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3 Network K. Burdis
4 Internet-Draft Rhodes University
5 Expires: February 10, 2003 R. Naffah
6 Forge Research
7 August 12, 2002
10 Secure Remote Password SASL Mechanism
11 draft-burdis-cat-srp-sasl-07
13 Status of this Memo
15 This document is an Internet-Draft and is in full conformance with
16 all provisions of Section 10 of RFC2026.
18 Internet-Drafts are working documents of the Internet Engineering
19 Task Force (IETF), its areas, and its working groups. Note that
20 other groups may also distribute working documents as Internet-
21 Drafts.
23 Internet-Drafts are draft documents valid for a maximum of six months
24 and may be updated, replaced, or obsoleted by other documents at any
25 time. It is inappropriate to use Internet-Drafts as reference
26 material or to cite them other than as "work in progress."
28 The list of current Internet-Drafts can be accessed at http://
29 www.ietf.org/ietf/1id-abstracts.txt.
31 The list of Internet-Draft Shadow Directories can be accessed at
32 http://www.ietf.org/shadow.html.
34 This Internet-Draft will expire on February 10, 2003.
36 Copyright Notice
38 Copyright (C) The Internet Society (2002). All Rights Reserved.
40 Abstract
42 This document describes a SASL mechanism based on the Secure Remote
43 Password protocol. This mechanism performs mutual authentication and
44 can provide a security layer with replay detection, integrity
45 protection and/or confidentiality protection.
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60 Table of Contents
62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
63 2. Conventions Used in this Document . . . . . . . . . . . . . 4
64 3. Data Element Formats . . . . . . . . . . . . . . . . . . . . 5
65 3.1 Scalar numbers . . . . . . . . . . . . . . . . . . . . . . . 5
66 3.2 Multi-Precision Integers . . . . . . . . . . . . . . . . . . 5
67 3.3 Octet Sequences . . . . . . . . . . . . . . . . . . . . . . 6
68 3.4 Extended Octet Sequences . . . . . . . . . . . . . . . . . . 6
69 3.5 Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
70 3.6 Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . 6
71 3.7 Data Element Size Limits . . . . . . . . . . . . . . . . . . 7
72 4. Protocol Description . . . . . . . . . . . . . . . . . . . . 8
73 4.1 Client sends its identity . . . . . . . . . . . . . . . . . 9
74 4.2 Server sends initial protocol elements . . . . . . . . . . . 9
75 4.3 Client sends its ephemeral public key . . . . . . . . . . . 11
76 4.4 Server sends its ephemeral public key . . . . . . . . . . . 12
77 4.5 Client sends its evidence . . . . . . . . . . . . . . . . . 12
78 4.6 Server sends its evidence . . . . . . . . . . . . . . . . . 13
79 5. Security Layer . . . . . . . . . . . . . . . . . . . . . . . 15
80 5.1 Cryptographic primitives . . . . . . . . . . . . . . . . . . 17
81 5.1.1 Pseudo random number generators . . . . . . . . . . . . . . 17
82 5.1.2 Key derivation function . . . . . . . . . . . . . . . . . . 18
83 5.2 Confidentiality Protection . . . . . . . . . . . . . . . . . 19
84 5.3 Replay Detection . . . . . . . . . . . . . . . . . . . . . . 20
85 5.4 Integrity Protection . . . . . . . . . . . . . . . . . . . . 21
86 5.5 Summary of Security Layer Output . . . . . . . . . . . . . . 21
87 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 23
88 7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 26
89 7.1 Mandatory Algorithms . . . . . . . . . . . . . . . . . . . . 26
90 7.2 Modulus and generator values . . . . . . . . . . . . . . . . 26
91 7.3 Replay detection sequence number counters . . . . . . . . . 26
92 7.4 SASL Profile Considerations . . . . . . . . . . . . . . . . 27
93 8. Security Considerations . . . . . . . . . . . . . . . . . . 29
94 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
95 References . . . . . . . . . . . . . . . . . . . . . . . . . 31
96 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 33
97 A. Modulus and Generator values . . . . . . . . . . . . . . . . 34
98 B. Changes since the previous draft . . . . . . . . . . . . . . 36
99 Full Copyright Statement . . . . . . . . . . . . . . . . . . 38
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116 1. Introduction
118 The Secure Remote Password (SRP) is a password-based, zero-knowledge,
119 authentication and key-exchange protocol developed by Thomas Wu. It
120 has good performance, is not plaintext-equivalent and maintains
121 perfect forward secrecy. It provides authentication (optionally
122 mutual authentication) and the negotiation of a session key [SRP].
124 The mechanism described herein is based on the optimised SRP protocol
125 described at the end of section 3 in [RFC-2945], since this reduces
126 the total number of messages exchanged by grouping together pieces of
127 information that do not depend on earlier messages. Due to the
128 design of the mechanism, mutual authentication is MANDATORY.
130 The SASL mechanism name associated with this protocol is "SRP".
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172 2. Conventions Used in this Document
174 o A hex digit is an element of the set:
176 {0, 1, 2, 3, 4, 5, 6, 7, 8 , 9, A, B, C, D, E, F}
178 A hex digit is the representation of a 4-bit string. Examples:
180 7 = 0111
182 A = 1010
184 o An octet is an 8-bit string. In this document an octet may be
185 written as a pair of hex digits. Examples:
187 7A = 01111010
189 02 = 00000010
191 o All data is encoded and sent in network byte order (big-endian).
193 o The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
194 NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
195 in this document are to be interpreted as described in [RFC-2119].
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228 3. Data Element Formats
230 This section describes the encoding of the data elements used by the
231 SASL mechanism described in this document.
233 3.1 Scalar numbers
235 Scalar numbers are unsigned quantities. Using b[k] to refer to the
236 k-th octet being processed, the value of a two-octet scalar is:
238 ((b[0] << 8) + b[1]),
240 where << is the bit left-shift operator. The value of a four-octet
241 scalar is:
243 ((b[0] << 24) + (b[1] << 16) + (b[2] << 8) + b[3]).
246 3.2 Multi-Precision Integers
248 Multi-Precision Integers, or MPIs, are positive integers used to hold
249 large integers used in cryptographic computations.
251 MPIs are encoded using a scheme inspired by that used by OpenPGP -
252 [RFC-2440] (section 3.2) - for encoding such entities:
254 The encoded form of an MPI SHALL consist of two pieces: a two-
255 octet scalar that represents the length of the entity, in octets,
256 followed by a sequence of octets that contain the actual integer.
258 These octets form a big-endian number; A big-endian number can be
259 encoded by prefixing it with the appropriate length.
261 Examples: (all numbers are in hexadecimal)
263 The sequence of octets [00 01 01] encodes an MPI with the value
264 1, while the sequence [00 02 01 FF] encodes an MPI with the
265 value of 511.
267 Additional rule:
269 * The length field of an encoded MPI describes the octet count
270 starting from the MPI's first non-zero octet, containing the
271 most significant non-zero bit. Thus, the encoding [00 02 01]
272 is not formed correctly; It should be [00 01 01].
274 We shall use the syntax mpi(A) to denote the encoded form of the
275 multi-precision integer A. Furthermore, we shall use the syntax
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284 bytes(A) to denote the big-endian sequence of octets forming the
285 multi-precision integer with the most significant octet being the
286 first non-zero octet containing the most significant bit of A.
