Network Working Group K.R. Burdis Internet-Draft Rhodes University Expires: July 2, 2001 R. Naffah Forge Research January 2001 Secure Remote Password SASL Mechanism draft-burdis-cat-srp-sasl-04 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on July 2, 2001. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract This document describes a family of SASL mechanisms based on the Secure Remote Password protocol. These mechanisms perform mutual authentication and can provide a security layer with replay detection, integrity protection and/or confidentiality protection. Burdis & Naffah Expires July 2, 2001 [Page 1] Internet-Draft Secure Remote Password SASL Mechanism January 2001 Table of Contents 1. Mechanism Names . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in this Document . . . . . . . . . . . . . . 4 3. Data Element Formats . . . . . . . . . . . . . . . . . . . . . 5 3.1 Scalar numbers . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2 Multi-Precision Integers . . . . . . . . . . . . . . . . . . . 5 3.3 Octet Sequences . . . . . . . . . . . . . . . . . . . . . . . 6 3.4 Extended Octet Sequences . . . . . . . . . . . . . . . . . . . 6 3.5 Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.6 Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.7 Data Element Size Limits . . . . . . . . . . . . . . . . . . . 7 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 8 4.1 Client sends its authentication identity . . . . . . . . . . . 9 4.2 Server sends initial protocol elements . . . . . . . . . . . . 9 4.3 Client sends its ephemeral public key . . . . . . . . . . . . 10 4.4 Server sends its ephemeral public key . . . . . . . . . . . . 11 4.5 Client sends its evidence . . . . . . . . . . . . . . . . . . 11 4.6 Server sends its evidence . . . . . . . . . . . . . . . . . . 11 5. Security Layer . . . . . . . . . . . . . . . . . . . . . . . . 13 5.1 Confidentiality Protection . . . . . . . . . . . . . . . . . . 14 5.2 Replay Detection . . . . . . . . . . . . . . . . . . . . . . . 16 5.3 Integrity Protection . . . . . . . . . . . . . . . . . . . . . 16 5.4 Summary of Security Layer Output . . . . . . . . . . . . . . . 16 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 19 8. Security Considerations . . . . . . . . . . . . . . . . . . . 20 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21 References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23 A. Modulus and Generator values . . . . . . . . . . . . . . . . . 24 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 26 Burdis & Naffah Expires July 2, 2001 [Page 2] Internet-Draft Secure Remote Password SASL Mechanism January 2001 1. Mechanism Names The family of SASL mechanisms associated with the protocol described in this document are named "SRP-" where is the canonical name of a Message Digest Algorithm. For example, "SRP-SHA-160" shall denote the SASL mechanism using the protocol described in this document with SHA-1 (20-octet output length, or 160 bits) being used to compute both client-side and server-side digests. Similarly, "SRP-RIPEMD-160" shall denote the SASL mechanism using the protocol described in this document with RIPEMD-160 as the underlying Message Digest Algorithm. Burdis & Naffah Expires July 2, 2001 [Page 3] Internet-Draft Secure Remote Password SASL Mechanism January 2001 2. Conventions Used in this Document o A hex digit is an element of the set: {0, 1, 2, 3, 4, 5, 6, 7, 8 , 9, A, B, C, D, E, F} A hex digit is the representation of a 4-bit string. Examples: 7 = 0111 A = 1010 o An octet is an 8-bit string. In this document an octet may be written as a pair of hex digits. Examples: 7A = 01111010 02 = 00000010 o All data is encoded and sent in network byte order (big-endian). o The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [1]. Burdis & Naffah Expires July 2, 2001 [Page 4] Internet-Draft Secure Remote Password SASL Mechanism January 2001 3. Data Element Formats This section describes the encoding of the data elements used by the SASL mechanisms described in this document. 