7 INTERNET-DRAFT P. Leach
8 Obsoletes: 2831 Microsoft
9 Intended category: Standards track C. Newman
15 Using Digest Authentication as a SASL Mechanism
16 draft-ietf-sasl-rfc2831bis-00.txt
20 This document is an Internet-Draft and is in full conformance with
21 all provisions of Section 10 of RFC 2026.
23 Internet-Drafts are working documents of the Internet Engineering
24 Task Force (IETF), its areas, and its working groups. Note that other
25 groups may also distribute working documents as Internet-Drafts.
27 Internet-Drafts are draft documents valid for a maximum of six months
28 and may be updated, replaced, or obsoleted by other documents at any
29 time. It is inappropriate to use Internet-Drafts as reference
30 material or to cite them other than as "work in progress."
32 The list of current Internet-Drafts can be accessed at
33 http://www.ietf.org/ietf/1id-abstracts.txt
35 The list of Internet-Draft Shadow Directories can be accessed at
36 http://www.ietf.org/shadow.html.
40 Copyright (C) The Internet Society (2003). All Rights Reserved.
44 This specification defines how HTTP Digest Authentication [Digest]
45 can be used as a SASL [RFC 2222] mechanism for any protocol that has
46 a SASL profile. It is intended both as an improvement over CRAM-MD5
47 [RFC 2195] and as a convenient way to support a single authentication
48 mechanism for web, mail, LDAP, and other protocols.
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64 INTERNET DRAFT Digest SASL Mechanism February 2003
69 1 INTRODUCTION.....................................................3
70 1.1 CONVENTIONS AND NOTATION......................................3
71 1.2 REQUIREMENTS..................................................4
72 2 AUTHENTICATION...................................................5
73 2.1 INITIAL AUTHENTICATION........................................5
74 2.1.1 Step One...................................................5
75 2.1.2 Step Two...................................................9
76 2.1.3 Step Three................................................15
77 2.2 SUBSEQUENT AUTHENTICATION....................................16
78 2.2.1 Step one..................................................16
79 2.2.2 Step Two..................................................16
80 2.3 INTEGRITY PROTECTION.........................................17
81 2.4 CONFIDENTIALITY PROTECTION...................................17
82 3 SECURITY CONSIDERATIONS.........................................19
83 3.1 AUTHENTICATION OF CLIENTS USING DIGEST AUTHENTICATION........19
84 3.2 COMPARISON OF DIGEST WITH PLAINTEXT PASSWORDS................19
85 3.3 REPLAY ATTACKS...............................................19
86 3.4 ONLINE DICTIONARY ATTACKS....................................19
87 3.5 OFFLINE DICTIONARY ATTACKS...................................20
88 3.6 MAN IN THE MIDDLE............................................20
89 3.7 CHOSEN PLAINTEXT ATTACKS.....................................20
90 3.8 SPOOFING BY COUNTERFEIT SERVERS..............................21
91 3.9 STORING PASSWORDS............................................21
92 3.10 MULTIPLE REALMS.............................................21
93 3.11 SUMMARY.....................................................22
94 4 EXAMPLE.........................................................22
95 5 REFERENCES......................................................24
96 5.1 NORMATIVE REFERENCES.........................................24
97 5.2 INFORMATIVE REFERENCES.......................................25
98 6 AUTHORS' ADDRESSES..............................................26
99 7 ABNF............................................................27
100 7.1 AUGMENTED BNF................................................27
101 7.2 BASIC RULES..................................................29
102 8 SAMPLE CODE.....................................................31
103 9 INTEROPERABILITY CONSIDERATIONS.................................32
104 9.1 Implementing DES cipher in CBC mode..........................32
105 10 ACKNOWLEDGEMENTS..............................................32
106 11 FULL COPYRIGHT STATEMENT.......................................33
107 Appendix A: Changes from 2831.....................................34
108 Appendix B: Open Issues...........................................34
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129 This specification describes the use of HTTP Digest Access
130 Authentication as a SASL mechanism. The authentication type
131 associated with the Digest SASL mechanism is "DIGEST-MD5".
133 This specification is intended to be upward compatible with the
134 "md5-sess" algorithm of HTTP/1.1 Digest Access Authentication
135 specified in [Digest]. The only difference in the "md5-sess"
136 algorithm is that some directives not needed in a SASL mechanism have
137 had their values defaulted.
139 There is one new feature for use as a SASL mechanism: integrity
140 protection on application protocol messages after an authentication
143 Also, compared to CRAM-MD5, DIGEST-MD5 prevents chosen plaintext
144 attacks, and permits the use of third party authentication servers,
145 mutual authentication, and optimized reauthentication if a client has
146 recently authenticated to a server.
148 1.1 Conventions and Notation
150 This specification uses the same ABNF notation and lexical
151 conventions as HTTP/1.1 specification; see section 7.
153 Let { a, b, ... } be the concatenation of the octet strings a, b, ...
155 Let ** denote the power operation.
157 Let H(s) be the 16 octet MD5 hash [RFC 1321] of the octet string s.
159 Let KD(k, s) be H({k, ":", s}), i.e., the 16 octet hash of the string
160 k, a colon and the string s.
162 Let HEX(n) be the representation of the 16 octet MD5 hash n as a
163 string of 32 hex digits (with alphabetic characters always in lower
164 case, since MD5 is case sensitive).
166 Let HMAC(k, s) be the 16 octet HMAC-MD5 [RFC 2104] of the octet
167 string s using the octet string k as a key.
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187 Let unq(X) be the value of the quoted-string X without the
188 surrounding quotes and with all escape characters "\\" removed. For
189 example for the quoted-string "Babylon" the value of unq("Babylon")
190 is Babylon; for the quoted string "ABC\"123\\" the value of
191 unq("ABC\"123\\") is ABC"123\.
193 The value of a quoted string constant as an octet string does not
194 include any terminating null character.
198 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
199 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
200 document are to be interpreted as described in RFC 2119 [RFC 2119].
202 An implementation is not compliant if it fails to satisfy one or more
203 of the MUST level requirements for the protocols it implements. An
204 implementation that satisfies all the MUST level and all the SHOULD
205 level requirements for its protocols is said to be "unconditionally
206 compliant"; one that satisfies all the MUST level requirements but
207 not all the SHOULD level requirements for its protocols is said to be
208 "conditionally compliant."
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249 The following sections describe how to use Digest as a SASL
250 authentication mechanism.
252 2.1 Initial Authentication
254 If the client has not recently authenticated to the server, then it
255 must perform "initial authentication", as defined in this section. If
256 it has recently authenticated, then a more efficient form is
257 available, defined in the next section.