288 3.3 Octet Sequences
290 This mechanism generates, uses and exchanges sequences of octets;
291 e.g. output values of message digest algorithm functions. When such
292 entities travel on the wire, they shall be preceded by a one-octet
293 scalar quantity representing the count of following octets.
295 We shall use the syntax os(s) to denote the encoded form of the octet
296 sequence. Furthermore, we shall use the syntax bytes(s) to denote
297 the sequence of octets s, in big-endian order.
299 3.4 Extended Octet Sequences
301 Extended sequences of octets are exchanged when using the security
302 layer. When these sequences travel on the wire, they shall be
303 preceded by a four-octet scalar quantity representing the count of
304 following octets.
306 We shall use the syntax eos(s) to denote the encoded form of the
307 extended octet sequence. Furthermore, we shall use the syntax
308 bytes(s) to denote the sequence of octets s, in big-endian order.
310 3.5 Text
312 The only character set for text is the UTF-8 encoding [RFC-2279] of
313 Unicode characters [ISO-10646]. All text MUST be in Unicode
314 Normalization Form KC [UNICODE-KC] without NUL characters.
316 We shall use the syntax utf8(L) to denote the string L in UTF-8
317 encoding, preceded by a two-octet scalar quantity representing the
318 count of following octets. Furthermore, we shall use the syntax
319 bytes(L) to denote the sequence of octets representing the UTF-8
320 encoding of L, in big-endian order.
322 3.6 Buffers
324 In this SASL mechanism data is exchanged between the client and
325 server using buffers. A buffer acts as an envelope for the sequence
326 of data elements sent by one end-point of the exchange, and expected
327 by the other.
329 A buffer MAY NOT contain other buffers. It may only contain zero,
330 one or more data elements.
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340 A buffer shall be encoded as two fields: a four-octet scalar quantity
341 representing the count of following octets, and the concatenation of
342 the octets of the data element(s) contained in the buffer.
344 We shall use the syntax {A|B|C} to denote a buffer containing A, B
345 and C in that order. For example:
347 { mpi(N) | mpi(g) | utf8(L) }
349 is a buffer containing, in the designated order, the encoded forms of
350 an MPI N, an MPI g and a Text L.
352 3.7 Data Element Size Limits
354 The following table details the size limit, in number of octets, for
355 each of the SASL data element encodings described earlier.
357 Data element type Header Size limit in octets
358 (octets) (excluding header)
359 ------------------------------------------------------------
360 Octet Sequence 1 255
361 MPI 2 65,535
362 Text 2 65,535
363 Extended Octet Sequence 4 2,147,483,383
364 Buffer 4 2,147,483,643
366 An implementation MUST signal an exception if any size constraint is
367 violated.
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396 4. Protocol Description
398 The following sections describe the sequence of data transmitted
399 between the client and server for the SRP SASL mechanism, as well as
400 the extra control information exchanged to enable a client to request
401 whether or not replay detection, integrity protection and/or
402 confidentiality protection should be provided by a security layer.
404 Mechanism data exchanges, during the authentication phase, are shown
405 below:
407 Client Server
409 --- { utf8(U) | utf8(I) } ------------------------>
411 <-------- { mpi(N) | mpi(g) | os(s) | utf8(L) } ---
413 --- { mpi(A) | utf8(o) } ------------------------->
415 <----------------------------------- { mpi(B) } ---
417 --- { os(M1) } ----------------------------------->
419 ( no confidentiality protection )
421 <----------------------------------- { os(M2) } ---
423 where:
425 U is the authentication identity (username),
427 I is the authorisation identity,
429 N is the safe prime modulus,
431 g is the generator,
433 s is the user's password salt,
435 L is the options list indicating available security services,
437 A is the client's ephemeral public key,
439 o is the options list indicating chosen security services,
441 B is the server's ephemeral public key,
443 M1 is the client's evidence that the shared key K is known,
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452 M2 is the server's evidence that the shared key K is known.
455 4.1 Client sends its identity
457 The client determines its authentication identity U and authorisation
458 identity I, encodes them and sends them to the server.
460 The client sends:
462 { utf8(U) | utf8(I) }
465 4.2 Server sends initial protocol elements
467 The server receives U, and looks up the safe prime modulus N, the
468 generator g, and the salt s to be used for that identity.
470 The server also creates an options list L, which consists of a comma-
471 separated list of option strings that specify the options the server
472 supports. This options list MUST NOT contain any whitespace
473 characters and all alphabetic characters MUST be in lowercase. When
474 used in digest calculations by the client the options list MUST be
475 used as received.
477 The following option strings are defined:
479 o "mda=<message digest algorithm name>" indicates that the server
480 supports the designated hash function as the underlying Message
481 Digest Algorithm for the designated user to be used for all SRP
482 calculations - to compute both client-side and server-side
483 digests. The specified algorithm MUST meet the requirements
484 specified in section 3.2 of [RFC-2945]:
486 "Any hash function used with SRP should produce an output of at
487 least 16 bytes and have the property that small changes in the
488 input cause significant nonlinear changes in the output."
490 Note that in the interests of interoperability between client and
491 server implementations and with other SRP-based tools, both the
492 client and the server MUST support SHA-160 as an underlying
493 Message Digest Algorithm. While the server is not required to
494 list SHA-160 as an available underlying Message Digest Algorithm,
495 it must be able to do so.
497 o "integrity=hmac-<MDA-name>" indicates that the server supports
498 integrity protection using the HMAC algorithm [RFC-2104] with
499 <MDA-name> as the underlying Message Digest Algorithm. Acceptable
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508 MDA names are chosen from [SCAN] under the MessageDigest section.
509 A server SHOULD send such an option string for each HMAC algorithm
510 it supports. The server MUST advertise at least one integrity
511 protection algorithm and in the interest of interoperability the
512 server SHOULD advertise support for the HMAC-SHA-160 algorithm.
514 o "replay_detection" indicates that the server supports replay
515 detection using sequence numbers. Replay detection SHALL NOT be
516 activated without also activating integrity protection. If the
517 replay detection option is offered (by the server) and/or chosen
518 (by the client) without explicitly specifying an integrity
519 protection option, then the default integrity protection option
520 "integrity=hmac-sha-160" is implied and SHALL be activated.
522 o "confidentiality=<cipher name>" indicates that the server supports
523 confidentiality protection using the symmetric key block cipher
524 algorithm <cipher name>. The server SHOULD send such an option
525 string for each confidentiality protection algorithm it supports.
526 Note that in the interest of interoperability, if the server
527 offers confidentiality protection, it MUST send the option string
528 "confidentiality=aes" since it is then MANDATORY for it to provide
529 support for the [AES] algorithm.
531 o "mandatory=[integrity|replay_detection|confidentiality]" is an
532 option only available to the server that indicates that the
533 specified security layer option is MANDATORY and MUST be chosen by
534 the client for use in the resulting security layer. If a server
535 specifies an option as mandatory in this way, it MUST abort the
536 connection if the specified option is not chosen by the client.
537 It doesn't make sense for the client to send this option since it
538 is only able to choose options that the server advertises. The
539 client SHOULD abort the connection if the server does not offer an
540 option that it requires. If this option is not specified then
541 this implies that no options are mandatory. The server SHOULD
542 always send the "mandatory=integrity" option indicating that
543 integrity protection is required.
