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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 NETWORK WORKING GROUP A. Menon-Sen 3 Internet-Draft Oryx Mail Systems GmbH 4 Intended status: Standards Track A. Melnikov 5 Expires: March 14, 2010 Isode Ltd 6 C. Newman 7 N. Williams 8 Sun Microsystems 9 September 10, 2009 11 Salted Challenge Response (SCRAM) SASL and GSS-API Mechanism 12 draft-ietf-sasl-scram-07.txt 14 Status of this Memo 16 This Internet-Draft is submitted to IETF in full conformance with the 17 provisions of BCP 78 and BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 This Internet-Draft will expire on March 14, 2010. 37 Copyright Notice 39 Copyright (c) 2009 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents in effect on the date of 44 publication of this document (http://trustee.ietf.org/license-info). 45 Please review these documents carefully, as they describe your rights 46 and restrictions with respect to this document. 48 Abstract 50 The secure authentication mechanism most widely deployed and used by 51 Internet application protocols is the transmission of clear-text 52 passwords over a channel protected by Transport Layer Security (TLS). 53 There are some significant security concerns with that mechanism, 54 which could be addressed by the use of a challenge response 55 authentication mechanism protected by TLS. Unfortunately, the 56 challenge response mechanisms presently on the standards track all 57 fail to meet requirements necessary for widespread deployment, and 58 have had success only in limited use. 60 This specification describes a family of Simple Authentication and 61 Security Layer (SASL, RFC 4422) authentication mechanisms called the 62 Salted Challenge Response Authentication Mechanism (SCRAM), which 63 addresses the security concerns and meets the deployability 64 requirements. When used in combination with TLS or an equivalent 65 security layer, a mechanism from this family could improve the 66 status-quo for application protocol authentication and provide a 67 suitable choice for a mandatory-to-implement mechanism for future 68 application protocol standards. 70 Table of Contents 72 1. Conventions Used in This Document . . . . . . . . . . 4 73 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 4 74 1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . 5 75 2. Introduction . . . . . . . . . . . . . . . . . . . . . 7 76 3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . 9 77 4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . 10 78 5. SCRAM Authentication Exchange . . . . . . . . . . . . 11 79 5.1. SCRAM Attributes . . . . . . . . . . . . . . . . . . . 12 80 5.2. Compliance with SASL mechanism requirements . . . . . 15 81 6. Channel Binding . . . . . . . . . . . . . . . . . . . 16 82 6.1. Default Channel Binding . . . . . . . . . . . . . . . 17 83 7. Formal Syntax . . . . . . . . . . . . . . . . . . . . 18 84 8. SCRAM as a GSS-API Mechanism . . . . . . . . . . . . . 21 85 8.1. GSS-API Principal Name Types for SCRAM . . . . . . . . 21 86 8.2. GSS-API Per-Message Tokens for SCRAM . . . . . . . . . 21 87 8.3. GSS_Pseudo_random() for SCRAM . . . . . . . . . . . . 22 88 9. Security Considerations . . . . . . . . . . . . . . . 23 89 10. IANA Considerations . . . . . . . . . . . . . . . . . 25 90 11. Acknowledgements . . . . . . . . . . . . . . . . . . . 27 91 Appendix A. Other Authentication Mechanisms . . . . . . . . . . . 28 92 Appendix B. Design Motivations . . . . . . . . . . . . . . . . . . 29 93 Appendix C. Internet-Draft Change History . . . . . . . . . . . . 30 94 12. References . . . . . . . . . . . . . . . . . . . . . . 32 95 12.1. Normative References . . . . . . . . . . . . . . . . . 32 96 12.2. Normative References for GSS-API implementors . . . . 32 97 12.3. Informative References . . . . . . . . . . . . . . . . 33 98 Authors' Addresses . . . . . . . . . . . . . . . . . . 35 100 1. Conventions Used in This Document 102 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 103 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 104 document are to be interpreted as described in [RFC2119]. 106 Formal syntax is defined by [RFC5234] including the core rules 107 defined in Appendix B of [RFC5234]. 109 Example lines prefaced by "C:" are sent by the client and ones 110 prefaced by "S:" by the server. If a single "C:" or "S:" label 111 applies to multiple lines, then the line breaks between those lines 112 are for editorial clarity only, and are not part of the actual 113 protocol exchange. 115 1.1. Terminology 117 This document uses several terms defined in [RFC4949] ("Internet 118 Security Glossary") including the following: authentication, 119 authentication exchange, authentication information, brute force, 120 challenge-response, cryptographic hash function, dictionary attack, 121 eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, 122 one-way encryption function, password, replay attack and salt. 123 Readers not familiar with these terms should use that glossary as a 124 reference. 126 Some clarifications and additional definitions follow: 128 o Authentication information: Information used to verify an identity 129 claimed by a SCRAM client. The authentication information for a 130 SCRAM identity consists of salt, iteration count, the "StoredKey" 131 and "ServerKey" (as defined in the algorithm overview) for each 132 supported cryptographic hash function. 134 o Authentication database: The database used to look up the 135 authentication information associated with a particular identity. 136 For application protocols, LDAPv3 (see [RFC4510]) is frequently 137 used as the authentication database. For network-level protocols 138 such as PPP or 802.11x, the use of RADIUS is more common. 140 o Base64: An encoding mechanism defined in [RFC4648] which converts 141 an octet string input to a textual output string which can be 142 easily displayed to a human. The use of base64 in SCRAM is 143 restricted to the canonical form with no whitespace. 145 o Octet: An 8-bit byte. 147 o Octet string: A sequence of 8-bit bytes. 149 o Salt: A random octet string that is combined with a password 150 before applying a one-way encryption function. This value is used 151 to protect passwords that are stored in an authentication 152 database. 