3.1 Scalar numbers Scalar numbers are unsigned quantities. Using b[k] to refer to the k-th octet being processed, the value of a two-octet scalar is: ((b[0] << 8) + b[1]), where << is the bit left-shift operator. The value of a four-octet scalar is: ((b[0] << 24) + (b[1] << 16) + (b[2] << 8) + b[3]). 3.2 Multi-Precision Integers Multi-Precision Integers, or MPIs, are positive integers used to hold large integers used in cryptographic computations. MPIs are encoded using a scheme inspired by that used by OpenPGP - RFC2440 (section 3.2) [2] - for encoding such entities: The encoded form of an MPI SHALL consist of two pieces: a two-octet scalar that represents the length of the entity, in octets, followed by a sequence of octets that contain the actual integer. These octets form a big-endian number; A big-endian number can be encoded by prefixing it with the appropriate length. Examples: (all numbers are in hexadecimal) The sequence of octets [00 01 01] encodes an MPI with the value 1, while the sequence [00 02 01 FF] encodes an MPI with the value of 511 Additional rule: * The length field of an encoded MPI describes the octet count starting from the MPI's first non-zero octet, containing the most significant non-zero bit. Thus, the encoding [00 02 01] is not formed correctly; It should be [00 01 01]. We shall use the syntax mpi(A) to denote the encoded form of the multi-precision integer A. Furthermore, we shall use the syntax bytes(A) to denote the big-endian sequence of octets forming the Burdis & Naffah Expires July 2, 2001 [Page 5] Internet-Draft Secure Remote Password SASL Mechanism January 2001 multi-precision integer with the most significant octet being the first non-zero octet containing the most significant bit of A. 3.3 Octet Sequences These mechanisms generate, use and exchange sequences of octets; e.g. output values of message digest algorithm functions. When such entities travel on the wire, they shall be preceded by a one-octet scalar quantity representing the count of following octets. We shall use the syntax os(s) to denote the encoded form of the octet sequence. Furthermore, we shall use the syntax bytes(s) to denote the sequence of octets s, in big-endian order. 3.4 Extended Octet Sequences Extended sequences of octets are exchanged when using the security layer. When these sequences travel on the wire, they shall be preceded by a four-octet scalar quantity representing the count of following octets. We shall use the syntax eos(s) to denote the encoded form of the extended octet sequence. Furthermore, we shall use the syntax bytes(s) to denote the sequence of octets s, in big-endian order. 3.5 Text The only character set for text is the UTF-8 [3] encoding of Unicode characters [4]. We shall use the syntax utf8(L) to denote the string L in UTF-8 encoding, preceded by a two-octet scalar quantity representing the count of following octets. Furthermore, we shall use the syntax bytes(L) to denote the sequence of octets representing the UTF-8 encoding of L, in big-endian order. 3.6 Buffers In these SASL mechanisms data is exchanged between the client and server using buffers. A buffer acts as an envelope for the sequence of data elements sent by one end-point of the exchange, and expected by the other. A buffer MAY NOT contain other buffers. It may only contain zero, one or more data elements. A buffer shall be encoded as two fields: a four-octet scalar quantity representing the count of following octets, and the concatenation of the octets of the data element(s) contained in the Burdis & Naffah Expires July 2, 2001 [Page 6] Internet-Draft Secure Remote Password SASL Mechanism January 2001 buffer. We shall use the syntax {A|B|C} to denote a buffer containing A, B and C in that order. For example: { mpi(N) | mpi(g) | utf8(L) } is a buffer containing, in the designated order, the encoded forms of an MPI N, an MPI g and a Text L. 3.7 Data Element Size Limits The