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7Internet Engineering Task Force (IETF) C. Newman
8Request for Comments: 5802 Oracle
9Category: Standards Track A. Menon-Sen
10ISSN: 2070-1721 Oryx Mail Systems GmbH
11 A. Melnikov
12 Isode, Ltd.
13 N. Williams
14 Oracle
15 July 2010
16
17
18 Salted Challenge Response Authentication Mechanism (SCRAM)
19 SASL and GSS-API Mechanisms
20
21Abstract
22
23 The secure authentication mechanism most widely deployed and used by
24 Internet application protocols is the transmission of clear-text
25 passwords over a channel protected by Transport Layer Security (TLS).
26 There are some significant security concerns with that mechanism,
27 which could be addressed by the use of a challenge response
28 authentication mechanism protected by TLS. Unfortunately, the
29 challenge response mechanisms presently on the standards track all
30 fail to meet requirements necessary for widespread deployment, and
31 have had success only in limited use.
32
33 This specification describes a family of Simple Authentication and
34 Security Layer (SASL; RFC 4422) authentication mechanisms called the
35 Salted Challenge Response Authentication Mechanism (SCRAM), which
36 addresses the security concerns and meets the deployability
37 requirements. When used in combination with TLS or an equivalent
38 security layer, a mechanism from this family could improve the status
39 quo for application protocol authentication and provide a suitable
40 choice for a mandatory-to-implement mechanism for future application
41 protocol standards.
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58Newman, et al. Standards Track [Page 1]
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60RFC 5802 SCRAM July 2010
61
62
63Status of This Memo
64
65 This is an Internet Standards Track document.
66
67 This document is a product of the Internet Engineering Task Force
68 (IETF). It represents the consensus of the IETF community. It has
69 received public review and has been approved for publication by the
70 Internet Engineering Steering Group (IESG). Further information on
71 Internet Standards is available in Section 2 of RFC 5741.
72
73 Information about the current status of this document, any errata,
74 and how to provide feedback on it may be obtained at
75 http://www.rfc-editor.org/info/rfc5802.
76
77Copyright Notice
78
79 Copyright (c) 2010 IETF Trust and the persons identified as the
80 document authors. All rights reserved.
81
82 This document is subject to BCP 78 and the IETF Trust's Legal
83 Provisions Relating to IETF Documents
84 (http://trustee.ietf.org/license-info) in effect on the date of
85 publication of this document. Please review these documents
86 carefully, as they describe your rights and restrictions with respect
87 to this document. Code Components extracted from this document must
88 include Simplified BSD License text as described in Section 4.e of
89 the Trust Legal Provisions and are provided without warranty as
90 described in the Simplified BSD License.
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114Newman, et al. Standards Track [Page 2]
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116RFC 5802 SCRAM July 2010
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118
119Table of Contents
120
121 1. Introduction ....................................................4
122 2. Conventions Used in This Document ...............................5
123 2.1. Terminology ................................................5
124 2.2. Notation ...................................................6
125 3. SCRAM Algorithm Overview ........................................7
126 4. SCRAM Mechanism Names ...........................................8
127 5. SCRAM Authentication Exchange ...................................9
128 5.1. SCRAM Attributes ..........................................10
129 5.2. Compliance with SASL Mechanism Requirements ...............13
130 6. Channel Binding ................................................14
131 6.1. Default Channel Binding ...................................15
132 7. Formal Syntax ..................................................15
133 8. SCRAM as a GSS-API Mechanism ...................................19
134 8.1. GSS-API Principal Name Types for SCRAM ....................19
135 8.2. GSS-API Per-Message Tokens for SCRAM ......................20
136 8.3. GSS_Pseudo_random() for SCRAM .............................20
137 9. Security Considerations ........................................20
138 10. IANA Considerations ...........................................22
139 11. Acknowledgements ..............................................23
140 12. References ....................................................24
141 12.1. Normative References .....................................24
142 12.2. Normative References for GSS-API Implementors ............24
143 12.3. Informative References ...................................25
144 Appendix A. Other Authentication Mechanisms .......................27
145 Appendix B. Design Motivations ....................................27
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170Newman, et al. Standards Track [Page 3]
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172RFC 5802 SCRAM July 2010
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174
1751. Introduction
176
177 This specification describes a family of authentication mechanisms
178 called the Salted Challenge Response Authentication Mechanism (SCRAM)
179 which addresses the requirements necessary to deploy a challenge-
180 response mechanism more widely than past attempts (see Appendix A and
181 Appendix B). When used in combination with Transport Layer Security
182 (TLS; see [RFC5246]) or an equivalent security layer, a mechanism
183 from this family could improve the status quo for application
184 protocol authentication and provide a suitable choice for a
185 mandatory-to-implement mechanism for future application protocol
186 standards.
187
188 For simplicity, this family of mechanisms does not presently include
189 negotiation of a security layer [RFC4422]. It is intended to be used
190 with an external security layer such as that provided by TLS or SSH,
191 with optional channel binding [RFC5056] to the external security
192 layer.
193
194 SCRAM is specified herein as a pure Simple Authentication and
195 Security Layer (SASL) [RFC4422] mechanism, but it conforms to the new
196 bridge between SASL and the Generic Security Service Application
197 Program Interface (GSS-API) called "GS2" [RFC5801]. This means that
198 this document defines both, a SASL mechanism and a GSS-API mechanism.
199
200 SCRAM provides the following protocol features:
201
202 o The authentication information stored in the authentication
203 database is not sufficient by itself to impersonate the client.
204 The information is salted to prevent a pre-stored dictionary
205 attack if the database is stolen.
206
207 o The server does not gain the ability to impersonate the client to
208 other servers (with an exception for server-authorized proxies).
209
210 o The mechanism permits the use of a server-authorized proxy without
211 requiring that proxy to have super-user rights with the back-end
212 server.
213
214 o Mutual authentication is supported, but only the client is named
215 (i.e., the server has no name).
216
217 o When used as a SASL mechanism, SCRAM is capable of transporting
218 authorization identities (see [RFC4422], Section 2) from the
219 client to the server.
220
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226Newman, et al. Standards Track [Page 4]
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228RFC 5802 SCRAM July 2010
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230
231 A separate document defines a standard LDAPv3 [RFC4510] attribute
232 that enables storage of the SCRAM authentication information in LDAP.