154 1.2. Notation 156 The pseudocode description of the algorithm uses the following 157 notations: 159 o ":=": The variable on the left hand side represents the octet 160 string resulting from the expression on the right hand side. 162 o "+": Octet string concatenation. 164 o "[ ]": A portion of an expression enclosed in "[" and "]" may not 165 be included in the result under some circumstances. See the 166 associated text for a description of those circumstances. 168 o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in 169 [RFC2104]) using the octet string represented by "key" as the key 170 and the octet string "str" as the input string. The size of the 171 result is the hash result size for the hash function in use. For 172 example, it is 20 octets for SHA-1 (see [RFC3174]). 174 o H(str): Apply the cryptographic hash function to the octet string 175 "str", producing an octet string as a result. The size of the 176 result depends on the hash result size for the hash function in 177 use. 179 o XOR: Apply the exclusive-or operation to combine the octet string 180 on the left of this operator with the octet string on the right of 181 this operator. The length of the output and each of the two 182 inputs will be the same for this use. 184 o Hi(str, salt): 186 U0 := HMAC(str, salt + INT(1)) 187 U1 := HMAC(str, U0) 188 U2 := HMAC(str, U1) 189 ... 190 Ui-1 := HMAC(str, Ui-2) 191 Ui := HMAC(str, Ui-1) 193 Hi := U0 XOR U1 XOR U2 XOR ... XOR Ui 194 where "i" is the iteration count, "+" is the string concatenation 195 operator and INT(g) is a four-octet encoding of the integer g, 196 most significant octet first. 198 o This is, essentially, PBKDF2 [RFC2898] with HMAC() as the PRF and 199 with dkLen == output length of HMAC() == output length of H(). 201 2. Introduction 203 This specification describes a family of authentication mechanisms 204 called the Salted Challenge Response Authentication Mechanism (SCRAM) 205 which addresses the requirements necessary to deploy a challenge- 206 response mechanism more widely than past attempts. When used in 207 combination with Transport Layer Security (TLS, see [RFC5246]) or an 208 equivalent security layer, a mechanism from this family could improve 209 the status-quo for application protocol authentication and provide a 210 suitable choice for a mandatory-to-implement mechanism for future 211 application protocol standards. 213 For simplicity, this family of mechanisms does not presently include 214 negotiation of a security layer [RFC4422]. It is intended to be used 215 with an external security layer such as that provided by TLS or SSH, 216 with optional channel binding [RFC5056] to the external security 217 layer. 219 SCRAM is specified herein as a pure Simple Authentication and 220 Security Layer (SASL) [RFC4422] mechanism, but it conforms to the new 221 bridge between SASL and the Generic Security Services Application 222 Programming Interface (GSS-API) called "GS2" [I-D.ietf-sasl-gs2]. 223 This means that this document defines both, a SASL mechanism and a 224 GSS-API mechanism. 226 SCRAM provides the following protocol features: 228 o The authentication information stored in the authentication 229 database is not sufficient by itself to impersonate the client. 230 The information is salted to prevent a pre-stored dictionary 231 attack if the database is stolen. 233 o The server does not gain the ability to impersonate the client to 234 other servers (with an exception for server-authorized proxies). 236 o The mechanism permits the use of a server-authorized proxy without 237 requiring that proxy to have super-user rights with the back-end 238 server. 240 o Mutual authentication is supported, but only the client is named 241 (i.e., the server has no name). 243 o When used as a SASL mechanism, SCRAM is capable of transporting 244 authorization identities (see [RFC4422], Section 2) from the 245 client to the server. 247 A separate document defines a standard LDAPv3 [RFC4510] attribute 248 that enables storage of the SCRAM authentication information in LDAP. 250 See [I-D.melnikov-sasl-scram-ldap]. 252 For an in-depth discussion of why other challenge response mechanisms 253 are not considered sufficient, see appendix A. For more information 254 about the motivations behind the design of this mechanism, see 255 appendix B. 257 Comments regarding this draft may be sent either to the sasl@ietf.org 258 mailing list or to the authors. 260 3. SCRAM Algorithm Overview 262 Note that this section omits some details, such as client and server 263 nonces. See Section 5 for more details. 265 To begin with, the SCRAM client is in possession of a username and 266 password. It sends the username to the server, which retrieves the 267 corresponding authentication information, i.e. a salt, StoredKey, 268 ServerKey and the iteration count i. (Note that a server 269 implementation may choose to use the same iteration count for all 270 accounts.) The server sends the salt and the iteration count to the 271 client, which then computes the following values and sends a 272 ClientProof to the server: 274 SaltedPassword := Hi(password, salt) 275 ClientKey := HMAC(SaltedPassword, "Client Key") 276 StoredKey := H(ClientKey) 277 AuthMessage := client-first-message-bare + "," + 278 server-first-message + "," + 279 client-final-message-without-proof 280 ClientSignature := HMAC(StoredKey, AuthMessage) 281 ClientProof := ClientKey XOR ClientSignature 282 ServerKey := HMAC(SaltedPassword, "Server Key") 283 ServerSignature := HMAC(ServerKey, AuthMessage) 285 The server authenticates the client by computing the ClientSignature, 286 exclusive-ORing that with the ClientProof to recover the ClientKey 287 and verifying the correctness of the ClientKey by applying the hash 288 function and comparing the result to the StoredKey. If the ClientKey 289 is correct, this proves that the client has access to the user's 290 password. 292 Similarly, the client authenticates the server by computing the 293 ServerSignature and comparing it to the value sent by the server. If 294 the two are equal, it proves that the server had access to the user's 295 ServerKey. 297 The AuthMessage is computed by concatenating messages from the 298 authentication exchange. The format of these messages is defined in 299 Section 7. 