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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: November 27, 2009 Isode Ltd 6 C. Newman 7 N. Williams 8 Sun Microsystems 9 May 26, 2009 11 Salted Challenge Response (SCRAM) SASL Mechanism 12 draft-ietf-sasl-scram-01.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 November 27, 2009. 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 6. Channel Binding . . . . . . . . . . . . . . . . . . . 15 81 6.1. Channel Binding to TLS Channels . . . . . . . . . . . 16 82 7. Formal Syntax . . . . . . . . . . . . . . . . . . . . 17 83 8. SCRAM as a GSS-API Mechanism . . . . . . . . . . . . . 20 84 8.1. GSS-API Principal Name Types for SCRAM . . . . . . . . 20 85 8.2. GSS-API Per-Message Tokens for SCRAM . . . . . . . . . 20 86 8.3. GSS_Pseudo_random() for SCRAM . . . . . . . . . . . . 21 87 9. Security Considerations . . . . . . . . . . . . . . . 22 88 10. IANA Considerations . . . . . . . . . . . . . . . . . 24 89 11. Acknowledgements . . . . . . . . . . . . . . . . . . . 25 90 Appendix A. Other Authentication Mechanisms . . . . . . . . . . . 26 91 Appendix B. Design Motivations . . . . . . . . . . . . . . . . . . 27 92 Appendix C. Internet-Draft Change History . . . . . . . . . . . . 28 93 12. References . . . . . . . . . . . . . . . . . . . . . . 30 94 12.1. Normative References . . . . . . . . . . . . . . . . . 30 95 12.2. Normative References for GSS-API implementors . . . . 30 96 12.3. Informative References . . . . . . . . . . . . . . . . 31 97 Authors' Addresses . . . . . . . . . . . . . . . . . . 33 99 1. Conventions Used in This Document 101 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 102 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 103 document are to be interpreted as described in [RFC2119]. 105 Formal syntax is defined by [RFC5234] including the core rules 106 defined in Appendix B of [RFC5234]. 108 Example lines prefaced by "C:" are sent by the client and ones 109 prefaced by "S:" by the server. If a single "C:" or "S:" label 110 applies to multiple lines, then the line breaks between those lines 111 are for editorial clarity only, and are not part of the actual 112 protocol exchange. 114 1.1. Terminology 116 This document uses several terms defined in [RFC4949] ("Internet 117 Security Glossary") including the following: authentication, 118 authentication exchange, authentication information, brute force, 119 challenge-response, cryptographic hash function, dictionary attack, 120 eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, 121 one-way encryption function, password, replay attack and salt. 122 Readers not familiar with these terms should use that glossary as a 123 reference. 125 Some clarifications and additional definitions follow: 127 o Authentication information: Information used to verify an identity 128 claimed by a SCRAM client. The authentication information for a 129 SCRAM identity consists of salt, iteration count, the "StoredKey" 130 and "ServerKey" (as defined in the algorithm overview) for each 131 supported cryptographic hash function. 133 o Authentication database: The database used to look up the 134 authentication information associated with a particular identity. 135 For application protocols, LDAPv3 (see [RFC4510]) is frequently 136 used as the authentication database. For network-level protocols 137 such as PPP or 802.11x, the use of RADIUS is more common. 139 o Base64: An encoding mechanism defined in [RFC4648] which converts 140 an octet string input to a textual output string which can be 141 easily displayed to a human. The use of base64 in SCRAM is 142 restricted to the canonical form with no whitespace. 144 o Octet: An 8-bit byte. 146 o Octet string: A sequence of 8-bit bytes. 148 o Salt: A random octet string that is combined with a password 149 before applying a one-way encryption function. This value is used 150 to protect passwords that are stored in an authentication 151 database. 153 1.2. Notation 155 The pseudocode description of the algorithm uses the following 156 notations: 158 o ":=": The variable on the left hand side represents the octet 159 string resulting from the expression on the right hand side. 161 o "+": Octet string concatenation. 163 o "[ ]": A portion of an expression enclosed in "[" and "]" may not 164 be included in the result under some circumstances. See the 165 associated text for a description of those circumstances. 167 o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in 168 [RFC2104]) using the octet string represented by "key" as the key 169 and the octet string "str" as the input string. The size of the 170 result is the hash result size for the hash function in use. For 171 example, it is 20 octets for SHA-1 (see [RFC3174]). 173 o H(str): Apply the cryptographic hash function to the octet string 174 "str", producing an octet string as a result. The size of the 175 result depends on the hash result size for the hash function in 176 use. 178 o XOR: Apply the exclusive-or operation to combine the octet string 179 on the left of this operator with the octet string on the right of 180 this operator. The length of the output and each of the two 181 inputs will be the same for this use. 183 o Hi(str, salt): 185 U0 := HMAC(str, salt + INT(1)) 186 U1 := HMAC(str, U0) 187 U2 := HMAC(str, U1) 188 ... 