261 The server starts by sending a challenge. The data encoded in the
262 challenge contains a string formatted according to the rules for a
263 "digest-challenge" defined as follows:
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308 1#( realm | nonce | qop-options | stale | maxbuf | charset
309 algorithm | cipher-opts | auth-param )
311 realm = "realm" "=" <"> realm-value <">
312 realm-value = qdstr-val
313 nonce = "nonce" "=" <"> nonce-value <">
314 nonce-value = *qdtext
315 qop-options = "qop" "=" <"> qop-list <">
316 qop-list = 1#qop-value
317 qop-value = "auth" | "auth-int" | "auth-conf" |
319 stale = "stale" "=" "true"
320 maxbuf = "maxbuf" "=" maxbuf-value
321 maxbuf-value = 1*DIGIT
322 charset = "charset" "=" "utf-8"
323 algorithm = "algorithm" "=" "md5-sess"
324 cipher-opts = "cipher" "=" <"> 1#cipher-value <">
325 cipher-value = "3des" | "des" | "rc4-40" | "rc4" |
327 auth-param = token "=" ( token | quoted-string )
329 The meanings of the values of the directives used above are as
333 Mechanistically, a string which can enable users to know which
334 username and password to use, in case they might have different
335 ones for different servers. Conceptually, it is the name of a
336 collection of accounts that might include the user's account. This
337 string should contain at least the name of the host performing the
338 authentication and might additionally indicate the collection of
339 users who might have access. An example might be
340 "registered_users@gotham.news.example.com". This directive is
341 optional; if not present, the client SHOULD solicit it from the
342 user or be able to compute a default; a plausible default might be
343 the realm supplied by the user when they logged in to the client
344 system. Multiple realm directives are allowed, in which case the
345 user or client must choose one as the realm for which to supply to
346 username and password.
349 A server-specified data string which MUST be different each time a
350 digest-challenge is sent as part of initial authentication. It is
351 recommended that this string be base64 or hexadecimal data. Note
352 that since the string is passed as a quoted string, the
353 double-quote character is not allowed unless escaped (see section
354 7.2). The contents of the nonce are implementation dependent. The
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367 security of the implementation depends on a good choice. It is
368 RECOMMENDED that it contain at least 64 bits of entropy. The nonce
369 is opaque to the client. This directive is required and MUST
370 appear exactly once; if not present, or if multiple instances are
371 present, the client should abort the authentication exchange.
374 A quoted string of one or more tokens indicating the "quality of
375 protection" values supported by the server. The value "auth"
376 indicates authentication; the value "auth-int" indicates
377 authentication with integrity protection; the value "auth-conf"
378 indicates authentication with integrity protection and encryption.
379 This directive is optional; if not present it defaults to "auth".
380 The client MUST ignore unrecognized options; if the client
381 recognizes no option, it should abort the authentication exchange.
384 The "stale" directive is not used in initial authentication. See
385 the next section for its use in subsequent authentications. This
386 directive may appear at most once; if multiple instances are
387 present, the client should abort the authentication exchange.
389 maxbuf ("maximal ciphertext buffer size")
390 A number indicating the size of the largest buffer the server is
391 able to receive when using "auth-int" or "auth-conf". The value
392 MUST be bigger than 16 and smaller or equal to 16777215 (i.e.
393 2**24-1). If this directive is missing, the default value is
394 65536. This directive may appear at most once; if multiple
395 instances are present, the client should abort the authentication
398 Let call "maximal cleartext buffer size" (or "maximal sender
399 size") the maximal size of a cleartext buffer that, after being
400 transformed by integrity (section 2.3) or confidentiality (section
401 2.4) protection function, will produce a SASL block of the maxbuf
402 size. The "maximal sender size" for the client can be calculated
403 by subtracting 16 from the maxbuf value. As it should be clear
404 from the name, the sender MUST never pass a block of data bigger
405 than the "maximal sender size" through the selected protection
406 function. This will guaranty that the receiver will never get a
407 block bigger than the maxbuf.
410 This directive, if present, specifies that the server supports
411 UTF-8 [UTF-8] encoding for the username, realm and password. If
412 present, the username, realm and password MUST be in Unicode
413 Normalization Form KC [UNICODE-NORMALIZATION] (without NUL
414 character) encoded as UTF-8 [UTF-8]. If not present, the username,
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427 realm and password must be encoded in ISO 8859-1 [ISO-8859] (of
428 which US-ASCII [USASCII] is a subset). The directive is needed for
429 backwards compatibility with HTTP Digest, which only supports ISO
430 8859-1. This directive may appear at most once; if multiple
431 instances are present, the client should abort the authentication
434 Note, that this directive doesn't affect authorization id
435 ("authzid") which is always in UTF-8.
438 This directive is required for backwards compatibility with HTTP
439 Digest, which supports other algorithms. This directive is
440 required and MUST appear exactly once; if not present, or if
441 multiple instances are present, the client should abort the
442 authentication exchange.
445 A list of ciphers that the server supports. This directive must be
446 present exactly once if "auth-conf" is offered in the
447 "qop-options" directive, in which case the "3des" cipher is
448 mandatory-to-implement. The client MUST ignore unrecognized
449 options; if the client recognizes no option, it should abort the
450 authentication exchange.
453 the Data Encryption Standard (DES) cipher [FIPS] in cipher
454 block chaining (CBC) mode with a 56 bit key.
457 the "triple DES" cipher in CBC mode with EDE with the same key
458 for each E stage (aka "two keys mode") for a total key length
462 the RC4 cipher with a 128 bit, 40 bit, and 56 bit key,
465 auth-param This construct allows for future extensions; it may appear
466 more than once. The client MUST ignore any unrecognized
469 For use as a SASL mechanism, note that the following changes are made
470 to "digest-challenge" from HTTP: the following Digest options (called
471 "directives" in HTTP terminology) are unused (i.e., MUST NOT be sent,
472 and MUST be ignored if received):
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489 The size of a digest-challenge MUST be less than 2048 bytes.
493 The client makes note of the "digest-challenge" and then responds
494 with a string formatted and computed according to the rules for a
495 "digest-response" defined as follows:
497 digest-response = 1#( username | realm | nonce | cnonce |
498 nonce-count | qop | digest-uri | response |
499 maxbuf | charset | cipher | authzid |
502 username = "username" "=" <"> username-value <">
503 username-value = qdstr-val
504 cnonce = "cnonce" "=" <"> cnonce-value <">
505 cnonce-value = *qdtext
506 nonce-count = "nc" "=" nc-value
508 qop = "qop" "=" qop-value
509 digest-uri = "digest-uri" "=" <"> digest-uri-value <">
510 digest-uri-value = serv-type "/" host [ "/" serv-name ]
513 response = "response" "=" response-value
514 response-value = 32LHEX
515 LHEX = "0" | "1" | "2" | "3" |
516 "4" | "5" | "6" | "7" |
517 "8" | "9" | "a" | "b" |
518 "c" | "d" | "e" | "f"
519 cipher = "cipher" "=" cipher-value
520 authzid = "authzid" "=" <"> authzid-value <">
521 authzid-value = qdstr-val
523 The 'host' non-terminal is defined in [RFC 2732] as
525 host = hostname | IPv4address | IPv6reference
526 ipv6reference = "[" IPv6address "]"
528 where IPv6address and IPv4address are defined in [RFC 2373]
529 and 'hostname' is defined in [RFC 2396].