following table details the size limit, in number of octets, for each of the SASL data element encodings described earlier. Data element type Header Size limit in octets (octets) (excluding header) ------------------------------------------------------------ Octet Sequence 1 255 MPI 2 65,535 Text 2 65,535 Extended Octet Sequence 4 2,147,483,383 Buffer 4 2,147,483,643 An implementation SHOULD signal an exception if any size constraint is violated. Burdis & Naffah Expires July 2, 2001 [Page 7] Internet-Draft Secure Remote Password SASL Mechanism January 2001 4. Protocol Description SRP is a password-based, zero-knowledge, authentication and key-exchange protocol developed by Thomas Wu. It has good performance, is not plaintext-equivalent and maintains perfect forward secrecy. It provides authentication (optionally mutual authentication) and the negotiation of a session key [12]. The mechanisms described herein are based on the optimised SRP protocol described at the end of section 3 in [13], since this reduces the total number of messages exchanged by grouping together pieces of information that do not depend on earlier messages. Due to the design of the mechanism, mutual authentication is MANDATORY. This document describes the sequence of data transmitted between the client and server, and it adds extra control information to enable the client to request whether or not replay detection, integrity protection and/or confidentiality protection should be provided by a security layer. Mechanism data exchanges, during the authentication phase, are shown below: Client Server ----- { utf8(U) } --------------------------------> <-------------- { mpi(N) | mpi(g) | utf8(L) } ----- ----- { utf8(I) | mpi(A) | utf8(o) } -------------> <------------------------- { os(s) | mpi(B) } ----- ----- { os(M1) } ---------------------------------> <--------------------------------- { os(M2) } ----- where: U is the authentication identity (username), N is the safe prime modulus, g is the generator, L is the options list indicating available security services, Burdis & Naffah Expires July 2, 2001 [Page 8] Internet-Draft Secure Remote Password SASL Mechanism January 2001 I is the authorisation identity, A is the client's ephemeral public key, o is the options list indicating chosen security services, s is the user's password salt, B is the server's ephemeral public key, M1 is the client's evidence that the shared key K is known, M2 is the server's evidence that the shared key K is known. 4.1 Client sends its authentication identity The client determines its authentication identity U, encodes it and sends it to the server. The client sends: { utf8(U) } 4.2 Server sends initial protocol elements The server receives U, and looks up the safe prime modulus N and the generator g to be used for that identity. The server also creates an options list L, which consists of a comma-separated list of option strings that specify the security service options the server supports. The following security service options strings are defined: o "integrity=HMAC-" indicates that the server supports integrity protection using the HMAC algorithm [9] with as the underlying Message Digest Algorithm. Acceptable MDA names are chosen from [15] under the MessageDigest section. A server SHOULD send such an option string for each HMAC algorithm it supports. Note that in the interest of interoperability, if the server offers integrity protection it MUST, as a minimum, send the option string "integrity=HMAC-MD5" since support for this algorithm is then MANDATORY. o "replay detection" indicates that the server supports replay detection using sequence numbers. o "confidentiality=" indicates that the server supports confidentiality protection using the symmetric block cipher algorithm . The server SHOULD send such an Burdis & Naffah Expires July 2, 2001 [Page 9] Internet-Draft Secure Remote Password SASL Mechanism January 2001 option string for each confidentiality protection algorithm it