233 See [RFC5803].
234
235 For an in-depth discussion of why other challenge response mechanisms
236 are not considered sufficient, see Appendix A. For more information
237 about the motivations behind the design of this mechanism, see
238 Appendix B.
239
2402. Conventions Used in This Document
241
242 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
243 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
244 document are to be interpreted as described in [RFC2119].
245
246 Formal syntax is defined by [RFC5234] including the core rules
247 defined in Appendix B of [RFC5234].
248
249 Example lines prefaced by "C:" are sent by the client and ones
250 prefaced by "S:" by the server. If a single "C:" or "S:" label
251 applies to multiple lines, then the line breaks between those lines
252 are for editorial clarity only, and are not part of the actual
253 protocol exchange.
254
2552.1. Terminology
256
257 This document uses several terms defined in [RFC4949] ("Internet
258 Security Glossary") including the following: authentication,
259 authentication exchange, authentication information, brute force,
260 challenge-response, cryptographic hash function, dictionary attack,
261 eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,
262 one-way encryption function, password, replay attack, and salt.
263 Readers not familiar with these terms should use that glossary as a
264 reference.
265
266 Some clarifications and additional definitions follow:
267
268 o Authentication information: Information used to verify an identity
269 claimed by a SCRAM client. The authentication information for a
270 SCRAM identity consists of salt, iteration count, "StoredKey" and
271 "ServerKey" (as defined in the algorithm overview) for each
272 supported cryptographic hash function.
273
274 o Authentication database: The database used to look up the
275 authentication information associated with a particular identity.
276 For application protocols, LDAPv3 (see [RFC4510]) is frequently
277
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284RFC 5802 SCRAM July 2010
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287 used as the authentication database. For network-level protocols
288 such as PPP or 802.11x, the use of RADIUS [RFC2865] is more
289 common.
290
291 o Base64: An encoding mechanism defined in [RFC4648] that converts
292 an octet string input to a textual output string that can be
293 easily displayed to a human. The use of base64 in SCRAM is
294 restricted to the canonical form with no whitespace.
295
296 o Octet: An 8-bit byte.
297
298 o Octet string: A sequence of 8-bit bytes.
299
300 o Salt: A random octet string that is combined with a password
301 before applying a one-way encryption function. This value is used
302 to protect passwords that are stored in an authentication
303 database.
304
3052.2. Notation
306
307 The pseudocode description of the algorithm uses the following
308 notations:
309
310 o ":=": The variable on the left-hand side represents the octet
311 string resulting from the expression on the right-hand side.
312
313 o "+": Octet string concatenation.
314
315 o "[ ]": A portion of an expression enclosed in "[" and "]" may not
316 be included in the result under some circumstances. See the
317 associated text for a description of those circumstances.
318
319 o Normalize(str): Apply the SASLprep profile [RFC4013] of the
320 "stringprep" algorithm [RFC3454] as the normalization algorithm to
321 a UTF-8 [RFC3629] encoded "str". The resulting string is also in
322 UTF-8. When applying SASLprep, "str" is treated as a "stored
323 strings", which means that unassigned Unicode codepoints are
324 prohibited (see Section 7 of [RFC3454]). Note that
325 implementations MUST either implement SASLprep or disallow use of
326 non US-ASCII Unicode codepoints in "str".
327
328 o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in
329 [RFC2104]) using the octet string represented by "key" as the key
330 and the octet string "str" as the input string. The size of the
331 result is the hash result size for the hash function in use. For
332 example, it is 20 octets for SHA-1 (see [RFC3174]).
333
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340RFC 5802 SCRAM July 2010
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342
343 o H(str): Apply the cryptographic hash function to the octet string
344 "str", producing an octet string as a result. The size of the
345 result depends on the hash result size for the hash function in
346 use.
347
348 o XOR: Apply the exclusive-or operation to combine the octet string
349 on the left of this operator with the octet string on the right of
350 this operator. The length of the output and each of the two
351 inputs will be the same for this use.
352
353 o Hi(str, salt, i):
354
355 U1 := HMAC(str, salt + INT(1))
356 U2 := HMAC(str, U1)
357 ...
358 Ui-1 := HMAC(str, Ui-2)
359 Ui := HMAC(str, Ui-1)
360
361 Hi := U1 XOR U2 XOR ... XOR Ui
362
363 where "i" is the iteration count, "+" is the string concatenation
364 operator, and INT(g) is a 4-octet encoding of the integer g, most
365 significant octet first.
366
367 Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the
368 pseudorandom function (PRF) and with dkLen == output length of
369 HMAC() == output length of H().
370
3713. SCRAM Algorithm Overview
372
373 The following is a description of a full, uncompressed SASL SCRAM
374 authentication exchange. Nothing in SCRAM prevents either sending
375 the client-first message with the SASL authentication request defined
376 by an application protocol ("initial client response"), or sending
377 the server-final message as additional data of the SASL outcome of
378 authentication exchange defined by an application protocol. See
379 [RFC4422] for more details.
380
381 Note that this section omits some details, such as client and server
382 nonces. See Section 5 for more details.
383
384 To begin with, the SCRAM client is in possession of a username and
385 password (*) (or a ClientKey/ServerKey, or SaltedPassword). It sends
386 the username to the server, which retrieves the corresponding
387 authentication information, i.e., a salt, StoredKey, ServerKey, and
388 the iteration count i. (Note that a server implementation may choose
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399 to use the same iteration count for all accounts.) The server sends
400 the salt and the iteration count to the client, which then computes
401 the following values and sends a ClientProof to the server:
402
403 (*) Note that both the username and the password MUST be encoded in
404 UTF-8 [RFC3629].
405
406 Informative Note: Implementors are encouraged to create test cases
407 that use both usernames and passwords with non-ASCII codepoints. In
408 particular, it's useful to test codepoints whose "Unicode
409 Normalization Form C" and "Unicode Normalization Form KC" are
410 different. Some examples of such codepoints include Vulgar Fraction
411 One Half (U+00BD) and Acute Accent (U+00B4).
412
413 SaltedPassword := Hi(Normalize(password), salt, i)
414 ClientKey := HMAC(SaltedPassword, "Client Key")
415 StoredKey := H(ClientKey)
416 AuthMessage := client-first-message-bare + "," +
417 server-first-message + "," +
418 client-final-message-without-proof
419 ClientSignature := HMAC(StoredKey, AuthMessage)
420 ClientProof := ClientKey XOR ClientSignature
421 ServerKey := HMAC(SaltedPassword, "Server Key")
422 ServerSignature := HMAC(ServerKey, AuthMessage)
423
424 The server authenticates the client by computing the ClientSignature,
425 exclusive-ORing that with the ClientProof to recover the ClientKey
426 and verifying the correctness of the ClientKey by applying the hash
427 function and comparing the result to the StoredKey. If the ClientKey
428 is correct, this proves that the client has access to the user's
429 password.
