1 2 3 4NETWORK WORKING GROUP A. Menon-Sen 5Internet-Draft Oryx Mail Systems GmbH 6Intended status: Standards Track A. Melnikov 7Expires: February 1, 2010 Isode Ltd 8 C. Newman 9 N. Williams 10 Sun Microsystems 11 July 31, 2009 12 13 14 Salted Challenge Response (SCRAM) SASL Mechanism 15 draft-ietf-sasl-scram-04.txt 16 17Status of this Memo 18 19 This Internet-Draft is submitted to IETF in full conformance with the 20 provisions of BCP 78 and BCP 79. 21 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF), its areas, and its working groups. Note that 24 other groups may also distribute working documents as Internet- 25 Drafts. 26 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 31 32 The list of current Internet-Drafts can be accessed at 33 http://www.ietf.org/ietf/1id-abstracts.txt. 34 35 The list of Internet-Draft Shadow Directories can be accessed at 36 http://www.ietf.org/shadow.html. 37 38 This Internet-Draft will expire on February 1, 2010. 39 40Copyright Notice 41 42 Copyright (c) 2009 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 44 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents in effect on the date of 47 publication of this document (http://trustee.ietf.org/license-info). 48 Please review these documents carefully, as they describe your rights 49 and restrictions with respect to this document. 50 51 52 53 54 55Menon-Sen, et al. Expires February 1, 2010 [Page 1] 56 57Internet-Draft SCRAM July 2009 58 59 60Abstract 61 62 The secure authentication mechanism most widely deployed and used by 63 Internet application protocols is the transmission of clear-text 64 passwords over a channel protected by Transport Layer Security (TLS). 65 There are some significant security concerns with that mechanism, 66 which could be addressed by the use of a challenge response 67 authentication mechanism protected by TLS. Unfortunately, the 68 challenge response mechanisms presently on the standards track all 69 fail to meet requirements necessary for widespread deployment, and 70 have had success only in limited use. 71 72 This specification describes a family of Simple Authentication and 73 Security Layer (SASL, RFC 4422) authentication mechanisms called the 74 Salted Challenge Response Authentication Mechanism (SCRAM), which 75 addresses the security concerns and meets the deployability 76 requirements. When used in combination with TLS or an equivalent 77 security layer, a mechanism from this family could improve the 78 status-quo for application protocol authentication and provide a 79 suitable choice for a mandatory-to-implement mechanism for future 80 application protocol standards. 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 111Menon-Sen, et al. Expires February 1, 2010 [Page 2] 112 113Internet-Draft SCRAM July 2009 114 115 116Table of Contents 117 118 1. Conventions Used in This Document . . . . . . . . . . 4 119 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 4 120 1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . 5 121 2. Introduction . . . . . . . . . . . . . . . . . . . . . 7 122 3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . 9 123 4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . 10 124 5. SCRAM Authentication Exchange . . . . . . . . . . . . 11 125 5.1. SCRAM Attributes . . . . . . . . . . . . . . . . . . . 12 126 6. Channel Binding . . . . . . . . . . . . . . . . . . . 15 127 6.1. Default Channel Binding . . . . . . . . . . . . . . . 16 128 7. Formal Syntax . . . . . . . . . . . . . . . . . . . . 17 129 8. SCRAM as a GSS-API Mechanism . . . . . . . . . . . . . 20 130 8.1. GSS-API Principal Name Types for SCRAM . . . . . . . . 20 131 8.2. GSS-API Per-Message Tokens for SCRAM . . . . . . . . . 20 132 8.3. GSS_Pseudo_random() for SCRAM . . . . . . . . . . . . 21 133 9. Security Considerations . . . . . . . . . . . . . . . 22 134 10. IANA Considerations . . . . . . . . . . . . . . . . . 24 135 11. Acknowledgements . . . . . . . . . . . . . . . . . . . 26 136 Appendix A. Other Authentication Mechanisms . . . . . . . . . . . 27 137 Appendix B. Design Motivations . . . . . . . . . . . . . . . . . . 28 138 Appendix C. Internet-Draft Change History . . . . . . . . . . . . 29 139 12. References . . . . . . . . . . . . . . . . . . . . . . 31 140 12.1. Normative References . . . . . . . . . . . . . . . . . 31 141 12.2. Normative References for GSS-API implementors . . . . 31 142 12.3. Informative References . . . . . . . . . . . . . . . . 32 143 Authors' Addresses . . . . . . . . . . . . . . . . . . 34 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167Menon-Sen, et al. Expires February 1, 2010 [Page 3] 168 169Internet-Draft SCRAM July 2009 170 171 1721. Conventions Used in This Document 173 174 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 175 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 176 document are to be interpreted as described in [RFC2119]. 177 178 Formal syntax is defined by [RFC5234] including the core rules 179 defined in Appendix B of [RFC5234]. 180 181 Example lines prefaced by "C:" are sent by the client and ones 182 prefaced by "S:" by the server. If a single "C:" or "S:" label 183 applies to multiple lines, then the line breaks between those lines 184 are for editorial clarity only, and are not part of the actual 185 protocol exchange. 186 1871.1. Terminology 188 189 This document uses several terms defined in [RFC4949] ("Internet 190 Security Glossary") including the following: authentication, 191 authentication exchange, authentication information, brute force, 192 challenge-response, cryptographic hash function, dictionary attack, 193 eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, 194 one-way encryption function, password, replay attack and salt. 195 Readers not familiar with these terms should use that glossary as a 196 reference. 197 198 Some clarifications and additional definitions follow: 199 200 o Authentication information: Information used to verify an identity 201 claimed by a SCRAM client. The authentication information for a 202 SCRAM identity consists of salt, iteration count, the "StoredKey" 203 and "ServerKey" (as defined in the algorithm overview) for each 204 supported cryptographic hash function. 205 206 o Authentication database: The database used to look up the 207 authentication information associated with a particular identity. 208 For application protocols, LDAPv3 (see [RFC4510]) is frequently 209 used as the authentication database. For network-level protocols 210 such as PPP or 802.11x, the use of RADIUS is more common. 211 212 o Base64: An encoding mechanism defined in [RFC4648] which converts 213 an octet string input to a textual output string which can be 214 easily displayed to a human. The use of base64 in SCRAM is 215 restricted to the canonical form with no whitespace. 