189 Ui-1 := HMAC(str, Ui-2) 190 Ui := HMAC(str, Ui-1) 192 Hi := U0 XOR U1 XOR U2 XOR ... XOR Ui 193 where "i" is the iteration count, "+" is the string concatenation 194 operator and INT(g) is a four-octet encoding of the integer g, 195 most significant octet first. 197 o This is, essentially, PBKDF2 [RFC2898] with HMAC() as the PRF and 198 with dkLen == output length of HMAC() == output length of H(). 200 2. Introduction 202 This specification describes a family of authentication mechanisms 203 called the Salted Challenge Response Authentication Mechanism (SCRAM) 204 which addresses the requirements necessary to deploy a challenge- 205 response mechanism more widely than past attempts. When used in 206 combination with Transport Layer Security (TLS, see [RFC5246]) or an 207 equivalent security layer, a mechanism from this family could improve 208 the status-quo for application protocol authentication and provide a 209 suitable choice for a mandatory-to-implement mechanism for future 210 application protocol standards. 212 For simplicity, this family of mechanism does not presently include 213 negotiation of a security layer. It is intended to be used with an 214 external security layer such as that provided by TLS or SSH, with 215 optional channel binding [RFC5056] to the external security layer. 217 SCRAM is specified herein as a pure Simple Authentication and 218 Security Layer (SASL) [RFC4422] mechanism, but it conforms to the new 219 bridge between SASL and the Generic Security Services Application 220 Programming Interface (GSS-API) called "GS2" [I-D.ietf-sasl-gs2]. 221 This means that SCRAM is actually both, a GSS-API and SASL mechanism. 223 SCRAM provides the following protocol features: 225 o The authentication information stored in the authentication 226 database is not sufficient by itself to impersonate the client. 227 The information is salted to prevent a pre-stored dictionary 228 attack if the database is stolen. 230 o The server does not gain the ability to impersonate the client to 231 other servers (with an exception for server-authorized proxies). 233 o The mechanism permits the use of a server-authorized proxy without 234 requiring that proxy to have super-user rights with the back-end 235 server. 237 o A standard attribute is defined to enable storage of the 238 authentication information in LDAPv3 (see [RFC4510]). 240 o Mutual authentication is supported, but only the client is named 241 (i.e., the server has no name). 243 For an in-depth discussion of why other challenge response mechanisms 244 are not considered sufficient, see appendix A. For more information 245 about the motivations behind the design of this mechanism, see 246 appendix B. 248 Comments regarding this draft may be sent either to the 249 ietf-sasl@imc.org mailing list or to the authors. 251 3. SCRAM Algorithm Overview 253 Note that this section omits some details, such as client and server 254 nonces. See Section 5 for more details. 256 To begin with, the client is in possession of a username and 257 password. It sends the username to the server, which retrieves the 258 corresponding authentication information, i.e. a salt, StoredKey, 259 ServerKey and the iteration count i. (Note that a server 260 implementation may chose to use the same iteration count for all 261 account.) The server sends the salt and the iteration count to the 262 client, which then computes the following values and sends a 263 ClientProof to the server: 265 SaltedPassword := Hi(password, salt) 266 ClientKey := HMAC(SaltedPassword, "Client Key") 267 StoredKey := H(ClientKey) 268 AuthMessage := client-first-message + "," + 269 server-first-message + "," + 270 client-final-message-without-proof 271 ClientSignature := HMAC(StoredKey, AuthMessage) 272 ClientProof := ClientKey XOR ClientSignature 273 ServerKey := HMAC(SaltedPassword, "Server Key") 274 ServerSignature := HMAC(ServerKey, AuthMessage) 276 The server authenticates the client by computing the ClientSignature, 277 exclusive-ORing that with the ClientProof to recover the ClientKey 278 and verifying the correctness of the ClientKey by applying the hash 279 function and comparing the result to the StoredKey. If the ClientKey 280 is correct, this proves that the client has access to the user's 281 password. 283 Similarly, the client authenticates the server by computing the 284 ServerSignature and comparing it to the value sent by the server. If 285 the two are equal, it proves that the server had access to the user's 286 ServerKey. 288 The AuthMessage is computed by concatenating messages from the 289 authentication exchange. The format of these messages is defined in 290 Section 7. 