supports. Note that in the interest of interoperability, if the server offers confidentiality protection, it MUST send the option string "confidentiality=aes" since it is then MANDATORY for it to provide support for this algorithm. (Rijndael [5] is synonymous with AES [6].) Additional rules: o Replay detection SHALL NOT be activated without also activating integrity protection. If the replay detection option is offered (by the server) and/or chosen (by the client) without explicitely specifying an integrity protection option, then the default integrity protection option "integrity=HMAC-MD5" is implied and shall be activated. o The options list SHOULD NOT be interpreted in a case-sensitive manner, and whitespace characters SHOULD be ignored. For example, if the server supports integrity protection using the HMAC-MD5 and HMAC-SHA-160 algorithms, replay detection and no confidentiality protection, the options list would be: integrity=HMAC-MD5,integrity=HMAC-SHA-160,replay detection The server sends: { mpi(N) | mpi(g) | utf8(L) } 4.3 Client sends its ephemeral public key The client receives the options list L from the server that specifies the security service options the server supports. The client selects options from this list and creates a new options list o that specifies the security services that will be used in the security layer. At most one available integrity protection algorithm and one available confidentiality protection algorithm may be selected. The client determines its authorisation identity I, and generates its ephemeral public key A. The client sends: { utf8(I) | mpi(A) | utf8(o) } Burdis & Naffah Expires July 2, 2001 [Page 10] Internet-Draft Secure Remote Password SASL Mechanism January 2001 4.4 Server sends its ephemeral public key The server reads the client's salt s, calculates the shared context key K and generates its ephemeral public key B. The server sends: { os(s) | mpi(B) } 4.5 Client sends its evidence The client calculates the shared context key K, and calculates the evidence M1 that proves to the server that it knows the shared context key K, including L as part of the calculation. M1 is computed as: H( bytes(H( bytes(N) )) ^ bytes( H( bytes(g) ))) | bytes(H( bytes(U) )) | bytes(s) | bytes(H( bytes(L) )) | bytes(A) | bytes(B) | bytes(K) ) where: H() is the result of digesting the designated input/data with the underlying Message Digest Algorithm function (see Section 1). ^ is the bitwise XOR operator. The client sends: { os(M1) } 4.6 Server sends its evidence The server calculates the evidence M2 that proves to the client that it knows the shared context key K, as well as U, I, and o. M2 is computed as: Burdis & Naffah Expires July 2, 2001 [Page 11] Internet-Draft Secure Remote Password SASL Mechanism January 2001 H( bytes(A) | bytes(H( bytes(U) )) | bytes(H( bytes(I) )) | bytes(H( bytes(o) )) | bytes(M1) | bytes(K) ) where: H() is the result of digesting the designated input/data with the underlying Message Digest Algorithm function (see Section 1) The server sends: { os(M2) } Burdis & Naffah Expires July 2, 2001 [Page 12] Internet-Draft Secure Remote Password SASL Mechanism January 2001 5. Security Layer Depending on the options offered by the server and specified by the client, the security layer may provide integrity protection, replay detection, and/or confidentiality protection. The security layer can be thought of as a three-stage filter through which the data flows from the output of one stage to the input of the following one. The first input is the original data, while the last output is the data after being subject to the transformations of this filter. The data always passes through this three-stage filter, though any of the stages may be inactive. Only when a stage is active would the output be different from the input. In other words, if a stage is inactive, the octet sequence at the output side is an exact duplicate of the same sequence at the input side. Schematically, the three-stage