532 The user's name in the specified realm, encoded according to the
533 value of the "charset" directive. This directive is required and
534 MUST be present exactly once; otherwise, authentication fails.
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548 The realm containing the user's account, encoded according to the
549 value of the "charset" directive. This directive is required if
550 the server provided any realms in the
551 "digest-challenge", in which case it may appear exactly once and
552 its value SHOULD be one of those realms. If the directive is
553 missing, "realm-value" will set to the empty string when computing
554 A1 (see below for details).
557 The server-specified data string received in the preceding digest-
558 challenge. This directive is required and MUST be present exactly
559 once; otherwise, authentication fails.
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608 A client-specified data string which MUST be different each time a
609 digest-response is sent as part of initial authentication. The
610 cnonce-value is an opaque quoted string value provided by the
611 client and used by both client and server to avoid chosen
612 plaintext attacks, and to provide mutual authentication. The
613 security of the implementation depends on a good choice. It is
614 RECOMMENDED that it contain at least 64 bits of entropy. This
615 directive is required and MUST be present exactly once; otherwise,
616 authentication fails.
619 The nc-value is the hexadecimal count of the number of requests
620 (including the current request) that the client has sent with the
621 nonce value in this request. For example, in the first request
622 sent in response to a given nonce value, the client sends
623 "nc=00000001". The purpose of this directive is to allow the
624 server to detect request replays by maintaining its own copy of
625 this count - if the same nc-value is seen twice, then the request
626 is a replay. See the description below of the construction of the
627 response value. This directive is required and MUST be present
628 exactly once; otherwise, authentication fails.
631 Indicates what "quality of protection" the client accepted. If
632 present, it may appear exactly once and its value MUST be one of
633 the alternatives in qop-options. If not present, it defaults to
634 "auth". These values affect the computation of the response. Note
635 that this is a single token, not a quoted list of alternatives.
638 Indicates the type of service, such as "http" for web service,
639 "ftp" for FTP service, "smtp" for mail delivery service, etc. The
640 service name as defined in the SASL profile for the protocol see
641 section 4 of [RFC 2222], registered in the IANA registry of
642 "service" elements for the GSSAPI host-based service name form
646 The DNS host name or IP (IPv4 or IPv6) address for the service
647 requested. The DNS host name must be the fully-qualified
648 canonical name of the host. The DNS host name is the preferred
649 form; see notes on server processing of the digest-uri.
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668 Indicates the name of the service if it is replicated. The service
669 is considered to be replicated if the client's service-location
670 process involves resolution using standard DNS lookup operations,
671 and if these operations involve DNS records (such as SRV [RFC
672 2052], or MX) which resolve one DNS name into a set of other DNS
673 names. In this case, the initial name used by the client is the
674 "serv-name", and the final name is the "host" component. For
675 example, the incoming mail service for "example.com" may be
676 replicated through the use of MX records stored in the DNS, one of
677 which points at an SMTP server called "mail3.example.com"; it's
678 "serv-name" would be "example.com", it's "host" would be
679 "mail3.example.com". If the service is not replicated, or the
680 serv-name is identical to the host, then the serv-name component
684 Indicates the principal name of the service with which the client
685 wishes to connect, formed from the serv-type, host, and serv-name.
686 For example, the FTP service on "ftp.example.com" would have a
687 "digest-uri" value of "ftp/ftp.example.com"; the SMTP server from
688 the example above would have a "digest-uri" value of
689 "SMTP/mail3.example.com/example.com".
691 Servers SHOULD check that the supplied value is correct. This will
692 detect accidental connection to the incorrect server, as well as some
693 redirection attacks. It is also so that clients will be trained to
694 provide values that will work with implementations that use a shared
695 back-end authentication service that can provide server
698 The serv-type component should match the service being offered. The
699 host component should match one of the host names of the host on
700 which the service is running, or it's IP address. Servers SHOULD NOT
701 normally support the IP address form, because server authentication
702 by IP address is not very useful; they should only do so if the DNS
703 is unavailable or unreliable. The serv-name component should match
704 one of the service's configured service names.
706 This directive may appear at most once; if multiple instances are
707 present, the client should abort the authentication exchange.
709 Note: In the HTTP use of Digest authentication, the digest-uri is the
710 URI (usually a URL) of the resource requested -- hence the name of
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728 A string of 32 hex digits computed as defined below, which proves
729 that the user knows a password. This directive is required and
730 MUST be present exactly once; otherwise, authentication fails.
733 A number indicating the size of the largest buffer the client is
734 able to receive. If this directive is missing, the default value
735 is 65536. This directive may appear at most once; if multiple
736 instances are present, the server should abort the authentication
740 This directive, if present, specifies that the client has used
741 UTF-8 [UTF-8] encoding for the username, realm and password. If
742 present, the username, realm and password MUST be in Unicode
743 Normalization Form KC [UNICODE-NORMALIZATION] (without NUL
744 character) encoded as UTF-8 [UTF-8]. If not present, the username
745 and password must be encoded in ISO 8859-1 [ISO-8859] (of which
746 US-ASCII [USASCII] is a subset). The client should send this
747 directive only if the server has indicated it supports UTF-8
748 [UTF-8]. The directive is needed for backwards compatibility with
749 HTTP Digest, which only supports ISO 8859-1.
751 Note, that this directive doesn't affect authorization id
752 ("authzid") which is always in UTF-8.
755 32 hex digits, where the alphabetic characters MUST be lower case,
756 because MD5 is not case insensitive.
759 The cipher chosen by the client. This directive MUST appear
760 exactly once if "auth-conf" is negotiated; if required and not
761 present, authentication fails.
764 The "authorization ID" in Unicode Normalization Form KC [UNICODE-
765 NORMALIZATION] without NUL character, encoded in UTF-8 [UTF-8].
766 This directive is optional. If present, and the authenticating
767 user has sufficient privilege, and the server supports it, then
768 after authentication the server will use this identity for making
769 all accesses and access checks. If the client specifies it, and
770 the server does not support it, then the response-value calculated
771 on the server will not match the one calculated on the client and
772 authentication will fail.
774 The size of a digest-response MUST be less than 4096 bytes.
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787 2.1.2.1 Response-value
789 The definition of "response-value" above indicates the encoding for
790 its value -- 32 lower case hex characters. The following definitions
791 show how the value is computed.