216 217 o Octet: An 8-bit byte. 218 219 o Octet string: A sequence of 8-bit bytes. 220 221 222 223Menon-Sen, et al. Expires February 1, 2010 [Page 4] 224 225Internet-Draft SCRAM July 2009 226 227 228 o Salt: A random octet string that is combined with a password 229 before applying a one-way encryption function. This value is used 230 to protect passwords that are stored in an authentication 231 database. 232 2331.2. Notation 234 235 The pseudocode description of the algorithm uses the following 236 notations: 237 238 o ":=": The variable on the left hand side represents the octet 239 string resulting from the expression on the right hand side. 240 241 o "+": Octet string concatenation. 242 243 o "[ ]": A portion of an expression enclosed in "[" and "]" may not 244 be included in the result under some circumstances. See the 245 associated text for a description of those circumstances. 246 247 o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in 248 [RFC2104]) using the octet string represented by "key" as the key 249 and the octet string "str" as the input string. The size of the 250 result is the hash result size for the hash function in use. For 251 example, it is 20 octets for SHA-1 (see [RFC3174]). 252 253 o H(str): Apply the cryptographic hash function to the octet string 254 "str", producing an octet string as a result. The size of the 255 result depends on the hash result size for the hash function in 256 use. 257 258 o XOR: Apply the exclusive-or operation to combine the octet string 259 on the left of this operator with the octet string on the right of 260 this operator. The length of the output and each of the two 261 inputs will be the same for this use. 262 263 o Hi(str, salt): 264 265 266 267 U0 := HMAC(str, salt + INT(1)) 268 U1 := HMAC(str, U0) 269 U2 := HMAC(str, U1) 270 ... 271 Ui-1 := HMAC(str, Ui-2) 272 Ui := HMAC(str, Ui-1) 273 274 Hi := U0 XOR U1 XOR U2 XOR ... XOR Ui 275 276 277 278 279Menon-Sen, et al. Expires February 1, 2010 [Page 5] 280 281Internet-Draft SCRAM July 2009 282 283 284 where "i" is the iteration count, "+" is the string concatenation 285 operator and INT(g) is a four-octet encoding of the integer g, 286 most significant octet first. 287 288 o This is, essentially, PBKDF2 [RFC2898] with HMAC() as the PRF and 289 with dkLen == output length of HMAC() == output length of H(). 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335Menon-Sen, et al. Expires February 1, 2010 [Page 6] 336 337Internet-Draft SCRAM July 2009 338 339 3402. Introduction 341 342 This specification describes a family of authentication mechanisms 343 called the Salted Challenge Response Authentication Mechanism (SCRAM) 344 which addresses the requirements necessary to deploy a challenge- 345 response mechanism more widely than past attempts. When used in 346 combination with Transport Layer Security (TLS, see [RFC5246]) or an 347 equivalent security layer, a mechanism from this family could improve 348 the status-quo for application protocol authentication and provide a 349 suitable choice for a mandatory-to-implement mechanism for future 350 application protocol standards. 351 352 For simplicity, this family of mechanisms does not presently include 353 negotiation of a security layer [RFC4422]. It is intended to be used 354 with an external security layer such as that provided by TLS or SSH, 355 with optional channel binding [RFC5056] to the external security 356 layer. 357 358 SCRAM is specified herein as a pure Simple Authentication and 359 Security Layer (SASL) [RFC4422] mechanism, but it conforms to the new 360 bridge between SASL and the Generic Security Services Application 361 Programming Interface (GSS-API) called "GS2" [I-D.ietf-sasl-gs2]. 362 This means that this document defines both, a SASL mechanism and a 363 GSS-API mechanism. 364 365 SCRAM provides the following protocol features: 366 367 o The authentication information stored in the authentication 368 database is not sufficient by itself to impersonate the client. 369 The information is salted to prevent a pre-stored dictionary 370 attack if the database is stolen. 371 372 o The server does not gain the ability to impersonate the client to 373 other servers (with an exception for server-authorized proxies). 374 375 o The mechanism permits the use of a server-authorized proxy without 376 requiring that proxy to have super-user rights with the back-end 377 server. 378 379 o Mutual authentication is supported, but only the client is named 380 (i.e., the server has no name). 381 382 A separate document defines a standard LDAPv3 [RFC4510] attribute 383 that enables storage of the SCRAM authentication information in LDAP. 384 See [I-D.melnikov-sasl-scram-ldap]. 385 386 For an in-depth discussion of why other challenge response mechanisms 387 are not considered sufficient, see appendix A. For more information 388 389 390 391Menon-Sen, et al. Expires February 1, 2010 [Page 7] 392 393Internet-Draft SCRAM July 2009 394 395 396 about the motivations behind the design of this mechanism, see 397 appendix B. 398 399 Comments regarding this draft may be sent either to the 400 ietf-sasl@imc.org mailing list or to the authors. 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447Menon-Sen, et al. Expires February 1, 2010 [Page 8] 448 449Internet-Draft SCRAM July 2009 450 451 4523. SCRAM Algorithm Overview 453 454 Note that this section omits some details, such as client and server 455 nonces. See Section 5 for more details. 456 457 To begin with, the SCRAM client is in possession of a username and 458 password. It sends the username to the server, which retrieves the 459 corresponding authentication information, i.e. a salt, StoredKey, 460 ServerKey and the iteration count i. (Note that a server 461 implementation may chose to use the same iteration count for all 462 accounts.) The server sends the salt and the iteration count to the 463 client, which then computes the following values and sends a 464 ClientProof to the server: 465 466 467 SaltedPassword := Hi(password, salt) 468 ClientKey := HMAC(SaltedPassword, "Client Key") 469 StoredKey := H(ClientKey) 470 AuthMessage := client-first-message-bare + "," + 471 server-first-message + "," + 472 client-final-message-without-proof 473 ClientSignature := HMAC(StoredKey, AuthMessage) 474 ClientProof := ClientKey XOR ClientSignature 475 ServerKey := HMAC(SaltedPassword, "Server Key") 476 ServerSignature := HMAC(ServerKey, AuthMessage) 477 478 479 The server authenticates the client by computing the ClientSignature, 480 exclusive-ORing that with the ClientProof to recover the ClientKey 481 and verifying the correctness of the ClientKey by applying the hash 482 function and comparing the result to the StoredKey. If the ClientKey 483 is correct, this proves that the client has access to the user's 484 password. 