430
431 Similarly, the client authenticates the server by computing the
432 ServerSignature and comparing it to the value sent by the server. If
433 the two are equal, it proves that the server had access to the user's
434 ServerKey.
435
436 The AuthMessage is computed by concatenating messages from the
437 authentication exchange. The format of these messages is defined in
438 Section 7.
439
4404. SCRAM Mechanism Names
441
442 A SCRAM mechanism name is a string "SCRAM-" followed by the
443 uppercased name of the underlying hash function taken from the IANA
444 "Hash Function Textual Names" registry (see http://www.iana.org),
445 optionally followed by the suffix "-PLUS" (see below). Note that
446 SASL mechanism names are limited to 20 octets, which means that only
447
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454
455 hash function names with lengths shorter or equal to 9 octets
456 (20-length("SCRAM-")-length("-PLUS") can be used. For cases when the
457 underlying hash function name is longer than 9 octets, an alternative
458 9-octet (or shorter) name can be used to construct the corresponding
459 SCRAM mechanism name, as long as this alternative name doesn't
460 conflict with any other hash function name from the IANA "Hash
461 Function Textual Names" registry. In order to prevent future
462 conflict, such alternative names SHOULD be registered in the IANA
463 "Hash Function Textual Names" registry.
464
465 For interoperability, all SCRAM clients and servers MUST implement
466 the SCRAM-SHA-1 authentication mechanism, i.e., an authentication
467 mechanism from the SCRAM family that uses the SHA-1 hash function as
468 defined in [RFC3174].
469
470 The "-PLUS" suffix is used only when the server supports channel
471 binding to the external channel. If the server supports channel
472 binding, it will advertise both the "bare" and "plus" versions of
473 whatever mechanisms it supports (e.g., if the server supports only
474 SCRAM with SHA-1, then it will advertise support for both SCRAM-SHA-1
475 and SCRAM-SHA-1-PLUS). If the server does not support channel
476 binding, then it will advertise only the "bare" version of the
477 mechanism (e.g., only SCRAM-SHA-1). The "-PLUS" exists to allow
478 negotiation of the use of channel binding. See Section 6.
479
4805. SCRAM Authentication Exchange
481
482 SCRAM is a SASL mechanism whose client response and server challenge
483 messages are text-based messages containing one or more attribute-
484 value pairs separated by commas. Each attribute has a one-letter
485 name. The messages and their attributes are described in
486 Section 5.1, and defined in Section 7.
487
488 SCRAM is a client-first SASL mechanism (see [RFC4422], Section 5,
489 item 2a), and returns additional data together with a server's
490 indication of a successful outcome.
491
492 This is a simple example of a SCRAM-SHA-1 authentication exchange
493 when the client doesn't support channel bindings (username 'user' and
494 password 'pencil' are used):
495
496 C: n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL ../scram/scram_test.go:35
497 S: r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,s=QSXCR+Q6sek8bf92,
498 i=4096
499 C: c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
500 p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
501 S: v=rmF9pqV8S7suAoZWja4dJRkFsKQ=
502
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510
511 First, the client sends the "client-first-message" containing:
512
513 o a GS2 header consisting of a flag indicating whether channel
514 binding is supported-but-not-used, not supported, or used, and an
515 optional SASL authorization identity;
516
517 o SCRAM username and a random, unique nonce attributes.
518
519 Note that the client's first message will always start with "n", "y",
520 or "p"; otherwise, the message is invalid and authentication MUST
521 fail. This is important, as it allows for GS2 extensibility (e.g.,
522 to add support for security layers).
523
524 In response, the server sends a "server-first-message" containing the
525 user's iteration count i and the user's salt, and appends its own
526 nonce to the client-specified one.
527
528 The client then responds by sending a "client-final-message" with the
529 same nonce and a ClientProof computed using the selected hash
530 function as explained earlier.
531
532 The server verifies the nonce and the proof, verifies that the
533 authorization identity (if supplied by the client in the first
534 message) is authorized to act as the authentication identity, and,
535 finally, it responds with a "server-final-message", concluding the
536 authentication exchange.
537
538 The client then authenticates the server by computing the
539 ServerSignature and comparing it to the value sent by the server. If
540 the two are different, the client MUST consider the authentication
541 exchange to be unsuccessful, and it might have to drop the
542 connection.
543
5445.1. SCRAM Attributes
545
546 This section describes the permissible attributes, their use, and the
547 format of their values. All attribute names are single US-ASCII ../scram/scram.go:193
548 letters and are case-sensitive.
549
550 Note that the order of attributes in client or server messages is
551 fixed, with the exception of extension attributes (described by the
552 "extensions" ABNF production), which can appear in any order in the
553 designated positions. See Section 7 for authoritative reference.
554
555 o a: This is an optional attribute, and is part of the GS2 [RFC5801]
556 bridge between the GSS-API and SASL. This attribute specifies an
557 authorization identity. A client may include it in its first
558 message to the server if it wants to authenticate as one user, but
559
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567 subsequently act as a different user. This is typically used by
568 an administrator to perform some management task on behalf of
569 another user, or by a proxy in some situations.
570
571 Upon the receipt of this value the server verifies its
572 correctness according to the used SASL protocol profile.
573 Failed verification results in failed authentication exchange.
574
575 If this attribute is omitted (as it normally would be), the
576 authorization identity is assumed to be derived from the
577 username specified with the (required) "n" attribute.
578
579 The server always authenticates the user specified by the "n"
580 attribute. If the "a" attribute specifies a different user,
581 the server associates that identity with the connection after
582 successful authentication and authorization checks.
583
584 The syntax of this field is the same as that of the "n" field
585 with respect to quoting of '=' and ','.
586
587 o n: This attribute specifies the name of the user whose password is
588 used for authentication (a.k.a. "authentication identity"
589 [RFC4422]). A client MUST include it in its first message to the
590 server. If the "a" attribute is not specified (which would
591 normally be the case), this username is also the identity that
592 will be associated with the connection subsequent to
593 authentication and authorization.
594
595 Before sending the username to the server, the client SHOULD
596 prepare the username using the "SASLprep" profile [RFC4013] of
597 the "stringprep" algorithm [RFC3454] treating it as a query
598 string (i.e., unassigned Unicode code points are allowed). If
599 the preparation of the username fails or results in an empty
600 string, the client SHOULD abort the authentication exchange
601 (*).