301 4. SCRAM Mechanism Names 303 A SCRAM mechanism name is a string "SCRAM-" followed by the 304 uppercased name of the underlying hash function taken from the IANA 305 "Hash Function Textual Names" registry (see http://www.iana.org), 306 optionally followed by the suffix "-PLUS" (see below). Note that 307 SASL mechanism names are limited to 20 characters, which means that 308 only hash function names with lengths shorter or equal to 9 309 characters (20-length("SCRAM-")-length("-PLUS") can be used. For 310 cases when the underlying hash function name is longer than 9 311 characters, an alternative 9 character (or shorter) name can be used 312 to construct the corresponding SCRAM mechanism name, as long as this 313 alternative name doesn't conflict with any other hash function name 314 from the IANA "Hash Function Textual Names" registry. 316 For interoperability, all SCRAM clients and servers MUST implement 317 the SCRAM-SHA-1 authentication mechanism, i.e. an authentication 318 mechanism from the SCRAM family that uses the SHA-1 hash function as 319 defined in [RFC3174]. 321 The "-PLUS" suffix is used only when the server supports channel 322 binding to the external channel. If the server supports channel 323 binding, it will advertise both the "bare" and "plus" versions of 324 whatever mechanisms it supports (e.g., if the server supports only 325 SCRAM with SHA-1 then it will advertise support for both SCRAM-SHA-1 326 and SCRAM-SHA-1-PLUS); if the server does not support channel 327 binding, then it will advertise only the "bare" version of the 328 mechanism (e.g., only SCRAM-SHA-1). The "-PLUS" exists to allow 329 negotiation of the use of channel binding. See Section 6. 331 5. SCRAM Authentication Exchange 333 SCRAM is a SASL mechanism whose client response and server challenge 334 messages are text-based messages containing one or more attribute- 335 value pairs separated by commas. Each attribute has a one-letter 336 name. The messages and their attributes are described in 337 Section 5.1, and defined in Section 7. 339 SCRAM is a client-first SASL mechanism (See [RFC4422], Section 5, 340 item 2a), and returns additional data together with a server's 341 indication of a successful outcome. 343 This is a simple example of a SCRAM-SHA-1 authentication exchange 344 when the client doesn't support channel bindings: 346 C: n,,n=Chris Newman,r=ClientNonce 347 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 348 C: c=biwsCg==,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4 349 S: v=WxPv/siO5l+qxN4 351 [[anchor5: Note that the all hashes above are fake and will be fixed 352 during AUTH48.]] 354 With channel-binding data sent by the client this might look like 355 this (see [tls-server-end-point] for the definition of tls-server- 356 end-point TLS channel binding): 358 C: p=tls-server-end-point,,n=Chris Newman,r=ClientNonce 359 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 360 C: c=cD10bHMtc2VydmVyLWVuZC1wb2ludCwsy1hFtXOnZ+ySrQM6srFp 361 l/77uqvtxrg7nBY1BetEr/g=,r=ClientNonceServerNonce,p=Wx 362 Pv/siO5l+qxN4 363 S: v=WxPv/siO5l+qxN4 365 [[anchor6: Note that all hashes above are fake and will be fixed 366 during AUTH48.]] 368 First, the client sends the "client-first-message" containing: 370 o a GS2 header consisting of a flag indicating whether channel 371 binding is supported-but-not-used, not supported, or used, and an 372 optional SASL authorization identity; 374 o SCRAM username and a random, unique nonce attributes. 376 Note that the client's first message will always start with "n", "y" 377 or "p", otherwise the message is invalid and authentication MUST 378 fail. This is important, as it allows for GS2 extensibility (e.g., 379 to add support for security layers). 381 In response, the server sends a "server-first-message" containing the 382 user's iteration count i, the user's salt, and appends its own nonce 383 to the client-specified one. 385 The client then responds by sending "client-final-message" with the 386 same nonce and a ClientProof computed using the selected hash 387 function as explained earlier. 389 The server verifies the nonce and the proof, verifies that the 390 authorization identity (if supplied by the client in the first 391 message) is authorized to act as the authentication identity, and, 392 finally, it responds with a "server-final-message", concluding the 393 authentication exchange. 395 The client then authenticates the server by computing the 396 ServerSignature and comparing it to the value sent by the server. If 397 the two are different, the client MUST consider the authentication 398 exchange to be unsuccessful and it might have to drop the connection. 400 5.1. SCRAM Attributes 402 This section describes the permissible attributes, their use, and the 403 format of their values. All attribute names are single US-ASCII 404 letters and are case-sensitive. 406 Note that the order of attributes in client or server messages is 407 fixed, with the exception of extension attributes (described by the 408 "extensions" ABNF production), which can appear in any order in the 409 designated positions. See the ABNF section for authoritative 410 reference. 412 o a: This is an optional attribute, and is part of the GS2 413 [I-D.ietf-sasl-gs2] bridge between the GSS-API and SASL. This 414 attribute specifies an authorization identity. A client may 415 include it in its first message to the server if it wants to 416 authenticate as one user, but subsequently act as a different 417 user. This is typically used by an administrator to perform some 418 management task on behalf of another user, or by a proxy in some 419 situations. 421 Upon the receipt of this value the server verifies its 422 correctness according to the used SASL protocol profile. 423 Failed verification results in failed authentication exchange. 425 If this attribute is omitted (as it normally would be), the 426 authorization identity is assumed to be derived from the 427 username specified with the (required) "n" attribute. 429 The server always authenticates the user specified by the "n" 430 attribute. If the "a" attribute specifies a different user, 431 the server associates that identity with the connection after 432 successful authentication and authorization checks. 