545 o "maxbuffersize=<number of bytes>" indicates to the peer the
546 maximum number of raw bytes (excluding the SASL buffer 4-byte
547 length header) to be processed by the security layer at a time, if
548 one is negotiated. The value of <number of bytes> MUST NOT exceed
549 the Buffer size limit defined in section 3.7. If this option is
550 not detected by a client or server mechanism, then it shall
551 operate its security layer on the assumption that the maximum
552 number of bytes that may be sent, to the peer server or client
553 mechanism respectively, is the Buffer data size limit indicated in
554 section 3.7. On the other hand, if a recipient detects this
555 option, it shall break any octet-sequence longer than the
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564 designated limit into two or more fragments, each wrapped in a
565 SASL buffer, before sending them, in sequence, to the peer.
567 For example, if the server supports integrity protection using the
568 HMAC-SHA-160 and HMAC-MD5 algorithms, replay detection and no
569 confidentiality protection, the options list would be:
571 mda=sha-1,integrity=hmac-sha-160,integrity=hmac-
572 md5,replay_detection
574 The server sends:
576 { mpi(N) | mpi(g) | os(s) | utf8(L) }
579 4.3 Client sends its ephemeral public key
581 The client receives the options list L from the server that specifies
582 the Message Digest Algorithm(s) available to be used for all SRP
583 calculations, the security service options the server supports, and
584 the maximum buffer size the server can handle. The client selects
585 options from this list and creates a new options list o that
586 specifies the selected Message Digest Algorithm to be used for SRP
587 calculations and the security services that will be used in the
588 security layer. At most one available Message Digest Algorithm name,
589 one available integrity protection algorithm and one available
590 confidentiality protection algorithm may be selected. In addition
591 the client may specify the maximum buffer size it can handle. The
592 client MUST include any option specified by the mandatory option.
594 The client SHOULD always select an integrity protection algorithm
595 even if the server does not make it mandatory to do so. If the
596 client selects a confidentiality protection algorithm it SHOULD then
597 also select an integrity protection algorithm.
599 This options list MUST NOT contain any whitespace characters and all
600 alphabetic characters MUST be in lowercase. When used in digest
601 calculations by the server the options list MUST be used as received.
603 The client generates its ephemeral public key A as follows:
605 a = prng();
607 A = g**a % N;
609 where:
611 prng() is a random number generation function,
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620 a is the MPI that will act as the client's private key,
622 ** is the exponentiation operator,
624 % is the modulus operator,
626 The client sends:
628 { mpi(A) | utf8(o) }
631 4.4 Server sends its ephemeral public key
633 The server reads the client's verifier v, calculates the shared
634 context key K and generates its ephemeral public key B as follows:
636 b = prng();
638 B = (v + g**b) % N;
640 K = H2((A * v**u) ** b % N);
642 where:
644 b is the MPI that will act as the server's private key,
646 v is the stored password verifier value,
648 u is a 32-bit unsigned integer which takes its value from the
649 first 32 bits of the hash of B, MSB first,
651 H2() is the "Interleaved SHA" function, as described in [RFC-
652 2945], but generalised to any message digest algorithm, and
653 applied using the underlying Message Digest Algorithm (see Section
654 4.2).
656 The server sends:
658 { mpi(B) }
661 4.5 Client sends its evidence
663 The client calculates the shared context key K, and calculates the
664 evidence M1 that proves to the server that it knows the shared
665 context key K, including I and L as part of the calculation.
667 K, on the client's side is computed as follows:
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676 x = H(s | H(U | ":" | p));
678 K = H2((B - g**x) ** (a + u * x) % N);
680 where:
682 H() is the result of digesting the designated input/data with the
683 underlying Message Digest Algorithm function (see Section 4.2).
685 p is the password value.
687 M1 is computed as:
689 H( bytes(H( bytes(N) )) ^ bytes( H( bytes(g) ))
690 | bytes(H( bytes(U) ))
691 | bytes(s)
692 | bytes(A)
693 | bytes(B)
694 | bytes(K)
695 | bytes(H( bytes(I) )
696 | bytes(H( bytes(L) ))
697 )
699 where:
701 ^ is the bitwise XOR operator.
703 All parameters received from the server that are used as input to a
704 digest operation MUST be used as received.
706 The client sends:
708 { os(M1) }
711 4.6 Server sends its evidence
713 When the Confidentiality Protection service is advertised by the
714 server and chosen by the client, the server MUST NOT send M2 but
715 instead conclude the SASL exchange after receiving and verifying the
716 client's M1. Otherwise, M2 MUST be sent.
718 When the server has to send its evidence M2, which proves to the
719 client that it knows the shared context key K, as well as U, I and o,
720 it shall compute it as follows:
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732 H( bytes(A)
733 | bytes(M1)
734 | bytes(K)
735 | bytes(H( bytes(U) ))
736 | bytes(H( bytes(I) ))
737 | bytes(H( bytes(o) ))
738 )
740 All parameters received from the client that are used as input to a
741 digest operation MUST be used as received.
743 If Confidentiality Protection was not negotiated the server sends:
745 { os(M2) }
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788 5. Security Layer
790 Section 3 of [RFC-2222] describes the operation of the security
791 layer:
793 "The security layer takes effect immediately following the last
794 response of the authentication exchange for data sent by the
795 client and the completion indication for data sent by the server.
796 Once the security layer is in effect, the protocol stream is
797 processed by the security layer into buffers of cipher-text. Each
798 buffer is transferred over the connection as a stream of octets
799 prepended with a four octet field in network byte order that
800 represents the length of the following buffer. The length of the
801 cipher-text buffer must be no larger than the maximum size that
802 was defined or negotiated by the other side."
804 Depending on the options offered by the server and chosen by the
805 client, the security layer may provide integrity protection, replay
806 detection, and/or confidentiality protection.
808 The security layer can be thought of as a three-stage filter through
809 which the data flows from the output of one stage to the input of the
810 following one. The first input is the original data, while the last
811 output is the data after being subject to the transformations of this
812 filter.
814 The data always passes through this three-stage filter, though any of
815 the stages may be inactive. Only when a stage is active would the
816 output be different from the input. In other words, if a stage is
817 inactive, the octet sequence at the output side is an exact duplicate
818 of the same sequence at the input side.
820 Schematically, the three-stage filter security layer appears as
821 follows:
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844 +----------------------------+
845 | | I/ p1
846 p1 --->| Confidentiality protection |---+
847 | | | A/ c
848 +----------------------------+ |
849 |
850 +------------------------------------+
851 |
852 | +----------------------------+
853 | | | I/ p2
854 p2 +-->| Replay detection |---+
855 | | | A/ p2 | q
856 +----------------------------+ |
857 |
858 +------------------------------------+
859 |
860 | +----------------------------+
861 | | | I/ p3
862 p3 +-->| Integrity protection |--->
863 | | A/ p3 | C
864 +----------------------------+
866 where:
868 p1, p2 and p3 are the input octet sequences at each stage,
870 I/ denotes the output at the end of one stage if/when the stage is
871 inactive or disabled,
873 A/ denotes the output at the end of one stage if/when the stage is
874 active or enabled,
876 c is the encrypted (sender-side) or decrypted (receiver-side)
877 octet sequence. c1 shall denote the value computed by the sender,
878 while c2 shall denote the value computed by the receiver.
880 q is a four-octet scalar quantity representing a sequence number,
882 C is the Message Authentication Code. C1 shall denote the value
883 of the MAC as computed by the sender, while C2 shall denote the
884 value computed by the receiver.
886 It is worth noting here that both client and server have their own
887 distinct security contexts, including distinct encryption and
888 decryption sub-contexts. In principal, nothing in this specification
889 should prevent an implementation from supporting asynchronous
890 connections.