602
603 (*) An interactive client can request a repeated entry of the
604 username value.
605
606 Upon receipt of the username by the server, the server MUST
607 either prepare it using the "SASLprep" profile [RFC4013] of the
608 "stringprep" algorithm [RFC3454] treating it as a query string
609 (i.e., unassigned Unicode codepoints are allowed) or otherwise
610 be prepared to do SASLprep-aware string comparisons and/or
611 index lookups. If the preparation of the username fails or
612 results in an empty string, the server SHOULD abort the
613
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623 authentication exchange. Whether or not the server prepares
624 the username using "SASLprep", it MUST use it as received in
625 hash calculations.
626
627 The characters ',' or '=' in usernames are sent as '=2C' and
628 '=3D' respectively. If the server receives a username that
629 contains '=' not followed by either '2C' or '3D', then the
630 server MUST fail the authentication.
631
632 o m: This attribute is reserved for future extensibility. In this ../scram/scram.go:238 ../scram/scram.go:421
633 version of SCRAM, its presence in a client or a server message
634 MUST cause authentication failure when the attribute is parsed by
635 the other end.
636
637 o r: This attribute specifies a sequence of random printable ASCII
638 characters excluding ',' (which forms the nonce used as input to
639 the hash function). No quoting is applied to this string. As
640 described earlier, the client supplies an initial value in its
641 first message, and the server augments that value with its own
642 nonce in its first response. It is important that this value be
643 different for each authentication (see [RFC4086] for more details
644 on how to achieve this). The client MUST verify that the initial
645 part of the nonce used in subsequent messages is the same as the
646 nonce it initially specified. The server MUST verify that the
647 nonce sent by the client in the second message is the same as the
648 one sent by the server in its first message.
649
650 o c: This REQUIRED attribute specifies the base64-encoded GS2 header
651 and channel binding data. It is sent by the client in its second
652 authentication message. The attribute data consist of:
653
654 * the GS2 header from the client's first message (recall that the
655 GS2 header contains a channel binding flag and an optional
656 authzid). This header is going to include channel binding type
657 prefix (see [RFC5056]), if and only if the client is using
658 channel binding;
659
660 * followed by the external channel's channel binding data, if and
661 only if the client is using channel binding.
662
663 o s: This attribute specifies the base64-encoded salt used by the
664 server for this user. It is sent by the server in its first
665 message to the client.
666
667 o i: This attribute specifies an iteration count for the selected
668 hash function and user, and MUST be sent by the server along with
669 the user's salt.
670
671
672
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678
679 For the SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism, servers
680 SHOULD announce a hash iteration-count of at least 4096. Note
681 that a client implementation MAY cache ClientKey&ServerKey (or
682 just SaltedPassword) for later reauthentication to the same
683 service, as it is likely that the server is going to advertise
684 the same salt value upon reauthentication. This might be
685 useful for mobile clients where CPU usage is a concern.
686
687 o p: This attribute specifies a base64-encoded ClientProof. The
688 client computes this value as described in the overview and sends
689 it to the server.
690
691 o v: This attribute specifies a base64-encoded ServerSignature. It
692 is sent by the server in its final message, and is used by the
693 client to verify that the server has access to the user's
694 authentication information. This value is computed as explained
695 in the overview.
696
697 o e: This attribute specifies an error that occurred during
698 authentication exchange. It is sent by the server in its final
699 message and can help diagnose the reason for the authentication
700 exchange failure. On failed authentication, the entire server-
701 final-message is OPTIONAL; specifically, a server implementation
702 MAY conclude the SASL exchange with a failure without sending the
703 server-final-message. This results in an application-level error
704 response without an extra round-trip. If the server-final-message
705 is sent on authentication failure, then the "e" attribute MUST be
706 included.
707
708 o As-yet unspecified mandatory and optional extensions. Mandatory
709 extensions are encoded as values of the 'm' attribute (see ABNF
710 for reserved-mext in section 7). Optional extensions use as-yet
711 unassigned attribute names.
712
713 Mandatory extensions sent by one peer but not understood by the
714 other MUST cause authentication failure (the server SHOULD send
715 the "extensions-not-supported" server-error-value).
716
717 Unknown optional extensions MUST be ignored upon receipt.
718
7195.2. Compliance with SASL Mechanism Requirements
720
721 This section describes compliance with SASL mechanism requirements
722 specified in Section 5 of [RFC4422].
723
724 1) "SCRAM-SHA-1" and "SCRAM-SHA-1-PLUS".
725
726 2a) SCRAM is a client-first mechanism.
727
728
729
730Newman, et al. Standards Track [Page 13]
731
732RFC 5802 SCRAM July 2010
733
734
735 2b) SCRAM sends additional data with success.
736
737 3) SCRAM is capable of transferring authorization identities from
738 the client to the server.
739
740 4) SCRAM does not offer any security layers (SCRAM offers channel
741 binding instead).
742
743 5) SCRAM has a hash protecting the authorization identity.
744
7456. Channel Binding
746
747 SCRAM supports channel binding to external secure channels, such as
748 TLS. Clients and servers may or may not support channel binding,
749 therefore the use of channel binding is negotiable. SCRAM does not
750 provide security layers, however, therefore it is imperative that
751 SCRAM provide integrity protection for the negotiation of channel
752 binding.
753
754 Use of channel binding is negotiated as follows:
755
756 o Servers that support the use of channel binding SHOULD advertise
757 both the non-PLUS (SCRAM-<hash-function>) and PLUS-variant (SCRAM-
758 <hash-function>-PLUS) mechanism name. If the server cannot
759 support channel binding, it SHOULD advertise only the non-PLUS-
760 variant. If the server would never succeed in the authentication
761 of the non-PLUS-variant due to policy reasons, it MUST advertise
762 only the PLUS-variant.
763
764 o If the client supports channel binding and the server does not
765 appear to (i.e., the client did not see the -PLUS name advertised
766 by the server), then the client MUST NOT use an "n" gs2-cbind-
767 flag.
768
769 o Clients that support mechanism negotiation and channel binding
770 MUST use a "p" gs2-cbind-flag when the server offers the PLUS-
771 variant of the desired GS2 mechanism.
