1 2 3 4NETWORK WORKING GROUP L. Zhu 5Internet-Draft K. Jaganathan 6Expires: March 17, 2006 K. Lauter 7 Microsoft Corporation 8 September 13, 2005 9 10 11 ECC Support for PKINIT 12 draft-zhu-pkinit-ecc-00 13 14Status of this Memo 15 16 By submitting this Internet-Draft, each author represents that any 17 applicable patent or other IPR claims of which he or she is aware 18 have been or will be disclosed, and any of which he or she becomes 19 aware will be disclosed, in accordance with Section 6 of BCP 79. 20 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that 23 other groups may also distribute working documents as Internet- 24 Drafts. 25 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 30 31 The list of current Internet-Drafts can be accessed at 32 http://www.ietf.org/ietf/1id-abstracts.txt. 33 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html. 36 37 This Internet-Draft will expire on March 17, 2006. 38 39Copyright Notice 40 41 Copyright (C) The Internet Society (2005). 42 43Abstract 44 45 This document describes the use of Elliptic Curve certificates, 46 Elliptic Curve signature schemes and Elliptic Curve Diffie-Hellman 47 (ECDH) key agreement within the framework of PKINIT - the Kerberos 48 Version 5 extension that provides for the use of public key 49 cryptography. 50 51 52 53 54 55Zhu, et al. Expires March 17, 2006 [Page 1] 56 57Internet-Draft ECC Support for PKINIT September 2005 58 59 60Table of Contents 61 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 64 3. Using Elliptic Curve Certificates and Elliptic Curve 65 Signature Schemes . . . . . . . . . . . . . . . . . . . . . . 3 66 4. Using ECDH Key Exchange . . . . . . . . . . . . . . . . . . . 4 67 5. Choosing the Domain Parameters and the Key Size . . . . . . . 6 68 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 69 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 70 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 71 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 72 9.1. Normative References . . . . . . . . . . . . . . . . . . . 8 73 9.2. Informative References . . . . . . . . . . . . . . . . . . 9 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 75 Intellectual Property and Copyright Statements . . . . . . . . . . 11 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111Zhu, et al. Expires March 17, 2006 [Page 2] 112 113Internet-Draft ECC Support for PKINIT September 2005 114 115 1161. Introduction 117 118 Elliptic Curve Cryptography (ECC) is emerging as an attractive 119 public-key cryptosystem that provides security equivalent to 120 currently popular public-key mechanisms such as RSA and DSA with 121 smaller key sizes [LENSTRA] [NISTSP80057]. 122 123 Currently [PKINIT] permits the use of ECC algorithms but it does not 124 specify how ECC parameters are chosen and how to derive the shared 125 key for key delivery using Elliptic Curve Diffie-Hellman (ECDH) 126 [IEEE1363]. 127 128 This document describes how to use Elliptic Curve certificates, 129 Elliptic Curve signature schemes, and ECDH with [PKINIT]. However, 130 it should be noted that there is no syntactic or semantic change to 131 the existing [PKINIT] messages. Both the client and the KDC 132 contribute one ECDH key pair using the key agrement protocol 133 described in this document. 134 135 1362. Conventions Used in This Document 137 138 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 139 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 140 document are to be interpreted as described in [RFC2119]. 141 142 1433. Using Elliptic Curve Certificates and Elliptic Curve Signature 144 Schemes 145 146 ECC certificates and signature schemes can be used in the 147 Cryptographic Message Syntax (CMS) [RFC3369] content type 148 'SignedData'. 149 150 X.509 certificates [RFC3280] containing ECC public keys or signed 151 using ECC signature schemes MUST comply with [RFC3279]. 