434 The syntax of this field is the same as that of the "n" field 435 with respect to quoting of '=' and ','. 437 o n: This attribute specifies the name of the user whose password is 438 used for authentication (a.k.a. "authentication identity" 439 [RFC4422]). A client MUST include it in its first message to the 440 server. If the "a" attribute is not specified (which would 441 normally be the case), this username is also the identity which 442 will be associated with the connection subsequent to 443 authentication and authorization. 445 Before sending the username to the server, the client SHOULD 446 prepare the username using the "SASLPrep" profile [RFC4013] of 447 the "stringprep" algorithm [RFC3454] treating it as a query 448 string (i.e., unassigned Unicode code points are allowed). If 449 the preparation of the username fails or results in an empty 450 string, the client SHOULD abort the authentication exchange 451 (*). 453 (*) An interactive client can request a repeated entry of the 454 username value. 456 Upon receipt of the username by the server, the server SHOULD 457 prepare it using the "SASLPrep" profile [RFC4013] of the 458 "stringprep" algorithm [RFC3454] treating it as a query string 459 (i.e., unassigned Unicode code points are allowed). If the 460 preparation of the username fails or results in an empty 461 string, the server SHOULD abort the authentication exchange. 462 Whether or not the server prepares the username using 463 "SASLPrep", it MUST use it as received in hash calculations. 465 The characters ',' or '=' in usernames are sent as '=2C' and 466 '=3D' respectively. If the server receives a username which 467 contains '=' not followed by either '2C' or '3D', then the 468 server MUST fail the authentication. 470 o m: This attribute is reserved for future extensibility. In this 471 version of SCRAM, its presence in a client or a server message 472 MUST cause authentication failure when the attribute is parsed by 473 the other end. 475 o r: This attribute specifies a sequence of random printable ASCII 476 characters excluding ',' which forms the nonce used as input to 477 the hash function. No quoting is applied to this string. As 478 described earlier, the client supplies an initial value in its 479 first message, and the server augments that value with its own 480 nonce in its first response. It is important that this value be 481 different for each authentication. The client MUST verify that 482 the initial part of the nonce used in subsequent messages is the 483 same as the nonce it initially specified. The server MUST verify 484 that the nonce sent by the client in the second message is the 485 same as the one sent by the server in its first message. 487 o c: This REQUIRED attribute specifies the base64-encoded GS2 header 488 and channel-binding data. It is sent by the client in its second 489 authentication message. The attribute data consist of: 491 * the GS2 header from the client's first message (recall: a 492 channel binding flag and an optional authzid). This header is 493 going to include channel binding type prefix (see [RFC5056]), 494 if and only if the client is using channel binding; 496 * followed by the external channel's channel binding data, if and 497 only if the client is using channel binding. 499 o s: This attribute specifies the base64-encoded salt used by the 500 server for this user. It is sent by the server in its first 501 message to the client. 503 o i: This attribute specifies an iteration count for the selected 504 hash function and user, and MUST be sent by the server along with 505 the user's salt. 507 For SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism servers SHOULD 508 announce a hash iteration-count of at least 4096. Note that a 509 client implementation MAY cache SaltedPassword/ClientKey for 510 later reauthentication to the same service, as it is likely 511 that the server is going to advertise the same salt value upon 512 reauthentication. This might be useful for mobile clients 513 where CPU usage is a concern. 515 o p: This attribute specifies a base64-encoded ClientProof. The 516 client computes this value as described in the overview and sends 517 it to the server. 519 o v: This attribute specifies a base64-encoded ServerSignature. It 520 is sent by the server in its final message, and is used by the 521 client to verify that the server has access to the user's 522 authentication information. This value is computed as explained 523 in the overview. 525 5.2. Compliance with SASL mechanism requirements 527 This section describes compliance with SASL mechanism requirements 528 specified in Section 5 of [RFC4422]. 530 1) "SCRAM-SHA-1" and "SCRAM-SHA-1-PLUS". 532 2a) SCRAM is a client-first mechanism. 534 2b) SCRAM sends additional data with success. 536 3) SCRAM is capable of transferring authorization identities from the 537 client to the server. 539 4) SCRAM does not offer any security layers (SCRAM offers channel 540 binding instead). 542 5) SCRAM has a hash protecting the authorization identity. 544 6. Channel Binding 546 SCRAM supports channel binding to external secure channels, such as 547 TLS. Clients and servers may or may not support channel binding, 548 therefore the use of channel binding is negotiable. SCRAM does not 549 provide security layers, however, therefore it is imperative that 550 SCRAM provide integrity protection for the negotiation of channel 551 binding. 553 Use of channel binding is negotiated as follows: 555 o Servers SHOULD advertise both non-PLUS (SCRAM-) and 556 the PLUS-variant (SCRAM--PLUS) SASL mechanism 557 names. If the server cannot support channel binding, it MAY 558 advertise only the non-PLUS variant. If the server would never 559 succeed authentication of the non-PLUS variant due to policy 560 reasons, it MAY advertise only the PLUS-variant. 562 o If the client negotiates mechanisms then the client MUST select 563 SCRAM--PLUS if offered by the server and the client 564 wants to select SCRAM with the given hash function. Otherwise 565 (the client does not negotiate mechanisms), if the client has no 566 prior knowledge about mechanisms supported by the server and 567 wasn't explicitly configured to use a particular variant of the 568 SCRAM mechanism, then it MUST select only SCRAM- 569 (not suffixed with "-PLUS"). 