 416 [B96] Bellovin, S., "Problem areas for the IP security protocols 417 (Proceedings of the Sixth Usenix Unix Security 418 Symposium)", 1996. 420 [DP07] Degabriele, J. and K. Paterson, "Attacking the IPsec 421 Standards in Encryption-only Configurations (IEEE 422 Symposium on Privacy and Security)", 2007. 424 [H10] Hoban, A., "Using Intel AES New Instructions and PCLMULQDQ 425 to Significantly Improve IPSec Performance on Linux", 426 2010. 428 [KKGEGD] Kounavis, M., Kang, X., Grewal, K., Eszenyi, M., Gueron, 429 S., and D. Durham, "Encrypting the Internet (SIGCOMM)", 430 2010. 432 [M13] McGrew, D., "Impossible plaintext cryptanalysis and 433 probable-plaintext collision attacks of 64-bit block 434 cipher modes", 2012. 436 [PD10] Paterson, K. and J. Degabriele, "On the (in)security of 437 IPsec in MAC-then-encrypt configurations (ACM Conference 438 on Computer and Communications Security, ACM CCS)", 2010. 440 [RFC4305] Eastlake, D., "Cryptographic Algorithm Implementation 441 Requirements for Encapsulating Security Payload (ESP) and 442 Authentication Header (AH)", RFC 4305, December 2005. 444 [RFC4307] Schiller, J., "Cryptographic Algorithms for Use in the 445 Internet Key Exchange Version 2 (IKEv2)", RFC 4307, 446 December 2005. 448 [RFC4835] Manral, V., "Cryptographic Algorithm Implementation 449 Requirements for Encapsulating Security Payload (ESP) and 450 Authentication Header (AH)", RFC 4835, April 2007. 452 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 453 4949, August 2007. 455 [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated 456 Encryption", RFC 5116, January 2008. 458 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 459 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 460 RFC 6151, March 2011. 462 [RFC6379] Law, L. and J. Solinas, "Suite B Cryptographic Suites for 463 IPsec", RFC 6379, October 2011. 465 [V02] Vaudenay, S., "Security Flaws Induced by CBC Padding - 466 Applications to SSL, IPSEC, WTLS ... (EUROCRYPT)", 2002. 468 Authors' Addresses 469 David McGrew 470 Cisco Systems 471 13600 Dulles Technology Drive 472 Herndon, Virginia 20171 473 USA 475 Phone: 408 525 8651 476 Email: mcgrew@cisco.com 478 Wajdi Feghali 479 Intel Corp. 480 75 Reed Road 481 Hudson, Massachusetts 482 USA 484 Email: wajdi.k.feghali@intel.com 486 Paul Hoffman 487 VPN Consortium 489 Email: paul.hoffman@vpnc.org