485 486 Similarly, the client authenticates the server by computing the 487 ServerSignature and comparing it to the value sent by the server. If 488 the two are equal, it proves that the server had access to the user's 489 ServerKey. 490 491 The AuthMessage is computed by concatenating messages from the 492 authentication exchange. The format of these messages is defined in 493 Section 7. 494 495 496 497 498 499 500 501 502 503Menon-Sen, et al. Expires February 1, 2010 [Page 9] 504 505Internet-Draft SCRAM July 2009 506 507 5084. SCRAM Mechanism Names 509 510 A SCRAM mechanism name is a string "SCRAM-" followed by the 511 uppercased name of the underlying hash function taken from the IANA 512 "Hash Function Textual Names" registry (see http://www.iana.org), 513 optionally followed by the suffix "-PLUS" (see below). Note that 514 SASL mechanism names are limited to 20 characters, which means that 515 only hash function names with lengths shorter or equal to 9 516 characters (20-length("SCRAM-")-length("-PLUS") can be used. For 517 cases when the underlying hash function name is longer than 9 518 characters, an alternative 9 character (or shorter) name can be used 519 to construct the corresponding SCRAM mechanism name, as long as this 520 alternative name doesn't conflict with any other hash function name 521 from the IANA "Hash Function Textual Names" registry. 522 523 For interoperability, all SCRAM clients and servers MUST implement 524 the SCRAM-SHA-1 authentication mechanism, i.e. an authentication 525 mechanism from the SCRAM family that uses the SHA-1 hash function as 526 defined in [RFC3174]. 527 528 The "-PLUS" suffix is used only when the server supports channel 529 binding to the external channel. If the server supports channel 530 binding, it will advertise both the "bare" and "plus" versions of 531 whatever mechanisms it supports (e.g., if the server supports only 532 SCRAM with SHA-1 then it will advertise support for both SCRAM-SHA-1 533 and SCRAM-SHA-1-PLUS); if the server does not support channel 534 binding, then it will advertise only the "bare" version of the 535 mechanism (e.g., only SCRAM-SHA-1). The "-PLUS" exists to allow 536 negotiation of the use of channel binding. See Section 6. 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559Menon-Sen, et al. Expires February 1, 2010 [Page 10] 560 561Internet-Draft SCRAM July 2009 562 563 5645. SCRAM Authentication Exchange 565 566 SCRAM is a SASL mechanism whose client response and server challenge 567 messages are text-based messages containing one or more attribute- 568 value pairs separated by commas. Each attribute has a one-letter 569 name. The messages and their attributes are described in 570 Section 5.1, and defined in Section 7. 571 572 This is a simple example of a SCRAM-SHA-1 authentication exchange 573 when the client doesn't support channel bindings: 574 575 576 C: n,,n=Chris Newman,r=ClientNonce 577 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 578 C: c=biwsCg==,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4 579 S: v=WxPv/siO5l+qxN4 580 581 582 [[anchor5: Note that the all hashes above are fake and will be fixed 583 during AUTH48.]] 584 585 With channel-binding data sent by the client this might look like 586 this (see [tls-server-end-point] for the definition of tls-server- 587 end-point TLS channel binding): 588 589 590 C: p=tls-server-end-point,,n=Chris Newman,r=ClientNonce 591 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 592 C: c=cD10bHMtc2VydmVyLWVuZC1wb2ludCwsy1hFtXOnZ+ySrQM6srFp 593 l/77uqvtxrg7nBY1BetEr/g=,r=ClientNonceServerNonce,p=Wx 594 Pv/siO5l+qxN4 595 S: v=WxPv/siO5l+qxN4 596 597 598 [[anchor6: Note that all hashes above are fake and will be fixed 599 during AUTH48.]] 600 601 First, the client sends a message containing: 602 603 o a GS2 header consisting of a flag indicating whether channel 604 binding is supported-but-not-used, not supported, or used, and an 605 optional SASL authorization identity; 606 607 o SCRAM username and a random, unique nonce attributes. 608 609 Note that the client's first message will always start with "n", "y" 610 or "p", otherwise the message is invalid and authentication MUST 611 fail. This is important, as it allows for GS2 extensibility (e.g., 612 613 614 615Menon-Sen, et al. Expires February 1, 2010 [Page 11] 616 617Internet-Draft SCRAM July 2009 618 619 620 to add support for security layers). 621 622 In response, the server sends the user's iteration count i, the 623 user's salt, and appends its own nonce to the client-specified one. 624 The client then responds with the same nonce and a ClientProof 625 computed using the selected hash function as explained earlier. The 626 server verifies the nonce and the proof, verifies that the 627 authorization identity (if supplied by the client in the first 628 message) is authorized to act as the authentication identity, and, 629 finally, it responds with a ServerSignature, concluding the 630 authentication exchange. The client then authenticates the server by 631 computing the ServerSignature and comparing it to the value sent by 632 the server. If the two are different, the client MUST consider the 633 authentication exchange to be unsuccessful and it might have to drop 634 the connection. 635 6365.1. SCRAM Attributes 637 638 This section describes the permissible attributes, their use, and the 639 format of their values. All attribute names are single US-ASCII 640 letters and are case-sensitive. 641 642 Note that the order of attributes in client or server messages is 643 fixed, with the exception of extension attributes (described by the 644 "extensions" ABNF production), which can appear in any order in the 645 designated positions. See the ABNF section for authoritative 646 reference. 647 648 o a: This is an optional attribute, and is part of the GS2 649 [I-D.ietf-sasl-gs2] bridge between the GSS-API and SASL. This 650 attribute specifies an authorization identity. A client may 651 include it in its first message to the server if it wants to 652 authenticate as one user, but subsequently act as a different 653 user. This is typically used by an administrator to perform some 654 management task on behalf of another user, or by a proxy in some 655 situations. 656 657 Upon the receipt of this value the server verifies its 658 correctness according to the used SASL protocol profile. 659 Failed verification results in failed authentication exchange. 660 661 If this attribute is omitted (as it normally would be), the 662 authorization identity is assumed to be derived from the 663 username specified with the (required) "n" attribute. 664 665 The server always authenticates the user specified by the "n" 666 attribute. If the "a" attribute specifies a different user, 667 the server associates that identity with the connection after 668 669 670 671Menon-Sen, et al. Expires February 1, 2010 [Page 12] 672 673Internet-Draft SCRAM July 2009 674 675 676 successful authentication and authorization checks. 