T. Wu
Internet-Draft Stanford University
Expire in six months June 1997
Telnet Authentication: SRP
<draft-wu-telnet-auth-option-00.txt>
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 <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.
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,
computed against the parameters exchanged earlier.
IAC SB AUTHENTICATION REPLY <authentication-type-pair> EXP
<server's exponential residue> 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 <authentication-type-pair> CHALLENGE
<challenge from server> IAC SE
This command presents the server's challenge to the client.
This is usually 128 bits (16 bytes) long.
IAC SB AUTHENTICATION IS <authentication-type-pair> RESPONSE
<response from client and optional counterchallenge> 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 <authentication-type-pair> ACCEPT
<server's response to counterchallenge> 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 <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.
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