T. Wu Internet-Draft Stanford University Expire in six months June 1997 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 (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 1. Command Names and Codes Authentication Types SRP 5 Suboption Commands AUTH 0 REJECT 1 ACCEPT 2 CHALLENGE 3 RESPONSE 4 EXP 5 PARAMS 6 2. Command Meanings IAC SB AUTHENTICATION IS 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 PARAMS IAC SE This command is used to pass the three parameter values used in the exponentiation to the client. IAC SB AUTHENTICATION IS EXP IAC SE This command is used to pass the client's exponential residue, computed against the parameters exchanged earlier. IAC SB AUTHENTICATION REPLY EXP IAC SE This command is used to pass the server's exponential residue, computed against the same parameters. Note that the same sub-option is used for both sides of the transmission. IAC SB AUTHENTICATION REPLY CHALLENGE IAC SE This command presents the server's challenge to the client. This is usually 128 bits (16 bytes) long. IAC SB AUTHENTICATION IS RESPONSE IAC SE This command answers the server's challenge with a 128-bit (16 bit) response. The client is also permitted to issue its own challenge, which follows the response. IAC SB AUTHENTICATION REPLY ACCEPT IAC SE This command indicates that the authentication was successful. If the client included a counterchallenge in its response, the server will construct a response to that challenge and include it as sub-option data. IAC SB AUTHENTICATION REPLY REJECT 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. For the PARAMS command, since three pieces of data are being transmitted, each parameter is preceded by a byte count, two bytes long with the high-order byte first. 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 can effectively perform mutual authentication through its counterchallenge, 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 which has yet to be defined. 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 server would send a challenge, the client would respond to it with both a response and an optional counterchallenge, and the server would either send back an ACCEPT or a REJECT. If the server accepts authentication, it also responds to the client's counterchallenge if one was issued. 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. The ordering of the two EXP messages may vary with the relative speeds of the two computers. ] IAC SB AUTHENTICATION IS SRP CLIENT|ONE_WAY EXP ww ww ww ww ww ww ww ww ... IAC SE IAC SB AUTHENTICATION REPLY SRP CLIENT|ONE_WAY EXP yy yy yy yy yy yy yy yy ... IAC SE [ The server sends a 16-byte challenge to the client. ] IAC SB AUTHENTICATION REPLY SRP CLIENT|ONE_WAY CHALLENGE xx xx xx xx xx xx xx xx ... IAC SE [ The client sends its response along with a counterchallenge to the server. ] IAC SB AUTHENTICATION IS SRP CLIENT|ONE_WAY RESPONSE rr rr rr rr rr rr rr rr ... zz zz zz zz zz zz zz zz ... IAC SE [ The server accepts the response and answers the counterchallenge. ] IAC SB AUTHENTICATION REPLY SRP CLIENT|ONE_WAY ACCEPT aa aa aa aa aa aa aa aa ... IAC SE 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. Author's Address Thomas Wu Stanford University Stanford, CA 94305 Phone: (415) 725-6969 EMail: tjw@cs.Stanford.EDU