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900 5.1 Cryptographic primitives
902 5.1.1 Pseudo random number generators
904 This mechanism requires random data to be generated for use in:
906 1. The CALG key material and the cipher initial vectors (IVs) for
907 both the client and server when the Confidentiality Protection
908 service is enabled.
910 2. The IALG key material for both the client and server when the
911 Integrity Protection service is enabled.
913 The PRNG used in this specification is based on the "UMacGenerator"
914 algorithm described in [UMAC]. It uses the [AES] algorithm, in its
915 256-bit key size variant, as the underlying symmetric key block
916 cipher for its operations.
918 A formal description of this PRNG follows:
920 o Initialisation
922 * SK: a 32-octet sequence (seeding key to AES)
924 o Input
926 * n: a positive integer
928 o Output
930 * Y: an n-octet sequence
932 o Algorithm
934 * (initialisation)
936 1. Initialise an AES instance for encryption with the first 32
937 octets of SK as its user-supplied key material. Let "aes"
938 be that instance; i.e. aes = AES(SK, ENCRYPTION);
940 2. Initialise T to be an all-zero 16-octet long sequence;
942 * (for every input)
944 1. Initialise "remaining" to n;
946 2. Initialise Y to be a 0-length octet sequence;
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956 3. while (remaining > 0) do
958 1. T = aes(T);
960 2. Append m octets from T to Y, where m is the minimum of
961 16 and remaining;
963 3. Subtract 16 from remaining;
965 4. return Y;
967 In the rest of this document, "PRNG" will refer to this algorithm
968 with the initialisation parameter SK set to be the shared context key
969 K computed by the SRP calculations (see Section 4.4 and Section 4.5).
971 This algorithm MAY also be used as part of the SRP calculations to
972 generate the required "a" and "b" parameters used in creating the
973 client and server ephemeral private keys ("A" and "B"). In this case
974 the initialisation parameter SK can be any 32-octet sequence (e.g.
975 multiple representations of the time-of-day).
977 If the same PRNG instance is used for both the SRP calculations and
978 the calculations in this specification, it MUST be re-initialised
979 with the shared context key K before any of the latter calculations
980 are performed.
982 5.1.2 Key derivation function
984 During the authentication phase, both parties compute the shared
985 context key K (see Section 4.4 for the server, and Section 4.5 for
986 the client sides respectively). The length of K is 2*s bits, where
987 "s" is the output length of the underlying Message Digest Algorithm
988 used in the SRP calculations (see "mda" option in Section 4.2).
990 When Confidentiality Protection is required, and the length of K is
991 not equal to the length of the user-supplied key material needed to
992 initialise the chosen Confidentiality Algorithm (CALG), the peers
993 MUST apply the Key Derivation Function (KDF) in order to obtain
994 enough data for this purpose.
996 Similarly, when Integrity Protection is required, and the length of K
997 is not equal to the required length of the key material needed to
998 initialise the chosen Integrity Algorithm (IALG), the peers MUST
999 apply the Key Derivation Function (KDF) in order to obtain enough
1000 data for this purpose too.
1002 We define this KDF as:
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1012 Km = KDF(K, n)
1014 where:
1016 Km: is the required key material,
1018 K: is the shared context key, and
1020 n: is the required length of Km.
1022 The following steps describe the KDF algorithm:
1024 If length of K is greater than or equal to n, then
1026 Let Km be the first n bytes of K;
1028 Else
1030 Let Km = PRNG(n);
1032 return Km
1035 5.2 Confidentiality Protection
1037 The plaintext data octet sequence p1 is encrypted using the chosen
1038 confidentiality algorithm (CALG) initialised for encryption with the
1039 key material Km obtained by applying the KDF to K (the shared context
1040 key K), and m (the key size of the chosen CALG) - see Section 5.1.2.
1042 Km = KDF(K, m)
1044 c1 = CALG(Km, ENCRYPTION)( bytes(p1) )
1046 On the receiving side, the ciphertext data octet sequence p1 is
1047 decrypted using the chosen confidentiality algorithm (CALG)
1048 initialised for decryption, with the key Km obtained by a similar
1049 process.
1051 Km = KDF(K, m)
1053 c2 = CALG(Km, DECRYPTION)( bytes(p1) )
1055 The designated CALG block cipher MUST be used in OFB (Output Feedback
1056 Block) mode in the ISO variant, as described in [HAC], algorithm
1057 7.20.
1059 Let k be the block size of the chosen symmetric key block cipher
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1068 algorithm; e.g. for AES this is 128 bits or 16 octets. The OFB mode
1069 used shall have a block size of k.
1071 It is recommended that Block ciphers operating in OFB mode be used
1072 with an Initial Vector (the mode's IV). In such a mode of operation
1073 - OFB with key re-use - the IV need not be secret. For the SASL
1074 mechanism described in this document, the IVs shall be:
1076 IV = PRNG(n);
1078 where n is the block size of the negotiated CALG.
1080 The input data to the confidentiality protection algorithm shall be a
1081 multiple of the symmetric key block cipher block size k. When the
1082 input length is not a multiple of k octets, the data shall be padded
1083 according to the following scheme (described in [PKCS7] which itself
1084 is based on [RFC-1423]):
1086 Assuming the length of the input is l octets, (k - (l mod k))
1087 octets, all having the value (k - (l mod k)), shall be appended to
1088 the original data. In other words, the input is padded at the
1089 trailing end with one of the following sequences:
1091 01 -- if l mod k = k-1
1092 02 02 -- if l mod k = k-2
1093 ...
1094 ...
1095 ...
1096 k k ... k k -- if l mod k = 0
1098 The padding can be removed unambiguously since all input is padded
1099 and no padding sequence is a suffix of another. This padding
1100 method is well-defined if and only if k < 256 octets, which is the
1101 case with symmetric block ciphers today, and in the forseeable
1102 future.
1104 The output of this stage, when it is active, is:
1106 at the sending side: CALG(Km, ENCRYPT)( bytes(p1) )
1108 at the receiving side: CALG(Km, DECRYPT)( bytes(p1) )
1111 5.3 Replay Detection
1113 A sequence number q is incremented every time a message is sent to
1114 the peer.
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1124 The output of this stage, when it is active, is:
1126 p2 | q
1128 At the other end, the receiver increments its instance of the
1129 sequence number. This new value of the sequence number is then used
1130 in the integrity protection transformation, which must also be active
1131 as described in Section 4.2. See Section 7.3 for more details.
1133 5.4 Integrity Protection
1135 When the Integrity Protection stage is active, a message
1136 authentication code C is computed using the chosen integrity
1137 protection algorithm (IALG) as follows:
1139 o the IALG is initialised (once) with the key material Kn obtained
1140 by applying the KDF to K (the shared context key K), and n (the
1141 required key size of the chosen IALG) - see Section 5.1.2; i.e.
1142 Kn = KDF(K, n),
1144 o the IALG is updated with every exchange of the sequence p3,
1145 yielding the value C and a new IALG context for use in the
1146 following exchange.
1148 At the other end, the receiver computes its version of C, using the
1149 same transformation, and checks that its value is equal to that
1150 received. If the two values do not agree, the receiver MUST signal
1151 an exception and abort.
