Internet Draft T. Wu draft-wu-telnet-auth-srp-01.txt Stanford University Expires 31 September 1998 March 1998 Telnet Authentication: SRP Status of this Memo This document is an Internet-Draft. 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." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document specifies an authentication scheme for the Telnet protocol under the framework described in RFC 1416, using the SRP authentication mechanism. The specific mechanism, SRP-SHA1, is described in [SRP-DRAFT]. 1. Command Names and Codes Authentication Types SRP 5 Suboption Commands AUTH 0 REJECT 1 ACCEPT 2 CHALLENGE 3 RESPONSE 4 EXP 8 PARAMS 9 draft-wu-telnet-auth-srp-01.txt [Page 2]
2. Command Meanings IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH IAC SE This command indicates that the client has supplied the username and is ready to receive that user's field parameters. There is no authentication information to be sent to the remote side of the connection yet. This should only be sent after the IAC SB AUTHENTICATION NAME command has been issued. IAC SB AUTHENTICATION REPLY <authentication-type-pair> PARAMS <values of modulus, generator, and salt> IAC SE This command is used to pass the three parameter values used in the exponentiation to the client. These values are often called n, g, and s. IAC SB AUTHENTICATION IS <authentication-type-pair> EXP <client's exponential residue> IAC SE This command is used to pass the client's exponential residue, otherwise known as A, computed against the parameters exchanged earlier. IAC SB AUTHENTICATION REPLY <authentication-type-pair> CHALLENGE <server's exponential residue> IAC SE This command is used to pass the server's exponential residue, computed against the same parameters. This quantity is actually the sum of two residues, i.e. g^x + g^b. For details see [SRP] and [SRP-DRAFT]. IAC SB AUTHENTICATION IS <authentication-type-pair> RESPONSE <response from client> IAC SE This command gives the server proof of the client's authenticity with a 160-bit (20 byte) response. IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT <server's response> IAC SE This command indicates that the authentication was successful. The server will construct its own proof of authenticity and include it as sub-option data. IAC SB AUTHENTICATION REPLY <authentication-type-pair> REJECT <optional reason for rejection> IAC SE This command indicates that the authentication was not successful, and if there is any more data in the sub-option, it is an ASCII text message of the reason for the rejection. draft-wu-telnet-auth-srp-01.txt [Page 3]
For the PARAMS command, since three pieces of data are being transmitted, each parameter is preceded by a 16-bit (two byte) length specifier in network byte order. The EXP commands do not have a count in front of the data because there is only one piece of data in that suboption. The CHALLENGE, RESPONSE, and ACCEPT data also do not have a count because they are all fixed in size. 3. Implementation Rules Currently, only AUTH_CLIENT_TO_SERVER mode is supported. Although the SRP protocol effectively performs implicit mutual authentication as a result of the two-way proofs, only the AUTH_HOW_ONE_WAY authentication mode is currently defined. The AUTH_HOW_MUTUAL setting is being reserved for an explicit mutual-authentication variant of the SRP protocol to be defined in future specifications. All large number data sent in the arguments of the PARAMS and EXP commands must be in network byte order, i.e. most significant byte first. No padding is used. The SRP-SHA1 mechanism, as described in [SRP-DRAFT] generates a 40-byte session key, which is much longer than the 8-byte key generated by conventional authentication mechanisms like Kerberos [KERBEROS-AUTH]. This allows implementations to use different keys for incoming and outgoing traffic, increasing the security of the encrypted session. 4. Examples User "tjw" may wish to log in on machine "foo". The client would send IAC SB AUTHENTICATION NAME "tjw" IAC SE IAC SB AUTHENTICATION IS SRP AUTH IAC SE. The server would look up the field and salt parameters for "tjw" from its password file and send them back to the client. Client and server would then exchange exponential residues and calculate their session keys (after the client prompted "tjw" for his password). Then, the client would send the server its proof that it knows the session key. The server would either send back an ACCEPT or a REJECT. If the server accepts authentication, it also sends its own proof that it knows the session key to the client. draft-wu-telnet-auth-srp-01.txt [Page 4]
Client Server IAC DO AUTHENTICATION IAC WILL AUTHENTICATION [ The server is now free to request authentication information. ] IAC SB AUTHENTICATION SEND SRP CLIENT|ONE_WAY IAC SE [ The server has requested SRP authentication. This is the only mode currently supported. The client will now respond with the name of the user that it wants to log in as. ] IAC SB AUTHENTICATION NAME "tjw" IAC SE IAC SB AUTHENTICATION IS SRP CLIENT|ONE_WAY AUTH IAC SE [ The server looks up the appropriate information for "tjw" and sends back the parameters in a PARAMS command. ] IAC SB AUTHENTICATION REPLY SRP CLIENT|ONE_WAY PARAMS ss ss nn nn nn nn ... ss ss gg gg gg gg ... ss ss tt tt tt tt ... IAC SE [ Both sides send their exponential residues. In SRP-3, the CHALLENGE message may be computed but not sent before the EXP command. ] IAC SB AUTHENTICATION IS SRP CLIENT|ONE_WAY EXP aa aa aa aa aa aa aa aa ... IAC SE IAC SB AUTHENTICATION REPLY SRP CLIENT|ONE_WAY CHALLENGE bb bb bb bb bb bb bb bb ... IAC SE [ The client sends its response to the server. ] IAC SB AUTHENTICATION IS SRP CLIENT|ONE_WAY RESPONSE xx xx xx xx xx xx xx xx ... IAC SE [ The server accepts the response and sends its own proof. ] IAC SB AUTHENTICATION REPLY SRP CLIENT|ONE_WAY ACCEPT yy yy yy yy yy yy yy yy ... IAC SE draft-wu-telnet-auth-srp-01.txt [Page 5]
5. Security Considerations The ability to negotiate a common authentication mechanism between client and server is a feature of the authentication option that should be used with caution. When the negotiation is performed, no authentication has yet occurred. Therefore, each system has no way of knowing whether or not it is talking to the system it intends. An intruder could attempt to negotiate the use of an authentication system which is either weak, or already compromised by the intruder. Since SRP relies on the security of the underlying public-key cryptosystem, the modulus "n" should be large enough to resist brute-force attack. A length of at least 1024 bits is recommended, and implementations should reject attempts to use moduli that are shorter than 512 bits. Because SRP is believed to offer greater protection against intruders than previous Telnet authentication mechanisms, it is recommended that it be placed ahead of alternatives when negotiating a common authentication mechanism. Some sites may wish to disable other weaker mechanisms completely for maximal security; implementations should permit this policy to be set easily on a site-by-site basis. 6. References [RFC1416] D. Borman, "Telnet Authentication Option", RFC 1416, February 1993. [SRP] T. Wu, "The Secure Remote Password Protocol", In Proceedings of the 1998 ISOC Network and Distributed System Security Symposium, San Diego, CA, pp. 97-111. [SRP-DRAFT] T. Wu, "The SRP Authentication and Key Exchange System", draft-wu-srp-auth-XX.txt, Stanford University. 7. Author's Address Thomas Wu Stanford University Stanford, CA 94305 EMail: tjw@cs.Stanford.EDU