filter security layer appears as follows: +----------------------------+ | | I/ p1 p1 --->| Confidentiality protection |---+ | | | A/ c +----------------------------+ | | +------------------------------------+ | | +----------------------------+ | | | I/ p2 p2 +-->| Replay detection |---+ | | | A/ p2 | q +----------------------------+ | | +------------------------------------+ | | +----------------------------+ | | | I/ p3 p3 +-->| Integrity protection |---> | | A/ p3 | C +----------------------------+ where: p1, p2 and p3 are the input octet sequences at each stage, Burdis & Naffah Expires July 2, 2001 [Page 13] Internet-Draft Secure Remote Password SASL Mechanism January 2001 I/ denotes the output at the end of one stage if/when the stage is inactive or disabled, A/ denotes the output at the end of one stage if/when the stage is active or enabled, c is the encrypted (sender-side) or decrypted (receiver-side) octet sequence. c1 shall denote the value computed by the sender, while c2 shall denote the value computed by the receiver. q is a four-octet scalar quantity representing a sequence number, C is the Message Authentication Code. C1 shall denote the value of the MAC as computed by the sender, while C2 shall denote the value computed by the receiver. The following paragraphs detail each of the transformations mentioned above. 5.1 Confidentiality Protection The plaintext data octet sequence p1 is encrypted using the chosen confidentiality algorithm (CALG) initialised for encryption with the shared context key K. c1 = CALG(K, ENCRYPTION)( bytes(p1) ) On the receiving side, the ciphertext data octet sequence p1 is decrypted using the chosen confidentiality algorithm (CALG) initialised for decryption, with the shared context key K. c2 = CALG(K, DECRYPTION)( bytes(p1) ) The designated CALG block cipher should be used in OFB (Output Feedback Block) mode in the ISO variant, as described in [16], algorithm 7.20. Let k be the block size of the chosen symmetric cipher algorithm; e.g. for AES this is 128 bits or 16 octets. The OFB mode used shall be of length/size k. It is recommended that Block ciphers operating in OFB mode be used with an Initial Vector (the mode's IV). For the SASL mechanisms described in this document, the IV shall be an all-zero octet sequence of size k. In such a mode of operation - OFB with key re-use - the IV, which need not be secret, must be changed. Otherwise an identical keystream results; and, by XORing corresponding ciphertexts, an Burdis & Naffah Expires July 2, 2001 [Page 14] Internet-Draft Secure Remote Password SASL Mechanism January 2001 adversary may reduce cryptanalysis to that of a running-key cipher with one plaintext as the running key. To counter the effect of fixing the IV to an all-zero octet sequence, the sender should use a one k-octet sequence as the value of its first block, constructed as follows: o the first (most significant) (k-2) octets are random, o the octets at position #k-1 and #k, assuming the first octet is at position #1, are exact copies of those at positions #1 and #2 respectively. The input data to the confidentiality protection algorithm shall be a multiple of the symmetric cipher block size k. When the input length is not a multiple of k octets, the data shall be padded according to the following scheme (described in [17] which itself is based on RFC1423 [18]): Assuming the length of the input is l octets, (k - (l mod k)) octets, all having the value (k - (l mod k)), shall be appended to the original data. In other words, the input is padded at the trailing end with one of the following sequences: 01 -- if l mod k = k-1 02 02 -- if l mod k = k-2 ... ... ... k k ... k k -- if l mod k = 0 The padding can be removed unambiguously since all input is padded and no padding sequence is a suffix of another. This padding method is well-defined if and only if k < 256 octets, which is the case with symmetric block ciphers today, and in the forseeable future. The output of this