1153 The output of this stage, when it is active, is then:
1155 IALG(Kn)( bytes(p3) )
1158 5.5 Summary of Security Layer Output
1160 The following table shows the data exchanged by the security layer
1161 peers, depending on the possible legal combinations of the three
1162 security services in operation:
1164 CP IP RD Peer sends/receives
1166 I I I { eos(p) }
1167 I A I { eos(p) | os( IALG(K)( bytes(p) ) ) }
1168 I A A { eos(p) | os( IALG(K)( bytes(p) | bytes(q)) ) }
1169 A I I { eos(c) }
1170 A A I { eos(c) | os( IALG(K)( bytes(c) ) ) }
1171 A A A { eos(c) | os( IALG(K)((bytes(c) | bytes(q)) ) }
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1180 where
1182 CP Confidentiality protection,
1184 IP Integrity protection,
1186 RD Replay detection,
1188 I Security service is Inactive/disabled,
1190 A Security service is Active/enabled,
1192 p The original plaintext,
1194 q The sequence number.
1196 c The enciphered input obtained by either:
1198 CALG(Km, ENCRYPT)( bytes(p) ) at the sender's side, or
1200 CALG(Km, DECRYPT)( bytes(p) ) at the receiver's side
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1236 6. Example
1238 The example below uses SMTP authentication [RFC-2554]. The base64
1239 encoding of challenges and responses, as well as the reply codes
1240 preceding the responses are part of the SMTP authentication
1241 specification, not part of this SASL mechanism itself.
1243 "C:" and "S:" indicate lines sent by the client and server
1244 respectively.
1247 S: 220 smtp.example.com ESMTP server ready
1249 C: EHLO zaau.example.com
1251 S: 250-smtp.example.com
1252 S: 250 AUTH SRP CRAM-MD5 DIGEST-MD5
1254 C: AUTH SRP AAAADAAEdGVzdAAEdGVzdA==
1256 with:
1258 U = "test"
1260 I = "test"
1262 S: 334 AAABygEArGvbQTJKmpvxZt5eE4lYL69ytmUZh+4H/DGSlD21YFCjcynLtKCZ
1263 7YGT4HV3Z6E91SMSq0sDMQ3Nf0ip2gT9UOgIOWntt2ewz2CVF5oWOrNmGgX71fqq6Ck
1264 YqZYvC5O4Vfl5k+yXXuqoDXQK2/T/dHNZ0EHVwz6nHSgeRGsUdzvKl7Q6I/uAFna9IH
1265 pDbGSB8dK5B4cXRhpbnTLmiPh3SFRFI7UksNV9Xqd6J3XS7PoDLPvb9S+zeGFgJ5AE5
1266 Xrmr4dOcwPOUymczAQce8MI2CpWmPOo0MOCca41+Onb+7aUtcgD2J965DXeI21SX1R1
1267 m2XjcvzWjvIPpxEfnkr/cwABAgqsi3AvmIqdEbREALhtZGE9U0hBLTEsbWFuZGF0b3J
1268 5PXJlcGxheSBkZXRlY3Rpb24scmVwbGF5IGRldGVjdGlvbixpbnRlZ3JpdHk9aG1hYy
1269 1zaGExLGludGVncml0eT1obWFjLW1kNSxjb25maWRlbnRpYWxpdHk9YWVzLGNvbmZpZ
1270 GVudGlhbGl0eT1jYXN0NSxjb25maWRlbnRpYWxpdHk9Ymxvd2Zpc2gsbWF4YnVmZmVy
1271 c2l6ZT0yMTQ3NDgzNjQz
1273 with:
1275 N = "21766174458617435773191008891802753781907668374255538511144
1276 6432246898862353838409572109090130860564015713997172358072665816
1277 4960647214841029141336415219736447718088739565548373811507267740
1278 2235101762521901569820740293149529620419333266262073471054548368
1279 7360395197024862265062488610602569718029849535611214426801576680
1280 0076142998822245709041387397397017192709399211475176516806361476
1281 1119615476233422096442783117971236371647333871414335895773474667
1282 3089670508070055093204247996784170368679283167612722742303140675
1283 4829113358247958306143957755934710196177140617368437852270348349
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1292 5337037655006751328447510550299250924469288819"
1294 g = "2"
1296 s = "814819216327401865851972"
1298 L = "mda=sha-1,mandatory=replay_detection,replay_detection,integ
1299 rity=hmac-sha1,integrity=hmac-md5,confidentiality=aes,confidenti
1300 ality=cast5,confidentiality=blowfish,maxbuffersize=2147483643"
1302 C: AAABYwEAAp5q/4zhXoTUzXBscozN97SWgfDcAImIk3lNHNvd0b+Dr7jEm6upXblZ
1303 T5sL9mPgFsejlIh+B/eCu/HvzWCrXj6ylPZv8dy3LCH3LIORqQ45S7Lsbmrrg/dukDh
1304 4tZCJMLD4r3evzaY8KVhtJeLMVbeXuh4JljKP42Ll59Lzwf8jfPh4+4Lae1rpWUCL9D
1305 ueKcY+nN+xNHTit/ynLATxwL93P6+GoGY4TkUbUBfjiI1+rAMvyMDMw5XozGy07FOEc
1306 ++U0iPeXCQP4MT5FipOUoz8CYX7J1LbaXp2WJuFHlkyVXF7oCoyHbhld/5CfR3o6q/B
1307 /x9+yZRqaHH+JfllOgBfbWRhPVNIQS0xLHJlcGxheSBkZXRlY3Rpb24saW50ZWdyaXR
1308 5PWhtYWMtbWQ1LGNvbmZpZGVudGlhbGl0eT1ibG93ZmlzaCxtYXhidWZmZXJzaXplPT
1309 IxNDc0ODM2NDM=
1311 with:
1313 A = "33059541846712102497463123211304342021934496372587869281515
1314 9695658237779884462777478850394977744553746930451895815615888405
1315 0562780707370878253753979367019077142882237029766166623275718227
1316 6555389834190840322081091599089081947324537907613924707058150037
1317 7802790776231793962143786411792516760030102436603621046541729396
1318 6890613394379900527412007068242559299422872893332111365840536495
1319 1858834742328835373387573188369956379881606380890675411966073665
1320 1106922002294035533470301541999274557200666703389531481794516625
1321 4757418442215980634933876533189969562613241499465295849832999091
1322 40398081321840949606581251320320995783959866"
1324 o = mda=sha-1,replay_detection,integrity=hmac-md5,confidentialit
1325 y=blowfish,maxbuffersize=2147483643"
1327 S: 334 AAABAgEAOUKbXpnzMhziivGgMwm+FS8sKGSvjh5M3D+80RF/5z9rm0oPoi4+
1328 pF83fueWn4Hz9M+muF/22PHHZkHtlutDrtapj4OtirdxC21fS9bMtEh3F0whTX+3mPv
1329 thw5sk11turandHiLvcUZOgcrAGIoDKcBPoGyBud+8bMgpkf/uGfyBM2nEX/hV+oGgg
1330 X+LiHjmkxAJ3kewfQPH0eV9ffEuuyu8BUcBXkJsS6l7eWkuERSCttVOi/jS031c+CD/
1331 nuecUXYiF8IYzW03rbcwYhZzifmTi3VK9C8zG2K1WmGU+cDKlZMkyCPMmtCsxlbgE8z
1332 SHCuCiOgQ35XhcA0Qa0C3Q==
1334 with:
1336 B: "722842847565031844205403087285424428589273458129750231766015
1337 4465607827529853239240118185263492617243523916106658696965596526
1338 8585300845435562962039149169549800169184521786717633959469278439
1339 8771344445002432579509292115598435685062882631760796416554562980
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1348 8475896198325835507901319556929511421472132184990365213059654962
1349 7218189966140113906545856088040473723048909402258929560823932725
1350 2022154114087913895411927676707073040281136096806681758265221209
1351 8822374723416364340410020172215773934302794679034424699999611678
1352 9730443114919539575466941344964841591072763617954717789621871251
1353 71089179399349194452686682517183909017223901"