772
773 o If the client does not support channel binding, then it MUST use
774 an "n" gs2-cbind-flag. Conversely, if the client requires the use
775 of channel binding then it MUST use a "p" gs2-cbind-flag. Clients
776 that do not support mechanism negotiation never use a "y" gs2-
777 cbind-flag, they use either "p" or "n" according to whether they
778 require and support the use of channel binding or whether they do
779 not, respectively.
780
781 o Upon receipt of the client-first message, the server checks the
782 channel binding flag (gs2-cbind-flag).
783
784
785
786Newman, et al. Standards Track [Page 14]
787
788RFC 5802 SCRAM July 2010
789
790
791 * If the flag is set to "y" and the server supports channel
792 binding, the server MUST fail authentication. This is because
793 if the client sets the channel binding flag to "y", then the
794 client must have believed that the server did not support
795 channel binding -- if the server did in fact support channel
796 binding, then this is an indication that there has been a
797 downgrade attack (e.g., an attacker changed the server's
798 mechanism list to exclude the -PLUS suffixed SCRAM mechanism
799 name(s)).
800
801 * If the channel binding flag was "p" and the server does not
802 support the indicated channel binding type, then the server
803 MUST fail authentication.
804
805 The server MUST always validate the client's "c=" field. The server
806 does this by constructing the value of the "c=" attribute and then
807 checking that it matches the client's c= attribute value.
808
809 For more discussions of channel bindings, and the syntax of channel
810 binding data for various security protocols, see [RFC5056].
811
8126.1. Default Channel Binding
813
814 A default channel binding type agreement process for all SASL
815 application protocols that do not provide their own channel binding
816 type agreement is provided as follows.
817
818 'tls-unique' is the default channel binding type for any application
819 that doesn't specify one.
820
821 Servers MUST implement the "tls-unique" [RFC5929] channel binding
822 type, if they implement any channel binding. Clients SHOULD
823 implement the "tls-unique" [RFC5929] channel binding type, if they
824 implement any channel binding. Clients and servers SHOULD choose the
825 highest-layer/innermost end-to-end TLS channel as the channel to
826 which to bind.
827
828 Servers MUST choose the channel binding type indicated by the client,
829 or fail authentication if they don't support it.
830
8317. Formal Syntax
832
833 The following syntax specification uses the Augmented Backus-Naur
834 form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3",
835 and "UTF8-4" non-terminal are defined in [RFC3629].
836
837
838
839
840
841
842Newman, et al. Standards Track [Page 15]
843
844RFC 5802 SCRAM July 2010
845
846
847 ALPHA = <as defined in RFC 5234 appendix B.1>
848 DIGIT = <as defined in RFC 5234 appendix B.1>
849 UTF8-2 = <as defined in RFC 3629 (STD 63)>
850 UTF8-3 = <as defined in RFC 3629 (STD 63)>
851 UTF8-4 = <as defined in RFC 3629 (STD 63)>
852
853 attr-val = ALPHA "=" value
854 ;; Generic syntax of any attribute sent
855 ;; by server or client
856
857 value = 1*value-char
858
859 value-safe-char = %x01-2B / %x2D-3C / %x3E-7F /
860 UTF8-2 / UTF8-3 / UTF8-4
861 ;; UTF8-char except NUL, "=", and ",".
862
863 value-char = value-safe-char / "="
864
865 printable = %x21-2B / %x2D-7E
866 ;; Printable ASCII except ",".
867 ;; Note that any "printable" is also
868 ;; a valid "value".
869
870 base64-char = ALPHA / DIGIT / "/" / "+"
871
872 base64-4 = 4base64-char
873
874 base64-3 = 3base64-char "="
875
876 base64-2 = 2base64-char "=="
877
878 base64 = *base64-4 [base64-3 / base64-2]
879
880 posit-number = %x31-39 *DIGIT
881 ;; A positive number.
882
883 saslname = 1*(value-safe-char / "=2C" / "=3D")
884 ;; Conforms to <value>.
885
886 authzid = "a=" saslname
887 ;; Protocol specific.
888
889 cb-name = 1*(ALPHA / DIGIT / "." / "-") ../scram/parse.go:226
890 ;; See RFC 5056, Section 7.
891 ;; E.g., "tls-server-end-point" or
892 ;; "tls-unique".
893
894
895
896
897
898Newman, et al. Standards Track [Page 16]
899
900RFC 5802 SCRAM July 2010
901
902
903 gs2-cbind-flag = ("p=" cb-name) / "n" / "y" ../scram/scram.go:384
904 ;; "n" -> client doesn't support channel binding.
905 ;; "y" -> client does support channel binding
906 ;; but thinks the server does not.
907 ;; "p" -> client requires channel binding.
908 ;; The selected channel binding follows "p=".
909
910 gs2-header = gs2-cbind-flag "," [ authzid ] "," 5802:949 ../scram/scram.go:171
911 ;; GS2 header for SCRAM
912 ;; (the actual GS2 header includes an optional
913 ;; flag to indicate that the GSS mechanism is not
914 ;; "standard", but since SCRAM is "standard", we
915 ;; don't include that flag).
916
917 username = "n=" saslname
918 ;; Usernames are prepared using SASLprep.
919
920 reserved-mext = "m=" 1*(value-char)
921 ;; Reserved for signaling mandatory extensions.
922 ;; The exact syntax will be defined in
923 ;; the future.
924
925 channel-binding = "c=" base64 5802:1015 ../scram/scram.go:454
926 ;; base64 encoding of cbind-input.
927
928 proof = "p=" base64
929
930 nonce = "r=" c-nonce [s-nonce]
931 ;; Second part provided by server.
932
933 c-nonce = printable
934
935 s-nonce = printable
936
937 salt = "s=" base64
938
939 verifier = "v=" base64
940 ;; base-64 encoded ServerSignature.
941
942 iteration-count = "i=" posit-number
943 ;; A positive number.
944
945 client-first-message-bare =
946 [reserved-mext ","] ../scram/scram.go:239
947 username "," nonce ["," extensions]
948
949 client-first-message = 5802:910 ../scram/scram.go:171
950 gs2-header client-first-message-bare
951
952
953
954Newman, et al. Standards Track [Page 17]
955
956RFC 5802 SCRAM July 2010
957
958
959 server-first-message = ../scram/scram.go:262 ../scram/scram.go:422
960 [reserved-mext ","] nonce "," salt ","
961 iteration-count ["," extensions]
962
963 client-final-message-without-proof =
964 channel-binding "," nonce [","
965 extensions]
966
967 client-final-message =
968 client-final-message-without-proof "," proof
969
970 server-error = "e=" server-error-value
971
972 server-error-value = "invalid-encoding" /
973 "extensions-not-supported" / ; unrecognized 'm' value ../scram/scram.go:241 ../scram/scram.go:424
974 "invalid-proof" /
975 "channel-bindings-dont-match" /
976 "server-does-support-channel-binding" /
977 ; server does not support channel binding
978 "channel-binding-not-supported" /
979 "unsupported-channel-binding-type" /
980 "unknown-user" /
981 "invalid-username-encoding" /
982 ; invalid username encoding (invalid UTF-8 or
983 ; SASLprep failed)
984 "no-resources" /
985 "other-error" /
986 server-error-value-ext
987 ; Unrecognized errors should be treated as "other-error".