793 Although qop-value and components of digest-uri-value may be
794 case-insensitive, the case which the client supplies in step two is
795 preserved for the purpose of computing and verifying the
799 HEX( KD ( HEX(H(A1)),
800 { nonce-value, ":" nc-value, ":",
801 cnonce-value, ":", qop-value, ":", HEX(H(A2)) }))
803 If authzid is specified, then A1 is
806 A1 = { H( { unq(username-value), ":", unq(realm-value), ":", passwd } ),
807 ":", nonce-value, ":", cnonce-value, ":", unq(authzid-value) }
809 If authzid is not specified, then A1 is
812 A1 = { H( { unq(username-value), ":", unq(realm-value), ":", passwd } ),
813 ":", nonce-value, ":", cnonce-value }
819 The "username-value", "realm-value" and "passwd" are encoded
820 according to the value of the "charset" directive. If "charset=UTF-8"
821 is present, and all the characters of "username-value" are, after
822 converting to Unicode Normalization Form KC [UNICODE-NORMALIZATION],
823 in the ISO 8859-1 character set, then it must be converted to ISO
824 8859-1 before being hashed. The same transformation has to be done
825 for "realm-value" and "passwd". This is so that authentication
826 databases that store the hashed username, realm and password (which
827 is common) can be shared compatibly with HTTP, which specifies ISO
828 8859-1. A sample implementation of this conversion is in section 8.
830 If the "qop" directive's value is "auth", then A2 is:
832 A2 = { "AUTHENTICATE:", digest-uri-value }
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847 If the "qop" value is "auth-int" or "auth-conf" then A2 is:
849 A2 = { "AUTHENTICATE:", digest-uri-value,
850 ":00000000000000000000000000000000" }
852 Note that "AUTHENTICATE:" must be in upper case, and the second
853 string constant is a string with a colon followed by 32 zeros.
855 These apparently strange values of A2 are for compatibility with
856 HTTP; they were arrived at by setting "Method" to "AUTHENTICATE" and
857 the hash of the entity body to zero in the HTTP digest calculation of
860 Also, in the HTTP usage of Digest, several directives in the
861 "digest-challenge" sent by the server have to be returned by the
862 client in the "digest-response". These are:
867 These directives are not needed when Digest is used as a SASL
868 mechanism (i.e., MUST NOT be sent, and MUST be ignored if received).
872 The server receives and validates the "digest-response". The server
873 checks that the nonce-count is "00000001". If it supports subsequent
874 authentication (see section 2.2), it saves the value of the nonce and
875 the nonce-count. It sends a message formatted as follows:
877 response-auth = "rspauth" "=" response-value
879 where response-value is calculated as above, using the values sent in
880 step two, except that if qop is "auth", then A2 is
882 A2 = { ":", digest-uri-value }
884 And if qop is "auth-int" or "auth-conf" then A2 is
886 A2 = { ":", digest-uri-value, ":00000000000000000000000000000000" }
888 Compared to its use in HTTP, the following Digest directives in the
889 "digest-response" are unused:
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907 2.2 Subsequent Authentication
909 If the client has previously authenticated to the server, and
910 remembers the values of username, realm, nonce, nonce-count, cnonce,
911 and qop that it used in that authentication, and the SASL profile for
912 a protocol permits an initial client response, then it MAY perform
913 "subsequent authentication", as defined in this section.
917 The client uses the values from the previous authentication and sends
918 an initial response with a string formatted and computed according to
919 the rules for a "digest-response", as defined above, but with a
920 nonce-count one greater than used in the last "digest-response".
924 The server receives the "digest-response". If the server does not
925 support subsequent authentication, then it sends a
926 "digest-challenge", and authentication proceeds as in initial
927 authentication. If the server has no saved nonce and nonce-count from
928 a previous authentication, then it sends a "digest-challenge", and
929 authentication proceeds as in initial authentication. Otherwise, the
930 server validates the "digest-response", checks that the nonce-count
931 is one greater than that used in the previous authentication using
932 that nonce, and saves the new value of nonce-count.
934 If the response is invalid, then the server sends a
935 "digest-challenge", and authentication proceeds as in initial
936 authentication (and should be configurable to log an authentication
937 failure in some sort of security audit log, since the failure may be
938 a symptom of an attack). The nonce-count MUST NOT be incremented in
939 this case: to do so would allow a denial of service attack by sending
940 an out-of-order nonce-count.
942 If the response is valid, the server MAY choose to deem that
943 authentication has succeeded. However, if it has been too long since
944 the previous authentication, or for any other reason, the server MAY
945 send a new "digest-challenge" with a new value for nonce. The
946 challenge MAY contain a "stale" directive with value "true", which
947 says that the client may respond to the challenge using the password
948 it used in the previous response; otherwise, the client must solicit
949 the password anew from the user. This permits the server to make sure
950 that the user has presented their password recently. (The directive
951 name refers to the previous nonce being stale, not to the last use of
952 the password.) Except for the handling of "stale", after sending the
953 "digest-challenge" authentication proceeds as in the case of initial
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967 2.3 Integrity Protection
969 If the server offered "qop=auth-int" and the client responded
970 "qop=auth-int", then subsequent messages, up to but not including the
971 next subsequent authentication, between the client and the server
972 MUST be integrity protected. Using as a base session key the value of
973 H(A1) as defined above the client and server calculate a pair of
974 message integrity keys as follows.
976 The key for integrity protecting messages from client to server is:
979 "Digest session key to client-to-server signing key magic constant"})
981 The key for integrity protecting messages from server to client is:
984 "Digest session key to server-to-client signing key magic constant"})
986 where MD5 is as specified in [RFC 1321]. If message integrity is
987 negotiated, a MAC block for each message is appended to the message.
988 The MAC block is 16 bytes: the first 10 bytes of the HMAC-MD5 [RFC
989 2104] of the message, a 2-byte message type number in network byte
990 order with value 1, and the 4-byte sequence number in network byte
991 order. The message type is to allow for future extensions such as
994 MAC(Ki, SeqNum, msg) = (HMAC(Ki, {SeqNum, msg})[0..9], 0x0001,
997 where Ki is Kic for messages sent by the client and Kis for those
998 sent by the server. The sequence number is an unsigned number
999 initialized to zero after initial or subsequent authentication, and
1000 incremented by one for each message sent/successfully verified.
1001 (Note, that there are two independent counters for sending and
1002 receiving.) The sequence number wraps around to 0 after 2**32-1.
1004 Upon receipt, MAC(Ki, SeqNum, msg) is computed and compared with the
1005 received value; the message is discarded if they differ. The
1006 receiver's sequence counter is incremented if they match.
1009 2.4 Confidentiality Protection
1011 If the server sent a "cipher-opts" directive and the client responded
1012 with a "cipher" directive, then subsequent messages between the
1013 client and the server MUST be confidentiality protected. Using as a
1014 base session key the value of H(A1) as defined above the client and
1018 Leach & Newman Expires: August 2003 [Page 17]
1024 INTERNET DRAFT Digest SASL Mechanism February 2003
1027 server calculate a pair of message integrity keys as follows.