292 4. SCRAM Mechanism Names 294 A SCRAM mechanism name is a string "SCRAM-" followed by the 295 uppercased name of the underlying hashed function taken from the IANA 296 "Hash Function Textual Names" registry (see http://www.iana.org), 297 optionally followed by the suffix "-PLUS" (see below). Note that 298 SASL mechanism names are limited to 20 characters, which means that 299 only hash function names with lengths shorter or equal to 9 300 characters (20-length("SCRAM-")-lenght("-PLUS") can be used. For 301 cases when the underlying hash function name is longer than 9 302 characters, an alternative 9 character (or shorter) name can be used 303 to construct the corresponding SCRAM mechanism name, as long as this 304 alternative name doesn't conflict with any other hash function name 305 from the IANA "Hash Function Textual Names" registry. 307 For interoperability, all SCRAM clients and servers MUST implement 308 the SCRAM-SHA-1 authentication mechanism, i.e. an authentication 309 mechanism from the SCRAM family that uses the SHA-1 hash function as 310 defined in [RFC3174]. 312 The "-PLUS" suffix is used only when the server supports channel 313 binding to the external channel. In this case the server will 314 advertise both, SCRAM-SHA-1 and SCRAM-SHA-1-PLUS, otherwise the 315 server will advertise only SCRAM-SHA-1. The "-PLUS" exists to allow 316 negotiation of the use of channel binding. See Section 6. 318 5. SCRAM Authentication Exchange 320 SCRAM is a text protocol where the client and server exchange 321 messages containing one or more attribute-value pairs separated by 322 commas. Each attribute has a one-letter name. The messages and 323 their attributes are described in Section 5.1, and defined in 324 Section 7. 326 This is a simple example of a SCRAM-SHA-1 authentication exchange 327 when the client doesn't support channel bindings: 329 C: n,n=Chris Newman,r=ClientNonce 330 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 331 C: c=biwK,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4 332 S: v=WxPv/siO5l+qxN4 334 [[anchor5: Note that the all hashes above are fake and will be fixed 335 during AUTH48.]] 337 With channel-binding data sent by the client this might look like 338 this: 340 C: p,n=Chris Newman,r=ClientNonce 341 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 342 C: c=cCx0bHMtc2VydmVyLWVuZC1wb2ludDrLWEW1c6dn7JKtAzqysWmX/ 343 vu6q+3GuDucFjUF60Sv+A==,r=ClientNonceServerNonce,p=Wx 344 Pv/siO5l+qxN4 345 S: v=WxPv/siO5l+qxN4 347 [[anchor6: Note that all hashes above are fake and will be fixed 348 during AUTH48.]] 350 First, the client sends a message containing: 352 o a GS2 header consisting of a flag indicating whether channel 353 binding is supported-but-not-used, not supported, or used, and an 354 optional SASL authorization identity; 356 o SCRAM username and a random, unique nonce attributes. 358 Note that the client's first message will always start with "n", "y" 359 or "p", otherwise the message is invalid and authentication MUST 360 fail. This is important, as it allows for GS2 extensibility (e.g., 361 to add support for security layers). 363 In response, the server sends the user's iteration count i, the 364 user's salt, and appends its own nonce to the client-specified one. 365 The client then responds with the same nonce and a ClientProof 366 computed using the selected hash function as explained earlier. The 367 server verifies the nonce and the proof, verifies that the 368 authorization identity (if supplied by the client in the first 369 message) is authorized to act as the authentication identity, and, 370 finally, it responds with a ServerSignature, concluding the 371 authentication exchange. The client then authenticates the server by 372 computing the ServerSignature and comparing it to the value sent by 373 the server. If the two are different, the client MUST consider the 374 authentication exchange to be unsuccessful and it might have to drop 375 the connection. 377 5.1. SCRAM Attributes 379 This section describes the permissible attributes, their use, and the 380 format of their values. All attribute names are single US-ASCII 381 letters and are case-sensitive. 383 Note that the order of attributes in client or server messages is 384 fixed, with the exception of extension attributes (described by the 385 "extensions" ABNF production), which can appear in any order in the 386 designated positions. See the ABNF section for authoritative 387 reference. 389 o a: This is an optional attribute, and is part of the GS2 390 [I-D.ietf-sasl-gs2] bridge between the GSS-API and SASL. This 391 attribute specifies an authorization identity. A client may 392 include it in its first message to the server if it wants to 393 authenticate as one user, but subsequently act as a different 394 user. This is typically used by an administrator to perform some 395 management task on behalf of another user, or by a proxy in some 396 situations. 398 Upon the receipt of this value the server verifies its 399 correctness according to the used SASL protocol profile. 400 Failed verification results in failed authentication exchange. 402 If this attribute is omitted (as it normally would be), or 403 specified with an empty value, the authorization identity is 404 assumed to be derived from the username specified with the 405 (required) "n" attribute. 407 The server always authenticates the user specified by the "n" 408 attribute. If the "a" attribute specifies a different user, 409 the server associates that identity with the connection after 410 successful authentication and authorization checks. 