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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 24, 2009 Isode Ltd 6 C. Newman 7 N. Williams 8 Sun Microsystems 9 May 23, 2009 11 Salted Challenge Response (SCRAM) SASL Mechanism 12 draft-ietf-sasl-scram-00.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 24, 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: 328 C: n,n=Chris Newman,r=ClientNonce 329 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 330 C: c=0123456789ABCDEF,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4 331 S: v=WxPv/siO5l+qxN4 333 With channel-binding data sent by the client this might look like 334 this: 336 C: p,n=Chris Newman,r=ClientNonce 337 S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128 338 C: c=0123456789ABCDEF,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4 339 S: v=WxPv/siO5l+qxN4 341 First, the client sends a message containing: 343 o a GS2 header consisting of a flag indicating whether channel 344 binding is supported-but-not-used, not supported, or used, and an 345 optional SASL authorization identity; 347 o SCRAM username and a random, unique nonce attributes. 349 Note that the client's first message will always start with "n", "y" 350 or "p", otherwise the message is invalid and authentication MUST 351 fail. This is important, as it allows for GS2 extensibility (e.g., 352 to add support for security layers). 354 In response, the server sends the user's iteration count i, the 355 user's salt, and appends its own nonce to the client-specified one. 356 The client then responds with the same nonce and a ClientProof 357 computed using the selected hash function as explained earlier. The 358 server verifies the nonce and the proof, verifies that the 359 authorization identity (if supplied by the client in the first 360 message) is authorized to act as the authentication identity, and, 361 finally, it responds with a ServerSignature, concluding the 362 authentication exchange. The client then authenticates the server by 363 computing the ServerSignature and comparing it to the value sent by 364 the server. If the two are different, the client MUST consider the 365 authentication exchange to be unsuccessful and it might have to drop 366 the connection. 368 5.1. SCRAM Attributes 370 This section describes the permissible attributes, their use, and the 371 format of their values. All attribute names are single US-ASCII 372 letters and are case-sensitive. 374 Note that the order of attributes in client or server messages is 375 fixed, with the exception of extension attributes (described by the 376 "extensions" ABNF production), which can appear in any order in the 377 designated positions. See the ABNF section for authoritative 378 reference. 380 o a: This is an optional attribute, and is part of the GS2 381 [I-D.ietf-sasl-gs2] bridge between the GSS-API and SASL. This 382 attribute specifies an authorization identity. A client may 383 include it in its first message to the server if it wants to 384 authenticate as one user, but subsequently act as a different 385 user. This is typically used by an administrator to perform some 386 management task on behalf of another user, or by a proxy in some 387 situations. 389 Upon the receipt of this value the server verifies its 390 correctness according to the used SASL protocol profile. 391 Failed verification results in failed authentication exchange. 393 If this attribute is omitted (as it normally would be), or 394 specified with an empty value, the authorization identity is 395 assumed to be derived from the username specified with the 396 (required) "n" attribute. 398 The server always authenticates the user specified by the "n" 399 attribute. If the "a" attribute specifies a different user, 400 the server associates that identity with the connection after 401 successful authentication and authorization checks. 403 The syntax of this field is the same as that of the "n" field 404 with respect to quoting of '=' and ','. 406 o n: This attribute specifies the name of the user whose password is 407 used for authentication. A client must include it in its first 408 message to the server. If the "a" attribute is not specified 409 (which would normally be the case), this username is also the 410 identity which will be associated with the connection subsequent 411 to authentication and authorization. 