RFC1411 - Telnet Authentication: Kerberos Version 4

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　　Network Working Group D. Borman, Editor
Request for Comments: 1411 Cray Research, Inc.
January 1993

Telnet Authentication: Kerberos Version 4

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

KERBEROS_V4 1

Suboption Commands

AUTH 0
REJECT 1
ACCEPT 2
CHALLENGE 3
RESPONSE 4

2. Command Meanings

IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH <kerberos
ticket and authenticator> IAC SE

This is used to pass the Kerberos ticket to the remote side of the
connection. The first octet of the <authentication-type-pair>
value is KERBEROS_V4, to indicate the usage of Kerberos version 4.

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

This command indicates that the authentication was successful.

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.

IAC SB AUTHENTICATION IS <authentication-type-pair> CHALLENGE
<encrypted challenge> IAC SE
IAC SB AUTHENTICATION REPLY <authentication-type-pair> RESPONSE
<encrypted response> IAC SE

These two commands are used to perform mutual authentication.
They are only used when the AUTH_HOW_MUTUAL bit is set in the
second octet of the authentication-type-pair. After successfully
sending an AUTH and receiving an ACCEPT, a CHALLENGE is sent. The
challenge is a random 8 byte number with the most significant byte
first, and the least significant byte last. When the CHALLENGE
command is sent, the "encrypted challenge" is the 8-byte-challenge
encrypted in the session key. When the CHALLENGE command is
received, the contents are decrypted to get the original 8-byte-
challenge, this value is then incremented by one, re-encrypted
with the session key, and returned as the "encrypted response" in
the RESPONSE command. The receiver of the RESPONSE command
decrypts the "encrypted response", and verifies that the resultant
value is the original 8-byte-challenge incremented by one.

The "encrypted challenge" value sent/received in the CHALLENGE
command is also encrypted with the session key on both sides of
the session, to produce a random 8-byte key to be used as the
default key for the ENCRYPTION option.

3. Implementation Rules

If the second octet of the authentication-type-pair has the AUTH_WHO
bit set to AUTH_CLIENT_TO_SERVER, then the client sends the initial
AUTH command, and the server responds with either ACCEPT or REJECT.
In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, and the
server responds with ACCEPT, then the client then sends a CHALLENGE,
and the server sends a RESPONSE.

If the second octet of the authentication-type-pair has the AUTH_WHO
bit set to AUTH_SERVER_TO_CLIENT, then the server sends the initial
AUTH command, and the client responds with either ACCEPT or REJECT.
In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, and the
client responds with ACCEPT, then the server then sends a CHALLENGE,
and the client sends a RESPONSE.

The authenticator (Kerberos Principal) used is of the form
"rcmd.host@realm".

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 KERBEROS_V4 AUTH <joe's kerberos
ticket> IAC SE The server would then authenticate the user as "joe"
from the ticket information, and since "pete" is allowing "joe" to
use his account, the server would send back ACCEPT. If mutual
authentication is being used, the the client would send a CHALLENGE,
and verify the RESPONSE that the server sends back.

Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KERBEROS_V4 CLIENT|MUTUAL
KERBEROS_V4 CLIENT|ONE_WAY IAC
SE
[ The server has requested mutual Version 4 Kerberos
authentication. If mutual authentication is not supported,
then the server is willing to do one-way authentication.

The client will now respond with the name of the user that it
wants to log in as, and the Kerberos ticket. ]
IAC SB AUTHENTICATION NAME
"pete" IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V4 CLIENT|MUTUAL AUTH
<kerberos ticket information>
IAC SE
[ The server responds with an ACCEPT command to state that the
authentication was successful. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V4 CLIENT|MUTUAL ACCEPT
IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
really talking to the right server. ]
IAC SB AUTHENTICATION IS
KERBEROS_V4 CLIENT|MUTUAL
CHALLENGE xx xx xx xx xx xx xx
xx IAC SE
[ Lastly, the server sends across a RESPONSE to prove that it
really is the right server.
IAC SB AUTHENTICATION REPLY
KERBEROS_V4 CLIENT|MUTUAL
RESPONSE yy yy yy yy yy yy yy yy
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

David A. Borman, Editor
Cray Research, Inc.
655F Lone Oak Drive
Eagan, MN 55123

Phone: (612) 452-6650
EMail: dab@CRAY.COM

Mailing List: telnet-ietf@CRAY.COM

Chair's Address

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

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