RFC1411 - Telnet Authentication: Kerberos Version 4

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 CLIENTMUTUAL

KERBEROS_V4 CLIENTONE_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 CLIENTMUTUAL 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 CLIENTMUTUAL 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 CLIENTMUTUAL

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 CLIENTMUTUAL

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