413 Before sending the username to the server, the client MUST 414 prepare the username using the "SASLPrep" profile [RFC4013] of 415 the "stringprep" algorithm [RFC3454]. If the preparation of 416 the username fails or results in an empty string, the client 417 SHOULD abort the authentication exchange (*). 419 (*) An interactive client can request a repeated entry of the 420 username value. 422 Upon receipt of the username by the server, the server SHOULD 423 prepare it using the "SASLPrep" profile [RFC4013] of the 424 "stringprep" algorithm [RFC3454]. If the preparation of the 425 username fails or results in an empty string, the server SHOULD 426 abort the authentication exchange. 428 The characters ',' or '=' in usernames are sent as '=2C' and 429 '=3D' respectively. If the server receives a username which 430 contains '=' not followed by either '2C' or '3D', then the 431 server MUST fail the authentication. 433 o m: This attribute is reserved for future extensibility. In this 434 version of SCRAM, its presence in a client or a server message 435 MUST cause authentication failure when the attribute is parsed by 436 the other end. 438 o r: This attribute specifies a sequence of random printable 439 characters excluding ',' which forms the nonce used as input to 440 the hash function. No quoting is applied to this string. As 441 described earlier, the client supplies an initial value in its 442 first message, and the server augments that value with its own 443 nonce in its first response. It is important that this be value 444 different for each authentication. The client MUST verify that 445 the initial part of the nonce used in subsequent messages is the 446 same as the nonce it initially specified. The server MUST verify 447 that the nonce sent by the client in the second message is the 448 same as the one sent by the server in its first message. 450 o c: This REQUIRED attribute specifies base64-encoded of a header 451 and the channel-binding data. It is sent by the client in its 452 second authentication message. The header consist of: 454 * the GS2 header from the client's first message (recall: a 455 channel binding flag and an optional authzid); 457 * followed by the external channel's channel binding type prefix 458 (see [RFC5056], if and only if the client is using channel 459 binding; 461 * followed by the external channel's channel binding data, if and 462 only if the client is using channel binding. 464 o s: This attribute specifies the base64-encoded salt used by the 465 server for this user. It is sent by the server in its first 466 message to the client. 468 o i: This attribute specifies an iteration count for the selected 469 hash function and user, and must be sent by the server along with 470 the user's salt. 472 For SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism servers SHOULD 473 announce a hash iteration-count of at least 4096. Note that a 474 client implementation MAY cache SaltedPassword/ClientKey for 475 later reauthentication to the same service, as it is likely 476 that the server is going to advertise the same salt value upon 477 reauthentication. This might be useful for mobile clients 478 where CPU usage is a concern. 480 o p: This attribute specifies a base64-encoded ClientProof. The 481 client computes this value as described in the overview and sends 482 it to the server. 484 o v: This attribute specifies a base64-encoded ServerSignature. It 485 is sent by the server in its final message, and is used by the 486 client to verify that the server has access to the user's 487 authentication information. This value is computed as explained 488 in the overview. 490 6. Channel Binding 492 SCRAM supports channel binding to external secure channels, such as 493 TLS. Clients and servers may or may not support channel binding, 494 therefore the use of channel binding is negotiable. SCRAM does not 495 provide security layers, however, therefore it is imperative that 496 SCRAM provide integrity protection for the negotiation of channel 497 binding. 499 Use of channel binding is negotiated as follows: 501 o The server advertises support for channel binding by advertising 502 both, SCRAM- and SCRAM--PLUS. 504 o If the client negotiates mechanisms then client MUST select SCRAM- 505 -PLUS if offered by the server. Otherwise, if the 506 client does not negotiate mechanisms then it MUST select only 507 SCRAM- (not suffixed with "-PLUS"). 