412 The syntax of this field is the same as that of the "n" field 413 with respect to quoting of '=' and ','. 415 o n: This attribute specifies the name of the user whose password is 416 used for authentication. A client must include it in its first 417 message to the server. If the "a" attribute is not specified 418 (which would normally be the case), this username is also the 419 identity which will be associated with the connection subsequent 420 to authentication and authorization. 422 Before sending the username to the server, the client MUST 423 prepare the username using the "SASLPrep" profile [RFC4013] of 424 the "stringprep" algorithm [RFC3454]. If the preparation of 425 the username fails or results in an empty string, the client 426 SHOULD abort the authentication exchange (*). 428 (*) An interactive client can request a repeated entry of the 429 username value. 431 Upon receipt of the username by the server, the server SHOULD 432 prepare it using the "SASLPrep" profile [RFC4013] of the 433 "stringprep" algorithm [RFC3454]. If the preparation of the 434 username fails or results in an empty string, the server SHOULD 435 abort the authentication exchange. 437 The characters ',' or '=' in usernames are sent as '=2C' and 438 '=3D' respectively. If the server receives a username which 439 contains '=' not followed by either '2C' or '3D', then the 440 server MUST fail the authentication. 442 o m: This attribute is reserved for future extensibility. In this 443 version of SCRAM, its presence in a client or a server message 444 MUST cause authentication failure when the attribute is parsed by 445 the other end. 447 o r: This attribute specifies a sequence of random printable 448 characters excluding ',' which forms the nonce used as input to 449 the hash function. No quoting is applied to this string. As 450 described earlier, the client supplies an initial value in its 451 first message, and the server augments that value with its own 452 nonce in its first response. It is important that this be value 453 different for each authentication. The client MUST verify that 454 the initial part of the nonce used in subsequent messages is the 455 same as the nonce it initially specified. The server MUST verify 456 that the nonce sent by the client in the second message is the 457 same as the one sent by the server in its first message. 459 o c: This REQUIRED attribute specifies base64-encoded of a header 460 and the channel-binding data. It is sent by the client in its 461 second authentication message. The header consist of: 463 * the GS2 header from the client's first message (recall: a 464 channel binding flag and an optional authzid); 466 * followed by the external channel's channel binding type prefix 467 (see [RFC5056], if and only if the client is using channel 468 binding; 470 * followed by the external channel's channel binding data, if and 471 only if the client is using channel binding. 473 o s: This attribute specifies the base64-encoded salt used by the 474 server for this user. It is sent by the server in its first 475 message to the client. 477 o i: This attribute specifies an iteration count for the selected 478 hash function and user, and must be sent by the server along with 479 the user's salt. 481 For SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism servers SHOULD 482 announce a hash iteration-count of at least 4096. Note that a 483 client implementation MAY cache SaltedPassword/ClientKey for 484 later reauthentication to the same service, as it is likely 485 that the server is going to advertise the same salt value upon 486 reauthentication. This might be useful for mobile clients 487 where CPU usage is a concern. 489 o p: This attribute specifies a base64-encoded ClientProof. The 490 client computes this value as described in the overview and sends 491 it to the server. 493 o v: This attribute specifies a base64-encoded ServerSignature. It 494 is sent by the server in its final message, and is used by the 495 client to verify that the server has access to the user's 496 authentication information. This value is computed as explained 497 in the overview. 499 6. Channel Binding 501 SCRAM supports channel binding to external secure channels, such as 502 TLS. Clients and servers may or may not support channel binding, 503 therefore the use of channel binding is negotiable. SCRAM does not 504 provide security layers, however, therefore it is imperative that 505 SCRAM provide integrity protection for the negotiation of channel 506 binding. 508 Use of channel binding is negotiated as follows: 510 o The server advertises support for channel binding by advertising 511 both, SCRAM- and SCRAM--PLUS. 513 o If the client negotiates mechanisms then client MUST select SCRAM- 514 -PLUS if offered by the server. Otherwise, if the 515 client does not negotiate mechanisms then it MUST select only 516 SCRAM- (not suffixed with "-PLUS"). 518 o If the client and server both support channel binding, or if the 519 client wishes to use channel binding but the client does not 520 negotiate mechanisms, the client MUST set the GS2 channel binding 521 flag to "p" and MUST include channel binding data for the external 522 channel in the computation of the "c=" attribute (see 523 Section 5.1). 