1355 C: AAAAFRTkoju6xGP+zH89iaDWIFjfIKt5Kg==
1357 S: 235 Authentication successful.
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1404 7. Discussion
1406 7.1 Mandatory Algorithms
1408 The algorithms specified as mandatory were chosen for utility and
1409 availablity. We felt that a mandatory confidentiality and integrity
1410 protection algorithm for the security layer and a mandatory Message
1411 Digest Algorithm for SRP calculations should be specified to ensure
1412 interoperability between implementations of this mechanism:
1414 o The SHA-160 Message Digest Algorithm was chosen as an underlying
1415 algorithm for SRP calculations because this allows for easy
1416 interoperability with other SRP-based tools that use the SRP-SHA1
1417 protocol described in section 3 of [RFC-2945] and create their
1418 password files using this algorithm.
1420 o The HMAC algorithm was chosen as an integrity algorithm because it
1421 is faster than MAC algorithms based on secret key encryption
1422 algorithms [RFC-2847].
1424 o AES was chosen as a cipher because it has undergone thorough
1425 scrutiny by the best cryptographers in the world.
1427 Since confidentiality protection is optional, this mechanism should
1428 be usable in countries that have strict controls on the use of
1429 cryptography.
1431 7.2 Modulus and generator values
1433 It is RECOMMENDED that the server use values for the modulus (N) and
1434 generator (g) chosen from those listed in Appendix A so that the
1435 client can avoid expensive constraint checks, since these predefined
1436 values already meet the constraints described in [RFC-2945]:
1438 "For maximum security, N should be a safe prime (i.e. a number of
1439 the form N = 2q + 1, where q is also prime). Also, g should be a
1440 generator modulo N (see [SRP] for details), which means that for
1441 any X where 0 < X < N, there exists a value x for which g**x % N
1442 == X."
1445 7.3 Replay detection sequence number counters
1447 The mechanism described in this document allows the use of a Replay
1448 Detection security service that works by including sequence number
1449 counters in the message authentication code (MAC) created by the
1450 Integrity Protection service. As noted in Section 4.2 integrity
1451 protection is always activated when the Replay Detection service is
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1460 activated.
1462 Both the client and the server keep two sequence number counters.
1463 Each of these counters is a 32-bit unsigned integer initialised with
1464 a Starting Value and incremented by an Increment Value with every
1465 successful transmission of an SASL buffer through the security layer.
1466 The Sent counter is incremented for each buffer sent through the
1467 security layer. The Received counter is incremented for each buffer
1468 received through the security layer. If the value of a sequence
1469 number counter exceeds 2**32-1 it wraps around and starts from zero
1470 again.
1472 When a sender sends a buffer it includes the value of its Sent
1473 counter in the computation of the MAC accompanying each integrity
1474 protected message. When a recipient receives a buffer it uses the
1475 value of it's Received counter in its computation of the integrity
1476 protection MAC for the received message. The recipient's Received
1477 counter must be the same as the sender's Sent counter in order for
1478 the received and computed MACs to match.
1480 This specification assumes that for each sequence number counter the
1481 Starting Value is ZERO, and that the Increment Value is ONE. These
1482 values do not affect the security or the intended objective of the
1483 replay detection service, since they never travel on the wire.
1485 7.4 SASL Profile Considerations
1487 As mentioned briefly in [RFC-2222], and detailed in [SASL] a SASL
1488 specification has three layers: (a) a protocol definition using SASL
1489 known as the "Profile", (b) a SASL mechanism definition, and (c) the
1490 SASL framework.
1492 Point (3) in section 5 of [SASL] ("Protocol profile requirements")
1493 clearly states that it is the responsibility of the Profile to define
1494 "...how the challenges and responses are encoded, how the server
1495 indicates completion or failure of the exchange, how the client
1496 aborts an exchange, and how the exchange method interacts with any
1497 line length limits in the protocol."
1499 The username entity, referenced as "U" throughout this document, and
1500 used by the server to locate the password data, is assumed to travel
1501 "in the clear," meaning that no transformation is applied to its
1502 contents. This assumption was made to allow the same SRP password
1503 files to be used in this mechanism, as those used with other SRP
1504 applications and tools.
1506 A Profile may decide, for privacy or other reason, to disallow such
1507 information to travel in the clear, and instead use a hashed version
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1516 of U, or more generally a transformation function applied to U; i.e.
1517 f(U). Such a Profile would require additional tools to add the
1518 required entries to the SRP password files for the new value(s) of
1519 f(U). It is worth noting too that if this is the case, and the same
1520 user shall access the server through this mechanism as well as
1521 through other SRP tools, then at least two entries, one with U and
1522 the other with f(U) need to be present in the SRP password files if
1523 those same files are to be used for both types of access.
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1572 8. Security Considerations
1574 This mechanism relies on the security of SRP, which bases its
1575 security on the difficulty of solving the Diffie-Hellman problem in
1576 the multiplicative field modulo a large safe prime. See section 4
1577 "Security Considerations" of [RFC-2945] and section 4 "Security
1578 analysis" of [SRP].
1580 B, the server's ephemeral public key, is computed as g**b + v = g**b
1581 + g**x, which is symmetric and allows two guesses per *active
1582 attack*. In practical terms, this makes no difference to the
1583 security of SRP, since the number of active attacks needed is still
1584 linearly proportional to the number of guesses needed; only the
1585 constant factor (2 vs. 1) has changed.
1587 This mechanism also relies on the security of the HMAC algorithm and
1588 the underlying hash function when integrity protection is used.
1589 Section 6 "Security" of [RFC-2104] discusses these security issues in
1590 detail. Weaknesses found in MD5 do not impact HMAC-MD5 [DOBBERTIN].
1592 U, A, I and o, sent from the client to the server, and N, g, L, s and
1593 B, sent from the server to the client could be modified by an
1594 attacker before reaching the other party. For this reason, these
1595 values are included in the respective calculations of evidence (M1
1596 and M2) to prove that each party knows the session key K. This
1597 allows each party to verify that these values were received
1598 unmodified.
1600 The use of integrity protection is RECOMMENDED to detect message
1601 tampering and to avoid session hijacking after authentication has
1602 taken place.
1604 Replay attacks may be avoided through the use of sequence numbers,
1605 because sequence numbers make each integrity protected message
1606 exchanged during a session different, and each session uses a
1607 different key.
1609 Research [KRAWCZYK] shows that the order and way of combining message
1610 encryption (Confidentiality Protection) and message authentication
1611 (Integrity Protection) are important. This mechanism follows the EtA
1612 (encrypt-then-authenticate) method and is "generically secure."
1614 This mechanism uses a Pseudo-Random Number Generator (PRNG) for
1615 generating some of its parameters. Section 5.1.1 describes a
1616 securely seeded, cryptographically strong PRNG implementation for
1617 this purpose.
