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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 13, 2010 Isode Ltd 6 C. Newman 7 N. Williams 8 Sun Microsystems 9 September 9, 2009 11 Salted Challenge Response (SCRAM) SASL and GSS-API Mechanism 12 draft-ietf-sasl-scram-06.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 13, 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 A separate document defines a standard LDAPv3 [RFC4510] attribute 244 that enables storage of the SCRAM authentication information in LDAP. 245 See [I-D.melnikov-sasl-scram-ldap]. 247 For an in-depth discussion of why other challenge response mechanisms 248 are not considered sufficient, see appendix A. For more information 249 about the motivations behind the design of this mechanism, see 250 appendix B. 252 Comments regarding this draft may be sent either to the sasl@ietf.org 253 mailing list or to the authors. 255 3. SCRAM Algorithm Overview 257 Note that this section omits some details, such as client and server 258 nonces. See Section 5 for more details. 260 To begin with, the SCRAM client is in possession of a username and 261 password. It sends the username to the server, which retrieves the 262 corresponding authentication information, i.e. a salt, StoredKey, 263 ServerKey and the iteration count i. (Note that a server 264 implementation may choose to use the same iteration count for all 265 accounts.) The server sends the salt and the iteration count to the 266 client, which then computes the following values and sends a 267 ClientProof to the server: 269 SaltedPassword := Hi(password, salt) 270 ClientKey := HMAC(SaltedPassword, "Client Key") 271 StoredKey := H(ClientKey) 272 AuthMessage := client-first-message-bare + "," + 273 server-first-message + "," + 274 client-final-message-without-proof 275 ClientSignature := HMAC(StoredKey, AuthMessage) 276 ClientProof := ClientKey XOR ClientSignature 277 ServerKey := HMAC(SaltedPassword, "Server Key") 278 ServerSignature := HMAC(ServerKey, AuthMessage) 280 The server authenticates the client by computing the ClientSignature, 281 exclusive-ORing that with the ClientProof to recover the ClientKey 282 and verifying the correctness of the ClientKey by applying the hash 283 function and comparing the result to the StoredKey. If the ClientKey 284 is correct, this proves that the client has access to the user's 285 password. 287 Similarly, the client authenticates the server by computing the 288 ServerSignature and comparing it to the value sent by the server. If 289 the two are equal, it proves that the server had access to the user's 290 ServerKey. 292 The AuthMessage is computed by concatenating messages from the 293 authentication exchange. The format of these messages is defined in 294 Section 7. 296 4. SCRAM Mechanism Names 298 A SCRAM mechanism name is a string "SCRAM-" followed by the 299 uppercased name of the underlying hash function taken from the IANA 300 "Hash Function Textual Names" registry (see http://www.iana.org), 301 optionally followed by the suffix "-PLUS" (see below). Note that 302 SASL mechanism names are limited to 20 characters, which means that 303 only hash function names with lengths shorter or equal to 9 304 characters (20-length("SCRAM-")-length("-PLUS") can be used. For 305 cases when the underlying hash function name is longer than 9 306 characters, an alternative 9 character (or shorter) name can be used 307 to construct the corresponding SCRAM mechanism name, as long as this 308 alternative name doesn't conflict with any other hash function name 309 from the IANA "Hash Function Textual Names" registry. 311 For interoperability, all SCRAM clients and servers MUST implement 312 the SCRAM-SHA-1 authentication mechanism, i.e. an authentication 313 mechanism from the SCRAM family that uses the SHA-1 hash function as 314 defined in [RFC3174]. 316 The "-PLUS" suffix is used only when the server supports channel 317 binding to the external channel. If the server supports channel 318 binding, it will advertise both the "bare" and "plus" versions of 319 whatever mechanisms it supports (e.g., if the server supports only 320 SCRAM with SHA-1 then it will advertise support for both SCRAM-SHA-1 321 and SCRAM-SHA-1-PLUS); if the server does not support channel 322 binding, then it will advertise only the "bare" version of the 323 mechanism (e.g., only SCRAM-SHA-1). The "-PLUS" exists to allow 324 negotiation of the use of channel binding. See Section 6. 326 5. SCRAM Authentication Exchange 328 SCRAM is a SASL mechanism whose client response and server challenge 329 messages are text-based messages containing one or more attribute- 330 value pairs separated by commas. Each attribute has a one-letter 331 name. The messages and their attributes are described in 332 Section 5.1, and defined in Section 7. 