677 678 The syntax of this field is the same as that of the "n" field 679 with respect to quoting of '=' and ','. 680 681 o n: This attribute specifies the name of the user whose password is 682 used for authentication (a.k.a. "authentication identity" 683 [RFC4422]). A client MUST include it in its first message to the 684 server. If the "a" attribute is not specified (which would 685 normally be the case), this username is also the identity which 686 will be associated with the connection subsequent to 687 authentication and authorization. 688 689 Before sending the username to the server, the client MUST 690 prepare the username using the "SASLPrep" profile [RFC4013] of 691 the "stringprep" algorithm [RFC3454]. If the preparation of 692 the username fails or results in an empty string, the client 693 SHOULD abort the authentication exchange (*). 694 695 (*) An interactive client can request a repeated entry of the 696 username value. 697 698 Upon receipt of the username by the server, the server SHOULD 699 prepare it using the "SASLPrep" profile [RFC4013] of the 700 "stringprep" algorithm [RFC3454]. If the preparation of the 701 username fails or results in an empty string, the server SHOULD 702 abort the authentication exchange. 703 704 The characters ',' or '=' in usernames are sent as '=2C' and 705 '=3D' respectively. If the server receives a username which 706 contains '=' not followed by either '2C' or '3D', then the 707 server MUST fail the authentication. 708 709 o m: This attribute is reserved for future extensibility. In this 710 version of SCRAM, its presence in a client or a server message 711 MUST cause authentication failure when the attribute is parsed by 712 the other end. 713 714 o r: This attribute specifies a sequence of random printable 715 characters excluding ',' which forms the nonce used as input to 716 the hash function. No quoting is applied to this string. As 717 described earlier, the client supplies an initial value in its 718 first message, and the server augments that value with its own 719 nonce in its first response. It is important that this value be 720 different for each authentication. The client MUST verify that 721 the initial part of the nonce used in subsequent messages is the 722 same as the nonce it initially specified. The server MUST verify 723 that the nonce sent by the client in the second message is the 724 725 726 727Menon-Sen, et al. Expires February 1, 2010 [Page 13] 728 729Internet-Draft SCRAM July 2009 730 731 732 same as the one sent by the server in its first message. 733 734 o c: This REQUIRED attribute specifies base64-encoded of a header 735 and the channel-binding data. It is sent by the client in its 736 second authentication message. The header consist of: 737 738 * the GS2 header from the client's first message (recall: a 739 channel binding flag and an optional authzid). This header is 740 going to include channel binding type prefix (see [RFC5056]), 741 if and only if the client is using channel binding; 742 743 * followed by the external channel's channel binding data, if and 744 only if the client is using channel binding. 745 746 o s: This attribute specifies the base64-encoded salt used by the 747 server for this user. It is sent by the server in its first 748 message to the client. 749 750 o i: This attribute specifies an iteration count for the selected 751 hash function and user, and MUST be sent by the server along with 752 the user's salt. 753 754 For SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism servers SHOULD 755 announce a hash iteration-count of at least 4096. Note that a 756 client implementation MAY cache SaltedPassword/ClientKey for 757 later reauthentication to the same service, as it is likely 758 that the server is going to advertise the same salt value upon 759 reauthentication. This might be useful for mobile clients 760 where CPU usage is a concern. 761 762 o p: This attribute specifies a base64-encoded ClientProof. The 763 client computes this value as described in the overview and sends 764 it to the server. 765 766 o v: This attribute specifies a base64-encoded ServerSignature. It 767 is sent by the server in its final message, and is used by the 768 client to verify that the server has access to the user's 769 authentication information. This value is computed as explained 770 in the overview. 771 772 773 774 775 776 777 778 779 780 781 782 783Menon-Sen, et al. Expires February 1, 2010 [Page 14] 784 785Internet-Draft SCRAM July 2009 786 787 7886. Channel Binding 789 790 SCRAM supports channel binding to external secure channels, such as 791 TLS. Clients and servers may or may not support channel binding, 792 therefore the use of channel binding is negotiable. SCRAM does not 793 provide security layers, however, therefore it is imperative that 794 SCRAM provide integrity protection for the negotiation of channel 795 binding. 796 797 Use of channel binding is negotiated as follows: 798 799 o Servers SHOULD advertise both non-PLUS (SCRAM-<hash-function>) and 800 the PLUS-variant (SCRAM-<hash-function>-PLUS) SASL mechanism 801 names. If the server cannot support channel binding, it MAY 802 advertise only the non-PLUS variant. If the server would never 803 succeed authentication of the non-PLUS variant due to policy 804 reasons, it MAY advertise only the PLUS-variant. 805 806 o If the client negotiates mechanisms then the client MUST select 807 SCRAM-<hash-function>-PLUS if offered by the server and the client 808 wants to select SCRAM with the given hash function. Otherwise 809 (the client does not negotiate mechanisms), if the client has no 810 prior knowledge about mechanisms supported by the server and 811 wasn't explicitly configured to use a particular variant of the 812 SCRAM mechanism, then it MUST select only SCRAM-<hash-function> 813 (not suffixed with "-PLUS"). 814 815 o If the client supports channel binding and the server appears to 816 support it (i.e., the client sees SCRAM-<hash-function>-PLUS), or 817 if the client wishes to use channel binding but the client does 818 not negotiate mechanisms, then the client MUST set the GS2 channel 819 binding flag to "p" in order to indicate the channel binding type 820 it is using and it MUST include the channel binding data for the 821 external channel in the computation of the "c=" attribute (see 822 Section 5.1). 823 824 o If the client supports channel binding but the server does not 825 appear to (i.e., the client did not see SCRAM-<hash-function>- 826 PLUS) then the client MUST either fail authentication or it MUST 827 choose the non-PLUS mechanism and set the GS2 channel binding flag 828 to "y" and MUST NOT include channel binding data for the external 829 channel in the computation of the "c=" attribute (see 830 Section 5.1). 