152 153 The elliptic curve domain parameters recommended in [X9.62], 154 [FIPS186-2], and [SECG] SHOULD be used. 155 156 The signatureAlgorithm field of the CMS data type SignerInfo can 157 contain one of the following ECC signature algorithm identifiers: 158 159 ecdsa-with-Sha1 [ECCPKALGS] 160 ecdsa-with-Sha256 [ECCPKALGS] 161 ecdsa-with-Sha384 [ECCPKALGS] 162 ecdsa-with-Sha512 [ECCPKALGS] 163 164 165 166 167Zhu, et al. Expires March 17, 2006 [Page 3] 168 169Internet-Draft ECC Support for PKINIT September 2005 170 171 172 The corresponding digestAlgorithm field contains one of the following 173 hash algorithm identifiers respectively: 174 175 id-sha1 [RFC3279] 176 id-sha256 [ECCPKALGS] 177 id-sha384 [ECCPKALGS] 178 id-sha512 [ECCPKALGS] 179 180 Namely id-sha1 MUST be used in conjunction with ecdsa-with-Sha1, id- 181 sha256 MUST be used in conjunction with ecdsa-with-Sha256, id-sha384 182 MUST be used in conjunction with ecdsa-with-Sha384, and id-sha512 183 MUST be used in conjunction with ecdsa-with-Sha512. 184 185 Implementations of this specfication MUST support ecdsa-with-Sha256 186 and SHOULD support ecdsa-with-Sha1. 187 188 1894. Using ECDH Key Exchange 190 191 This section describes how ECDH can be used as the AS reply key 192 delivery method [PKINIT]. Note that the protocol description is 193 similar to that of Modular Exponential Diffie-Hellman (MODP DH), as 194 described in [PKINIT]. 195 196 If the client wishes to use ECDH key agreement method, it encodes its 197 ECDH public key value and the domain parameters [IEEE1363] for its 198 ECDH public key in clientPublicValue of the PA-PK-AS-REQ message 199 [PKINIT]. 200 201 As described in [PKINIT], the ECDH domain parameters for the client's 202 public key are specified in the algorithm field of the type 203 SubjectPublicKeyInfo [RFC3279] and the client's ECDH public key value 204 is mapped to a subjectPublicKey (a BIT STRING) according to 205 [RFC3279]. 206 207 The following algorithm identifier is used to identify the client's 208 choice of the ECDH key agreement method for key delivery. 209 210 id-ecPublicKey (Elliptic Curve Diffie-Hellman [IEEE1363]) 211 212 If the domain parameters are not accepted by the KDC, the KDC sends 213 back an error message [RFC4120] with the code 214 KDC_ERR_DH_KEY_PARAMETERS_NOT_ACCEPTED [PKINIT]. This error message 215 contains the list of domain parameters acceptable to the KDC. This 216 list is encoded as TD-DH-PARAMETERS [PKINIT], and it is in the KDC's 217 decreasing preference order. The client can then pick a set of 218 domain parameters from the list and retry the authentication. 219 220 221 222 223Zhu, et al. Expires March 17, 2006 [Page 4] 224 225Internet-Draft ECC Support for PKINIT September 2005 226 227 228 Both the client and the KDC MUST have local policy that specifies 229 which set of domain parameters are acceptable if they do not have a 230 priori knowledge of the chosen domain parameters. The need for such 231 local policy is explained in Section 6. 232 233 If the ECDH domain parameters are accepted by the KDC, the KDC sends 234 back its ECDH public key value in the subjectPublicKey field of the 235 PA-PK-AS-REP message [PKINIT]. 236 237 As described in [PKINIT], the KDC's ECDH public key value is encoded 238 as a BIT STRING according to [RFC3279]. 239 240 Note that in the steps above, the client can indicate to the KDC that 241 it wishes to reuse ECDH keys or to allow the KDC to do so, by 242 including the clientDHNonce field in the request [PKINIT], and the 243 KDC can then reuse the ECDH keys and include serverDHNonce field in 244 the reply [PKINIT]. This logic is the same as that of the Modular 245 Exponential Diffie-Hellman key agreement method [PKINIT]. 246 247 If ECDH is negotiated as the key delivery method, both the KDC and 248 the client calculate the shared secret value and derive the reply key 249 as follows: 250 251 1) Let DHSharedSecret be the x-coordinate of the shared secret value 252 (an elliptic curve point). DHSharedSecret is the output of 253 operation ECSVDP-DH as described in Section 7.2.1 of [IEEE1363]. 254 255 2) DHSharedSecret is first padded with leading zeros such that the 256 size of DHSharedSecret in octets is the same as that of the 257 modulus, then represented as a string of octets in big-endian 258 order. 259 260 3) The DHSharedSecret value derived above is used as input to the 261 octetstring2key() function to derive the AS reply key k, as 262 described in Section 3.2.3.1 of [PKINIT]. 263 264 Both the client and KDC then proceed as described in [PKINIT] and 265 [RFC4120]. 266 267 Lastly it should be noted that ECDH can be used with any certificates 268 and signature schemes. However, a significant advantage of using 269 ECDH together with ECC certificates and signature schemes is that the 270 ECC domain parameters in the client or KDC certificates can be used. 271 This obviates the need of locally preconfigured domain parameters as 272 described in Section 6. 