334 This is a simple example of a SCRAM-SHA-1 authentication exchange 335 when the client doesn't support channel bindings: 337 C: n,,n=Chris Newman,r=ClientNonce 338 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 339 C: c=biwsCg==,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4 340 S: v=WxPv/siO5l+qxN4 342 [[anchor5: Note that the all hashes above are fake and will be fixed 343 during AUTH48.]] 345 With channel-binding data sent by the client this might look like 346 this (see [tls-server-end-point] for the definition of tls-server- 347 end-point TLS channel binding): 349 C: p=tls-server-end-point,,n=Chris Newman,r=ClientNonce 350 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 351 C: c=cD10bHMtc2VydmVyLWVuZC1wb2ludCwsy1hFtXOnZ+ySrQM6srFp 352 l/77uqvtxrg7nBY1BetEr/g=,r=ClientNonceServerNonce,p=Wx 353 Pv/siO5l+qxN4 354 S: v=WxPv/siO5l+qxN4 356 [[anchor6: Note that all hashes above are fake and will be fixed 357 during AUTH48.]] 359 First, the client sends the "client-first-message" containing: 361 o a GS2 header consisting of a flag indicating whether channel 362 binding is supported-but-not-used, not supported, or used, and an 363 optional SASL authorization identity; 365 o SCRAM username and a random, unique nonce attributes. 367 Note that the client's first message will always start with "n", "y" 368 or "p", otherwise the message is invalid and authentication MUST 369 fail. This is important, as it allows for GS2 extensibility (e.g., 370 to add support for security layers). 372 In response, the server sends a "server-first-message" containing the 373 user's iteration count i, the user's salt, and appends its own nonce 374 to the client-specified one. 376 The client then responds by sending "client-final-message" with the 377 same nonce and a ClientProof computed using the selected hash 378 function as explained earlier. 380 The server verifies the nonce and the proof, verifies that the 381 authorization identity (if supplied by the client in the first 382 message) is authorized to act as the authentication identity, and, 383 finally, it responds with a "server-final-message", concluding the 384 authentication exchange. 386 The client then authenticates the server by computing the 387 ServerSignature and comparing it to the value sent by the server. If 388 the two are different, the client MUST consider the authentication 389 exchange to be unsuccessful and it might have to drop the connection. 391 5.1. SCRAM Attributes 393 This section describes the permissible attributes, their use, and the 394 format of their values. All attribute names are single US-ASCII 395 letters and are case-sensitive. 397 Note that the order of attributes in client or server messages is 398 fixed, with the exception of extension attributes (described by the 399 "extensions" ABNF production), which can appear in any order in the 400 designated positions. See the ABNF section for authoritative 401 reference. 403 o a: This is an optional attribute, and is part of the GS2 404 [I-D.ietf-sasl-gs2] bridge between the GSS-API and SASL. This 405 attribute specifies an authorization identity. A client may 406 include it in its first message to the server if it wants to 407 authenticate as one user, but subsequently act as a different 408 user. This is typically used by an administrator to perform some 409 management task on behalf of another user, or by a proxy in some 410 situations. 412 Upon the receipt of this value the server verifies its 413 correctness according to the used SASL protocol profile. 414 Failed verification results in failed authentication exchange. 416 If this attribute is omitted (as it normally would be), the 417 authorization identity is assumed to be derived from the 418 username specified with the (required) "n" attribute. 420 The server always authenticates the user specified by the "n" 421 attribute. If the "a" attribute specifies a different user, 422 the server associates that identity with the connection after 423 successful authentication and authorization checks. 425 The syntax of this field is the same as that of the "n" field 426 with respect to quoting of '=' and ','. 428 o n: This attribute specifies the name of the user whose password is 429 used for authentication (a.k.a. "authentication identity" 430 [RFC4422]). A client MUST include it in its first message to the 431 server. If the "a" attribute is not specified (which would 432 normally be the case), this username is also the identity which 433 will be associated with the connection subsequent to 434 authentication and authorization. 436 Before sending the username to the server, the client SHOULD 437 prepare the username using the "SASLPrep" profile [RFC4013] of 438 the "stringprep" algorithm [RFC3454] treating it as a query 439 string (i.e., unassigned Unicode code points are allowed). If 440 the preparation of the username fails or results in an empty 441 string, the client SHOULD abort the authentication exchange 442 (*). 444 (*) An interactive client can request a repeated entry of the 445 username value. 