988 ; In order to prevent information disclosure, the server
989 ; may substitute the real reason with "other-error".
990
991 server-error-value-ext = value
992 ; Additional error reasons added by extensions
993 ; to this document.
994
995 server-final-message = (server-error / verifier)
996 ["," extensions]
997
998 extensions = attr-val *("," attr-val)
999 ;; All extensions are optional,
1000 ;; i.e., unrecognized attributes
1001 ;; not defined in this document
1002 ;; MUST be ignored.
1003
1004 cbind-data = 1*OCTET
1005
1006
1007
1008
1009
1010Newman, et al. Standards Track [Page 18]
1011
1012RFC 5802 SCRAM July 2010
1013
1014
1015 cbind-input = gs2-header [ cbind-data ] 5802:925 ../scram/scram.go:454
1016 ;; cbind-data MUST be present for
1017 ;; gs2-cbind-flag of "p" and MUST be absent
1018 ;; for "y" or "n".
1019
10208. SCRAM as a GSS-API Mechanism
1021
1022 This section and its sub-sections and all normative references of it
1023 not referenced elsewhere in this document are INFORMATIONAL for SASL
1024 implementors, but they are NORMATIVE for GSS-API implementors.
1025
1026 SCRAM is actually also a GSS-API mechanism. The messages are the
1027 same, but a) the GS2 header on the client's first message and channel
1028 binding data is excluded when SCRAM is used as a GSS-API mechanism,
1029 and b) the RFC2743 section 3.1 initial context token header is
1030 prefixed to the client's first authentication message (context
1031 token).
1032
1033 The GSS-API mechanism OID for SCRAM-SHA-1 is 1.3.6.1.5.5.14 (see
1034 Section 10).
1035
1036 SCRAM security contexts always have the mutual_state flag
1037 (GSS_C_MUTUAL_FLAG) set to TRUE. SCRAM does not support credential
1038 delegation, therefore SCRAM security contexts alway have the
1039 deleg_state flag (GSS_C_DELEG_FLAG) set to FALSE.
1040
10418.1. GSS-API Principal Name Types for SCRAM
1042
1043 SCRAM does not explicitly name acceptor principals. However, the use
1044 of acceptor principal names to find or prompt for passwords is
1045 useful. Therefore, SCRAM supports standard generic name syntaxes for
1046 acceptors such as GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section
1047 4.1). Implementations should use the target name passed to
1048 GSS_Init_sec_context(), if any, to help retrieve or prompt for SCRAM
1049 passwords.
1050
1051 SCRAM supports only a single name type for initiators:
1052 GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for
1053 SCRAM.
1054
1055 There is no name canonicalization procedure for SCRAM beyond applying
1056 SASLprep as described in Section 5.1.
1057
1058 The query, display, and exported name syntaxes for SCRAM principal
1059 names are all the same. There are no SCRAM-specific name syntaxes
1060 (SCRAM initiator principal names are free-form); -- applications
1061 should use generic GSS-API name types such as GSS_C_NT_USER_NAME and
1062
1063
1064
1065
1066Newman, et al. Standards Track [Page 19]
1067
1068RFC 5802 SCRAM July 2010
1069
1070
1071 GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section 4). The exported
1072 name token does, of course, conform to [RFC2743], Section 3.2, but
1073 the "NAME" part of the token is just a SCRAM user name.
1074
10758.2. GSS-API Per-Message Tokens for SCRAM
1076
1077 The per-message tokens for SCRAM as a GSS-API mechanism SHALL be the
1078 same as those for the Kerberos V GSS-API mechanism [RFC4121] (see
1079 Section 4.2 and sub-sections), using the Kerberos V "aes128-cts-hmac-
1080 sha1-96" enctype [RFC3962].
1081
1082 The replay_det_state (GSS_C_REPLAY_FLAG), sequence_state
1083 (GSS_C_SEQUENCE_FLAG), conf_avail (GSS_C_CONF_FLAG) and integ_avail
1084 (GSS_C_CONF_FLAG) security context flags are always set to TRUE.
1085
1086 The 128-bit session "protocol key" SHALL be derived by using the
1087 least significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API
1088 session key" || ClientKey || AuthMessage). "Specific keys" are then
1089 derived as usual as described in Section 2 of [RFC4121], [RFC3961],
1090 and [RFC3962].
1091
1092 The terms "protocol key" and "specific key" are Kerberos V5 terms
1093 [RFC3961].
1094
1095 SCRAM does support PROT_READY, and is PROT_READY on the initiator
1096 side first upon receipt of the server's reply to the initial security
1097 context token.
1098
10998.3. GSS_Pseudo_random() for SCRAM
1100
1101 The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for
1102 the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor-
1103 asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and
1104 GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random().
1105 The protocol key to be used for the GSS_Pseudo_random() SHALL be the
1106 same as the key defined in Section 8.2.
1107
11089. Security Considerations
1109
1110 If the authentication exchange is performed without a strong security
1111 layer (such as TLS with data confidentiality), then a passive
1112 eavesdropper can gain sufficient information to mount an offline
1113 dictionary or brute-force attack that can be used to recover the
1114 user's password. The amount of time necessary for this attack
1115 depends on the cryptographic hash function selected, the strength of
1116 the password, and the iteration count supplied by the server. An
1117 external security layer with strong encryption will prevent this
1118 attack.
1119
1120
1121
1122Newman, et al. Standards Track [Page 20]
1123
1124RFC 5802 SCRAM July 2010
1125
1126
1127 If the external security layer used to protect the SCRAM exchange
1128 uses an anonymous key exchange, then the SCRAM channel binding
1129 mechanism can be used to detect a man-in-the-middle attack on the
1130 security layer and cause the authentication to fail as a result.
1131 However, the man-in-the-middle attacker will have gained sufficient
1132 information to mount an offline dictionary or brute-force attack.