571 o If the client supports channel binding and the server appears to 572 support it (i.e., the client sees SCRAM--PLUS), or 573 if the client wishes to use channel binding but the client does 574 not negotiate mechanisms, then the client MUST set the GS2 channel 575 binding flag to "p" in order to indicate the channel binding type 576 it is using and it MUST include the channel binding data for the 577 external channel in the computation of the "c=" attribute (see 578 Section 5.1). 580 o If the client supports channel binding but the server does not 581 appear to (i.e., the client did not see SCRAM-- 582 PLUS) then the client MUST either fail authentication or it MUST 583 choose the non-PLUS mechanism and set the GS2 channel binding flag 584 to "y" and MUST NOT include channel binding data for the external 585 channel in the computation of the "c=" attribute (see 586 Section 5.1). 588 o If the client does not support channel binding then the client 589 MUST set the GS2 channel binding flag to "n" and MUST NOT include 590 channel binding data for the external channel in the computation 591 of the "c=" attribute (see Section 5.1). 593 o Upon receipt of the client first message the server checks the GS2 594 channel binding flag (gs2-cb-flag). 596 * If the flag is set to "y" and the server supports channel 597 binding the server MUST fail authentication. This is because 598 if the client sets the GS2 channel binding flag set to "y" then 599 the client must have believed that the server did not support 600 channel binding -- if the server did in fact support channel 601 binding then this is an indication that there has been a 602 downgrade attack (e.g., an attacker changed the server's 603 mechanism list to exclude the -PLUS suffixed SCRAM mechanism 604 name(s)). 606 * If the channel binding flag was "p" and the server does not 607 support the indicated channel binding type then the server MUST 608 fail authentication. 610 The server MUST always validate the client's "c=" field. The server 611 does this by constructing the value of the "c=" attribute and then 612 checking that it matches the client's c= attribute value. 614 For more discussions of channel bindings, and the syntax of the 615 channel binding data for various security protocols, see [RFC5056]. 617 6.1. Default Channel Binding 619 A default channel binding type agreement process for all SASL 620 application protocols that do not provide their own channel binding 621 type agreement is provided as follows. 623 'tls-unique' is the default channel binding type for any application 624 that doesn't specify one. 626 Servers MUST implement the "tls-unique" [tls-unique] 627 [I-D.altman-tls-channel-bindings] channel binding type, if they 628 implement any channel binding. Clients SHOULD implement the "tls- 629 unique" [tls-unique] [I-D.altman-tls-channel-bindings] channel 630 binding type, if they implement any channel binding. Clients and 631 servers SHOULD choose the highest- layer/innermost end-to-end TLS 632 channel as the channel to bind to. 634 Servers MUST choose the channel binding type indicated by the client, 635 or fail authentication if they don't support it. 637 7. Formal Syntax 639 The following syntax specification uses the Augmented Backus-Naur 640 Form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3" 641 and "UTF8-4" non-terminal are defined in [RFC3629]. 643 ALPHA = 644 DIGIT = 645 UTF8-2 = 646 UTF8-3 = 647 UTF8-4 = 649 attr-val = ALPHA "=" value 650 ;; Generic syntax of any attribute sent 651 ;; by server or client 653 value = 1*value-char 655 value-safe-char = %x01-2B / %x2D-3C / %x3E-7F / 656 UTF8-2 / UTF8-3 / UTF8-4 657 ;; UTF8-char except NUL, "=", and ",". 659 value-char = value-safe-char / "=" 661 printable = %x21-2B / %x2D-7E 662 ;; Printable ASCII except ",". 663 ;; Note that any "printable" is also 664 ;; a valid "value". 666 base64-char = ALPHA / DIGIT / "/" / "+" 668 base64-4 = 4base64-char 670 base64-3 = 3base64-char "=" 672 base64-2 = 2base64-char "==" 674 base64 = *base64-4 [base64-3 / base64-2] 676 posit-number = %x31-39 *DIGIT 677 ;; A positive number 679 saslname = 1*(value-safe-char / "=2C" / "=3D") 680 ;; Conforms to 682 authzid = "a=" saslname 683 ;; Protocol specific. 685 cb-name = 1*(ALPHA / DIGIT / "." / "-") 686 ;; See RFC 5056 section 7. 687 ;; E.g. "tls-server-end-point" or 688 ;; "tls-unique" 690 gs2-cbind-flag = "p=" cb-name / "n" / "y" 691 ;; "n" -> client doesn't support channel binding 692 ;; "y" -> client does support channel binding 693 ;; but thinks the server does not. 694 ;; "p" -> client requires channel binding. 695 ;; The selected channel binding follows "p=". 697 gs2-header = gs2-cbind-flag "," [ authzid ] "," 698 ;; GS2 header for SCRAM 699 ;; (the actual GS2 header includes an optional 700 ;; flag to indicate that the GSS mechanism is not 701 ;; "standard" but since SCRAM is "standard" we 702 ;; don't include that flag). 704 username = "n=" saslname 705 ;; Usernames are prepared using SASLPrep. 707 reserved-mext = "m=" 1*(value-char) 708 ;; Reserved for signalling mandatory extensions. 709 ;; The exact syntax will be defined in 710 ;; the future. 712 channel-binding = "c=" base64 713 ;; base64 encoding of cbind-input 715 proof = "p=" base64 717 nonce = "r=" c-nonce [s-nonce] 718 ;; Second part provided by server. 720 c-nonce = printable 722 s-nonce = printable 724 salt = "s=" base64 726 verifier = "v=" base64 727 ;; base-64 encoded ServerSignature. 729 iteration-count = "i=" posit-number 730 ;; A positive number 732 client-first-message-bare = 734 [reserved-mext ","] 735 username "," nonce ["," extensions] 737 client-first-message = 738 gs2-header client-first-message-bare 740 server-first-message = 741 [reserved-mext ","] nonce "," salt "," 742 iteration-count ["," extensions] 744 client-final-message-without-proof = 745 channel-binding "," nonce ["," 746 extensions] 748 client-final-message = 749 client-final-message-without-proof "," proof 751 gss-server-error = "e=" value 752 server-final-message = gss-server-error / 753 verifier ["," extensions] 754 ;; The error message is only for the GSS-API 755 ;; form of SCRAM, and it is OPTIONAL to 756 ;; implement it. 758 extensions = attr-val *("," attr-val) 759 ;; All extensions are optional, 760 ;; i.e. unrecognized attributes 761 ;; not defined in this document 762 ;; MUST be ignored. 