273 274 275 276 277 278 279Zhu, et al. Expires March 17, 2006 [Page 5] 280 281Internet-Draft ECC Support for PKINIT September 2005 282 283 2845. Choosing the Domain Parameters and the Key Size 285 286 The domain parameters and the key size should be chosen so as to 287 provide sufficient cryptographic security [RFC3766]. The following 288 table, based on table 2 on page 63 of NIST SP800-57 part 1 289 [NISTSP80057], gives approximate comparable key sizes for symmetric- 290 and asymmetric-key cryptosystems based on the best-known algorithms 291 for attacking them. 292 293 294 Symmetric | ECC | RSA 295 -------------+----------- +------------ 296 80 | 160 - 223 | 1024 297 112 | 224 - 255 | 2048 298 128 | 256 - 383 | 3072 299 192 | 384 - 511 | 7680 300 256 | 512+ | 15360 301 302 Table 1: Comparable key sizes (in bits) 303 304 Thus, for example, when securing a 128-bit symmetric key, it is 305 prudent to use 256-bit Elliptic Curve Cryptography (ECC), e.g. group 306 P-256 (secp256r1) as described below. 307 308 A set of ECDH domain parameters is also known as a curve. A curve is 309 a named curve if the domain paratmeters are well known and can be 310 identified by an Object Identifier, otherwise it is called a custom 311 curve. [PKINIT] supports both named curves and custom curves, see 312 Section 6 on the tradeoff of choosing between named curves and custom 313 curves. 314 315 The named curves recommended in this document are also recommended by 316 NIST [FIPS186-2]. These fifteen ECC curves are given in the 317 following table [FIPS186-2] [SECG]. 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335Zhu, et al. Expires March 17, 2006 [Page 6] 336 337Internet-Draft ECC Support for PKINIT September 2005 338 339 340 Description SEC 2 OID 341 ----------------- --------- 342 343 ECPRGF192Random group P-192 secp192r1 344 EC2NGF163Random group B-163 sect163r2 345 EC2NGF163Koblitz group K-163 sect163k1 346 347 ECPRGF224Random group P-224 secp224r1 348 EC2NGF233Random group B-233 sect233r1 349 EC2NGF233Koblitz group K-233 sect233k1 350 351 ECPRGF256Random group P-256 secp256r1 352 EC2NGF283Random group B-283 sect283r1 353 EC2NGF283Koblitz group K-283 sect283k1 354 355 ECPRGF384Random group P-384 secp384r1 356 EC2NGF409Random group B-409 sect409r1 357 EC2NGF409Koblitz group K-409 sect409k1 358 359 ECPRGF521Random group P-521 secp521r1 360 EC2NGF571Random group B-571 sect571r1 361 EC2NGF571Koblitz group K-571 sect571k1 362 363 3646. Security Considerations 365 366 Kerberos error messages are not integrity protected, as a result, the 367 domain parameters sent by the KDC as TD-DH-PARAMETERS can be tampered 368 with by an attacker so that the set of domain parameters selected 369 could be either weaker or not mutually preferred. Local policy can 370 configure sets of domain parameters acceptable locally, or disallow 371 the negotiation of ECDH domain parameters. 372 373 Beyond elliptic curve size, the main issue is elliptic curve 374 structure. As a general principle, it is more conservative to use 375 elliptic curves with as little algebraic structure as possible - thus 376 random curves are more conservative than special curves such as 377 Koblitz curves, and curves over F_p with p random are more 378 conservative than curves over F_p with p of a special form (and 379 curves over F_p with p random might be considered more conservative 380 than curves over F_2^m as there is no choice between multiple fields 381 of similar size for characteristic 2). Note, however, that algebraic 382 structure can also lead to implementation efficiencies and 383 implementors and users may, therefore, need to balance conservatism 384 against a need for efficiency. Concrete attacks are known against 385 only very few special classes of curves, such as supersingular 386 curves, and these classes are excluded from the ECC standards such as 387 [IEEE1363] and [X9.62]. 388 389 390 391Zhu, et al. Expires March 17, 2006 [Page 7] 392 393Internet-Draft ECC Support for PKINIT September 2005 394 395 396 Another issue is the potential for catastrophic failures when a 397 single elliptic curve is widely used. In this case, an attack on the 398 elliptic curve might result in the compromise of a large number of 399 keys. Again, this concern may need to be balanced against efficiency 400 and interoperability improvements associated with widely-used curves. 