1029 The key for confidentiality protecting messages from client to server
1032 Kcc = MD5({H(A1)[0..n-1],
1033 "Digest H(A1) to client-to-server sealing key magic constant"})
1035 The key for confidentiality protecting messages from server to client
1038 Kcs = MD5({H(A1)[0..n-1],
1039 "Digest H(A1) to server-to-client sealing key magic constant"})
1041 where MD5 is as specified in [RFC 1321]. For cipher "rc4-40" n is 5;
1042 for "rc4-56" n is 7; for the rest n is 16. The key for the "rc4-*"
1043 ciphers is all 16 bytes of Kcc or Kcs; the key for "des" is the first
1044 7 bytes; the key for "3des" is the first 14 bytes. The IV for "des"
1045 and "3des" is the last 8 bytes of Kcc or Kcs.
1047 The MAC block is a variable length padding prefix followed by 16
1048 bytes formatted as follows: the first 10 bytes of the HMAC-MD5 [RFC
1049 2104] of the message, a 2-byte message type number in network byte
1050 order with value 1, and the 4-byte sequence number in network byte
1051 order. If the blocksize of the chosen cipher is not 1 byte, the
1052 padding prefix is one or more octets each containing the number of
1053 padding bytes, such that total length of the encrypted part of the
1054 message is a multiple of the blocksize. The padding and first 10
1055 bytes of the MAC block are encrypted with the chosen cipher along
1058 SEAL(Ki, Kc, SeqNum, msg) =
1059 {CIPHER(Kc, {msg, pad, HMAC(Ki, {SeqNum, msg})[0..9]}), 0x0001,
1062 where CIPHER is the chosen cipher, Ki and Kc are Kic and Kcc for
1063 messages sent by the client and Kis and Kcs for those sent by the
1064 server. The sequence number is an unsigned number initialized to zero
1065 after initial or subsequent authentication, and incremented by one
1066 for each message sent/successfully verified. (Note, that there are
1067 two independent counters for sending and receiving.) The sequence
1068 number wraps around to 0 after 2**32-1.
1070 Upon receipt, the message is decrypted, HMAC(Ki, {SeqNum, msg}) is
1071 computed and compared with the received value; the message is
1072 discarded if they differ. The receiver's sequence counter is
1073 incremented if they match.
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1084 INTERNET DRAFT Digest SASL Mechanism February 2003
1087 3 Security Considerations
1089 3.1 Authentication of Clients using Digest Authentication
1091 Digest Authentication does not provide a strong authentication
1092 mechanism, when compared to public key based mechanisms, for example.
1093 However, since it prevents chosen plaintext attacks, it is stronger
1094 than (e.g.) CRAM-MD5, which has been proposed for use with ACAP [RFC
1095 2244], POP and IMAP [RFC 2195]. It is intended to replace the much
1096 weaker and even more dangerous use of plaintext passwords; however,
1097 since it is still a password based mechanism it avoids some of the
1098 potential deployabilty issues with public-key, OTP or similar
1101 Digest Authentication offers no confidentiality protection beyond
1102 protecting the actual password. All of the rest of the challenge and
1103 response are available to an eavesdropper, including the user's name
1104 and authentication realm.
1106 3.2 Comparison of Digest with Plaintext Passwords
1108 The greatest threat to the type of transactions for which these
1109 protocols are used is network snooping. This kind of transaction
1110 might involve, for example, online access to a mail service whose use
1111 is restricted to paying subscribers. With plaintext password
1112 authentication an eavesdropper can obtain the password of the user.
1113 This not only permits him to access anything in the database, but,
1114 often worse, will permit access to anything else the user protects
1115 with the same password.
1119 Replay attacks are defeated if the client or the server chooses a
1120 fresh nonce for each authentication, as this specification requires.
1122 As a security precaution, the server, when verifying a response from
1123 the client, must use the original server nonce ("nonce") it sent, not
1124 the one returned by the client in the response, as it might have been
1125 modified by an attacker.
1127 To prevent some redirection attacks it is recommended that the server
1128 verifies that the "serv-type" part of the "digest-uri" matches the
1129 service name and that the hostname/IP address belongs to the server.
1131 3.4 Online dictionary attacks
1133 If the attacker can eavesdrop, then it can test any overheard
1134 nonce/response pairs against a (potentially very large) list of
1138 Leach & Newman Expires: August 2003 [Page 19]
1144 INTERNET DRAFT Digest SASL Mechanism February 2003
1147 common words. Such a list is usually much smaller than the total
1148 number of possible passwords. The cost of computing the response for
1149 each password on the list is paid once for each challenge.
1151 The server can mitigate this attack by not allowing users to select
1152 passwords that are in a dictionary.
1154 3.5 Offline dictionary attacks
1156 If the attacker can choose the challenge, then it can precompute the
1157 possible responses to that challenge for a list of common words. Such
1158 a list is usually much smaller than the total number of possible
1159 passwords. The cost of computing the response for each password on
1160 the list is paid just once.
1162 Offline dictionary attacks are defeated if the client chooses a fresh
1163 nonce for each authentication, as this specification requires.
1165 3.6 Man in the Middle
1167 Digest authentication is vulnerable to "man in the middle" (MITM)
1168 attacks. Clearly, a MITM would present all the problems of
1169 eavesdropping. But it also offers some additional opportunities to
1172 A possible man-in-the-middle attack would be to substitute a weaker
1173 qop scheme for the one(s) sent by the server; the server will not be
1174 able to detect this attack. For this reason, the client should always
1175 use the strongest scheme that it understands from the choices
1176 offered, and should never choose a scheme that does not meet its
1177 minimum requirements.
1179 A man-in-the-middle attack may also make the client and the server
1180 that agreed to use confidentiality protection to use different (and
1181 possibly weaker) cipher's. This is because the chosen cipher is not
1182 used in the shared secret calculation.
1184 3.7 Chosen plaintext attacks
1186 A chosen plaintext attack is where a MITM or a malicious server can
1187 arbitrarily choose the challenge that the client will use to compute
1188 the response. The ability to choose the challenge is known to make
1189 cryptanalysis much easier [MD5].
1191 However, Digest does not permit the attack to choose the challenge as
1192 long as the client chooses a fresh nonce for each authentication, as
1193 this specification requires.
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1204 INTERNET DRAFT Digest SASL Mechanism February 2003
1207 3.8 Spoofing by Counterfeit Servers
1209 If a user can be led to believe that she is connecting to a host
1210 containing information protected by a password she knows, when in
1211 fact she is connecting to a hostile server, then the hostile server
1212 can obtain challenge/response pairs where it was able to partly
1213 choose the challenge. There is no known way that this can be
1216 3.9 Storing passwords
1218 Digest authentication requires that the authenticating agent (usually
1219 the server) store some data derived from the user's name and password
1220 in a "password file" associated with a given realm. Normally this
1221 might contain pairs consisting of username and H({ username-value,
1222 ":", realm-value, ":", passwd }), which is adequate to compute H(A1)
1223 as described above without directly exposing the user's password.