525 o If the client supports channel binding but the server does not 526 then the client MUST set the GS2 channel binding flag to "y" and 527 MUST NOT include channel binding data for the external channel in 528 the computation of the "c=" attribute (see Section 5.1). 530 o If the client does not support channel binding then the client 531 MUST set the GS2 channel binding flag to "n" and MUST NOT include 532 channel binding data for the external channel in the computation 533 of the "c=" attribute (see Section 5.1). 535 o If the server receives a client first message with the GS2 channel 536 binding flag set to "y" and the server supports channel binding 537 the server MUST fail authentication. This is because if the 538 client sets the GS2 channel binding flag set to "y" then the 539 client must have believed that the server did not support channel 540 binding -- if the server did in fact support channel binding then 541 this is an indication that there has been a downgrade attack 542 (e.g., an attacker changed the server's mechanism list to exclude 543 the -PLUS suffixed SCRAM mechanism name(s)). 545 The server MUST always validate the client's "c=" field. The server 546 does this by constructing the value of the "c=" attribute and then 547 checking that it matches the client's c= attribute value. 549 6.1. Channel Binding to TLS Channels 551 If an external TLS channel is to be bound into the authentication, 552 and if the channel supports channel bindings of type 'tls-server-end- 553 point', then those MUST be used, else if the channel supports channel 554 bindings of type 'tls-unique' type, then those MUST be used. 556 7. Formal Syntax 558 The following syntax specification uses the Augmented Backus-Naur 559 Form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3" 560 and "UTF8-4" non-terminal are defined in [RFC3629]. 562 ALPHA = 563 DIGIT = 564 UTF8-2 = 565 UTF8-3 = 566 UTF8-4 = 568 generic-message = attr-val *("," attr-val) 569 ;; Generic syntax of any server challenge 570 ;; or client response 572 attr-val = ALPHA "=" value 574 value = 1*value-char 576 value-safe-char = %x01-2B / %x2D-3C / %x3E-7F / 577 UTF8-2 / UTF8-3 / UTF8-4 578 ;; UTF8-char except NUL, "=", and ",". 580 value-char = value-safe-char / "=" 582 base64-char = ALPHA / DIGIT / "/" / "+" 584 base64-4 = 4base64-char 586 base64-3 = 3base64-char "=" 588 base64-2 = 2base64-char "==" 590 base64 = *base64-4 [base64-3 / base64-2] 592 posit-number = %x31-39 *DIGIT 593 ;; A positive number 595 saslname = 1*(value-safe-char / "=2C" / "=3D") 596 ;; Conforms to 598 authzid = "a=" saslname 599 ;; Protocol specific. 601 gs2-cbind-flag = "n" / "y" / "p" 602 ;; "n" -> client doesn't support channel binding 603 ;; "y" -> client does support channel binding 604 ;; but thinks the server does not. 605 ;; "p" -> client requires channel binding 607 gs2-header = gs2-cbind-flag [ authzid ] "," 608 ;; GS2 header for SCRAM 609 ;; (the actual GS2 header includes an optional 610 ;; flag to indicate that the GSS mechanism is not 611 ;; "standard" but since SCRAM is "standard" we 612 ;; don't include that flag). 614 username = "n=" saslname 615 ;; Usernames are prepared using SASLPrep. 617 reserved-mext = "m=" 1*(value-char) 618 ;; Reserved for signalling mandatory extensions. 619 ;; The exact syntax will be defined in 620 ;; the future. 622 channel-binding = "c=" base64 623 ;; base64 encoding of cbind-input 625 proof = "p=" base64 627 nonce = "r=" c-nonce [s-nonce] 628 ;; Second part provided by server. 630 c-nonce = value 632 s-nonce = value 634 salt = "s=" base64 636 verifier = "v=" base64 637 ;; base-64 encoded ServerSignature. 639 iteration-count = "i=" posit-number 640 ;; A positive number 642 client-first-message = 643 gs2-header [reserved-mext ","] 644 username "," nonce ["," extensions] 646 server-first-message = 647 [reserved-mext ","] nonce "," salt "," 648 iteration-count ["," extensions] 650 client-final-message-without-proof = 651 channel-binding "," nonce ["," 652 extensions] 654 client-final-message = 655 client-final-message-without-proof "," proof 657 gss-server-error = "e=" value 658 server-final-message = gss-server-error / 659 verifier ["," extensions] 660 ;; The error message is only for the GSS-API 661 ;; form of SCRAM, and it is OPTIONAL to 662 ;; implement it. 664 extensions = attr-val *("," attr-val) 665 ;; All extensions are optional, 666 ;; i.e. unrecognized attributes 667 ;; not defined in this document 668 ;; MUST be ignored. 670 cbind-data = *OCTET 671 cbind-type = value 672 ;; e.g. "tls-server-end-point" or 673 ;; "tls-unique" 675 cbind-input = gs2-header [ cbind-type ":" cbind-data ] 677 8. SCRAM as a GSS-API Mechanism 679 This section and its sub-sections and all normative references of it 680 not referenced elsewhere in this document are INFORMATIONAL for SASL 681 implementors, but they are NORMATIVE for GSS-API implementors. 683 SCRAM is actually also GSS-API mechanism. The messages are the same, 684 but a) the GS2 header on the client's first message and channel 685 binding data is excluded when SCRAM is used as a GSS-API mechanism, 686 and b) the RFC2743 section 3.1 initial context token header is 687 prefixed to the client's first authentication message (context 688 token). 