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1628 9. Acknowledgements
1630 The following people provided valuable feedback in the preparation of
1631 this document:
1633 Stephen Farrell <stephen.farrell@baltimore.ie>
1635 Timothy Martin <tmartin@andrew.cmu.edu>
1637 Alexey Melnikov <mel@messagingdirect.com>
1639 Ken Murchison <ken@oceana.com>
1641 Magnus Nystrom <magnus@rsasecurity.com>
1643 Thomas Wu <tom@arcot.com>
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1684 References
1686 [AES] National Institute of Standards and Technology,
1687 "Rijndael: NIST's Selection for the AES", December
1688 2000, <http://csrc.nist.gov/encryption/aes/rijndael/
1689 Rijndael.pdf>.
1691 [DOBBERTIN] Dobbertin, H., "The Status of MD5 After a Recent
1692 Attack", December 1996, <ftp://ftp.rsasecurity.com/pub/
1693 cryptobytes/crypto2n2.pdf>.
1695 [HAC] Menezes, A., van Oorschot, P. and S. Vanstone,
1696 "Handbook of Applied Cryptography", CRC Press, Inc.,
1697 ISBN 0-8493-8523-7, 1997, <http://
1698 www.cacr.math.uwaterloo.ca/hac/about/chap7.ps>.
1700 [ISO-10646] "International Standard --Information technology--
1701 Universal Multiple-Octet Coded Character Set (UCS) --
1702 Part 1 Architecture and Basic Multilingual Plane. UTF-8
1703 is described in Annex R, adopted but not yet published.
1704 UTF-16 is described in Annex Q, adopted but not yet
1705 published.", ISO/IEC 10646-1, 1993.
1707 [KRAWCZYK] Krawczyk, H., "The order of encryption and
1708 authentication for protecting communications (Or: how
1709 secure is SSL?)", June 2001, <http://eprint.iacr.org/
1710 2001/045/>.
1712 [PKCS7] RSA Data Security, Inc., "PKCS #7: Cryptographic
1713 Message Syntax Standard", Version 1.5, November 1993,
1714 <ftp://ftp.rsasecurity.com/pub/pkcs/ascii/pkcs-7.asc>.
1716 [RFC-1423] Balenson, D., "Privacy Enhancement for Internet
1717 Electronic Mail: Part III: Algorithms, Modes, and
1718 Identifiers", RFC 1423, February 1993, <http://
1719 www.ietf.org/rfc/rfc1423.txt>.
1721 [RFC-2104] Krawczyk, H., "HMAC: Keyed-Hashing for Message
1722 Authentication", RFC 2104, February 1997, <http://
1723 www.ietf.org/rfc/rfc2104.txt>.
1725 [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
1726 Requirement Levels", BCP 0014, RFC 2119, March 1997,
1727 <http://www.ietf.org/rfc/rfc2119.txt>.
1729 [RFC-2222] Myers, J., "Simple Authentication and Security Layer
1730 (SASL)", RFC 2222, October 1997, <http://www.ietf.org/
1731 rfc/rfc2222.txt>.
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1737 Internet-Draft SRP SASL Mechanism August 2002
1740 [RFC-2279] Yergeau, F., "UTF-8, a transformation format of Unicode
1741 and ISO 10646", RFC 2279, January 1998, <http://
1742 www.ietf.org/rfc/rfc2279.txt>.
1744 [RFC-2440] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer,
1745 "OpenPGP Message Format", RFC 2440, November 1998,
1746 <http://www.ietf.org/rfc/rfc2440.txt>.
1748 [RFC-2554] Myers, J., "SMTP Service Extension for Authentication",
1749 RFC 2554, March 1999.
1751 [RFC-2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
1752 June 1999, <http://www.ietf.org/rfc/rfc2629.txt>.
1754 [RFC-2847] Eisler, M., "LIPKEY - A Low Infrastructure Public Key
1755 Mechanism Using SPKM", RFC 2847, June 2000, <http://
1756 www.ietf.org/rfc/rfc2847.txt>.
1758 [RFC-2945] Wu, T., "The SRP Authentication and Key Exchange
1759 System", RFC 2945, September 2000, <http://
1760 www.ietf.org/rfc/rfc2945.txt>.
1762 [SASL] Myers, J., "Simple Authentication and Security Layer
1763 (SASL)", April 2001, <http://www.ietf.org/internet-
1764 drafts/draft-myers-saslrev-01.txt>.
1766 [SCAN] Hopwood, D., "Standard Cryptographic Algorithm Naming",
1767 June 2000, <http://www.eskimo.com/~weidai/scan-mirror/
1768 >.
1770 [SRP] Wu, T., "The Secure Remote Password Protocol", March
1771 1998, <http://srp.stanford.edu/ndss.html>.
1773 [SRP-impl] Wu, T., "SRP: The Open Source Password Authentication
1774 Standard", March 1998, <http://srp.stanford.edu/srp/>.
1776 [UMAC] Krovetz, T., Black, J., Halevi, S., Hevia, A.,
1777 Krawczyk, H. and P. Rogaway, "UMAC: Message
1778 Authentication Code using Universal Hashing", October
1779 2000, <http://www.ietf.org/internet-drafts/draft-
1780 krovetz-umac-01.txt>.
1782 [UNICODE-KC] Durst, D., "Unicode Standard Annex #15: Unicode
1783 Normalization Forms.", March 2001, <http://
1784 www.unicode.org/unicode/reports/tr15>.
1791 Burdis & Naffah Expires February 10, 2003 [Page 32]
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1793 Internet-Draft SRP SASL Mechanism August 2002
1796 Authors' Addresses
1798 Keith Burdis
1799 Rhodes University
1800 Computer Science Department
1801 Grahamstown 6139
1802 ZA
1804 EMail: keith@rucus.ru.ac.za
1807 Raif S. Naffah
1808 Forge Research Pty. Limited
1809 Suite 116, Bay 9
1810 Locomotive Workshop,
1811 Australian Technology Park
1812 Cornwallis Street
1813 Eveleigh, NSW 1430
1814 AU
1816 EMail: raif@forge.com.au
1847 Burdis & Naffah Expires February 10, 2003 [Page 33]
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1849 Internet-Draft SRP SASL Mechanism August 2002