stage, when it is active, is: at the sending side: CALG(K, ENCRYPT)( bytes(p1) ) at the receiving side: CALG(K, DECRYPT)( bytes(p1) ) If the receiver, after decrypting the first block, finds that the last two octets do not match the value of the first two, it MUST signal an exception and abort the exchange. Burdis & Naffah Expires July 2, 2001 [Page 15] Internet-Draft Secure Remote Password SASL Mechanism January 2001 5.2 Replay Detection A sequence number q is incremented every time a message is sent to the peer. The output of this stage, when it is active, is: p2 | q At the other end, the receiver increments its copy of the sequence number. This new value of the sequence number is then used in the integrity protection transformation, which must also be active as described in Section 4.2. 5.3 Integrity Protection A message authentication code C is computed using the chosen integrity protection algorithm (IALG) initialised with the shared context key K, and applied to the sequence p3. The output of this stage, when it is active, is: IALG(K)( bytes(p3) ) At the other end, the receiver computes its version of the MAC, using the same transformation, and checks if its value is equal to that received. If the two values do not agree, the receiver MUST signal an exception and abort the exchange. 5.4 Summary of Security Layer Output The following table shows the data exchanged by the security layer peers, depending on the possible legal combinations of the three security services in operation: CP IP RD Peer sends/receives I I I { eos(p) } I A I { eos(p) | os( IALG(K)( bytes(p) ) ) } I A A { eos(p) | os( IALG(K)( bytes(p) | bytes(q)) ) } A I I { eos(c) } A A I { eos(c) | os( IALG(K)( bytes(c) ) ) } A A A { eos(c) | os( IALG(K)((bytes(c) | bytes(q)) ) } where CP Confidentiality protection, Burdis & Naffah Expires July 2, 2001 [Page 16] Internet-Draft Secure Remote Password SASL Mechanism January 2001 IP Integrity protection, RD Replay detection, I Security service is Inactive/disabled, A Security service is Active/enabled, p The original plaintext, q The sequence number. c The enciphered input obtained by either: CALG(K, ENCRYPT)( bytes(p) ) at the sender's side, or CALG(K, DECRYPT)( bytes(p) ) at the receiver's side, or Burdis & Naffah Expires July 2, 2001 [Page 17] Internet-Draft Secure Remote Password SASL Mechanism January 2001 6. Example The example below uses SMTP authentication [19]. The base64 encoding of challenges and responses, as well as the reply codes preceding the responses are part of the SMTP authentication[19] specification, not part of this SASL mechanism itself. "C:" and "S:" indicate lines sent by the client and server respectively. S: 220 smtp.example.com ESMTP server ready C: EHLO zaau.example.com S: 250-smtp.example.com S: 250 AUTH SRP-SHA-160 CRAM-MD5 DIGEST-MD5 C: AUTH SRP-SHA-160 AAAABQADZm9v S: AAAAqgCA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxObIlFKCH mONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjftawv/XLb0Brf t7jhr+1qJn6WunyQRfEsf5kkoZlHs5lOB//////////8AAQUAI2ludGVncml0eT1obWFj LW1kNSxyZXBsYXkgZGV0ZWN0aW9u C: AAAArAADZm9vAIBFoAAiZ7mnsz2UBmAtV4t2nW973SBNLUdL9BC3AG0CC0TCtYjjwP dhobc02S9ERw7G+lPcmAFXGO6KDHc7AXe33xp+WwGGkIyB49oJB8VZ+sXqCr6OBMFvV1H okkzIyjhogn2OZVdn89FryqG4LwuEsypCPGQ+cgxYWUGTIuAMrwAjaW50ZWdyaXR5PWht YWMtbWQ1LHJlcGxheSBkZXRlY3Rpb24= S: AAAAjgqSCwkzSOiPQ1JnAIEAmkVIho/d/xckmrzp1nMEtkWKxlOOiX0V8u+a9y9/0V KgzKJlcT+QI/uQH9l23tnfOOK3CfDuaZMnQgMLNCsvRy22x6YhZW07zo39QhMWLWLSjVJ lWXgxSQyds1JvVAQzZN+XaFdZs5lMDfSJMiC8L7MzZyw8XmHh5v1DtueK9mc= C: AAAAFRS0T1/zTL9Idv9R5F7tuCFMtWrCGg== S: AAAAFRShvobx8ubyF8fUAuupQIfWYPdu4A== C: S: 235 Authentication successful. Burdis & Naffah Expires July 2, 2001 [Page 18] Internet-Draft Secure Remote Password SASL Mechanism January 2001 7. Discussion The algorithms specified as mandatory were chosen for utility and availablity. We felt that a mandatory confidentiality and integrity protection algorithm should be specified to ensure interoperability between