1225 The security implications of this are that if this password file is
1226 compromised, then an attacker gains immediate access to documents on
1227 the server using this realm. Unlike, say a standard UNIX password
1228 file, this information need not be decrypted in order to access
1229 documents in the server realm associated with this file. On the other
1230 hand, decryption, or more likely a brute force attack, would be
1231 necessary to obtain the user's password. This is the reason that the
1232 realm is part of the digested data stored in the password file. It
1233 means that if one Digest authentication password file is compromised,
1234 it does not automatically compromise others with the same username
1235 and password (though it does expose them to brute force attack).
1237 There are two important security consequences of this. First the
1238 password file must be protected as if it contained plaintext
1239 passwords, because for the purpose of accessing documents in its
1240 realm, it effectively does.
1242 A second consequence of this is that the realm string should be
1243 unique among all realms that any single user is likely to use. In
1244 particular a realm string should include the name of the host doing
1247 3.10 Multiple realms
1249 Use of multiple realms may mean both that compromise of a the
1250 security database for a single realm does not compromise all
1251 security, and that there are more things to protect in order to keep
1252 the whole system secure.
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1264 INTERNET DRAFT Digest SASL Mechanism February 2003
1269 By modern cryptographic standards Digest Authentication is weak,
1270 compared to (say) public key based mechanisms. But for a large range
1271 of purposes it is valuable as a replacement for plaintext passwords.
1272 Its strength may vary depending on the implementation.
1277 This example shows the use of the Digest SASL mechanism with the
1278 IMAP4 AUTHENTICATE command [RFC 2060].
1280 In this example, "C:" and "S:" represent a line sent by the client or
1281 server respectively including a CRLF at the end. Linebreaks and
1282 indentation within a "C:" or "S:" are editorial and not part of the
1283 protocol. The password in this example was "secret". Note that the
1284 base64 encoding of the challenges and responses is part of the IMAP4
1285 AUTHENTICATE command, not part of the Digest specification itself.
1287 S: * OK elwood.innosoft.com PMDF IMAP4rev1 V6.0-9
1289 S: * CAPABILITY IMAP4 IMAP4rev1 ACL LITERAL+ NAMESPACE QUOTA
1290 UIDPLUS AUTH=CRAM-MD5 AUTH=DIGEST-MD5 AUTH=PLAIN
1292 C: a AUTHENTICATE DIGEST-MD5
1293 S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0
1294 RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh
1296 C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2
1297 QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw
1298 MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9Im
1299 ltYXAvZWx3b29kLmlubm9zb2Z0LmNvbSIscmVzcG9uc2U9ZDM4OGRhZDkw
1300 ZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcscW9wPWF1dGg=
1301 S: + cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
1303 S: a OK User logged in
1306 The base64-decoded version of the SASL exchange is:
1308 S: realm="elwood.innosoft.com",nonce="OA6MG9tEQGm2hh",qop="auth",
1309 algorithm=md5-sess,charset=utf-8
1310 C: charset=utf-8,username="chris",realm="elwood.innosoft.com",
1311 nonce="OA6MG9tEQGm2hh",nc=00000001,cnonce="OA6MHXh6VqTrRk",
1312 digest-uri="imap/elwood.innosoft.com",
1313 response=d388dad90d4bbd760a152321f2143af7,qop=auth
1314 S: rspauth=ea40f60335c427b5527b84dbabcdfffd
1318 Leach & Newman Expires: August 2003 [Page 22]
1324 INTERNET DRAFT Digest SASL Mechanism February 2003
1327 The password in this example was "secret".
1329 This example shows the use of the Digest SASL mechanism with the
1330 ACAP, using the same notational conventions and password as in the
1331 previous example. Note that ACAP does not base64 encode and uses
1332 fewer round trips that IMAP4.
1334 S: * ACAP (IMPLEMENTATION "Test ACAP server") (SASL "CRAM-MD5"
1335 "DIGEST-MD5" "PLAIN")
1336 C: a AUTHENTICATE "DIGEST-MD5"
1338 S: realm="elwood.innosoft.com",nonce="OA9BSXrbuRhWay",qop="auth",
1339 algorithm=md5-sess,charset=utf-8
1341 C: charset=utf-8,username="chris",realm="elwood.innosoft.com",
1342 nonce="OA9BSXrbuRhWay",nc=00000001,cnonce="OA9BSuZWMSpW8m",
1343 digest-uri="acap/elwood.innosoft.com",
1344 response=6084c6db3fede7352c551284490fd0fc,qop=auth
1346 S: rspauth=2f0b3d7c3c2e486600ef710726aa2eae) "AUTHENTICATE
1350 The server uses the values of all the directives, plus knowledge of
1351 the users password (or the hash of the user's name, server's realm
1352 and the user's password) to verify the computations above. If they
1353 check, then the user has authenticated.
1378 Leach & Newman Expires: August 2003 [Page 23]
1384 INTERNET DRAFT Digest SASL Mechanism February 2003
1389 5.1 Normative references
1391 [Digest] Franks, J., et al., "HTTP Authentication: Basic and Digest
1392 Access Authentication", RFC 2617, June 1999.
1394 [ISO-8859] ISO-8859. International Standard--Information Processing--
1395 8-bit Single-Byte Coded Graphic Character Sets --
1396 Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
1397 Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
1398 Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
1399 Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
1400 Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
1401 Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
1402 Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
1403 Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
1404 Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
1406 [RFC 822] Crocker, D., "Standard for The Format of ARPA Internet
1407 Text Messages," STD 11, RFC 822, August 1982.
1409 [RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
1412 [RFC 2052] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
1413 location of services (DNS SRV)", RFC 2052, October 1996.
1415 [RFC 2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
1416 Hashing for Message Authentication", RFC 2104, February
1438 Leach & Newman Expires: August 2003 [Page 24]
1444 INTERNET DRAFT Digest SASL Mechanism February 2003
1447 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
1448 Requirement Levels", BCP 14, RFC 2119, March 1997.
1450 [RFC 2222] Myers, J., "Simple Authentication and Security Layer
1451 (SASL)", RFC 2222, October 1997.
1453 [UNICODE-NORMALIZATION] Davis, Durst, "Unicode Standard Annex #15:
1454 Unicode Normalization Forms",
1455 http://www.unicode.org/unicode/reports/tr15/, March 2001.
1457 [UTF-8] Yergeau, "UTF-8, a transformation format of ISO 10646", RFC
1460 [USASCII] US-ASCII. Coded Character Set - 7-Bit American Standard
1461 Code for Information Interchange. Standard ANSI X3.4-1986,
1464 [RFC 2732] Hinden, R., Carpenter, B., Masinter, L., "Format for
1465 Literal IPv6 Addresses in URL's", RFC 2732, December 1999.
1467 [RFC 2373] Hinden, R., Deering, S., "IP Version 6 Addressing
1468 Architecture", RFC 2373, July 1998.
1470 [RFC 2396] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform
1471 Resource Identifiers (URI): Generic Syntax", RFC 2396,
1475 5.2 Informative references
1477 [RFC 2195] Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP
1478 AUTHorize Extension for Simple Challenge/Response", RFC
1479 2195, September 1997.