1852 Appendix A. Modulus and Generator values
1854 Modulus (N) and generator (g) values for various modulus lengths are
1855 given below. In each case the modulus is a large safe prime and the
1856 generator is a primitve root of GF(n) [RFC-2945]. These values are
1857 taken from software developed by Tom Wu and Eugene Jhong for the
1858 Stanford SRP distribution [SRP-impl].
1861 [264 bits]
1862 Modulus (base 16) =
1863 115B8B692E0E045692CF280B436735C77A5A9E8A9E7ED56C965F87DB5B2A2
1864 ECE3
1865 Generator = 2
1867 [384 bits]
1868 Modulus (base 16) =
1869 8025363296FB943FCE54BE717E0E2958A02A9672EF561953B2BAA3BAACC3E
1870 D5754EB764C7AB7184578C57D5949CCB41B
1871 Generator = 2
1873 [512 bits]
1874 Modulus (base 16) =
1875 D4C7F8A2B32C11B8FBA9581EC4BA4F1B04215642EF7355E37C0FC0443EF75
1876 6EA2C6B8EEB755A1C723027663CAA265EF785B8FF6A9B35227A52D86633DB
1877 DFCA43
1878 Generator = 2
1880 [640 bits]
1881 Modulus (base 16) =
1882 C94D67EB5B1A2346E8AB422FC6A0EDAEDA8C7F894C9EEEC42F9ED250FD7F0
1883 046E5AF2CF73D6B2FA26BB08033DA4DE322E144E7A8E9B12A0E4637F6371F
1884 34A2071C4B3836CBEEAB15034460FAA7ADF483
1885 Generator = 2
1887 [768 bits]
1888 Modulus (base 16) =
1889 B344C7C4F8C495031BB4E04FF8F84EE95008163940B9558276744D91F7CC9
1890 F402653BE7147F00F576B93754BCDDF71B636F2099E6FFF90E79575F3D0DE
1891 694AFF737D9BE9713CEF8D837ADA6380B1093E94B6A529A8C6C2BE33E0867
1892 C60C3262B
1893 Generator = 2
1895 [1024 bits]
1896 Modulus (base 16) =
1897 EEAF0AB9ADB38DD69C33F80AFA8FC5E86072618775FF3C0B9EA2314C9C256
1898 576D674DF7496EA81D3383B4813D692C6E0E0D5D8E250B98BE48E495C1D60
1899 89DAD15DC7D7B46154D6B6CE8EF4AD69B15D4982559B297BCF1885C529F56
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1905 Internet-Draft SRP SASL Mechanism August 2002
1908 6660E57EC68EDBC3C05726CC02FD4CBF4976EAA9AFD5138FE8376435B9FC6
1909 1D2FC0EB06E3
1910 Generator = 2
1912 [1280 bits]
1913 Modulus (base 16) =
1914 D77946826E811914B39401D56A0A7843A8E7575D738C672A090AB1187D690
1915 DC43872FC06A7B6A43F3B95BEAEC7DF04B9D242EBDC481111283216CE816E
1916 004B786C5FCE856780D41837D95AD787A50BBE90BD3A9C98AC0F5FC0DE744
1917 B1CDE1891690894BC1F65E00DE15B4B2AA6D87100C9ECC2527E45EB849DEB
1918 14BB2049B163EA04187FD27C1BD9C7958CD40CE7067A9C024F9B7C5A0B4F5
1919 003686161F0605B
1920 Generator = 2
1922 [1536 bits]
1923 Modulus (base 16) =
1924 9DEF3CAFB939277AB1F12A8617A47BBBDBA51DF499AC4C80BEEEA9614B19C
1925 C4D5F4F5F556E27CBDE51C6A94BE4607A291558903BA0D0F84380B655BB9A
1926 22E8DCDF028A7CEC67F0D08134B1C8B97989149B609E0BE3BAB63D4754838
1927 1DBC5B1FC764E3F4B53DD9DA1158BFD3E2B9C8CF56EDF019539349627DB2F
1928 D53D24B7C48665772E437D6C7F8CE442734AF7CCB7AE837C264AE3A9BEB87
1929 F8A2FE9B8B5292E5A021FFF5E91479E8CE7A28C2442C6F315180F93499A23
1930 4DCF76E3FED135F9BB
1931 Generator = 2
1933 [2048 bits]
1934 Modulus (base 16) =
1935 AC6BDB41324A9A9BF166DE5E1389582FAF72B6651987EE07FC3192943DB56
1936 050A37329CBB4A099ED8193E0757767A13DD52312AB4B03310DCD7F48A9DA
1937 04FD50E8083969EDB767B0CF6095179A163AB3661A05FBD5FAAAE82918A99
1938 62F0B93B855F97993EC975EEAA80D740ADBF4FF747359D041D5C33EA71D28
1939 1E446B14773BCA97B43A23FB801676BD207A436C6481F1D2B9078717461A5
1940 B9D32E688F87748544523B524B0D57D5EA77A2775D2ECFA032CFBDBF52FB3
1941 786160279004E57AE6AF874E7303CE53299CCC041C7BC308D82A5698F3A8D
1942 0C38271AE35F8E9DBFBB694B5C803D89F7AE435DE236D525F54759B65E372
1943 FCD68EF20FA7111F9E4AFF73
1944 Generator = 2
1959 Burdis & Naffah Expires February 10, 2003 [Page 35]
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1961 Internet-Draft SRP SASL Mechanism August 2002
1964 Appendix B. Changes since the previous draft
1966 Removed the references to Rijndael since, strictly speaking it is not
1967 the AES. This should also eliminate any ambiguities as to the
1968 required block and key sizes this specification refers to when
1969 mentioning the AES.
1971 Removed the requirement for (a) an all-zero IV, and (b) a dummy first
1972 block in the operations of the Confidentiality Service filter.
1974 Included the description of a secure PRNG.
1976 Included the description of a Key Derivation Function (KDF) to ensure
1977 there will always be enough bytes to initialise both the CALG and
1978 IALG from the shared context key computed by the SRP calculations.
1980 Added a paragraph before the end of the "Security layer" section to
1981 clarify that this specification does not mandate nor imply a lockstep
1982 in operating the security services.
1984 Added a paragraph to the "Security considerations" section about the
1985 quality of the PRNG to use.
1987 Added the restriction that all text should be in Unicode
1988 Normalization form KC with NULs prohibited.
1990 Tightened up the restrictions on the options lists L and o by
1991 specifying that they must not contain any whitespace and must always
1992 be in lowercase. Changed "replay detection" to "replay_detection".
1993 This should simplify parsing of these lists.
1995 Added explicit notes that parameters received from the other party
1996 must be used as received in all digest calculations. Clearly any
1997 alteration of these input parameters (such as changing the case of
1998 text) will prevent the digest calculations on each side from
1999 producing the same result.
2001 Made it mandatory for the server to advertise at least one integrity
2002 protection algorithm and recommended that the HMAC-SHA-160 algorithm
2003 always be advertised. Recommended that the server always make
2004 integrity protection mandatory.
2006 Recommended that the client always select integrity protection, even
2007 if the server does not make it mandatory to do so. Also recommended
2008 that the client always select integrity protection when it selects
2009 confidentiality protection.
2011 Added Alexey Melnikov to Section 9.
2015 Burdis & Naffah Expires February 10, 2003 [Page 36]
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2017 Internet-Draft SRP SASL Mechanism August 2002
2020 Added a quote from section 3 of [RFC-2222] to the description of the
2021 security layer in Section 8 to describe the operation of the security
2022 layer in SASL.
2024 TODO: Amend the Cryptix SASL library and re-generate the example.
2071 Burdis & Naffah Expires February 10, 2003 [Page 37]
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2073 Internet-Draft SRP SASL Mechanism August 2002
2076 Full Copyright Statement
2078 Copyright (C) The Internet Society (2002). All Rights Reserved.
2080 This document and translations of it may be copied and furnished to
2081 others, and derivative works that comment on or otherwise explain it
2082 or assist in its implementation may be prepared, copied, published
2083 and distributed, in whole or in part, without restriction of any
2084 kind, provided that the above copyright notice and this paragraph are
2085 included on all such copies and derivative works. However, this
2086 document itself may not be modified in any way, such as by removing
2087 the copyright notice or references to the Internet Society or other
2088 Internet organizations, except as needed for the purpose of
2089 developing Internet standards in which case the procedures for
2090 copyrights defined in the Internet Standards process must be
2091 followed, or as required to translate it into languages other than
2092 English.
2094 The limited permissions granted above are perpetual and will not be
2095 revoked by the Internet Society or its successors or assigns.
2097 This document and the information contained herein is provided on an
2098 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
2099 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
2100 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
2101 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
2102 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
2104 Acknowledgement
2106 Funding for the RFC Editor function is currently provided by the
2107 Internet Society.
2127 Burdis & Naffah Expires February 10, 2003 [Page 38]
2128 \f