1133 For this reason, SCRAM allows to increase the iteration count over
1134 time. (Note that a server that is only in possession of "StoredKey"
1135 and "ServerKey" can't automatically increase the iteration count upon
1136 successful authentication. Such an increase would require resetting
1137 the user's password.)
1138
1139 If the authentication information is stolen from the authentication
1140 database, then an offline dictionary or brute-force attack can be
1141 used to recover the user's password. The use of salt mitigates this
1142 attack somewhat by requiring a separate attack on each password.
1143 Authentication mechanisms that protect against this attack are
1144 available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is
1145 an example of such technology. The WG elected not to use EKE like
1146 mechanisms as a basis for SCRAM.
1147
1148 If an attacker obtains the authentication information from the
1149 authentication repository and either eavesdrops on one authentication
1150 exchange or impersonates a server, the attacker gains the ability to
1151 impersonate that user to all servers providing SCRAM access using the
1152 same hash function, password, iteration count, and salt. For this
1153 reason, it is important to use randomly generated salt values.
1154
1155 SCRAM does not negotiate a hash function to use. Hash function
1156 negotiation is left to the SASL mechanism negotiation. It is
1157 important that clients be able to sort a locally available list of
1158 mechanisms by preference so that the client may pick the appropriate
1159 mechanism to use from a server's advertised mechanism list. This
1160 preference order is not specified here as it is a local matter. The
1161 preference order should include objective and subjective notions of
1162 mechanism cryptographic strength (e.g., SCRAM with a successor to
1163 SHA-1 may be preferred over SCRAM with SHA-1).
1164
1165 Note that to protect the SASL mechanism negotiation applications
1166 normally must list the server mechanisms twice: once before and once
1167 after authentication, the latter using security layers. Since SCRAM
1168 does not provide security layers, the only ways to protect the
1169 mechanism negotiation are a) use channel binding to an external
1170 channel, or b) use an external channel that authenticates a user-
1171 provided server name.
1172
1173
1174
1175
1176
1177
1178Newman, et al. Standards Track [Page 21]
1179
1180RFC 5802 SCRAM July 2010
1181
1182
1183 SCRAM does not protect against downgrade attacks of channel binding
1184 types. The complexities of negotiating a channel binding type, and
1185 handling down-grade attacks in that negotiation, were intentionally
1186 left out of scope for this document.
1187
1188 A hostile server can perform a computational denial-of-service attack
1189 on clients by sending a big iteration count value.
1190
1191 See [RFC4086] for more information about generating randomness.
1192
119310. IANA Considerations
1194
1195 IANA has added the following family of SASL mechanisms to the SASL
1196 Mechanism registry established by [RFC4422]:
1197
1198 To: iana@iana.org
1199 Subject: Registration of a new SASL family SCRAM
1200
1201 SASL mechanism name (or prefix for the family): SCRAM-*
1202 Security considerations: Section 7 of [RFC5802]
1203 Published specification (optional, recommended): [RFC5802]
1204 Person & email address to contact for further information:
1205 IETF SASL WG <sasl@ietf.org>
1206 Intended usage: COMMON
1207 Owner/Change controller: IESG <iesg@ietf.org>
1208 Note: Members of this family MUST be explicitly registered
1209 using the "IETF Review" [RFC5226] registration procedure.
1210 Reviews MUST be requested on the SASL mailing list
1211 <sasl@ietf.org> (or a successor designated by the responsible
1212 Security AD).
1213
1214 Note to future SCRAM-mechanism designers: each new SCRAM-SASL
1215 mechanism MUST be explicitly registered with IANA and MUST comply
1216 with SCRAM-mechanism naming convention defined in Section 4 of this
1217 document.
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234Newman, et al. Standards Track [Page 22]
1235
1236RFC 5802 SCRAM July 2010
1237
1238
1239 IANA has added the following entries to the SASL Mechanism registry
1240 established by [RFC4422]:
1241
1242 To: iana@iana.org
1243 Subject: Registration of a new SASL mechanism SCRAM-SHA-1
1244
1245 SASL mechanism name (or prefix for the family): SCRAM-SHA-1
1246 Security considerations: Section 7 of [RFC5802]
1247 Published specification (optional, recommended): [RFC5802]
1248 Person & email address to contact for further information:
1249 IETF SASL WG <sasl@ietf.org>
1250 Intended usage: COMMON
1251 Owner/Change controller: IESG <iesg@ietf.org>
1252 Note:
1253
1254 To: iana@iana.org
1255 Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS
1256
1257 SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS
1258 Security considerations: Section 7 of [RFC5802]
1259 Published specification (optional, recommended): [RFC5802]
1260 Person & email address to contact for further information:
1261 IETF SASL WG <sasl@ietf.org>
1262 Intended usage: COMMON
1263 Owner/Change controller: IESG <iesg@ietf.org>
1264 Note:
1265
1266 Per this document, IANA has assigned a GSS-API mechanism OID for
1267 SCRAM-SHA-1 from the iso.org.dod.internet.security.mechanisms prefix
1268 (see "SMI Security for Mechanism Codes" registry).
1269
127011. Acknowledgements
1271
1272 This document benefited from discussions on the SASL WG mailing list.
1273 The authors would like to specially thank Dave Cridland, Simon
1274 Josefsson, Jeffrey Hutzelman, Kurt Zeilenga, Pasi Eronen, Ben
1275 Campbell, Peter Saint-Andre, and Tobias Markmann for their
1276 contributions to this document. A special thank you to Simon
1277 Josefsson for shepherding this document and for doing one of the
1278 first implementations of this specification.
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290Newman, et al. Standards Track [Page 23]
1291
1292RFC 5802 SCRAM July 2010
1293
1294
129512. References
1296
129712.1. Normative References
1298
1299 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
1300 Hashing for Message Authentication", RFC 2104,
1301 February 1997.
1302
1303 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1304 Requirement Levels", BCP 14, RFC 2119, March 1997.
1305
1306 [RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
1307 (SHA1)", RFC 3174, September 2001.
1308
1309 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
1310 Internationalized Strings ("stringprep")", RFC 3454,
1311 December 2002.
1312
1313 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
1314 10646", STD 63, RFC 3629, November 2003.
1315
1316 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
1317 and Passwords", RFC 4013, February 2005.