509 o If the client and server both support channel binding, or if the 510 client wishes to use channel binding but the client does not 511 negotiate mechanisms, the client MUST set the GS2 channel binding 512 flag to "p" and MUST include channel binding data for the external 513 channel in the computation of the "c=" attribute (see 514 Section 5.1). 516 o If the client supports channel binding but the server does not 517 then the client MUST set the GS2 channel binding flag to "y" and 518 MUST NOT include channel binding data for the external channel in 519 the computation of the "c=" attribute (see Section 5.1). 521 o If the client does not support channel binding then the client 522 MUST set the GS2 channel binding flag to "n" and MUST NOT include 523 channel binding data for the external channel in the computation 524 of the "c=" attribute (see Section 5.1). 526 o If the server receives a client first message with the GS2 channel 527 binding flag set to "y" and the server supports channel binding 528 the server MUST fail authentication. This is because if the 529 client sets the GS2 channel binding flag set to "y" then the 530 client must have believed that the server did not support channel 531 binding -- if the server did in fact support channel binding then 532 this is an indication that there has been a downgrade attack 533 (e.g., an attacker changed the server's mechanism list to exclude 534 the -PLUS suffixed SCRAM mechanism name(s)). 536 The server MUST always validate the client's "c=" field. The server 537 does this by constructing the value of the "c=" attribute and then 538 checking that it matches the client's c= attribute value. 540 6.1. Channel Binding to TLS Channels 542 If an external TLS channel is to be bound into the SCRAM 543 authentication, and if the channel was established using a X.509 544 [RFC5280] server certificate to authenticate the server, then the 545 SCRAM client and server MUST use the 'tls-server-end-point' channel 546 binding type. See the IANA Channel Binding Types registry. 548 If an external TLS channel is to be bound into the SCRAM 549 authentication, and if the channel was established either without the 550 use of any X.509 server certificate to authenticate the server, or 551 with a non X.509 server certificate, then the SCRAM client and server 552 MUST use the 'tls-unique' channel binding type. 554 7. Formal Syntax 556 The following syntax specification uses the Augmented Backus-Naur 557 Form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3" 558 and "UTF8-4" non-terminal are defined in [RFC3629]. 560 ALPHA = 561 DIGIT = 562 UTF8-2 = 563 UTF8-3 = 564 UTF8-4 = 566 generic-message = attr-val *("," attr-val) 567 ;; Generic syntax of any server challenge 568 ;; or client response 570 attr-val = ALPHA "=" value 572 value = 1*value-char 574 value-safe-char = %x01-2B / %x2D-3C / %x3E-7F / 575 UTF8-2 / UTF8-3 / UTF8-4 576 ;; UTF8-char except NUL, "=", and ",". 578 value-char = value-safe-char / "=" 580 base64-char = ALPHA / DIGIT / "/" / "+" 582 base64-4 = 4base64-char 584 base64-3 = 3base64-char "=" 586 base64-2 = 2base64-char "==" 588 base64 = *base64-4 [base64-3 / base64-2] 590 posit-number = %x31-39 *DIGIT 591 ;; A positive number 593 saslname = 1*(value-safe-char / "=2C" / "=3D") 594 ;; Conforms to 596 authzid = "a=" saslname 597 ;; Protocol specific. 599 gs2-cbind-flag = "n" / "y" / "p" 600 ;; "n" -> client doesn't support channel binding 601 ;; "y" -> client does support channel binding 602 ;; but thinks the server does not. 603 ;; "p" -> client requires channel binding 605 gs2-header = gs2-cbind-flag [ authzid ] "," 606 ;; GS2 header for SCRAM 607 ;; (the actual GS2 header includes an optional 608 ;; flag to indicate that the GSS mechanism is not 609 ;; "standard" but since SCRAM is "standard" we 610 ;; don't include that flag). 612 username = "n=" saslname 613 ;; Usernames are prepared using SASLPrep. 615 reserved-mext = "m=" 1*(value-char) 616 ;; Reserved for signalling mandatory extensions. 617 ;; The exact syntax will be defined in 618 ;; the future. 620 channel-binding = "c=" base64 621 ;; base64 encoding of cbind-input 623 proof = "p=" base64 625 nonce = "r=" c-nonce [s-nonce] 626 ;; Second part provided by server. 