690 The GSS-API mechanism OID for SCRAM is (see Section 10). 692 8.1. GSS-API Principal Name Types for SCRAM 694 SCRAM does not name acceptors. Therefore only GSS_C_NO_NAME and 695 names of type GSS_C_NT_ANONYMOUS shall be allowed as the target name 696 input of GSS_Init_sec_context() when using a SCRAM mechanism. 698 SCRAM supports only a single name type for initiators: 699 GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for 700 SCRAM. 702 There is no name canonicalization procedure for SCRAM beyond applying 703 SASLprep as described in Section 5.1. 705 The query, display and exported name syntax for SCRAM principal names 706 is the same: there is no syntax -- SCRAM principal names are free- 707 form. (The exported name token does, of course, conform to [RFC2743] 708 section 3.2, but the "NAME" part of the token is just a SCRAM user 709 name.) 711 8.2. GSS-API Per-Message Tokens for SCRAM 713 The per-message tokens for SCRAM as a GSS-API mechanism SHALL BE the 714 same as those for the Kerberos V GSS-API mechanism [RFC4121], using 715 the Kerberos V "aes128-cts-hmac-sha1-96" enctype [RFC3962]. 717 The 128-bit session key SHALL be derived by using the least 718 significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API session 719 key" || ClientKey || AuthMessage). 721 SCRAM does support PROT_READY, and is PROT_READY on the initiator 722 side first upon receipt of the server's reply to the initial security 723 context token. 725 8.3. GSS_Pseudo_random() for SCRAM 727 The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for 728 the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor- 729 asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and 730 GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random(). 732 9. Security Considerations 734 If the authentication exchange is performed without a strong security 735 layer, then a passive eavesdropper can gain sufficient information to 736 mount an offline dictionary or brute-force attack which can be used 737 to recover the user's password. The amount of time necessary for 738 this attack depends on the cryptographic hash function selected, the 739 strength of the password and the iteration count supplied by the 740 server. An external security layer with strong encryption will 741 prevent this attack. 743 If the external security layer used to protect the SCRAM exchange 744 uses an anonymous key exchange, then the SCRAM channel binding 745 mechanism can be used to detect a man-in-the-middle attack on the 746 security layer and cause the authentication to fail as a result. 747 However, the man-in-the-middle attacker will have gained sufficient 748 information to mount an offline dictionary or brute-force attack. 749 For this reason, SCRAM includes the ability to increase the iteration 750 count over time. 752 If the authentication information is stolen from the authentication 753 database, then an offline dictionary or brute-force attack can be 754 used to recover the user's password. The use of salt mitigates this 755 attack somewhat by requiring a separate attack on each password. 756 Authentication mechanisms which protect against this attack are 757 available (e.g., the EKE class of mechanisms). 759 If an attacker obtains the authentication information from the 760 authentication repository and either eavesdrops on one authentication 761 exchange or impersonates a server, the attacker gains the ability to 762 impersonate that user to all servers providing SCRAM access using the 763 same hash function, password, iteration count and salt. For this 764 reason, it is important to use randomly-generated salt values. 766 SCRAM does not negotiate a hash function to use. Hash function 767 negotiation is left to the SASL mechanism negotiation. It is 768 important that clients be able to sort a locally available list of 769 mechanisms by preference so that the client may pick the most 770 preferred of a server's advertised mechanism list. This preference 771 order is not specified here as it is a local matter. The preference 772 order should include objective and subjective notions of mechanism 773 cryptographic strength (e.g., SCRAM with a successor to SHA-1 may be 774 preferred over SCRAM with SHA-1). 776 Note that to protect the SASL mechanism negotiation applications 777 normally must list the server mechs twice: once before and once after 778 authentication, the latter using security layers. Since SCRAM does 779 not provide security layers the only ways to protect the mechanism 780 negotiation are: a) use channel binding to an external channel, or b) 781 use an external channel that authenticates a user-provided server 782 name. 784 A hostile server can perform a computational denial-of-service attack 785 on clients by sending a big iteration count value. 