831 832 o If the client does not support channel binding then the client 833 MUST set the GS2 channel binding flag to "n" and MUST NOT include 834 channel binding data for the external channel in the computation 835 of the "c=" attribute (see Section 5.1). 836 837 838 839Menon-Sen, et al. Expires February 1, 2010 [Page 15] 840 841Internet-Draft SCRAM July 2009 842 843 844 o Upon receipt of the client first message the server checks the GS2 845 channel binding flag (gs2-cb-flag). 846 847 * If the flag is set to "y" and the server supports channel 848 binding the server MUST fail authentication. This is because 849 if the client sets the GS2 channel binding flag set to "y" then 850 the client must have believed that the server did not support 851 channel binding -- if the server did in fact support channel 852 binding then this is an indication that there has been a 853 downgrade attack (e.g., an attacker changed the server's 854 mechanism list to exclude the -PLUS suffixed SCRAM mechanism 855 name(s)). 856 857 * If the channel binding flag was "p" and the server does not 858 support the indicated channel binding type then the server MUST 859 fail authentication. 860 861 The server MUST always validate the client's "c=" field. The server 862 does this by constructing the value of the "c=" attribute and then 863 checking that it matches the client's c= attribute value. 864 865 For more discussions of channel bindings, and the syntax of the 866 channel binding data for various security protocols, see [RFC5056]. 867 8686.1. Default Channel Binding 869 870 A default channel binding type agreement process for all SASL 871 application protocols that do not provide their own channel binding 872 type agreement is provided as follows. 873 874 'tls-unique' is the default channel binding type for any application 875 that doesn't specify one. 876 877 Servers MUST implement the "tls-unique" [tls-unique] 878 [I-D.altman-tls-channel-bindings] channel binding type, if they 879 implement any channel binding. Clients SHOULD implement the "tls- 880 unique" [tls-unique] [I-D.altman-tls-channel-bindings] channel 881 binding type, if they implement any channel binding. Clients and 882 servers SHOULD choose the highest- layer/innermost end-to-end TLS 883 channel as the channel to bind to. 884 885 Servers MUST choose the channel binding type indicated by the client, 886 or fail authentication if they don't support it. 887 888 889 890 891 892 893 894 895Menon-Sen, et al. Expires February 1, 2010 [Page 16] 896 897Internet-Draft SCRAM July 2009 898 899 9007. Formal Syntax 901 902 The following syntax specification uses the Augmented Backus-Naur 903 Form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3" 904 and "UTF8-4" non-terminal are defined in [RFC3629]. 905 906 907 ALPHA = <as defined in RFC 5234 appendix B.1> 908 DIGIT = <as defined in RFC 5234 appendix B.1> 909 UTF8-2 = <as defined in RFC 3629 (STD 63)> 910 UTF8-3 = <as defined in RFC 3629 (STD 63)> 911 UTF8-4 = <as defined in RFC 3629 (STD 63)> 912 913 attr-val = ALPHA "=" value 914 ;; Generic syntax of any attribute sent 915 ;; by server or client 916 917 value = 1*value-char 918 919 value-safe-char = %x01-2B / %x2D-3C / %x3E-7F / 920 UTF8-2 / UTF8-3 / UTF8-4 921 ;; UTF8-char except NUL, "=", and ",". 922 923 value-char = value-safe-char / "=" 924 925 base64-char = ALPHA / DIGIT / "/" / "+" 926 927 base64-4 = 4base64-char 928 929 base64-3 = 3base64-char "=" 930 931 base64-2 = 2base64-char "==" 932 933 base64 = *base64-4 [base64-3 / base64-2] 934 935 posit-number = %x31-39 *DIGIT 936 ;; A positive number 937 938 saslname = 1*(value-safe-char / "=2C" / "=3D") 939 ;; Conforms to <value> 940 941 authzid = "a=" saslname 942 ;; Protocol specific. 943 944 cb-name = 1*(ALPHA / DIGIT / "." / "-") 945 ;; See RFC 5056 section 7. 946 ;; E.g. "tls-server-end-point" or 947 ;; "tls-unique" 948 949 950 951Menon-Sen, et al. Expires February 1, 2010 [Page 17] 952 953Internet-Draft SCRAM July 2009 954 955 956 gs2-cbind-flag = "p=" cb-name / "n" / "y" 957 ;; "n" -> client doesn't support channel binding 958 ;; "y" -> client does support channel binding 959 ;; but thinks the server does not. 960 ;; "p" -> client requires channel binding. 961 ;; The selected channel binding follows "p=". 962 963 gs2-header = gs2-cbind-flag "," [ authzid ] "," 964 ;; GS2 header for SCRAM 965 ;; (the actual GS2 header includes an optional 966 ;; flag to indicate that the GSS mechanism is not 967 ;; "standard" but since SCRAM is "standard" we 968 ;; don't include that flag). 969 970 username = "n=" saslname 971 ;; Usernames are prepared using SASLPrep. 972 973 reserved-mext = "m=" 1*(value-char) 974 ;; Reserved for signalling mandatory extensions. 975 ;; The exact syntax will be defined in 976 ;; the future. 977 978 channel-binding = "c=" base64 979 ;; base64 encoding of cbind-input 980 981 proof = "p=" base64 982 983 nonce = "r=" c-nonce [s-nonce] 984 ;; Second part provided by server. 985 986 c-nonce = value 987 988 s-nonce = value 989 990 salt = "s=" base64 991 992 verifier = "v=" base64 993 ;; base-64 encoded ServerSignature. 994 995 iteration-count = "i=" posit-number 996 ;; A positive number 997 998 client-first-message-bare = 999 [reserved-mext ","] 1000 username "," nonce ["," extensions] 1001 1002 client-first-message = 1003 gs2-header client-first-message-bare 1004 1005 1006 1007Menon-Sen, et al. Expires February 1, 2010 [Page 18] 1008 1009Internet-Draft SCRAM July 2009 1010 1011 1012 server-first-message = 1013 [reserved-mext ","] nonce "," salt "," 1014 iteration-count ["," extensions] 1015 1016 client-final-message-without-proof = 1017 channel-binding "," nonce ["," 1018 extensions] 1019 1020 client-final-message = 1021 client-final-message-without-proof "," proof 1022 1023 gss-server-error = "e=" value 1024 server-final-message = gss-server-error / 1025 verifier ["," extensions] 1026 ;; The error message is only for the GSS-API 1027 ;; form of SCRAM, and it is OPTIONAL to 1028 ;; implement it. 1029 1030 extensions = attr-val *("," attr-val) 1031 ;; All extensions are optional, 1032 ;; i.e. unrecognized attributes 1033 ;; not defined in this document 1034 ;; MUST be ignored. 1035 1036 cbind-data = 1*OCTET 1037 1038 cbind-input = gs2-header [ cbind-data ] 1039 ;; cbind-data MUST be present for 1040 ;; gs2-cbind-flag of "p" and MUST be absent 1041 ;; for "y" or "n". 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063Menon-Sen, et al. Expires February 1, 2010 [Page 19] 1064 1065Internet-Draft SCRAM July 2009 1066 1067 10688. SCRAM as a GSS-API Mechanism 1069 1070 This section and its sub-sections and all normative references of it 1071 not referenced elsewhere in this document are INFORMATIONAL for SASL 1072 implementors, but they are NORMATIVE for GSS-API implementors. 