628 c-nonce = value 630 s-nonce = value 632 salt = "s=" base64 634 verifier = "v=" base64 635 ;; base-64 encoded ServerSignature. 637 iteration-count = "i=" posit-number 638 ;; A positive number 640 client-first-message = 641 gs2-header [reserved-mext ","] 642 username "," nonce ["," extensions] 644 server-first-message = 645 [reserved-mext ","] nonce "," salt "," 646 iteration-count ["," extensions] 648 client-final-message-without-proof = 649 channel-binding "," nonce ["," 650 extensions] 652 client-final-message = 653 client-final-message-without-proof "," proof 655 gss-server-error = "e=" value 656 server-final-message = gss-server-error / 657 verifier ["," extensions] 658 ;; The error message is only for the GSS-API 659 ;; form of SCRAM, and it is OPTIONAL to 660 ;; implement it. 662 extensions = attr-val *("," attr-val) 663 ;; All extensions are optional, 664 ;; i.e. unrecognized attributes 665 ;; not defined in this document 666 ;; MUST be ignored. 668 cbind-data = *OCTET 669 cbind-type = value 670 ;; e.g. "tls-server-end-point" or 671 ;; "tls-unique" 673 cbind-input = gs2-header [ cbind-type ":" cbind-data ] 675 8. SCRAM as a GSS-API Mechanism 677 This section and its sub-sections and all normative references of it 678 not referenced elsewhere in this document are INFORMATIONAL for SASL 679 implementors, but they are NORMATIVE for GSS-API implementors. 681 SCRAM is actually also GSS-API mechanism. The messages are the same, 682 but a) the GS2 header on the client's first message and channel 683 binding data is excluded when SCRAM is used as a GSS-API mechanism, 684 and b) the RFC2743 section 3.1 initial context token header is 685 prefixed to the client's first authentication message (context 686 token). 688 The GSS-API mechanism OID for SCRAM is (see Section 10). 690 8.1. GSS-API Principal Name Types for SCRAM 692 SCRAM does not name acceptors. Therefore only GSS_C_NO_NAME and 693 names of type GSS_C_NT_ANONYMOUS shall be allowed as the target name 694 input of GSS_Init_sec_context() when using a SCRAM mechanism. 696 SCRAM supports only a single name type for initiators: 697 GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for 698 SCRAM. 700 There is no name canonicalization procedure for SCRAM beyond applying 701 SASLprep as described in Section 5.1. 703 The query, display and exported name syntax for SCRAM principal names 704 is the same: there is no syntax -- SCRAM principal names are free- 705 form. (The exported name token does, of course, conform to [RFC2743] 706 section 3.2, but the "NAME" part of the token is just a SCRAM user 707 name.) 709 8.2. GSS-API Per-Message Tokens for SCRAM 711 The per-message tokens for SCRAM as a GSS-API mechanism SHALL BE the 712 same as those for the Kerberos V GSS-API mechanism [RFC4121], using 713 the Kerberos V "aes128-cts-hmac-sha1-96" enctype [RFC3962]. 715 The 128-bit session key SHALL be derived by using the least 716 significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API session 717 key" || ClientKey || AuthMessage). 719 SCRAM does support PROT_READY, and is PROT_READY on the initiator 720 side first upon receipt of the server's reply to the initial security 721 context token. 723 8.3. GSS_Pseudo_random() for SCRAM 725 The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for 726 the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor- 727 asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and 728 GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random(). 730 9. Security Considerations 732 If the authentication exchange is performed without a strong security 733 layer, then a passive eavesdropper can gain sufficient information to 734 mount an offline dictionary or brute-force attack which can be used 735 to recover the user's password. The amount of time necessary for 736 this attack depends on the cryptographic hash function selected, the 737 strength of the password and the iteration count supplied by the 738 server. An external security layer with strong encryption will 739 prevent this attack. 741 If the external security layer used to protect the SCRAM exchange 742 uses an anonymous key exchange, then the SCRAM channel binding 743 mechanism can be used to detect a man-in-the-middle attack on the 744 security layer and cause the authentication to fail as a result. 