447 Upon receipt of the username by the server, the server SHOULD 448 prepare it using the "SASLPrep" profile [RFC4013] of the 449 "stringprep" algorithm [RFC3454] treating it as a stored string 450 (i.e., unassigned Unicode code points are forbidden). If the 451 preparation of the username fails or results in an empty 452 string, the server SHOULD abort the authentication exchange. 454 The characters ',' or '=' in usernames are sent as '=2C' and 455 '=3D' respectively. If the server receives a username which 456 contains '=' not followed by either '2C' or '3D', then the 457 server MUST fail the authentication. 459 o m: This attribute is reserved for future extensibility. In this 460 version of SCRAM, its presence in a client or a server message 461 MUST cause authentication failure when the attribute is parsed by 462 the other end. 464 o r: This attribute specifies a sequence of random printable 465 characters excluding ',' which forms the nonce used as input to 466 the hash function. No quoting is applied to this string. As 467 described earlier, the client supplies an initial value in its 468 first message, and the server augments that value with its own 469 nonce in its first response. It is important that this value be 470 different for each authentication. The client MUST verify that 471 the initial part of the nonce used in subsequent messages is the 472 same as the nonce it initially specified. The server MUST verify 473 that the nonce sent by the client in the second message is the 474 same as the one sent by the server in its first message. 476 o c: This REQUIRED attribute specifies base64-encoded of a header 477 and the channel-binding data. It is sent by the client in its 478 second authentication message. The header consist of: 480 * the GS2 header from the client's first message (recall: a 481 channel binding flag and an optional authzid). This header is 482 going to include channel binding type prefix (see [RFC5056]), 483 if and only if the client is using channel binding; 485 * followed by the external channel's channel binding data, if and 486 only if the client is using channel binding. 488 o s: This attribute specifies the base64-encoded salt used by the 489 server for this user. It is sent by the server in its first 490 message to the client. 492 o i: This attribute specifies an iteration count for the selected 493 hash function and user, and MUST be sent by the server along with 494 the user's salt. 496 For SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism servers SHOULD 497 announce a hash iteration-count of at least 4096. Note that a 498 client implementation MAY cache SaltedPassword/ClientKey for 499 later reauthentication to the same service, as it is likely 500 that the server is going to advertise the same salt value upon 501 reauthentication. This might be useful for mobile clients 502 where CPU usage is a concern. 504 o p: This attribute specifies a base64-encoded ClientProof. The 505 client computes this value as described in the overview and sends 506 it to the server. 508 o v: This attribute specifies a base64-encoded ServerSignature. It 509 is sent by the server in its final message, and is used by the 510 client to verify that the server has access to the user's 511 authentication information. This value is computed as explained 512 in the overview. 514 5.2. Compliance with SASL mechanism requirements 516 This section describes compliance with SASL mechanism requirements 517 specified in Section 5 of [RFC4422]. 519 1) "SCRAM-SHA-1" and "SCRAM-SHA-1-PLUS". 521 2a) SCRAM is a client-first mechanism. 523 2b) SCRAM sends additional data with success. 525 3) SCRAM is capable of transferring authorization identities from the 526 client to the server. 528 4) SCRAM does not offer any security layers (SCRAM offers channel 529 binding instead). 531 5) SCRAM has a hash protecting the authorization identity. 533 6. Channel Binding 535 SCRAM supports channel binding to external secure channels, such as 536 TLS. Clients and servers may or may not support channel binding, 537 therefore the use of channel binding is negotiable. SCRAM does not 538 provide security layers, however, therefore it is imperative that 539 SCRAM provide integrity protection for the negotiation of channel 540 binding. 542 Use of channel binding is negotiated as follows: 544 o Servers SHOULD advertise both non-PLUS (SCRAM-) and 545 the PLUS-variant (SCRAM--PLUS) SASL mechanism 546 names. If the server cannot support channel binding, it MAY 547 advertise only the non-PLUS variant. If the server would never 548 succeed authentication of the non-PLUS variant due to policy 549 reasons, it MAY advertise only the PLUS-variant. 551 o If the client negotiates mechanisms then the client MUST select 552 SCRAM--PLUS if offered by the server and the client 553 wants to select SCRAM with the given hash function. Otherwise 554 (the client does not negotiate mechanisms), if the client has no 555 prior knowledge about mechanisms supported by the server and 556 wasn't explicitly configured to use a particular variant of the 557 SCRAM mechanism, then it MUST select only SCRAM- 558 (not suffixed with "-PLUS"). 