764 cbind-data = 1*OCTET 766 cbind-input = gs2-header [ cbind-data ] 767 ;; cbind-data MUST be present for 768 ;; gs2-cbind-flag of "p" and MUST be absent 769 ;; for "y" or "n". 771 8. SCRAM as a GSS-API Mechanism 773 This section and its sub-sections and all normative references of it 774 not referenced elsewhere in this document are INFORMATIONAL for SASL 775 implementors, but they are NORMATIVE for GSS-API implementors. 777 SCRAM is actually also GSS-API mechanism. The messages are the same, 778 but a) the GS2 header on the client's first message and channel 779 binding data is excluded when SCRAM is used as a GSS-API mechanism, 780 and b) the RFC2743 section 3.1 initial context token header is 781 prefixed to the client's first authentication message (context 782 token). 784 The GSS-API mechanism OID for SCRAM is (see Section 10). 786 8.1. GSS-API Principal Name Types for SCRAM 788 SCRAM does not name acceptors. Therefore only GSS_C_NO_NAME and 789 names of type GSS_C_NT_ANONYMOUS shall be allowed as the target name 790 input of GSS_Init_sec_context() when using a SCRAM mechanism. 792 SCRAM supports only a single name type for initiators: 793 GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for 794 SCRAM. 796 There is no name canonicalization procedure for SCRAM beyond applying 797 SASLprep as described in Section 5.1. 799 The query, display and exported name syntax for SCRAM principal names 800 is the same: there is no syntax -- SCRAM principal names are free- 801 form. (The exported name token does, of course, conform to [RFC2743] 802 section 3.2, but the "NAME" part of the token is just a SCRAM user 803 name.) 805 8.2. GSS-API Per-Message Tokens for SCRAM 807 The per-message tokens for SCRAM as a GSS-API mechanism SHALL be the 808 same as those for the Kerberos V GSS-API mechanism [RFC4121] (see 809 Section 4.2 and sub-sections), using the Kerberos V "aes128-cts-hmac- 810 sha1-96" enctype [RFC3962]. 812 The 128-bit session "protocol key" SHALL be derived by using the 813 least significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API 814 session key" || ClientKey || AuthMessage). "Specific keys" are then 815 derived as usual as described in Section 2 of [RFC4121], [RFC3961] 816 and [RFC3962]. 818 The terms "protocol key" and "specific key" are Kerberos V5 terms 820 [RFC3961]. 822 SCRAM does support PROT_READY, and is PROT_READY on the initiator 823 side first upon receipt of the server's reply to the initial security 824 context token. 826 8.3. GSS_Pseudo_random() for SCRAM 828 The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for 829 the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor- 830 asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and 831 GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random(). 832 The protocol key to be used for the GSS_Pseudo_random() SHALL be the 833 same as the key defined in Section 8.2. 835 9. Security Considerations 837 If the authentication exchange is performed without a strong security 838 layer, then a passive eavesdropper can gain sufficient information to 839 mount an offline dictionary or brute-force attack which can be used 840 to recover the user's password. The amount of time necessary for 841 this attack depends on the cryptographic hash function selected, the 842 strength of the password and the iteration count supplied by the 843 server. An external security layer with strong encryption will 844 prevent this attack. 846 If the external security layer used to protect the SCRAM exchange 847 uses an anonymous key exchange, then the SCRAM channel binding 848 mechanism can be used to detect a man-in-the-middle attack on the 849 security layer and cause the authentication to fail as a result. 850 However, the man-in-the-middle attacker will have gained sufficient 851 information to mount an offline dictionary or brute-force attack. 852 For this reason, SCRAM includes the ability to increase the iteration 853 count over time. 855 If the authentication information is stolen from the authentication 856 database, then an offline dictionary or brute-force attack can be 857 used to recover the user's password. The use of salt mitigates this 858 attack somewhat by requiring a separate attack on each password. 859 Authentication mechanisms which protect against this attack are 860 available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is 861 an example of such technology. The WG selected not to use EKE like 862 mechanisms as basis for SCRAM. 864 If an attacker obtains the authentication information from the 865 authentication repository and either eavesdrops on one authentication 866 exchange or impersonates a server, the attacker gains the ability to 867 impersonate that user to all servers providing SCRAM access using the 868 same hash function, password, iteration count and salt. For this 869 reason, it is important to use randomly-generated salt values. 871 SCRAM does not negotiate a hash function to use. Hash function 872 negotiation is left to the SASL mechanism negotiation. It is 873 important that clients be able to sort a locally available list of 874 mechanisms by preference so that the client may pick the most 875 preferred of a server's advertised mechanism list. This preference 876 order is not specified here as it is a local matter. The preference 877 order should include objective and subjective notions of mechanism 878 cryptographic strength (e.g., SCRAM with a successor to SHA-1 may be 879 preferred over SCRAM with SHA-1). 881 Note that to protect the SASL mechanism negotiation applications 882 normally must list the server mechs twice: once before and once after 883 authentication, the latter using security layers. Since SCRAM does 884 not provide security layers the only ways to protect the mechanism 885 negotiation are: a) use channel binding to an external channel, or b) 886 use an external channel that authenticates a user-provided server 887 name. 889 SCRAM does not protect against downgrade attacks of channel binding 890 types. The complexities of negotiation a channel binding type, and 891 handling down-grade attacks in that negotiation, was intentionally 892 left out of scope for this document. 