745 However, the man-in-the-middle attacker will have gained sufficient 746 information to mount an offline dictionary or brute-force attack. 747 For this reason, SCRAM includes the ability to increase the iteration 748 count over time. 750 If the authentication information is stolen from the authentication 751 database, then an offline dictionary or brute-force attack can be 752 used to recover the user's password. The use of salt mitigates this 753 attack somewhat by requiring a separate attack on each password. 754 Authentication mechanisms which protect against this attack are 755 available (e.g., the EKE class of mechanisms). 757 If an attacker obtains the authentication information from the 758 authentication repository and either eavesdrops on one authentication 759 exchange or impersonates a server, the attacker gains the ability to 760 impersonate that user to all servers providing SCRAM access using the 761 same hash function, password, iteration count and salt. For this 762 reason, it is important to use randomly-generated salt values. 764 SCRAM does not negotiate a hash function to use. Hash function 765 negotiation is left to the SASL mechanism negotiation. It is 766 important that clients be able to sort a locally available list of 767 mechanisms by preference so that the client may pick the most 768 preferred of a server's advertised mechanism list. This preference 769 order is not specified here as it is a local matter. The preference 770 order should include objective and subjective notions of mechanism 771 cryptographic strength (e.g., SCRAM with a successor to SHA-1 may be 772 preferred over SCRAM with SHA-1). 774 Note that to protect the SASL mechanism negotiation applications 775 normally must list the server mechs twice: once before and once after 776 authentication, the latter using security layers. Since SCRAM does 777 not provide security layers the only ways to protect the mechanism 778 negotiation are: a) use channel binding to an external channel, or b) 779 use an external channel that authenticates a user-provided server 780 name. 782 A hostile server can perform a computational denial-of-service attack 783 on clients by sending a big iteration count value. 785 10. IANA Considerations 787 IANA is requested to add the following entries to the SASL Mechanism 788 registry established by [RFC4422]: 790 To: iana@iana.org 791 Subject: Registration of a new SASL mechanism SCRAM-SHA-1 793 SASL mechanism name (or prefix for the family): SCRAM-SHA-1 794 Security considerations: Section 7 of [RFCXXXX] 795 Published specification (optional, recommended): [RFCXXXX] 796 Person & email address to contact for further information: 797 IETF SASL WG 798 Intended usage: COMMON 799 Owner/Change controller: IESG 800 Note: 802 To: iana@iana.org 803 Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS 805 SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS 806 Security considerations: Section 7 of [RFCXXXX] 807 Published specification (optional, recommended): [RFCXXXX] 808 Person & email address to contact for further information: 809 IETF SASL WG 810 Intended usage: COMMON 811 Owner/Change controller: IESG 812 Note: 814 Note that even though this document defines a family of SCRAM- 815 mechanisms, it doesn't register a family of SCRAM- mechanisms in the 816 SASL Mechanisms registry. IANA is requested to prevent future 817 registrations of SASL mechanisms starting with SCRAM- without 818 consulting the SASL mailing list first. 820 Note to future SCRAM- mechanism designers: each new SCRAM- SASL 821 mechanism MUST be explicitly registered with IANA and MUST comply 822 with SCRAM- mechanism naming convention defined in Section 4 of this 823 document. 825 We hereby request that IANA assign a GSS-API mechanism OID for SCRAM. 827 11. Acknowledgements 829 The authors would like to thank Dave Cridland for his contributions 830 to this document. 832 Appendix A. Other Authentication Mechanisms 834 The DIGEST-MD5 [I-D.ietf-sasl-digest-to-historic] mechanism has 835 proved to be too complex to implement and test, and thus has poor 836 interoperability. The security layer is often not implemented, and 837 almost never used; everyone uses TLS instead. For a more complete 838 list of problems with DIGEST-MD5 which lead to the creation of SCRAM 839 see [I-D.ietf-sasl-digest-to-historic]. 