1073 1074 SCRAM is actually also GSS-API mechanism. The messages are the same, 1075 but a) the GS2 header on the client's first message and channel 1076 binding data is excluded when SCRAM is used as a GSS-API mechanism, 1077 and b) the RFC2743 section 3.1 initial context token header is 1078 prefixed to the client's first authentication message (context 1079 token). 1080 1081 The GSS-API mechanism OID for SCRAM is <TBD> (see Section 10). 1082 10838.1. GSS-API Principal Name Types for SCRAM 1084 1085 SCRAM does not name acceptors. Therefore only GSS_C_NO_NAME and 1086 names of type GSS_C_NT_ANONYMOUS shall be allowed as the target name 1087 input of GSS_Init_sec_context() when using a SCRAM mechanism. 1088 1089 SCRAM supports only a single name type for initiators: 1090 GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for 1091 SCRAM. 1092 1093 There is no name canonicalization procedure for SCRAM beyond applying 1094 SASLprep as described in Section 5.1. 1095 1096 The query, display and exported name syntax for SCRAM principal names 1097 is the same: there is no syntax -- SCRAM principal names are free- 1098 form. (The exported name token does, of course, conform to [RFC2743] 1099 section 3.2, but the "NAME" part of the token is just a SCRAM user 1100 name.) 1101 11028.2. GSS-API Per-Message Tokens for SCRAM 1103 1104 The per-message tokens for SCRAM as a GSS-API mechanism SHALL be the 1105 same as those for the Kerberos V GSS-API mechanism [RFC4121], using 1106 the Kerberos V "aes128-cts-hmac-sha1-96" enctype [RFC3962]. 1107 1108 The 128-bit session key SHALL be derived by using the least 1109 significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API session 1110 key" || ClientKey || AuthMessage). 1111 1112 SCRAM does support PROT_READY, and is PROT_READY on the initiator 1113 side first upon receipt of the server's reply to the initial security 1114 context token. 1115 1116 1117 1118 1119Menon-Sen, et al. Expires February 1, 2010 [Page 20] 1120 1121Internet-Draft SCRAM July 2009 1122 1123 11248.3. GSS_Pseudo_random() for SCRAM 1125 1126 The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for 1127 the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor- 1128 asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and 1129 GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random(). 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175Menon-Sen, et al. Expires February 1, 2010 [Page 21] 1176 1177Internet-Draft SCRAM July 2009 1178 1179 11809. Security Considerations 1181 1182 If the authentication exchange is performed without a strong security 1183 layer, then a passive eavesdropper can gain sufficient information to 1184 mount an offline dictionary or brute-force attack which can be used 1185 to recover the user's password. The amount of time necessary for 1186 this attack depends on the cryptographic hash function selected, the 1187 strength of the password and the iteration count supplied by the 1188 server. An external security layer with strong encryption will 1189 prevent this attack. 1190 1191 If the external security layer used to protect the SCRAM exchange 1192 uses an anonymous key exchange, then the SCRAM channel binding 1193 mechanism can be used to detect a man-in-the-middle attack on the 1194 security layer and cause the authentication to fail as a result. 1195 However, the man-in-the-middle attacker will have gained sufficient 1196 information to mount an offline dictionary or brute-force attack. 1197 For this reason, SCRAM includes the ability to increase the iteration 1198 count over time. 1199 1200 If the authentication information is stolen from the authentication 1201 database, then an offline dictionary or brute-force attack can be 1202 used to recover the user's password. The use of salt mitigates this 1203 attack somewhat by requiring a separate attack on each password. 1204 Authentication mechanisms which protect against this attack are 1205 available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is 1206 an example of such technology. There are IPR disclosures at 1207 http://datatracker.ietf.org/ipr/ that mention RFC 2945. 1208 1209 If an attacker obtains the authentication information from the 1210 authentication repository and either eavesdrops on one authentication 1211 exchange or impersonates a server, the attacker gains the ability to 1212 impersonate that user to all servers providing SCRAM access using the 1213 same hash function, password, iteration count and salt. For this 1214 reason, it is important to use randomly-generated salt values. 1215 1216 SCRAM does not negotiate a hash function to use. Hash function 1217 negotiation is left to the SASL mechanism negotiation. It is 1218 important that clients be able to sort a locally available list of 1219 mechanisms by preference so that the client may pick the most 1220 preferred of a server's advertised mechanism list. This preference 1221 order is not specified here as it is a local matter. The preference 1222 order should include objective and subjective notions of mechanism 1223 cryptographic strength (e.g., SCRAM with a successor to SHA-1 may be 1224 preferred over SCRAM with SHA-1). 1225 1226 Note that to protect the SASL mechanism negotiation applications 1227 normally must list the server mechs twice: once before and once after 1228 1229 1230 1231Menon-Sen, et al. Expires February 1, 2010 [Page 22] 1232 1233Internet-Draft SCRAM July 2009 1234 1235 1236 authentication, the latter using security layers. Since SCRAM does 1237 not provide security layers the only ways to protect the mechanism 1238 negotiation are: a) use channel binding to an external channel, or b) 1239 use an external channel that authenticates a user-provided server 1240 name. 1241 1242 SCRAM does not protect against downgrade attacks of channel binding 1243 types. The complexities of negotiation a channel binding type, and 1244 handling down-grade attacks in that negotiation, was intentionally 1245 left out of scope for this document. 1246 1247 A hostile server can perform a computational denial-of-service attack 1248 on clients by sending a big iteration count value. 1249 1250 See [RFC4086] for more information about generating randomness. 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287Menon-Sen, et al. Expires February 1, 2010 [Page 23] 1288 1289Internet-Draft SCRAM July 2009 1290 1291 129210. IANA Considerations 1293 1294 IANA is requested to add the following family of SASL mechanisms to 1295 the SASL Mechanism registry established by [RFC4422]: 1296 1297 1298 To: iana@iana.org 1299 Subject: Registration of a new SASL family SCRAM 1300 1301 SASL mechanism name (or prefix for the family): SCRAM-* 1302 Security considerations: Section 7 of [RFCXXXX] 1303 Published specification (optional, recommended): [RFCXXXX] 1304 Person & email address to contact for further information: 1305 IETF SASL WG <ietf-sasl@imc.org> 1306 Intended usage: COMMON 1307 Owner/Change controller: IESG <iesg@ietf.org> 1308 Note: Members of this family must be explicitly registered 1309 using the "IETF Consensus" registration procedure. 