787 10. IANA Considerations 789 IANA is requested to add the following entries to the SASL Mechanism 790 registry established by [RFC4422]: 792 To: iana@iana.org 793 Subject: Registration of a new SASL mechanism SCRAM-SHA-1 795 SASL mechanism name (or prefix for the family): SCRAM-SHA-1 796 Security considerations: Section 7 of [RFCXXXX] 797 Published specification (optional, recommended): [RFCXXXX] 798 Person & email address to contact for further information: 799 IETF SASL WG 800 Intended usage: COMMON 801 Owner/Change controller: IESG 802 Note: 804 To: iana@iana.org 805 Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS 807 SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS 808 Security considerations: Section 7 of [RFCXXXX] 809 Published specification (optional, recommended): [RFCXXXX] 810 Person & email address to contact for further information: 811 IETF SASL WG 812 Intended usage: COMMON 813 Owner/Change controller: IESG 814 Note: 816 Note that even though this document defines a family of SCRAM- 817 mechanisms, it doesn't register a family of SCRAM- mechanisms in the 818 SASL Mechanisms registry. IANA is requested to prevent future 819 registrations of SASL mechanisms starting with SCRAM- without 820 consulting the SASL mailing list first. 822 Note to future SCRAM- mechanism designers: each new SCRAM- SASL 823 mechanism MUST be explicitly registered with IANA and MUST comply 824 with SCRAM- mechanism naming convention defined in Section 4 of this 825 document. 827 We hereby request that IANA assign a GSS-API mechanism OID for SCRAM. 829 11. Acknowledgements 831 The authors would like to thank Dave Cridland for his contributions 832 to this document. 834 Appendix A. Other Authentication Mechanisms 836 The DIGEST-MD5 [I-D.ietf-sasl-digest-to-historic] mechanism has 837 proved to be too complex to implement and test, and thus has poor 838 interoperability. The security layer is often not implemented, and 839 almost never used; everyone uses TLS instead. For a more complete 840 list of problems with DIGEST-MD5 which lead to the creation of SCRAM 841 see [I-D.ietf-sasl-digest-to-historic]. 843 The CRAM-MD5 SASL mechanism, while widely deployed has also some 844 problems, in particular it is missing some modern SASL features such 845 as support for internationalized usernames and passwords, support for 846 passing of authorization identity, support for channel bindings. It 847 also doesn't support server authentication. For a more complete list 848 of problems with CRAM-MD5 see [I-D.ietf-sasl-crammd5-to-historic]. 850 The PLAIN [RFC4616] SASL mechanism allows a malicious server or 851 eavesdropper to impersonate the authenticating user to any other 852 server for which the user has the same password. It also sends the 853 password in the clear over the network, unless TLS is used. Server 854 authentication is not supported. 856 Appendix B. Design Motivations 858 The following design goals shaped this document. Note that some of 859 the goals have changed since the initial version of the document. 861 o The SASL mechanism has all modern SASL features: support for 862 internationalized usernames and passwords, support for passing of 863 authorization identity, support for channel bindings. 865 o The protocol supports mutual authentication. 867 o The authentication information stored in the authentication 868 database is not sufficient by itself to impersonate the client. 870 o The server does not gain the ability to impersonate the client to 871 other servers (with an exception for server-authorized proxies), 872 unless such other servers allow SCRAM authentication and use the 873 same salt and iteration count for the user. 875 o The mechanism is extensible, but [hopefully] not overengineered in 876 this respect. 878 o Easier to implement than DIGEST-MD5 in both clients and servers. 880 Appendix C. Internet-Draft Change History 882 (RFC Editor: Please delete everything after this point) 884 Changes since -10 886 o Converted the source for this I-D to XML. 888 o Added text to make SCRAM compliant with the new GS2 design. 890 o Added text on channel binding negotiation. 892 o Added text on channel binding, including a reference to RFC5056. 894 o Added text on SCRAM as a GSS-API mechanism. This noted as not 895 relevant to SASL-only implementors -- the normative references for 896 SCRAM as a GSS-API mechanism are segregated as well. 898 Changes since -07 900 o Updated References. 902 o Clarified purpose of the m= attribute. 904 o Fixed a problem with authentication/authorization identity's ABNF 905 not allowing for some characters. 907 o Updated ABNF for nonce to show client-generated and server- 908 generated parts. 910 o Only register SCRAM-SHA-1 with IANA and require explicit 911 registrations of all other SCRAM- mechanisms. 913 Changes since -06 915 o Removed hash negotiation from SCRAM and turned it into a family of 916 SASL mechanisms. 918 o Start using "Hash Function Textual Names" IANA registry for SCRAM 919 mechanism naming. 921 o Fixed definition of Hi(str, salt) to be consistent with [RFC2898]. 923 o Clarified extensibility of SCRAM: added m= attribute (for future 924 mandatory extensions) and specified that all unrecognized 925 attributes must be ignored. 927 Changes since -05 928 o Changed the mandatory to implement hash algorithm to SHA-1 (as per 929 WG consensus). 931 o Added text about use of SASLPrep for username canonicalization/ 932 validation. 934 o Clarified that authorization identity is canonicalized/verified 935 according to SASL protocol profile. 