1481 [MD5] Kaliski, B.,Robshaw, M., "Message Authentication with MD5",
1482 CryptoBytes, Sping 1995, RSA Inc,
1483 (http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm)
1485 [RFC 2078] Linn, J., "Generic Security Service Application Program
1486 Interface, Version 2", RFC 2078, January 1997.
1488 [RFC 2060] Crispin, M., "Internet Message Access Protocol - Version
1489 4rev1", RFC 2060, December 1996.
1491 [RFC 2244] Newman, C., Myers, J., "ACAP -- Application Configuration
1492 Access Protocol", RFC 2244, November 1997.
1498 Leach & Newman Expires: August 2003 [Page 25]
1504 INTERNET DRAFT Digest SASL Mechanism February 2003
1507 [RFC 2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1508 Masinter, L., Leach, P., Berners-Lee, T., "Hypertext
1509 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1512 6 Authors' Addresses
1519 EMail: paulle@microsoft.com
1523 Innosoft International, Inc.
1525 West Covina, CA 91790 USA
1527 EMail: chris.newman@innosoft.com
1531 ACI WorldWide/MessagingDirect
1532 59 Clarendon Road, Watford, Hertfordshire, WD17 1FQ, UK
1534 Email: mel@messagingdirect.com
1558 Leach & Newman Expires: August 2003 [Page 26]
1564 INTERNET DRAFT Digest SASL Mechanism February 2003
1569 What follows is the definition of the notation as is used in the
1570 HTTP/1.1 specification [RFC 2616] and the HTTP authentication
1571 specification [Digest]; it is reproduced here for ease of reference.
1572 Since it is intended that a single Digest implementation can support
1573 both HTTP and SASL-based protocols, the same notation is used in both
1574 to facilitate comparison and prevention of unwanted differences.
1575 Since it is cut-and-paste from the HTTP specifications, not all
1576 productions may be used in this specification. It is also not quite
1577 legal ABNF; again, the errors were copied from the HTTP
1582 All of the mechanisms specified in this document are described in
1583 both prose and an augmented Backus-Naur Form (BNF) similar to that
1584 used by RFC 822 [RFC 822]. Implementers will need to be familiar with
1585 the notation in order to understand this specification.
1587 The augmented BNF includes the following constructs:
1590 The name of a rule is simply the name itself (without any
1591 enclosing "<" and ">") and is separated from its definition by the
1592 equal "=" character. White space is only significant in that
1593 indentation of continuation lines is used to indicate a rule
1594 definition that spans more than one line. Certain basic rules are
1595 in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
1596 brackets are used within definitions whenever their presence will
1597 facilitate discerning the use of rule names.
1600 Quotation marks surround literal text. Unless stated otherwise,
1601 the text is case-insensitive.
1604 Elements separated by a bar ("|") are alternatives, e.g., "yes |
1605 no" will accept yes or no.
1608 Elements enclosed in parentheses are treated as a single element.
1609 Thus, "(elem (foo | bar) elem)" allows the token sequences
1610 "elem foo elem" and "elem bar elem".
1613 The character "*" preceding an element indicates repetition. The
1614 full form is "<n>*<m>element" indicating at least <n> and at most
1618 Leach & Newman Expires: August 2003 [Page 27]
1624 INTERNET DRAFT Digest SASL Mechanism February 2003
1627 <m> occurrences of element. Default values are 0 and infinity so
1628 that "*(element)" allows any number, including zero; "1*element"
1629 requires at least one; and "1*2element" allows one or two.
1632 Square brackets enclose optional elements; "[foo bar]" is
1633 equivalent to "*1(foo bar)".
1636 Specific repetition: "<n>(element)" is equivalent to
1637 "<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
1638 Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
1639 alphabetic characters.
1642 A construct "#" is defined, similar to "*", for defining lists of
1643 elements. The full form is "<n>#<m>element" indicating at least
1644 <n> and at most <m> elements, each separated by one or more commas
1645 (",") and OPTIONAL linear white space (LWS). This makes the usual
1646 form of lists very easy; a rule such as
1647 ( *LWS element *( *LWS "," *LWS element ))
1650 Wherever this construct is used, null elements are allowed, but do
1651 not contribute to the count of elements present. That is,
1652 "(element), , (element) " is permitted, but counts as only two
1653 elements. Therefore, where at least one element is required, at
1654 least one non-null element MUST be present. Default values are 0
1655 and infinity so that "#element" allows any number, including zero;
1656 "1#element" requires at least one; and "1#2element" allows one or
1660 A semi-colon, set off some distance to the right of rule text,
1661 starts a comment that continues to the end of line. This is a
1662 simple way of including useful notes in parallel with the
1666 The grammar described by this specification is word-based. Except
1667 where noted otherwise, linear white space (LWS) can be included
1668 between any two adjacent words (token or quoted-string), and
1669 between adjacent words and separators, without changing the
1670 interpretation of a field. At least one delimiter (LWS and/or
1671 separators) MUST exist between any two tokens (for the definition
1672 of "token" below), since they would otherwise be interpreted as a
1678 Leach & Newman Expires: August 2003 [Page 28]
1684 INTERNET DRAFT Digest SASL Mechanism February 2003
1689 The following rules are used throughout this specification to
1690 describe basic parsing constructs. The US-ASCII coded character set
1691 is defined by ANSI X3.4-1986 [USASCII].
1693 OCTET = <any 8-bit character>
1694 CHAR = <any US-ASCII character (octets 0 - 127)>
1695 UPALPHA = <any US-ASCII uppercase letter "A".."Z">
1696 LOALPHA = <any US-ASCII lowercase letter "a".."z">
1697 ALPHA = UPALPHA | LOALPHA
1698 DIGIT = <any US-ASCII digit "0".."9">
1699 CTL = <any US-ASCII control character
1700 (octets 0 - 31) and DEL (127)>
1701 CR = <US-ASCII CR, carriage return (13)>
1702 LF = <US-ASCII LF, linefeed (10)>
1703 SP = <US-ASCII SP, space (32)>
1704 HT = <US-ASCII HT, horizontal-tab (9)>
1705 <"> = <US-ASCII double-quote mark (34)>
1706 TEXTCHAR = <any OCTET except CTLs, but including HT>
1709 All linear white space, including folding, has the same semantics as
1710 SP. A recipient MAY replace any linear white space with a single SP
1711 before interpreting the field value or forwarding the message
1714 LWS = [CRLF] 1*( SP | HT )
1716 The TEXT rule is only used for descriptive field contents and values
1717 that are not intended to be interpreted by the message parser. Words
1718 of TEXT contains characters either from ISO-8859-1 [ISO-8859]
1719 character set or UTF-8 [UTF-8].