841 The CRAM-MD5 SASL mechanism, while widely deployed has also some 842 problems, in particular it is missing some modern SASL features such 843 as support for internationalized usernames and passwords, support for 844 passing of authorization identity, support for channel bindings. It 845 also doesn't support server authentication. For a more complete list 846 of problems with CRAM-MD5 see [I-D.ietf-sasl-crammd5-to-historic]. 848 The PLAIN [RFC4616] SASL mechanism allows a malicious server or 849 eavesdropper to impersonate the authenticating user to any other 850 server for which the user has the same password. It also sends the 851 password in the clear over the network, unless TLS is used. Server 852 authentication is not supported. 854 Appendix B. Design Motivations 856 The following design goals shaped this document. Note that some of 857 the goals have changed since the initial version of the document. 859 o The SASL mechanism has all modern SASL features: support for 860 internationalized usernames and passwords, support for passing of 861 authorization identity, support for channel bindings. 863 o The protocol supports mutual authentication. 865 o The authentication information stored in the authentication 866 database is not sufficient by itself to impersonate the client. 868 o The server does not gain the ability to impersonate the client to 869 other servers (with an exception for server-authorized proxies), 870 unless such other servers allow SCRAM authentication and use the 871 same salt and iteration count for the user. 873 o The mechanism is extensible, but [hopefully] not overengineered in 874 this respect. 876 o Easier to implement than DIGEST-MD5 in both clients and servers. 878 Appendix C. Internet-Draft Change History 880 (RFC Editor: Please delete everything after this point) 882 Open Issues 884 o Add proper examples and test vectors 886 Changes since -10 888 o Converted the source for this I-D to XML. 890 o Added text to make SCRAM compliant with the new GS2 design. 892 o Added text on channel binding negotiation. 894 o Added text on channel binding, including a reference to RFC5056. 896 o Added text on SCRAM as a GSS-API mechanism. This noted as not 897 relevant to SASL-only implementors -- the normative references for 898 SCRAM as a GSS-API mechanism are segregated as well. 900 Changes since -07 902 o Updated References. 904 o Clarified purpose of the m= attribute. 906 o Fixed a problem with authentication/authorization identity's ABNF 907 not allowing for some characters. 909 o Updated ABNF for nonce to show client-generated and server- 910 generated parts. 912 o Only register SCRAM-SHA-1 with IANA and require explicit 913 registrations of all other SCRAM- mechanisms. 915 Changes since -06 917 o Removed hash negotiation from SCRAM and turned it into a family of 918 SASL mechanisms. 920 o Start using "Hash Function Textual Names" IANA registry for SCRAM 921 mechanism naming. 923 o Fixed definition of Hi(str, salt) to be consistent with [RFC2898]. 925 o Clarified extensibility of SCRAM: added m= attribute (for future 926 mandatory extensions) and specified that all unrecognized 927 attributes must be ignored. 929 Changes since -05 931 o Changed the mandatory to implement hash algorithm to SHA-1 (as per 932 WG consensus). 934 o Added text about use of SASLPrep for username canonicalization/ 935 validation. 937 o Clarified that authorization identity is canonicalized/verified 938 according to SASL protocol profile. 940 o Clarified that iteration count is per-user. 942 o Clarified how clients select the authentication function. 944 o Added IANA registration for the new mechanism. 946 o Added missing normative references (UTF-8, SASLPrep). 948 o Various editorial changes based on comments from Hallvard B 949 Furuseth, Nico William and Simon Josefsson. 951 Changes since -04 953 o Update Base64 and Security Glossary references. 955 o Add Formal Syntax section. 957 o Don't bother with "v=". 959 o Make MD5 mandatory to implement. Suggest i=128. 961 Changes since -03 963 o Seven years have passed, in which it became clear that DIGEST-MD5 964 suffered from unacceptably bad interoperability, so SCRAM-MD5 is 965 now back from the dead. 967 o Be hash agnostic, so MD5 can be replaced more easily. 969 o General simplification. 