894 A hostile server can perform a computational denial-of-service attack 895 on clients by sending a big iteration count value. 897 See [RFC4086] for more information about generating randomness. 899 10. IANA Considerations 901 IANA is requested to add the following family of SASL mechanisms to 902 the SASL Mechanism registry established by [RFC4422]: 904 To: iana@iana.org 905 Subject: Registration of a new SASL family SCRAM 907 SASL mechanism name (or prefix for the family): SCRAM-* 908 Security considerations: Section 7 of [RFCXXXX] 909 Published specification (optional, recommended): [RFCXXXX] 910 Person & email address to contact for further information: 911 IETF SASL WG 912 Intended usage: COMMON 913 Owner/Change controller: IESG 914 Note: Members of this family must be explicitly registered 915 using the "IETF Review" [RFC5226] registration procedure. 916 Reviews must be requested on the SASL WG mailing list. 918 "IETF Review" [RFC5226] registration procedure MUST be used for 919 registering new mechanisms in this family. The SASL mailing list 920 (or a successor designated by the responsible 921 Security AD) MUST be used for soliciting reviews on such 922 registrations. 924 Note to future SCRAM- mechanism designers: each new SCRAM- SASL 925 mechanism MUST be explicitly registered with IANA and MUST comply 926 with SCRAM- mechanism naming convention defined in Section 4 of this 927 document. 929 IANA is requested to add the following entries to the SASL Mechanism 930 registry established by [RFC4422]: 932 To: iana@iana.org 933 Subject: Registration of a new SASL mechanism SCRAM-SHA-1 935 SASL mechanism name (or prefix for the family): SCRAM-SHA-1 936 Security considerations: Section 7 of [RFCXXXX] 937 Published specification (optional, recommended): [RFCXXXX] 938 Person & email address to contact for further information: 939 IETF SASL WG 940 Intended usage: COMMON 941 Owner/Change controller: IESG 942 Note: 944 To: iana@iana.org 945 Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS 947 SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS 948 Security considerations: Section 7 of [RFCXXXX] 949 Published specification (optional, recommended): [RFCXXXX] 950 Person & email address to contact for further information: 951 IETF SASL WG 952 Intended usage: COMMON 953 Owner/Change controller: IESG 954 Note: 956 This document also requests IANA to assign a GSS-API mechanism OID 957 for SCRAM from the iso.org.dod.internet.security.mechanisms prefix 958 (see "SMI Security for Mechanism Codes" registry). 960 11. Acknowledgements 962 This document benefited from discussions on the SASL WG mailing list. 963 The authors would like to specially thank Dave Cridland, Simon 964 Josefsson, Jeffrey Hutzelman, Kurt Zeilenga, Pasi Eronen and Peter 965 Saint-Andrefor their contributions to this document. 967 Appendix A. Other Authentication Mechanisms 969 The DIGEST-MD5 [I-D.ietf-sasl-digest-to-historic] mechanism has 970 proved to be too complex to implement and test, and thus has poor 971 interoperability. The security layer is often not implemented, and 972 almost never used; everyone uses TLS instead. For a more complete 973 list of problems with DIGEST-MD5 which lead to the creation of SCRAM 974 see [I-D.ietf-sasl-digest-to-historic]. 976 The CRAM-MD5 SASL mechanism, while widely deployed has also some 977 problems, in particular it is missing some modern SASL features such 978 as support for internationalized usernames and passwords, support for 979 passing of authorization identity, support for channel bindings. It 980 also doesn't support server authentication. For a more complete list 981 of problems with CRAM-MD5 see [I-D.ietf-sasl-crammd5-to-historic]. 983 The PLAIN [RFC4616] SASL mechanism allows a malicious server or 984 eavesdropper to impersonate the authenticating user to any other 985 server for which the user has the same password. It also sends the 986 password in the clear over the network, unless TLS is used. Server 987 authentication is not supported. 989 Appendix B. Design Motivations 991 The following design goals shaped this document. Note that some of 992 the goals have changed since the initial version of the document. 994 o The SASL mechanism has all modern SASL features: support for 995 internationalized usernames and passwords, support for passing of 996 authorization identity, support for channel bindings. 998 o The protocol supports mutual authentication. 1000 o The authentication information stored in the authentication 1001 database is not sufficient by itself to impersonate the client. 1003 o The server does not gain the ability to impersonate the client to 1004 other servers (with an exception for server-authorized proxies), 1005 unless such other servers allow SCRAM authentication and use the 1006 same salt and iteration count for the user. 1008 o The mechanism is extensible, but [hopefully] not overengineered in 1009 this respect. 1011 o Easier to implement than DIGEST-MD5 in both clients and servers. 1013 Appendix C. Internet-Draft Change History 1015 (RFC Editor: Please delete everything after this point) 1017 Changes since -10 1019 o Converted the source for this I-D to XML. 1021 o Added text to make SCRAM compliant with the new GS2 design. 1023 o Added text on channel binding negotiation. 1025 o Added text on channel binding, including a reference to RFC5056. 1027 o Added text on SCRAM as a GSS-API mechanism. This noted as not 1028 relevant to SASL-only implementors -- the normative references for 1029 SCRAM as a GSS-API mechanism are segregated as well. 1031 Changes since -07 1033 o Updated References. 1035 o Clarified purpose of the m= attribute. 1037 o Fixed a problem with authentication/authorization identity's ABNF 1038 not allowing for some characters. 1040 o Updated ABNF for nonce to show client-generated and server- 1041 generated parts. 1043 o Only register SCRAM-SHA-1 with IANA and require explicit 1044 registrations of all other SCRAM- mechanisms. 1046 Changes since -06 1048 o Removed hash negotiation from SCRAM and turned it into a family of 1049 SASL mechanisms. 