Network Working Group K. Alagappan
Request for Comments: 1412 Digital Equipment Corporation
January 1993

Telnet Authentication: SPX

Status of this Memo

This memo defines an EXPerimental Protocol for the Internet
community. Discussion and suggestions for improvement are requested.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.

1. Command Names and Codes

Authentication Types

SPX 3

Suboption Commands

AUTH 0
REJECT 1
ACCEPT 2

2. Command Meanings

IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH
<SPX authentication token> IAC SE

This is used to pass the SPX authentication token to the remote
side of the connection. (A document which describes the
authentication token syntax is forthcoming.) The first octet of
the <authentication-type-pair> value is SPX. The second octet is
a modifier to the SPX authentication type.

IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT
<mutual response> IAC SE

This command indicates that the authentication was sUCcessful.
After an SPX authentication exchange, both sides have securely
established a random 8-byte key to be used as the default key for
the ENCRYPTION option. If the AUTH_HOW_MUTUAL bit is set in the
second octet of the authentication-type-pair, the sender includes
the mutual response bytes. The receiver of the ACCEPT command
compares the "mutual response" with its expected mutual response.

(A document which describes the mutual response syntax is forth
coming.) If the AUTH_HOW_ONE_WAY bit is set in the second octet
of the authentication-type-pair, the sender includes zero bytes of
mutual response.

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.

3. Implementation Rules

Every command after the first AUTHENTICATION IS must carry the same
set of modifiers (e.g., CLIENTMUTUAL) for subsequent AUTHENTICATION
IS and AUTHENTICATION REPLY commands.

If the second octet of the authentication-type-pair has the AUTH_WHO
bit set to AUTH_WHO_CLIENT, then the client sends the initial AUTH
command, and the server responds with either ACCEPT or REJECT.

If the second octet of the authentication-type-pair has the AUTH_WHO
bit set to AUTH_WHO_SERVER, then the server sends the initial AUTH
command, and the client responds with either ACCEPT or REJECT.

4. Examples

User "joe" may wish to log in as user "pete" on machine "foo". If
"pete" has set things up on "foo" to allow "joe" Access to his
account, then the client would send IAC SB AUTHENTICATION NAME "pete"
IAC SE IAC SB AUTHENTICATION IS SPX AUTH <joe's spx authentication
token> IAC SE. The server would then authenticate the user as "joe"
from the token information, and the server would send back either
ACCEPT or REJECT. If mutual authentication is being used, the server
would include in the ACCEPT message, a mutual response. The
authorization check to see if "pete" is allowing "joe" to use his
account is made after the authentication exchange is complete.
Therefore, it is possible for the client to receive an ACCEPT
response (based on the authentication token), but for joe to be
denied access to log in to pete's account.

Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION

[ The server is now free to request authentication information.
]

IAC SB AUTHENTICATION SEND SPX
CLIENTMUTUAL SPX CLIENTONE_WAY
IAC SE

[ The server has requested mutual SPX authentication. If mutual
authentication is not supported, then the server is willing to
do one-way SPX authentication. ]

[ The client will now respond with the name of the user that it
wants to log in as, and the SPX authentication token. ]

IAC SB AUTHENTICATION NAME
"pete" IAC SE
IAC SB AUTHENTICATION IS SPX
CLIENTMUTUAL AUTH <spx
authentication token
information> IAC SE

[ The server responds with an ACCEPT command to state that the
authentication was successful. ]

[ If AUTH_HOW_MUTUAL, the server responds with the mutual
response so the client can verify that it is really talking to
the right server. ]

[ If AUTH_HOW_ONE_WAY, the server responds with a NULL mutual
response, since the client is willing to trust the server
already. ]

IAC SB AUTHENTICATION REPLY SPX
CLIENTMUTUAL ACCEPT <mutual
response> 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

Kannan Alagappan
Digital Equipment Corporation
550 King Street, LKG1-2/A19
Littleton, MA 01460

EMail: kannan@sejour.lkg.dec.com

Mailing List: telnet-ietf@CRAY.COM

The working group can be contacted via the current chair:

Steve Alexander
INTERACTIVE Systems Corporation
1901 North Naper Boulevard
Naperville, IL 60563-8895

Phone: (708) 505-9100 x256
EMail: stevea@isc.com