implementations of these mechanisms. o The HMAC-MD5 algorithm was chosen as an integrity algorithm because it is faster than both HMAC-SHA-160 and MAC algorithms based on secret key encryption algorithms [8]. o Rijndael was chosen as a cipher because it has undergone thorough scrutiny by the best cryptographers in the world and was chosen ahead of many other algorithms as the Advanced Encryption Standard. Since confidentiality protection is optional this mechanism should be usable in countries that have strict controls on the use of cryptography. It is RECOMMENDED that the server use values for the modulus (N) and generator (g) chosen from those listed in Appendix A so that the client can avoid expensive constraint checks, since these predefined values already meet the constraints described in [13]: "For maximum security, N should be a safe prime (i.e. a number of the form N = 2q + 1, where q is also prime). Also, g should be a generator modulo N (see [SRP] for details), which means that for any X where 0 < X < N, there exists a value x for which g^x % N == X." Burdis & Naffah Expires July 2, 2001 [Page 19] Internet-Draft Secure Remote Password SASL Mechanism January 2001 8. Security Considerations These mechanisms rely on the security of SRP, which bases its security on the difficulty of solving the Diffie-Hellman problem in the multiplicative field modulo a large safe prime. See section 4 "Security Considerations" of [13] and section 4 "Security analysis" of [12]. This mechanism also relies on the security of the HMAC algorithm and the underlying hash function. Section 6 "Security" of [9] discusses these security issues in detail. Weaknesses found in MD5 do not impact HMAC-MD5 [7]. U, I, A and o, sent from the client to the server, and N, g, L, s and B, sent from the server to the client could be modified by an attacker before reaching the other party. For this reason, these values are included in the respective calculations of evidence (M1 and M2) to prove that each party knows the session key. This allows each party to verify that these values were received unmodified. The use of integrity protection is RECOMMENDED to detect message tampering and to avoid session hijacking after authentication has taken place. Replay attacks may be avoided through the use of sequence numbers, because sequence numbers make each integrity protected message exchanged during a session different, and each session uses a different key. Burdis & Naffah Expires July 2, 2001 [Page 20] Internet-Draft Secure Remote Password SASL Mechanism January 2001 9. Acknowledgements The following people provided valuable feedback in the preparation of this document: Timothy Martin Burdis & Naffah Expires July 2, 2001 [Page 21] Internet-Draft Secure Remote Password SASL Mechanism January 2001 References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 0014, RFC 2119, March 1997. [2] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP Message Format", RFC 2440, November 1998. [3] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO 10646", RFC 2279, January 1998. [4] "International Standard --Information technology-- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1 Architecture and Basic Multilingual Plane", ISO/IEC 10646-1, 1993. [5] Daemen, Joan and Vincent Rijmen, "AES Proposal: Rijndael", September 1999, . [6] National Institute of Standards and Technology, "Rijndael: NIST's Selection for the AES", December 2000, . [7] Dobbertin, H., "The Status of MD5 After a Recent Attack", December 1996, . [8] Eisler, M., "LIPKEY - A Low Infrastructure Public Key Mechanism Using SPKM", RFC 2847, June 2000. [9] Krawczyk, H. et al, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [10] Myers, J.G., "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997. [11] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, June 1999. [12] Wu, T., "The Secure Remote Password Protocol", March 1998, . [13] Wu, T., "The SRP Authentication and Key Exchange System", RFC 