560 o If the client supports channel binding and the server appears to 561 support it (i.e., the client sees SCRAM--PLUS), or 562 if the client wishes to use channel binding but the client does 563 not negotiate mechanisms, then the client MUST set the GS2 channel 564 binding flag to "p" in order to indicate the channel binding type 565 it is using and it MUST include the channel binding data for the 566 external channel in the computation of the "c=" attribute (see 567 Section 5.1). 569 o If the client supports channel binding but the server does not 570 appear to (i.e., the client did not see SCRAM-- 571 PLUS) then the client MUST either fail authentication or it MUST 572 choose the non-PLUS mechanism and set the GS2 channel binding flag 573 to "y" and MUST NOT include channel binding data for the external 574 channel in the computation of the "c=" attribute (see 575 Section 5.1). 577 o If the client does not support channel binding then the client 578 MUST set the GS2 channel binding flag to "n" and MUST NOT include 579 channel binding data for the external channel in the computation 580 of the "c=" attribute (see Section 5.1). 582 o Upon receipt of the client first message the server checks the GS2 583 channel binding flag (gs2-cb-flag). 585 * If the flag is set to "y" and the server supports channel 586 binding the server MUST fail authentication. This is because 587 if the client sets the GS2 channel binding flag set to "y" then 588 the client must have believed that the server did not support 589 channel binding -- if the server did in fact support channel 590 binding then this is an indication that there has been a 591 downgrade attack (e.g., an attacker changed the server's 592 mechanism list to exclude the -PLUS suffixed SCRAM mechanism 593 name(s)). 595 * If the channel binding flag was "p" and the server does not 596 support the indicated channel binding type then the server MUST 597 fail authentication. 599 The server MUST always validate the client's "c=" field. The server 600 does this by constructing the value of the "c=" attribute and then 601 checking that it matches the client's c= attribute value. 603 For more discussions of channel bindings, and the syntax of the 604 channel binding data for various security protocols, see [RFC5056]. 606 6.1. Default Channel Binding 608 A default channel binding type agreement process for all SASL 609 application protocols that do not provide their own channel binding 610 type agreement is provided as follows. 612 'tls-unique' is the default channel binding type for any application 613 that doesn't specify one. 615 Servers MUST implement the "tls-unique" [tls-unique] 616 [I-D.altman-tls-channel-bindings] channel binding type, if they 617 implement any channel binding. Clients SHOULD implement the "tls- 618 unique" [tls-unique] [I-D.altman-tls-channel-bindings] channel 619 binding type, if they implement any channel binding. Clients and 620 servers SHOULD choose the highest- layer/innermost end-to-end TLS 621 channel as the channel to bind to. 623 Servers MUST choose the channel binding type indicated by the client, 624 or fail authentication if they don't support it. 626 7. Formal Syntax 628 The following syntax specification uses the Augmented Backus-Naur 629 Form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3" 630 and "UTF8-4" non-terminal are defined in [RFC3629]. 632 ALPHA = 633 DIGIT = 634 UTF8-2 = 635 UTF8-3 = 636 UTF8-4 = 638 attr-val = ALPHA "=" value 639 ;; Generic syntax of any attribute sent 640 ;; by server or client 642 value = 1*value-char 644 value-safe-char = %x01-2B / %x2D-3C / %x3E-7F / 645 UTF8-2 / UTF8-3 / UTF8-4 646 ;; UTF8-char except NUL, "=", and ",". 648 value-char = value-safe-char / "=" 650 base64-char = ALPHA / DIGIT / "/" / "+" 652 base64-4 = 4base64-char 654 base64-3 = 3base64-char "=" 656 base64-2 = 2base64-char "==" 658 base64 = *base64-4 [base64-3 / base64-2] 660 posit-number = %x31-39 *DIGIT 661 ;; A positive number 663 saslname = 1*(value-safe-char / "=2C" / "=3D") 664 ;; Conforms to 666 authzid = "a=" saslname 667 ;; Protocol specific. 669 cb-name = 1*(ALPHA / DIGIT / "." / "-") 670 ;; See RFC 5056 section 7. 671 ;; E.g. "tls-server-end-point" or 672 ;; "tls-unique" 674 gs2-cbind-flag = "p=" cb-name / "n" / "y" 675 ;; "n" -> client doesn't support channel binding 676 ;; "y" -> client does support channel binding 677 ;; but thinks the server does not. 678 ;; "p" -> client requires channel binding. 679 ;; The selected channel binding follows "p=". 681 gs2-header = gs2-cbind-flag "," [ authzid ] "," 682 ;; GS2 header for SCRAM 683 ;; (the actual GS2 header includes an optional 684 ;; flag to indicate that the GSS mechanism is not 685 ;; "standard" but since SCRAM is "standard" we 686 ;; don't include that flag). 688 username = "n=" saslname 689 ;; Usernames are prepared using SASLPrep. 