1310 Reviews must be requested on the SASL WG mailing list. 1311 1312 1313 "IETF Consensus" registration procedure MUST be used for registering 1314 new mechanisms in this family. The SASL mailing list 1315 <ietf-sasl@imc.org> (or a successor designated by the responsible 1316 Security AD) MUST be used for soliciting reviews on such 1317 registrations. 1318 1319 Note to future SCRAM- mechanism designers: each new SCRAM- SASL 1320 mechanism MUST be explicitly registered with IANA and MUST comply 1321 with SCRAM- mechanism naming convention defined in Section 4 of this 1322 document. 1323 1324 IANA is requested to add the following entries to the SASL Mechanism 1325 registry established by [RFC4422]: 1326 1327 1328 To: iana@iana.org 1329 Subject: Registration of a new SASL mechanism SCRAM-SHA-1 1330 1331 SASL mechanism name (or prefix for the family): SCRAM-SHA-1 1332 Security considerations: Section 7 of [RFCXXXX] 1333 Published specification (optional, recommended): [RFCXXXX] 1334 Person & email address to contact for further information: 1335 IETF SASL WG <ietf-sasl@imc.org> 1336 Intended usage: COMMON 1337 Owner/Change controller: IESG <iesg@ietf.org> 1338 Note: 1339 1340 1341 1342 1343Menon-Sen, et al. Expires February 1, 2010 [Page 24] 1344 1345Internet-Draft SCRAM July 2009 1346 1347 1348 To: iana@iana.org 1349 Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS 1350 1351 SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS 1352 Security considerations: Section 7 of [RFCXXXX] 1353 Published specification (optional, recommended): [RFCXXXX] 1354 Person & email address to contact for further information: 1355 IETF SASL WG <ietf-sasl@imc.org> 1356 Intended usage: COMMON 1357 Owner/Change controller: IESG <iesg@ietf.org> 1358 Note: 1359 1360 1361 This document also requests IANA to assign a GSS-API mechanism OID 1362 for SCRAM. 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399Menon-Sen, et al. Expires February 1, 2010 [Page 25] 1400 1401Internet-Draft SCRAM July 2009 1402 1403 140411. Acknowledgements 1405 1406 This document benefited from discussions on the SASL WG mailing list. 1407 The authors would like to specially thank Dave Cridland, Simon 1408 Josefsson and Jeffrey Hutzelman for their contributions to this 1409 document. 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455Menon-Sen, et al. Expires February 1, 2010 [Page 26] 1456 1457Internet-Draft SCRAM July 2009 1458 1459 1460Appendix A. Other Authentication Mechanisms 1461 1462 The DIGEST-MD5 [I-D.ietf-sasl-digest-to-historic] mechanism has 1463 proved to be too complex to implement and test, and thus has poor 1464 interoperability. The security layer is often not implemented, and 1465 almost never used; everyone uses TLS instead. For a more complete 1466 list of problems with DIGEST-MD5 which lead to the creation of SCRAM 1467 see [I-D.ietf-sasl-digest-to-historic]. 1468 1469 The CRAM-MD5 SASL mechanism, while widely deployed has also some 1470 problems, in particular it is missing some modern SASL features such 1471 as support for internationalized usernames and passwords, support for 1472 passing of authorization identity, support for channel bindings. It 1473 also doesn't support server authentication. For a more complete list 1474 of problems with CRAM-MD5 see [I-D.ietf-sasl-crammd5-to-historic]. 1475 1476 The PLAIN [RFC4616] SASL mechanism allows a malicious server or 1477 eavesdropper to impersonate the authenticating user to any other 1478 server for which the user has the same password. It also sends the 1479 password in the clear over the network, unless TLS is used. Server 1480 authentication is not supported. 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511Menon-Sen, et al. Expires February 1, 2010 [Page 27] 1512 1513Internet-Draft SCRAM July 2009 1514 1515 1516Appendix B. Design Motivations 1517 1518 The following design goals shaped this document. Note that some of 1519 the goals have changed since the initial version of the document. 1520 1521 o The SASL mechanism has all modern SASL features: support for 1522 internationalized usernames and passwords, support for passing of 1523 authorization identity, support for channel bindings. 1524 1525 o The protocol supports mutual authentication. 1526 1527 o The authentication information stored in the authentication 1528 database is not sufficient by itself to impersonate the client. 1529 1530 o The server does not gain the ability to impersonate the client to 1531 other servers (with an exception for server-authorized proxies), 1532 unless such other servers allow SCRAM authentication and use the 1533 same salt and iteration count for the user. 1534 1535 o The mechanism is extensible, but [hopefully] not overengineered in 1536 this respect. 1537 1538 o Easier to implement than DIGEST-MD5 in both clients and servers. 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567Menon-Sen, et al. Expires February 1, 2010 [Page 28] 1568 1569Internet-Draft SCRAM July 2009 1570 1571 1572Appendix C. Internet-Draft Change History 1573 1574 (RFC Editor: Please delete everything after this point) 1575 1576 Changes since -10 1577 1578 o Converted the source for this I-D to XML. 1579 1580 o Added text to make SCRAM compliant with the new GS2 design. 1581 1582 o Added text on channel binding negotiation. 1583 1584 o Added text on channel binding, including a reference to RFC5056. 1585 1586 o Added text on SCRAM as a GSS-API mechanism. This noted as not 1587 relevant to SASL-only implementors -- the normative references for 1588 SCRAM as a GSS-API mechanism are segregated as well. 1589 1590 Changes since -07 1591 1592 o Updated References. 1593 1594 o Clarified purpose of the m= attribute. 1595 1596 o Fixed a problem with authentication/authorization identity's ABNF 1597 not allowing for some characters. 1598 1599 o Updated ABNF for nonce to show client-generated and server- 1600 generated parts. 1601 1602 o Only register SCRAM-SHA-1 with IANA and require explicit 1603 registrations of all other SCRAM- mechanisms. 1604 1605 Changes since -06 1606 1607 o Removed hash negotiation from SCRAM and turned it into a family of 1608 SASL mechanisms. 1609 1610 o Start using "Hash Function Textual Names" IANA registry for SCRAM 1611 mechanism naming. 1612 1613 o Fixed definition of Hi(str, salt) to be consistent with [RFC2898]. 1614 1615 o Clarified extensibility of SCRAM: added m= attribute (for future 1616 mandatory extensions) and specified that all unrecognized 1617 attributes must be ignored. 