2945, September 2000. [14] Wu, T., "SRP: The Open Source Password Authentication Standard", March 1998, . Burdis & Naffah Expires July 2, 2001 [Page 22] Internet-Draft Secure Remote Password SASL Mechanism January 2001 [15] Hopwood, D., "Standard Cryptographic Algorithm Naming", June 2000, . [16] Menezes, A.J., van Oorschot, P.C. and S.A. Vanstone, "Handbook of Applied Cryptography", CRC Press, Inc., ISBN 0-8493-8523-7, 1997, . [17] RSA Data Security, Inc., "PKCS #7: Cryptographic Message Syntax Standard", Version 1.5, November 1993, . [18] Balenson, D., "Privacy Enhancement for Internet Electronic Mail: Part III: Algorithms, Modes, and Identifiers", RFC 1423, February 1993, . [19] Myers, J.G., "SMTP Service Extension for Authentication", RFC 2554, March 1999. Authors' Addresses K.R. Burdis Rhodes University Computer Science Department Grahamstown 6139 ZA EMail: keith@rucus.ru.ac.za URI: http://www.cryptix.org/~keith/ Raif S. Naffah Forge Research Pty. Limited Suite 116, Bay 9 Locomotive Workshop, Australian Technology Park Cornwallis Street Eveleigh, NSW 1430 AU EMail: raif@forge.com.au URI: http://www.cryptix.org/~raif/ Burdis & Naffah Expires July 2, 2001 [Page 23] Internet-Draft Secure Remote Password SASL Mechanism January 2001 Appendix A. Modulus and Generator values Modulus (N) and generator (g) values for various modulus lengths are given below. In each case the modulus is a large safe prime and the generator is a primitve root of GF(n) [12]. These values are taken from software developed by Tom Wu and Eugene Jhong for the Stanford SRP distribution [14]. [264 bits] Modulus (base 16) = 115B8B692E0E045692CF280B436735C77A5A9E8A9E7ED56C965F87DB5B2A2ECE 3 Generator = 2 [384 bits] Modulus (base 16) = 8025363296FB943FCE54BE717E0E2958A02A9672EF561953B2BAA3BAACC3ED57 54EB764C7AB7184578C57D5949CCB41B Generator = 2 [512 bits] Modulus (base 16) = D4C7F8A2B32C11B8FBA9581EC4BA4F1B04215642EF7355E37C0FC0443EF756EA 2C6B8EEB755A1C723027663CAA265EF785B8FF6A9B35227A52D86633DBDFCA43 Generator = 2 [640 bits] Modulus (base 16) = C94D67EB5B1A2346E8AB422FC6A0EDAEDA8C7F894C9EEEC42F9ED250FD7F0046 E5AF2CF73D6B2FA26BB08033DA4DE322E144E7A8E9B12A0E4637F6371F34A207 1C4B3836CBEEAB15034460FAA7ADF483 Generator = 2 [768 bits] Modulus (base 16) = B344C7C4F8C495031BB4E04FF8F84EE95008163940B9558276744D91F7CC9F40 2653BE7147F00F576B93754BCDDF71B636F2099E6FFF90E79575F3D0DE694AFF 737D9BE9713CEF8D837ADA6380B1093E94B6A529A8C6C2BE33E0867C60C3262B Generator = 2 [1024 bits] Modulus (base 16) = EEAF0AB9ADB38DD69C33F80AFA8FC5E86072618775FF3C0B9EA2314C9C256576 D674DF7496EA81D3383B4813D692C6E0E0D5D8E250B98BE48E495C1D6089DAD1 5DC7D7B46154D6B6CE8EF4AD69B15D4982559B297BCF1885C529F566660E57EC 68EDBC3C05726CC02FD4CBF4976EAA9AFD5138FE8376435B9FC61D2FC0EB06E3 Generator = 2 Burdis & Naffah Expires July 2, 2001 [Page 24] Internet-Draft Secure Remote Password SASL Mechanism January 2001 [1280 bits] Modulus (base 16) = D77946826E811914B39401D56A0A7843A8E7575D738C672A090AB1187D690DC4 3872FC06A7B6A43F3B95BEAEC7DF04B9D242EBDC481111283216CE816E004B78 6C5FCE856780D41837D95AD787A50BBE90BD3A9C98AC0F5FC0DE744B1CDE1891 690894BC1F65E00DE15B4B2AA6D87100C9ECC2527E45EB849DEB14BB2049B163 EA04187FD27C1BD9C7958CD40CE7067A9C024F9B7C5A0B4F5003686161F0605B Generator = 2 [1536 bits] Modulus (base 16) = 9DEF3CAFB939277AB1F12A8617A47BBBDBA51DF499AC4C80BEEEA9614B19CC4D 5F4F5F556E27CBDE51C6A94BE4607A291558903BA0D0F84380B655BB9A22E8DC DF028A7CEC67F0D08134B1C8B97989149B609E0BE3BAB63D47548381DBC5B1FC 764E3F4B53DD9DA1158BFD3E2B9C8CF56EDF019539349627DB2FD53D24B7C486 65772E437D6C7F8CE442734AF7CCB7AE837C264AE3A9BEB87F8A2FE9B8B5292E 5A021FFF5E91479E8CE7A28C2442C6F315180F93499A234DCF76E3FED135F9BB Generator = 2 [2048 bits] Modulus (base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enerator = 2 Burdis & Naffah Expires July 2, 2001 [Page 25] Internet-Draft Secure Remote Password SASL Mechanism January 2001 Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. 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