691 reserved-mext = "m=" 1*(value-char) 692 ;; Reserved for signalling mandatory extensions. 693 ;; The exact syntax will be defined in 694 ;; the future. 696 channel-binding = "c=" base64 697 ;; base64 encoding of cbind-input 699 proof = "p=" base64 701 nonce = "r=" c-nonce [s-nonce] 702 ;; Second part provided by server. 704 c-nonce = value 706 s-nonce = value 708 salt = "s=" base64 710 verifier = "v=" base64 711 ;; base-64 encoded ServerSignature. 713 iteration-count = "i=" posit-number 714 ;; A positive number 716 client-first-message-bare = 717 [reserved-mext ","] 718 username "," nonce ["," extensions] 720 client-first-message = 721 gs2-header client-first-message-bare 723 server-first-message = 724 [reserved-mext ","] nonce "," salt "," 725 iteration-count ["," extensions] 727 client-final-message-without-proof = 728 channel-binding "," nonce ["," 729 extensions] 731 client-final-message = 732 client-final-message-without-proof "," proof 734 gss-server-error = "e=" value 735 server-final-message = gss-server-error / 736 verifier ["," extensions] 737 ;; The error message is only for the GSS-API 738 ;; form of SCRAM, and it is OPTIONAL to 739 ;; implement it. 741 extensions = attr-val *("," attr-val) 742 ;; All extensions are optional, 743 ;; i.e. unrecognized attributes 744 ;; not defined in this document 745 ;; MUST be ignored. 747 cbind-data = 1*OCTET 749 cbind-input = gs2-header [ cbind-data ] 750 ;; cbind-data MUST be present for 751 ;; gs2-cbind-flag of "p" and MUST be absent 752 ;; for "y" or "n". 754 8. SCRAM as a GSS-API Mechanism 756 This section and its sub-sections and all normative references of it 757 not referenced elsewhere in this document are INFORMATIONAL for SASL 758 implementors, but they are NORMATIVE for GSS-API implementors. 760 SCRAM is actually also GSS-API mechanism. The messages are the same, 761 but a) the GS2 header on the client's first message and channel 762 binding data is excluded when SCRAM is used as a GSS-API mechanism, 763 and b) the RFC2743 section 3.1 initial context token header is 764 prefixed to the client's first authentication message (context 765 token). 767 The GSS-API mechanism OID for SCRAM is (see Section 10). 769 8.1. GSS-API Principal Name Types for SCRAM 771 SCRAM does not name acceptors. Therefore only GSS_C_NO_NAME and 772 names of type GSS_C_NT_ANONYMOUS shall be allowed as the target name 773 input of GSS_Init_sec_context() when using a SCRAM mechanism. 775 SCRAM supports only a single name type for initiators: 776 GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for 777 SCRAM. 779 There is no name canonicalization procedure for SCRAM beyond applying 780 SASLprep as described in Section 5.1. 782 The query, display and exported name syntax for SCRAM principal names 783 is the same: there is no syntax -- SCRAM principal names are free- 784 form. (The exported name token does, of course, conform to [RFC2743] 785 section 3.2, but the "NAME" part of the token is just a SCRAM user 786 name.) 788 8.2. GSS-API Per-Message Tokens for SCRAM 790 The per-message tokens for SCRAM as a GSS-API mechanism SHALL be the 791 same as those for the Kerberos V GSS-API mechanism [RFC4121] (see 792 Section 4.2 and sub-sections), using the Kerberos V "aes128-cts-hmac- 793 sha1-96" enctype [RFC3962]. 795 The 128-bit session "protocol key" SHALL be derived by using the 796 least significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API 797 session key" || ClientKey || AuthMessage). "Specific keys" are then 798 derived as usual as described in Section 2 of [RFC4121], [RFC3961] 799 and [RFC3962]. 801 The terms "protocol key" and "specific key" are Kerberos V5 terms 803 [RFC3961]. 805 SCRAM does support PROT_READY, and is PROT_READY on the initiator 806 side first upon receipt of the server's reply to the initial security 807 context token. 809 8.3. GSS_Pseudo_random() for SCRAM 811 The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for 812 the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor- 813 asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and 814 GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random(). 815 The protocol key to be used for the GSS_Pseudo_random() SHALL be the 816 same as the key defined in Section 8.2. 818 9. Security Considerations 820 If the authentication exchange is performed without a strong security 821 layer, then a passive eavesdropper can gain sufficient information to 822 mount an offline dictionary or brute-force attack which can be used 823 to recover the user's password. The amount of time necessary for 824 this attack depends on the cryptographic hash function selected, the 825 strength of the password and the iteration count supplied by the 826 server. An external security layer with strong encryption will 827 prevent this attack. 829 If the external security layer used to protect the SCRAM exchange 830 uses an anonymous key exchange, then the SCRAM channel binding 831 mechanism can be used to detect a man-in-the-middle attack on the 832 security layer and cause the authentication to fail as a result. 