937 o Clarified that iteration count is per-user. 939 o Clarified how clients select the authentication function. 941 o Added IANA registration for the new mechanism. 943 o Added missing normative references (UTF-8, SASLPrep). 945 o Various editorial changes based on comments from Hallvard B 946 Furuseth, Nico William and Simon Josefsson. 948 Changes since -04 950 o Update Base64 and Security Glossary references. 952 o Add Formal Syntax section. 954 o Don't bother with "v=". 956 o Make MD5 mandatory to implement. Suggest i=128. 958 Changes since -03 960 o Seven years have passed, in which it became clear that DIGEST-MD5 961 suffered from unacceptably bad interoperability, so SCRAM-MD5 is 962 now back from the dead. 964 o Be hash agnostic, so MD5 can be replaced more easily. 966 o General simplification. 968 12. References 970 12.1. Normative References 972 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 973 Hashing for Message Authentication", RFC 2104, 974 February 1997. 976 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 977 Requirement Levels", BCP 14, RFC 2119, March 1997. 979 [RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 980 (SHA1)", RFC 3174, September 2001. 982 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 983 Internationalized Strings ("stringprep")", RFC 3454, 984 December 2002. 986 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 987 10646", STD 63, RFC 3629, November 2003. 989 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 990 and Passwords", RFC 4013, February 2005. 992 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 993 Security Layer (SASL)", RFC 4422, June 2006. 995 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 996 Encodings", RFC 4648, October 2006. 998 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 999 Channels", RFC 5056, November 2007. 1001 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1002 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1004 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1005 Housley, R., and W. Polk, "Internet X.509 Public Key 1006 Infrastructure Certificate and Certificate Revocation List 1007 (CRL) Profile", RFC 5280, May 2008. 1009 12.2. Normative References for GSS-API implementors 1011 [I-D.ietf-sasl-gs2] 1012 Josefsson, S. and N. Williams, "Using GSS-API Mechanisms 1013 in SASL: The GS2 Mechanism Family", draft-ietf-sasl-gs2-12 1014 (work in progress), April 2009. 1016 [RFC2743] Linn, J., "Generic Security Service Application Program 1017 Interface Version 2, Update 1", RFC 2743, January 2000. 1019 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) 1020 Encryption for Kerberos 5", RFC 3962, February 2005. 1022 [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 1023 Version 5 Generic Security Service Application Program 1024 Interface (GSS-API) Mechanism: Version 2", RFC 4121, 1025 July 2005. 1027 [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API 1028 Extension for the Generic Security Service Application 1029 Program Interface (GSS-API)", RFC 4401, February 2006. 1031 [RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the 1032 Kerberos V Generic Security Service Application Program 1033 Interface (GSS-API) Mechanism", RFC 4402, February 2006. 1035 12.3. Informative References 1037 [I-D.ietf-sasl-crammd5-to-historic] 1038 Zeilenga, K., "CRAM-MD5 to Historic", 1039 draft-ietf-sasl-crammd5-to-historic-00 (work in progress), 1040 November 2008. 1042 [I-D.ietf-sasl-digest-to-historic] 1043 Melnikov, A., "Moving DIGEST-MD5 to Historic", 1044 draft-ietf-sasl-digest-to-historic-00 (work in progress), 1045 July 2008. 1047 [I-D.ietf-sasl-rfc2831bis] 1048 Melnikov, A., "Using Digest Authentication as a SASL 1049 Mechanism", draft-ietf-sasl-rfc2831bis-12 (work in 1050 progress), March 2007. 1052 [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP 1053 AUTHorize Extension for Simple Challenge/Response", 1054 RFC 2195, September 1997. 1056 [RFC2202] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and HMAC- 1057 SHA-1", RFC 2202, September 1997. 1059 [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography 1060 Specification Version 2.0", RFC 2898, September 2000. 1062 [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 1063 Requirements for Security", BCP 106, RFC 4086, June 2005. 1065 [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol 1066 (LDAP): Technical Specification Road Map", RFC 4510, 1067 June 2006. 1069 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and 1070 Security Layer (SASL) Mechanism", RFC 4616, August 2006. 1072 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 1073 RFC 4949, August 2007. 1075 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1076 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1078 Authors' Addresses 1080 Abhijit Menon-Sen 1081 Oryx Mail Systems GmbH 1083 Email: ams@oryx.com 1085 Alexey Melnikov 1086 Isode Ltd 1088 Email: Alexey.Melnikov@isode.com 1090 Chris Newman 1091 Sun Microsystems 1092 1050 Lakes Drive 1093 West Covina, CA 91790 1094 USA 1096 Email: chris.newman@sun.com 1098 Nicolas Williams 1099 Sun Microsystems 1100 5300 Riata Trace Ct 1101 Austin, TX 78727 1102 USA 1104 Email: Nicolas.Williams@sun.com