1051 o Start using "Hash Function Textual Names" IANA registry for SCRAM 1052 mechanism naming. 1054 o Fixed definition of Hi(str, salt) to be consistent with [RFC2898]. 1056 o Clarified extensibility of SCRAM: added m= attribute (for future 1057 mandatory extensions) and specified that all unrecognized 1058 attributes must be ignored. 1060 Changes since -05 1061 o Changed the mandatory to implement hash algorithm to SHA-1 (as per 1062 WG consensus). 1064 o Added text about use of SASLPrep for username canonicalization/ 1065 validation. 1067 o Clarified that authorization identity is canonicalized/verified 1068 according to SASL protocol profile. 1070 o Clarified that iteration count is per-user. 1072 o Clarified how clients select the authentication function. 1074 o Added IANA registration for the new mechanism. 1076 o Added missing normative references (UTF-8, SASLPrep). 1078 o Various editorial changes based on comments from Hallvard B 1079 Furuseth, Nico William and Simon Josefsson. 1081 Changes since -04 1083 o Update Base64 and Security Glossary references. 1085 o Add Formal Syntax section. 1087 o Don't bother with "v=". 1089 o Make MD5 mandatory to implement. Suggest i=128. 1091 Changes since -03 1093 o Seven years have passed, in which it became clear that DIGEST-MD5 1094 suffered from unacceptably bad interoperability, so SCRAM-MD5 is 1095 now back from the dead. 1097 o Be hash agnostic, so MD5 can be replaced more easily. 1099 o General simplification. 1101 12. References 1103 12.1. Normative References 1105 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1106 Hashing for Message Authentication", RFC 2104, 1107 February 1997. 1109 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1110 Requirement Levels", BCP 14, RFC 2119, March 1997. 1112 [RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 1113 (SHA1)", RFC 3174, September 2001. 1115 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 1116 Internationalized Strings ("stringprep")", RFC 3454, 1117 December 2002. 1119 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 1120 10646", STD 63, RFC 3629, November 2003. 1122 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 1123 and Passwords", RFC 4013, February 2005. 1125 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 1126 Security Layer (SASL)", RFC 4422, June 2006. 1128 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1129 Encodings", RFC 4648, October 2006. 1131 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 1132 Channels", RFC 5056, November 2007. 1134 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1135 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1137 12.2. Normative References for GSS-API implementors 1139 [I-D.ietf-sasl-gs2] 1140 Josefsson, S. and N. Williams, "Using GSS-API Mechanisms 1141 in SASL: The GS2 Mechanism Family", draft-ietf-sasl-gs2-12 1142 (work in progress), April 2009. 1144 [RFC2743] Linn, J., "Generic Security Service Application Program 1145 Interface Version 2, Update 1", RFC 2743, January 2000. 1147 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 1148 Kerberos 5", RFC 3961, February 2005. 1150 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) 1151 Encryption for Kerberos 5", RFC 3962, February 2005. 1153 [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 1154 Version 5 Generic Security Service Application Program 1155 Interface (GSS-API) Mechanism: Version 2", RFC 4121, 1156 July 2005. 1158 [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API 1159 Extension for the Generic Security Service Application 1160 Program Interface (GSS-API)", RFC 4401, February 2006. 1162 [RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the 1163 Kerberos V Generic Security Service Application Program 1164 Interface (GSS-API) Mechanism", RFC 4402, February 2006. 1166 [tls-unique] 1167 Zhu, L., "Registration of TLS unique channel binding 1168 (generic)", IANA http://www.iana.org/assignments/ 1169 channel-binding-types/tls-unique, July 2008. 1171 12.3. Informative References 1173 [I-D.altman-tls-channel-bindings] 1174 Altman, J., Williams, N., and L. Zhu, "Channel Bindings 1175 for TLS", draft-altman-tls-channel-bindings-06 (work in 1176 progress), August 2009. 1178 [I-D.ietf-sasl-crammd5-to-historic] 1179 Zeilenga, K., "CRAM-MD5 to Historic", 1180 draft-ietf-sasl-crammd5-to-historic-00 (work in progress), 1181 November 2008. 1183 [I-D.ietf-sasl-digest-to-historic] 1184 Melnikov, A., "Moving DIGEST-MD5 to Historic", 1185 draft-ietf-sasl-digest-to-historic-00 (work in progress), 1186 July 2008. 1188 [I-D.melnikov-sasl-scram-ldap] 1189 Melnikov, A., "LDAP schema for storing SCRAM secrets", 1190 draft-melnikov-sasl-scram-ldap-02 (work in progress), 1191 July 2009. 1193 [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography 1194 Specification Version 2.0", RFC 2898, September 2000. 1196 [RFC2945] Wu, T., "The SRP Authentication and Key Exchange System", 1197 RFC 2945, September 2000. 1199 [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 1200 Requirements for Security", BCP 106, RFC 4086, June 2005. 1202 [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol 1203 (LDAP): Technical Specification Road Map", RFC 4510, 1204 June 2006. 1206 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and 1207 Security Layer (SASL) Mechanism", RFC 4616, August 2006. 1209 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 1210 RFC 4949, August 2007. 1212 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1213 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1214 May 2008. 1216 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1217 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1219 [tls-server-end-point] 1220 Zhu, L., "Registration of TLS server end-point channel 1221 bindings", IANA http://www.iana.org/assignments/ 1222 channel-binding-types/tls-server-end-point, July 2008. 1224 Authors' Addresses 1226 Abhijit Menon-Sen 1227 Oryx Mail Systems GmbH 1229 Email: ams@oryx.com 1231 Alexey Melnikov 1232 Isode Ltd 1234 Email: Alexey.Melnikov@isode.com 1236 Chris Newman 1237 Sun Microsystems 1238 1050 Lakes Drive 1239 West Covina, CA 91790 1240 USA 1242 Email: chris.newman@sun.com 1244 Nicolas Williams 1245 Sun Microsystems 1246 5300 Riata Trace Ct 1247 Austin, TX 78727 1248 USA 1250 Email: Nicolas.Williams@sun.com