401 Substantial additional information on elliptic curve choice can be 402 found in [IEEE1363], [X9.62] and [FIPS186-2]. 403 404 4057. IANA Considerations 406 407 No IANA actions are required for this document. 408 409 4108. Acknowledgements 411 412 The following people have made significant contributions to this 413 draft: Paul Leach, Dan Simon, Kelvin Yiu, David Cross and Sam 414 Hartman. 415 416 4179. References 418 4199.1. Normative References 420 421 [ECCPKALGS] 422 RFC-Editor: To be replaced by RFC number for draft-ietf- 423 pkix-ecc-pkalgs. Work in Progress. 424 425 [FIPS186-2] 426 NIST, "Digital Signature Standard", FIPS 186-2, 2000. 427 428 [IEEE1363] 429 IEEE, "Standard Specifications for Public Key Cryptography", 430 IEEE 1363, 2000. 431 432 [NISTSP80057] 433 NIST, "Recommendation on Key Management", 434 http://csrc.nist.gov/publications/nistpubs/, SP 800-57, 435 August 2005. 436 437 [PKINIT] RFC-Editor: To be replaced by RFC number for draft-ietf- 438 cat-kerberos-pk-init. Work in Progress. 439 440 441Zhu, et al. Expires March 17, 2006 [Page 8] 442 443Internet-Draft ECC Support for PKINIT September 2005 444 445 446 447 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 448 Requirement Levels", BCP 14, RFC 2119, March 1997. 449 450 [RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and 451 Identifiers for the Internet X.509 Public Key 452 Infrastructure Certificate and Certificate Revocation List 453 (CRL) Profile", RFC 3279, April 2002. 454 455 [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet 456 X.509 Public Key Infrastructure Certificate and 457 Certificate Revocation List (CRL) Profile", RFC 3280, 458 April 2002. 459 460 [RFC3369] Housley, R., "Cryptographic Message Syntax (CMS)", 461 RFC 3369, August 2002. 462 463 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For 464 Public Keys Used For Exchanging Symmetric Keys", BCP 86, 465 RFC 3766, April 2004. 466 467 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 468 Kerberos Network Authentication Service (V5)", RFC 4120, 469 July 2005. 470 471 [X9.62] ANSI, "Public Key Cryptography For The Financial Services 472 Industry: The Elliptic Curve Digital Signature Algorithm 473 (ECDSA)", ANSI X9.62, 1998. 474 4759.2. Informative References 476 477 [LENSTRA] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key 478 Sizes", Journal of Cryptology 14 (2001) 255-293. 479 480 [SECG] SECG, "Elliptic Curve Cryptography", SEC 1, 2000, 481 <http://www.secg.org/>. 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496Zhu, et al. Expires March 17, 2006 [Page 9] 497 498Internet-Draft ECC Support for PKINIT September 2005 499 500 501Authors' Addresses 502 503 Larry Zhu 504 Microsoft Corporation 505 One Microsoft Way 506 Redmond, WA 98052 507 US 508 509 Email: lzhu@microsoft.com 510 511 512 Karthik Jaganathan 513 Microsoft Corporation 514 One Microsoft Way 515 Redmond, WA 98052 516 US 517 518 Email: karthikj@microsoft.com 519 520 521 Kristin Lauter 522 Microsoft Corporation 523 One Microsoft Way 524 Redmond, WA 98052 525 US 526 527 Email: klauter@microsoft.com 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552Zhu, et al. Expires March 17, 2006 [Page 10] 553 554Internet-Draft ECC Support for PKINIT September 2005 555 556 557Intellectual Property Statement 558 559 The IETF takes no position regarding the validity or scope of any 560 Intellectual Property Rights or other rights that might be claimed to 561 pertain to the implementation or use of the technology described in 562 this document or the extent to which any license under such rights 563 might or might not be available; nor does it represent that it has 564 made any independent effort to identify any such rights. 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This document is subject 596 to the rights, licenses and restrictions contained in BCP 78, and 597 except as set forth therein, the authors retain all their rights. 598 599 600Acknowledgment 601 602 Funding for the RFC Editor function is currently provided by the 603 Internet Society. 604 605 606 607 608Zhu, et al. Expires March 17, 2006 [Page 11] 609 610 611