833 However, the man-in-the-middle attacker will have gained sufficient 834 information to mount an offline dictionary or brute-force attack. 835 For this reason, SCRAM includes the ability to increase the iteration 836 count over time. 838 If the authentication information is stolen from the authentication 839 database, then an offline dictionary or brute-force attack can be 840 used to recover the user's password. The use of salt mitigates this 841 attack somewhat by requiring a separate attack on each password. 842 Authentication mechanisms which protect against this attack are 843 available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is 844 an example of such technology. The WG selected not to use EKE like 845 mechanisms as basis for SCRAM. 847 If an attacker obtains the authentication information from the 848 authentication repository and either eavesdrops on one authentication 849 exchange or impersonates a server, the attacker gains the ability to 850 impersonate that user to all servers providing SCRAM access using the 851 same hash function, password, iteration count and salt. For this 852 reason, it is important to use randomly-generated salt values. 854 SCRAM does not negotiate a hash function to use. Hash function 855 negotiation is left to the SASL mechanism negotiation. It is 856 important that clients be able to sort a locally available list of 857 mechanisms by preference so that the client may pick the most 858 preferred of a server's advertised mechanism list. This preference 859 order is not specified here as it is a local matter. The preference 860 order should include objective and subjective notions of mechanism 861 cryptographic strength (e.g., SCRAM with a successor to SHA-1 may be 862 preferred over SCRAM with SHA-1). 864 Note that to protect the SASL mechanism negotiation applications 865 normally must list the server mechs twice: once before and once after 866 authentication, the latter using security layers. Since SCRAM does 867 not provide security layers the only ways to protect the mechanism 868 negotiation are: a) use channel binding to an external channel, or b) 869 use an external channel that authenticates a user-provided server 870 name. 872 SCRAM does not protect against downgrade attacks of channel binding 873 types. The complexities of negotiation a channel binding type, and 874 handling down-grade attacks in that negotiation, was intentionally 875 left out of scope for this document. 877 A hostile server can perform a computational denial-of-service attack 878 on clients by sending a big iteration count value. 880 See [RFC4086] for more information about generating randomness. 882 10. IANA Considerations 884 IANA is requested to add the following family of SASL mechanisms to 885 the SASL Mechanism registry established by [RFC4422]: 887 To: iana@iana.org 888 Subject: Registration of a new SASL family SCRAM 890 SASL mechanism name (or prefix for the family): SCRAM-* 891 Security considerations: Section 7 of [RFCXXXX] 892 Published specification (optional, recommended): [RFCXXXX] 893 Person & email address to contact for further information: 894 IETF SASL WG 895 Intended usage: COMMON 896 Owner/Change controller: IESG 897 Note: Members of this family must be explicitly registered 898 using the "IETF Review" [RFC5226] registration procedure. 899 Reviews must be requested on the SASL WG mailing list. 901 "IETF Review" [RFC5226] registration procedure MUST be used for 902 registering new mechanisms in this family. The SASL mailing list 903 (or a successor designated by the responsible 904 Security AD) MUST be used for soliciting reviews on such 905 registrations. 907 Note to future SCRAM- mechanism designers: each new SCRAM- SASL 908 mechanism MUST be explicitly registered with IANA and MUST comply 909 with SCRAM- mechanism naming convention defined in Section 4 of this 910 document. 912 IANA is requested to add the following entries to the SASL Mechanism 913 registry established by [RFC4422]: 915 To: iana@iana.org 916 Subject: Registration of a new SASL mechanism SCRAM-SHA-1 918 SASL mechanism name (or prefix for the family): SCRAM-SHA-1 919 Security considerations: Section 7 of [RFCXXXX] 920 Published specification (optional, recommended): [RFCXXXX] 921 Person & email address to contact for further information: 922 IETF SASL WG 923 Intended usage: COMMON 924 Owner/Change controller: IESG 925 Note: 927 To: iana@iana.org 928 Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS 930 SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS 931 Security considerations: Section 7 of [RFCXXXX] 932 Published specification (optional, recommended): [RFCXXXX] 933 Person & email address to contact for further information: 934 IETF SASL WG 935 Intended usage: COMMON 936 Owner/Change controller: IESG 937 Note: 939 This document also requests IANA to assign a GSS-API mechanism OID 940 for SCRAM from the iso.org.dod.internet.security.mechanisms prefix 941 (see "SMI Security for Mechanism Codes" registry). 