1721 TEXT = <any *OCTET except CTLs,
1724 A CRLF is allowed in the definition of TEXT only as part of a header
1725 field continuation. It is expected that the folding LWS will be
1726 replaced with a single SP before interpretation of the TEXT value.
1728 Hexadecimal numeric characters are used in several protocol elements.
1730 HEX = "A" | "B" | "C" | "D" | "E" | "F"
1731 | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
1733 Many HTTP/1.1 header field values consist of words separated by LWS
1734 or special characters. These special characters MUST be in a quoted
1738 Leach & Newman Expires: August 2003 [Page 29]
1744 INTERNET DRAFT Digest SASL Mechanism February 2003
1747 string to be used within a parameter value.
1750 separators = "(" | ")" | "<" | ">" | "@"
1751 | "," | ";" | ":" | "\" | <">
1752 | "/" | "[" | "]" | "?" | "="
1753 | "{" | "}" | SP | HT
1754 TOKENCHAR = <any CHAR except CTLs or separators>
1756 A string of text is parsed as a single word if it is quoted using
1759 quoted-string = ( <"> qdstr-val <"> )
1760 qdstr-val = *( qdtext | quoted-pair )
1761 qdtext = <any TEXTCHAR except <"> and "\">
1763 Note that LWS is NOT implicit between the double-quote marks (<">)
1764 surrounding a qdstr-val and the qdstr-val; any LWS will be considered
1765 part of the qdstr-val. This is also the case for quotation marks
1766 surrounding any other construct.
1768 The backslash character ("\") MAY be used as a single-character
1769 quoting mechanism only within qdstr-val and comment constructs.
1771 quoted-pair = "\" CHAR
1773 The value of this construct is CHAR. Note that an effect of this rule
1774 is that backslash itself MUST be quoted.
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1804 INTERNET DRAFT Digest SASL Mechanism February 2003
1809 The sample implementation in [Digest] also applies to DIGEST-MD5.
1811 The following code implements the conversion from UTF-8 to 8859-1 if
1814 /* if the string is entirely in the 8859-1 subset of UTF-8, then
1815 * translate to 8859-1 prior to MD5
1817 void MD5_UTF8_8859_1(MD5_CTX *ctx, const unsigned char *base,
1820 const unsigned char *scan, *end;
1824 for (scan = base; scan < end; ++scan) {
1825 if (*scan > 0xC3) break; /* abort if outside 8859-1 */
1826 if (*scan >= 0xC0 && *scan <= 0xC3) {
1827 if (++scan == end || *scan < 0x80 || *scan > 0xBF)
1831 /* if we found a character outside 8859-1, don't alter string
1834 MD5Update(ctx, base, len);
1838 /* convert to 8859-1 prior to applying hash
1841 for (scan = base; scan < end && *scan < 0xC0; ++scan)
1843 if (scan != base) MD5Update(ctx, base, scan - base);
1844 if (scan + 1 >= end) break;
1845 cbuf = ((scan[0] & 0x3) << 6) | (scan[1] & 0x3f);
1846 MD5Update(ctx, &cbuf, 1);
1848 } while (base < end);
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1864 INTERNET DRAFT Digest SASL Mechanism February 2003
1867 9 Interoperability considerations
1869 9.1 Implementing DES cipher in CBC mode
1871 Several cryptographic libraries (Ebones, OpenSSL) provide a convenience
1872 function des_cbc_encrypt for implementing DES cipher in CBC mode.
1873 There is a documented bug in this function: the function doesn't update
1874 IV before returning. If an implementation uses this function to implement
1875 DES cipher in CBC mode, it MUST update IV by copying the last 8 bytes of
1876 the des_cbc_encrypt's output to the IV buffer.
1878 Note that the function des_ede2_cbc_encrypt that may be used to implement
1879 3DES (in "two keys mode") in CBC mode works as expected.
1884 The following people had substantial contributions to the development
1885 and refinement of this document:
1895 as well as other members of SASL mailing list.
1918 Leach & Newman Expires: August 2003 [Page 32]
1924 INTERNET DRAFT Digest SASL Mechanism February 2003
1927 11 Full Copyright Statement
1929 Copyright (C) The Internet Society (2003). All Rights Reserved.
1931 This document and translations of it may be copied and furnished to
1932 others, and derivative works that comment on or otherwise explain it
1933 or assist in its implementation may be prepared, copied, published
1934 and distributed, in whole or in part, without restriction of any
1935 kind, provided that the above copyright notice and this paragraph are
1936 included on all such copies and derivative works. However, this
1937 document itself may not be modified in any way, such as by removing
1938 the copyright notice or references to the Internet Society or other
1939 Internet organizations, except as needed for the purpose of
1940 developing Internet standards in which case the procedures for
1941 copyrights defined in the Internet Standards process must be
1942 followed, or as required to translate it into languages other than
1945 The limited permissions granted above are perpetual and will not be
1946 revoked by the Internet Society or its successors or assigns.
1948 This document and the information contained herein is provided on an
1949 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
1950 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
1951 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
1952 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
1953 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
1957 Funding for the RFC Editor function is currently provided by the
1978 Leach & Newman Expires: August 2003 [Page 33]
1984 INTERNET DRAFT Digest SASL Mechanism February 2003
1987 Appendix A: Changes from 2831
1989 1). Fixed various typos in formulas.
1991 2). Dropped DES as mandatory to implement cipher (3DES is mandatory
1994 3). Tighten ABNF. Fixed some bugs.
1996 4). Clarified nc-value verification and which side is aborting
1999 5). Added text saying that for interoperability username/password
2000 MUST be normalized according to Normalization form KC.
2002 6). Clarified that unquoted version of the username, etc. used in A1
2005 7). Various cleanup to References section. Split all references to
2006 Normative and Informative.
2008 8). Added minimal and maximal limits on maxbuf. Clarified how to
2009 calculate max sender size.
2011 9). Change ABNF for host to allow for IPv6 addresses. ABNF now
2012 references RFC 2373 and RFC 2396.
2014 10). Added DES cipher interoperability section.
2016 11). Added man-in-the-middle considerations for ciphers.
2018 12). Clarified how sequence counters are modified.
2020 13). Addition warnings about preventing reply/redirection attacks.
2022 14). Specified that "charset" directive affects "realm" and doesn't
2026 15). Minor text clarifications.
2028 Appendix B: Open Issues
2030 1). The latest revision prohibits escaped characters in nonce/cnonce.
2033 2). What to do about CBC mode attack that affects TLS document and
2038 Leach & Newman Expires: August 2003 [Page 34]
2044 INTERNET DRAFT Digest SASL Mechanism February 2003
2047 One of the proposals is to drop DES/3DES ciphers and define a new one
2048 (e.g. AES) in such a way that is not susceptible to this kind of
2051 3). Merge DIGEST-MD5 AES cipher with this document?
2053 4). Normative vs. Informative references must be carefully rechecked.
2098 Leach & Newman Expires: August 2003 [Page 35]