971 12. References 973 12.1. Normative References 975 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 976 Hashing for Message Authentication", RFC 2104, 977 February 1997. 979 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 980 Requirement Levels", BCP 14, RFC 2119, March 1997. 982 [RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 983 (SHA1)", RFC 3174, September 2001. 985 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 986 Internationalized Strings ("stringprep")", RFC 3454, 987 December 2002. 989 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 990 10646", STD 63, RFC 3629, November 2003. 992 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 993 and Passwords", RFC 4013, February 2005. 995 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 996 Security Layer (SASL)", RFC 4422, June 2006. 998 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 999 Encodings", RFC 4648, October 2006. 1001 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 1002 Channels", RFC 5056, November 2007. 1004 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1005 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1007 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1008 Housley, R., and W. Polk, "Internet X.509 Public Key 1009 Infrastructure Certificate and Certificate Revocation List 1010 (CRL) Profile", RFC 5280, May 2008. 1012 12.2. Normative References for GSS-API implementors 1014 [I-D.ietf-sasl-gs2] 1015 Josefsson, S. and N. Williams, "Using GSS-API Mechanisms 1016 in SASL: The GS2 Mechanism Family", draft-ietf-sasl-gs2-12 1017 (work in progress), April 2009. 1019 [RFC2743] Linn, J., "Generic Security Service Application Program 1020 Interface Version 2, Update 1", RFC 2743, January 2000. 1022 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) 1023 Encryption for Kerberos 5", RFC 3962, February 2005. 1025 [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 1026 Version 5 Generic Security Service Application Program 1027 Interface (GSS-API) Mechanism: Version 2", RFC 4121, 1028 July 2005. 1030 [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API 1031 Extension for the Generic Security Service Application 1032 Program Interface (GSS-API)", RFC 4401, February 2006. 1034 [RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the 1035 Kerberos V Generic Security Service Application Program 1036 Interface (GSS-API) Mechanism", RFC 4402, February 2006. 1038 12.3. Informative References 1040 [I-D.ietf-sasl-crammd5-to-historic] 1041 Zeilenga, K., "CRAM-MD5 to Historic", 1042 draft-ietf-sasl-crammd5-to-historic-00 (work in progress), 1043 November 2008. 1045 [I-D.ietf-sasl-digest-to-historic] 1046 Melnikov, A., "Moving DIGEST-MD5 to Historic", 1047 draft-ietf-sasl-digest-to-historic-00 (work in progress), 1048 July 2008. 1050 [I-D.ietf-sasl-rfc2831bis] 1051 Melnikov, A., "Using Digest Authentication as a SASL 1052 Mechanism", draft-ietf-sasl-rfc2831bis-12 (work in 1053 progress), March 2007. 1055 [RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP 1056 AUTHorize Extension for Simple Challenge/Response", 1057 RFC 2195, September 1997. 1059 [RFC2202] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and HMAC- 1060 SHA-1", RFC 2202, September 1997. 1062 [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography 1063 Specification Version 2.0", RFC 2898, September 2000. 1065 [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 1066 Requirements for Security", BCP 106, RFC 4086, June 2005. 1068 [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol 1069 (LDAP): Technical Specification Road Map", RFC 4510, 1070 June 2006. 1072 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and 1073 Security Layer (SASL) Mechanism", RFC 4616, August 2006. 1075 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 1076 RFC 4949, August 2007. 1078 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1079 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1081 Authors' Addresses 1083 Abhijit Menon-Sen 1084 Oryx Mail Systems GmbH 1086 Email: ams@oryx.com 1088 Alexey Melnikov 1089 Isode Ltd 1091 Email: Alexey.Melnikov@isode.com 1093 Chris Newman 1094 Sun Microsystems 1095 1050 Lakes Drive 1096 West Covina, CA 91790 1097 USA 1099 Email: chris.newman@sun.com 1101 Nicolas Williams 1102 Sun Microsystems 1103 5300 Riata Trace Ct 1104 Austin, TX 78727 1105 USA 1107 Email: Nicolas.Williams@sun.com