943 11. Acknowledgements 945 This document benefited from discussions on the SASL WG mailing list. 946 The authors would like to specially thank Dave Cridland, Simon 947 Josefsson, Jeffrey Hutzelman, Kurt Zeilenga, Pasi Eronen and Peter 948 Saint-Andrefor their contributions to this document. 950 Appendix A. Other Authentication Mechanisms 952 The DIGEST-MD5 [I-D.ietf-sasl-digest-to-historic] mechanism has 953 proved to be too complex to implement and test, and thus has poor 954 interoperability. The security layer is often not implemented, and 955 almost never used; everyone uses TLS instead. For a more complete 956 list of problems with DIGEST-MD5 which lead to the creation of SCRAM 957 see [I-D.ietf-sasl-digest-to-historic]. 959 The CRAM-MD5 SASL mechanism, while widely deployed has also some 960 problems, in particular it is missing some modern SASL features such 961 as support for internationalized usernames and passwords, support for 962 passing of authorization identity, support for channel bindings. It 963 also doesn't support server authentication. For a more complete list 964 of problems with CRAM-MD5 see [I-D.ietf-sasl-crammd5-to-historic]. 966 The PLAIN [RFC4616] SASL mechanism allows a malicious server or 967 eavesdropper to impersonate the authenticating user to any other 968 server for which the user has the same password. It also sends the 969 password in the clear over the network, unless TLS is used. Server 970 authentication is not supported. 972 Appendix B. Design Motivations 974 The following design goals shaped this document. Note that some of 975 the goals have changed since the initial version of the document. 977 o The SASL mechanism has all modern SASL features: support for 978 internationalized usernames and passwords, support for passing of 979 authorization identity, support for channel bindings. 981 o The protocol supports mutual authentication. 983 o The authentication information stored in the authentication 984 database is not sufficient by itself to impersonate the client. 986 o The server does not gain the ability to impersonate the client to 987 other servers (with an exception for server-authorized proxies), 988 unless such other servers allow SCRAM authentication and use the 989 same salt and iteration count for the user. 991 o The mechanism is extensible, but [hopefully] not overengineered in 992 this respect. 994 o Easier to implement than DIGEST-MD5 in both clients and servers. 996 Appendix C. Internet-Draft Change History 998 (RFC Editor: Please delete everything after this point) 1000 Changes since -10 1002 o Converted the source for this I-D to XML. 1004 o Added text to make SCRAM compliant with the new GS2 design. 1006 o Added text on channel binding negotiation. 1008 o Added text on channel binding, including a reference to RFC5056. 1010 o Added text on SCRAM as a GSS-API mechanism. This noted as not 1011 relevant to SASL-only implementors -- the normative references for 1012 SCRAM as a GSS-API mechanism are segregated as well. 1014 Changes since -07 1016 o Updated References. 1018 o Clarified purpose of the m= attribute. 1020 o Fixed a problem with authentication/authorization identity's ABNF 1021 not allowing for some characters. 1023 o Updated ABNF for nonce to show client-generated and server- 1024 generated parts. 1026 o Only register SCRAM-SHA-1 with IANA and require explicit 1027 registrations of all other SCRAM- mechanisms. 1029 Changes since -06 1031 o Removed hash negotiation from SCRAM and turned it into a family of 1032 SASL mechanisms. 1034 o Start using "Hash Function Textual Names" IANA registry for SCRAM 1035 mechanism naming. 1037 o Fixed definition of Hi(str, salt) to be consistent with [RFC2898]. 1039 o Clarified extensibility of SCRAM: added m= attribute (for future 1040 mandatory extensions) and specified that all unrecognized 1041 attributes must be ignored. 1043 Changes since -05 1044 o Changed the mandatory to implement hash algorithm to SHA-1 (as per 1045 WG consensus). 1047 o Added text about use of SASLPrep for username canonicalization/ 1048 validation. 1050 o Clarified that authorization identity is canonicalized/verified 1051 according to SASL protocol profile. 1053 o Clarified that iteration count is per-user. 1055 o Clarified how clients select the authentication function. 1057 o Added IANA registration for the new mechanism. 1059 o Added missing normative references (UTF-8, SASLPrep). 1061 o Various editorial changes based on comments from Hallvard B 1062 Furuseth, Nico William and Simon Josefsson. 1064 Changes since -04 1066 o Update Base64 and Security Glossary references. 1068 o Add Formal Syntax section. 1070 o Don't bother with "v=". 1072 o Make MD5 mandatory to implement. Suggest i=128. 