1618 1619 Changes since -05 1620 1621 1622 1623Menon-Sen, et al. Expires February 1, 2010 [Page 29] 1624 1625Internet-Draft SCRAM July 2009 1626 1627 1628 o Changed the mandatory to implement hash algorithm to SHA-1 (as per 1629 WG consensus). 1630 1631 o Added text about use of SASLPrep for username canonicalization/ 1632 validation. 1633 1634 o Clarified that authorization identity is canonicalized/verified 1635 according to SASL protocol profile. 1636 1637 o Clarified that iteration count is per-user. 1638 1639 o Clarified how clients select the authentication function. 1640 1641 o Added IANA registration for the new mechanism. 1642 1643 o Added missing normative references (UTF-8, SASLPrep). 1644 1645 o Various editorial changes based on comments from Hallvard B 1646 Furuseth, Nico William and Simon Josefsson. 1647 1648 Changes since -04 1649 1650 o Update Base64 and Security Glossary references. 1651 1652 o Add Formal Syntax section. 1653 1654 o Don't bother with "v=". 1655 1656 o Make MD5 mandatory to implement. Suggest i=128. 1657 1658 Changes since -03 1659 1660 o Seven years have passed, in which it became clear that DIGEST-MD5 1661 suffered from unacceptably bad interoperability, so SCRAM-MD5 is 1662 now back from the dead. 1663 1664 o Be hash agnostic, so MD5 can be replaced more easily. 1665 1666 o General simplification. 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679Menon-Sen, et al. Expires February 1, 2010 [Page 30] 1680 1681Internet-Draft SCRAM July 2009 1682 1683 168412. References 1685 168612.1. Normative References 1687 1688 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1689 Hashing for Message Authentication", RFC 2104, 1690 February 1997. 1691 1692 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1693 Requirement Levels", BCP 14, RFC 2119, March 1997. 1694 1695 [RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 1696 (SHA1)", RFC 3174, September 2001. 1697 1698 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 1699 Internationalized Strings ("stringprep")", RFC 3454, 1700 December 2002. 1701 1702 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 1703 10646", STD 63, RFC 3629, November 2003. 1704 1705 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 1706 and Passwords", RFC 4013, February 2005. 1707 1708 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 1709 Security Layer (SASL)", RFC 4422, June 2006. 1710 1711 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1712 Encodings", RFC 4648, October 2006. 1713 1714 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 1715 Channels", RFC 5056, November 2007. 1716 1717 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1718 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1719 172012.2. Normative References for GSS-API implementors 1721 1722 [I-D.ietf-sasl-gs2] 1723 Josefsson, S. and N. Williams, "Using GSS-API Mechanisms 1724 in SASL: The GS2 Mechanism Family", draft-ietf-sasl-gs2-12 1725 (work in progress), April 2009. 1726 1727 [RFC2743] Linn, J., "Generic Security Service Application Program 1728 Interface Version 2, Update 1", RFC 2743, January 2000. 1729 1730 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) 1731 Encryption for Kerberos 5", RFC 3962, February 2005. 1732 1733 1734 1735Menon-Sen, et al. Expires February 1, 2010 [Page 31] 1736 1737Internet-Draft SCRAM July 2009 1738 1739 1740 [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 1741 Requirements for Security", BCP 106, RFC 4086, June 2005. 1742 1743 [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 1744 Version 5 Generic Security Service Application Program 1745 Interface (GSS-API) Mechanism: Version 2", RFC 4121, 1746 July 2005. 1747 1748 [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API 1749 Extension for the Generic Security Service Application 1750 Program Interface (GSS-API)", RFC 4401, February 2006. 1751 1752 [RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the 1753 Kerberos V Generic Security Service Application Program 1754 Interface (GSS-API) Mechanism", RFC 4402, February 2006. 1755 1756 [tls-unique] 1757 Zhu, L., "Registration of TLS unique channel binding 1758 (generic)", IANA http://www.iana.org/assignments/ 1759 channel-binding-types/tls-unique, July 2008. 1760 176112.3. Informative References 1762 1763 [I-D.altman-tls-channel-bindings] 1764 Altman, J., Williams, N., and L. Zhu, "Channel Bindings 1765 for TLS", draft-altman-tls-channel-bindings-05 (work in 1766 progress), June 2009. 1767 1768 [I-D.ietf-sasl-crammd5-to-historic] 1769 Zeilenga, K., "CRAM-MD5 to Historic", 1770 draft-ietf-sasl-crammd5-to-historic-00 (work in progress), 1771 November 2008. 1772 1773 [I-D.ietf-sasl-digest-to-historic] 1774 Melnikov, A., "Moving DIGEST-MD5 to Historic", 1775 draft-ietf-sasl-digest-to-historic-00 (work in progress), 1776 July 2008. 1777 1778 [I-D.melnikov-sasl-scram-ldap] 1779 Melnikov, A., "LDAP schema for storing SCRAM secrets", 1780 draft-melnikov-sasl-scram-ldap-02 (work in progress), 1781 July 2009. 1782 1783 [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography 1784 Specification Version 2.0", RFC 2898, September 2000. 1785 1786 [RFC2945] Wu, T., "The SRP Authentication and Key Exchange System", 1787 RFC 2945, September 2000. 1788 1789 1790 1791Menon-Sen, et al. Expires February 1, 2010 [Page 32] 1792 1793Internet-Draft SCRAM July 2009 1794 1795 1796 [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol 1797 (LDAP): Technical Specification Road Map", RFC 4510, 1798 June 2006. 1799 1800 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and 1801 Security Layer (SASL) Mechanism", RFC 4616, August 2006. 1802 1803 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 1804 RFC 4949, August 2007. 1805 1806 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1807 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1808 1809 [tls-server-end-point] 1810 Zhu, L., "Registration of TLS server end-point channel 1811 bindings", IANA http://www.iana.org/assignments/ 1812 channel-binding-types/tls-server-end-point, July 2008. 1813 1814 1815 1816 1817 1818 1819 1820 1821 1822 1823 1824 1825 1826 1827 1828 1829 1830 1831 1832 1833 1834 1835 1836 1837 1838 1839 1840 1841 1842 1843 1844 1845 1846 1847Menon-Sen, et al. Expires February 1, 2010 [Page 33] 1848 1849Internet-Draft SCRAM July 2009 1850 1851 1852Authors' Addresses 1853 1854 Abhijit Menon-Sen 1855 Oryx Mail Systems GmbH 1856 1857 Email: ams@oryx.com 1858 1859 1860 Alexey Melnikov 1861 Isode Ltd 1862 1863 Email: Alexey.Melnikov@isode.com 1864 1865 1866 Chris Newman 1867 Sun Microsystems 1868 1050 Lakes Drive 1869 West Covina, CA 91790 1870 USA 1871 1872 Email: chris.newman@sun.com 1873 1874 1875 Nicolas Williams 1876 Sun Microsystems 1877 5300 Riata Trace Ct 1878 Austin, TX 78727 1879 USA 1880 1881 Email: Nicolas.Williams@sun.com 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900 1901 1902 1903Menon-Sen, et al. Expires February 1, 2010 [Page 34] 1904 1905 1906