1318
1319 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
1320 Security Layer (SASL)", RFC 4422, June 2006.
1321
1322 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
1323 Encodings", RFC 4648, October 2006.
1324
1325 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
1326 Channels", RFC 5056, November 2007.
1327
1328 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
1329 Specifications: ABNF", STD 68, RFC 5234, January 2008.
1330
1331 [RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
1332 for TLS", RFC 5929, July 2010.
1333
133412.2. Normative References for GSS-API Implementors
1335
1336 [RFC2743] Linn, J., "Generic Security Service Application Program
1337 Interface Version 2, Update 1", RFC 2743, January 2000.
1338
1339 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
1340 Kerberos 5", RFC 3961, February 2005.
1341
1342
1343
1344
1345
1346Newman, et al. Standards Track [Page 24]
1347
1348RFC 5802 SCRAM July 2010
1349
1350
1351 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
1352 Encryption for Kerberos 5", RFC 3962, February 2005.
1353
1354 [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
1355 Version 5 Generic Security Service Application Program
1356 Interface (GSS-API) Mechanism: Version 2", RFC 4121,
1357 July 2005.
1358
1359 [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
1360 Extension for the Generic Security Service Application
1361 Program Interface (GSS-API)", RFC 4401, February 2006.
1362
1363 [RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the
1364 Kerberos V Generic Security Service Application Program
1365 Interface (GSS-API) Mechanism", RFC 4402, February 2006.
1366
1367 [RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
1368 Service Application Program Interface (GSS-API) Mechanisms
1369 in Simple Authentication and Security Layer (SASL): The
1370 GS2 Mechanism Family", RFC 5801, July 2010.
1371
137212.3. Informative References
1373
1374 [CRAMHISTORIC]
1375 Zeilenga, K., "CRAM-MD5 to Historic", Work in Progress,
1376 November 2008.
1377
1378 [DIGESTHISTORIC]
1379 Melnikov, A., "Moving DIGEST-MD5 to Historic", Work
1380 in Progress, July 2008.
1381
1382 [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
1383 "Remote Authentication Dial In User Service (RADIUS)",
1384 RFC 2865, June 2000.
1385
1386 [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
1387 Specification Version 2.0", RFC 2898, September 2000.
1388
1389 [RFC2945] Wu, T., "The SRP Authentication and Key Exchange System",
1390 RFC 2945, September 2000.
1391
1392 [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
1393 Requirements for Security", BCP 106, RFC 4086, June 2005.
1394
1395 [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol
1396 (LDAP): Technical Specification Road Map", RFC 4510,
1397 June 2006.
1398
1399
1400
1401
1402Newman, et al. Standards Track [Page 25]
1403
1404RFC 5802 SCRAM July 2010
1405
1406
1407 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and
1408 Security Layer (SASL) Mechanism", RFC 4616, August 2006.
1409
1410 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
1411 RFC 4949, August 2007.
1412
1413 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
1414 IANA Considerations Section in RFCs", BCP 26, RFC 5226,
1415 May 2008.
1416
1417 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
1418 (TLS) Protocol Version 1.2", RFC 5246, August 2008.
1419
1420 [RFC5803] Melnikov, A., "Lightweight Directory Access Protocol
1421 (LDAP) Schema for Storing Salted Challenge Response
1422 Authentication Mechanism (SCRAM) Secrets", RFC 5803,
1423 July 2010.
1424
1425 [tls-server-end-point]
1426 IANA, "Registration of TLS server end-point channel
1427 bindings", available from http://www.iana.org, June 2008.
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
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1450
1451
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1455
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1457
1458Newman, et al. Standards Track [Page 26]
1459
1460RFC 5802 SCRAM July 2010
1461
1462
1463Appendix A. Other Authentication Mechanisms
1464
1465 The DIGEST-MD5 [DIGESTHISTORIC] mechanism has proved to be too
1466 complex to implement and test, and thus has poor interoperability.
1467 The security layer is often not implemented, and almost never used;
1468 everyone uses TLS instead. For a more complete list of problems with
1469 DIGEST-MD5 that led to the creation of SCRAM, see [DIGESTHISTORIC].
1470
1471 The CRAM-MD5 SASL mechanism, while widely deployed, also has some
1472 problems. In particular, it is missing some modern SASL features
1473 such as support for internationalized usernames and passwords,
1474 support for passing of authorization identity, and support for
1475 channel bindings. It also doesn't support server authentication.
1476 For a more complete list of problems with CRAM-MD5, see
1477 [CRAMHISTORIC].
1478
1479 The PLAIN [RFC4616] SASL mechanism allows a malicious server or
1480 eavesdropper to impersonate the authenticating user to any other
1481 server for which the user has the same password. It also sends the
1482 password in the clear over the network, unless TLS is used. Server
1483 authentication is not supported.
1484
1485Appendix B. Design Motivations
1486
1487 The following design goals shaped this document. Note that some of
1488 the goals have changed since the initial version of the document.
1489
1490 o The SASL mechanism has all modern SASL features: support for
1491 internationalized usernames and passwords, support for passing of
1492 authorization identity, and support for channel bindings.
1493
1494 o The protocol supports mutual authentication.
1495
1496 o The authentication information stored in the authentication
1497 database is not sufficient by itself to impersonate the client.
1498
1499 o The server does not gain the ability to impersonate the client to
1500 other servers (with an exception for server-authorized proxies),
1501 unless such other servers allow SCRAM authentication and use the
1502 same salt and iteration count for the user.
1503
1504 o The mechanism is extensible, but (hopefully) not over-engineered
1505 in this respect.
1506
1507 o The mechanism is easier to implement than DIGEST-MD5 in both
1508 clients and servers.
1509
1510
1511
1512
1513
1514Newman, et al. Standards Track [Page 27]
1515
1516RFC 5802 SCRAM July 2010
1517
1518
1519Authors' Addresses
1520
1521 Chris Newman
1522 Oracle
1523 800 Royal Oaks
1524 Monrovia, CA 91016
1525 USA
1526
1527 EMail: chris.newman@oracle.com
1528
1529
1530 Abhijit Menon-Sen
1531 Oryx Mail Systems GmbH
1532
1533 EMail: ams@toroid.org
1534
1535
1536 Alexey Melnikov
1537 Isode, Ltd.
1538
1539 EMail: Alexey.Melnikov@isode.com
1540
1541
1542 Nicolas Williams
1543 Oracle
1544 5300 Riata Trace Ct
1545 Austin, TX 78727
1546 USA
1547
1548 EMail: Nicolas.Williams@oracle.com
1549
1550
1551
1552
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1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570Newman, et al. Standards Track [Page 28]
1571
1572