1074 Changes since -03 1076 o Seven years have passed, in which it became clear that DIGEST-MD5 1077 suffered from unacceptably bad interoperability, so SCRAM-MD5 is 1078 now back from the dead. 1080 o Be hash agnostic, so MD5 can be replaced more easily. 1082 o General simplification. 1084 12. References 1086 12.1. Normative References 1088 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1089 Hashing for Message Authentication", RFC 2104, 1090 February 1997. 1092 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1093 Requirement Levels", BCP 14, RFC 2119, March 1997. 1095 [RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 1096 (SHA1)", RFC 3174, September 2001. 1098 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 1099 Internationalized Strings ("stringprep")", RFC 3454, 1100 December 2002. 1102 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 1103 10646", STD 63, RFC 3629, November 2003. 1105 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 1106 and Passwords", RFC 4013, February 2005. 1108 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 1109 Security Layer (SASL)", RFC 4422, June 2006. 1111 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 1112 Encodings", RFC 4648, October 2006. 1114 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 1115 Channels", RFC 5056, November 2007. 1117 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1118 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1120 12.2. Normative References for GSS-API implementors 1122 [I-D.ietf-sasl-gs2] 1123 Josefsson, S. and N. Williams, "Using GSS-API Mechanisms 1124 in SASL: The GS2 Mechanism Family", draft-ietf-sasl-gs2-12 1125 (work in progress), April 2009. 1127 [RFC2743] Linn, J., "Generic Security Service Application Program 1128 Interface Version 2, Update 1", RFC 2743, January 2000. 1130 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 1131 Kerberos 5", RFC 3961, February 2005. 1133 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) 1134 Encryption for Kerberos 5", RFC 3962, February 2005. 1136 [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 1137 Version 5 Generic Security Service Application Program 1138 Interface (GSS-API) Mechanism: Version 2", RFC 4121, 1139 July 2005. 1141 [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API 1142 Extension for the Generic Security Service Application 1143 Program Interface (GSS-API)", RFC 4401, February 2006. 1145 [RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the 1146 Kerberos V Generic Security Service Application Program 1147 Interface (GSS-API) Mechanism", RFC 4402, February 2006. 1149 [tls-unique] 1150 Zhu, L., "Registration of TLS unique channel binding 1151 (generic)", IANA http://www.iana.org/assignments/ 1152 channel-binding-types/tls-unique, July 2008. 1154 12.3. Informative References 1156 [I-D.altman-tls-channel-bindings] 1157 Altman, J., Williams, N., and L. Zhu, "Channel Bindings 1158 for TLS", draft-altman-tls-channel-bindings-06 (work in 1159 progress), August 2009. 1161 [I-D.ietf-sasl-crammd5-to-historic] 1162 Zeilenga, K., "CRAM-MD5 to Historic", 1163 draft-ietf-sasl-crammd5-to-historic-00 (work in progress), 1164 November 2008. 1166 [I-D.ietf-sasl-digest-to-historic] 1167 Melnikov, A., "Moving DIGEST-MD5 to Historic", 1168 draft-ietf-sasl-digest-to-historic-00 (work in progress), 1169 July 2008. 1171 [I-D.melnikov-sasl-scram-ldap] 1172 Melnikov, A., "LDAP schema for storing SCRAM secrets", 1173 draft-melnikov-sasl-scram-ldap-02 (work in progress), 1174 July 2009. 1176 [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography 1177 Specification Version 2.0", RFC 2898, September 2000. 1179 [RFC2945] Wu, T., "The SRP Authentication and Key Exchange System", 1180 RFC 2945, September 2000. 1182 [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 1183 Requirements for Security", BCP 106, RFC 4086, June 2005. 1185 [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol 1186 (LDAP): Technical Specification Road Map", RFC 4510, 1187 June 2006. 1189 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and 1190 Security Layer (SASL) Mechanism", RFC 4616, August 2006. 1192 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 1193 RFC 4949, August 2007. 1195 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1196 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1197 May 2008. 1199 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1200 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1202 [tls-server-end-point] 1203 Zhu, L., "Registration of TLS server end-point channel 1204 bindings", IANA http://www.iana.org/assignments/ 1205 channel-binding-types/tls-server-end-point, July 2008. 1207 Authors' Addresses 1209 Abhijit Menon-Sen 1210 Oryx Mail Systems GmbH 1212 Email: ams@oryx.com 1214 Alexey Melnikov 1215 Isode Ltd 1217 Email: Alexey.Melnikov@isode.com 1219 Chris Newman 1220 Sun Microsystems 1221 1050 Lakes Drive 1222 West Covina, CA 91790 1223 USA 1225 Email: chris.newman@sun.com 1227 Nicolas Williams 1228 Sun Microsystems 1229 5300 Riata Trace Ct 1230 Austin, TX 78727 1231 USA 1233 Email: Nicolas.Williams@sun.com