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Network Working Group Internet Engineering Task Force
Internet-Draft Telnet Working Group
Cray Research, Inc.
D. Borman, Editor
July 1992
Telnet Authentication Option with Encryption
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. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a "working
draft" or "work in progress."
Please check the 1id-abstracts.txt listing contained in the
internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net,
nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the
current status of any Internet Draft.
1. Command Names and Codes
AUTHENTICATION 39
IS 0
SEND 1
REPLY 2
NAME 3
AUTH 4
ENCRYPT 5
Authentication Types
NULL 0
KERBEROS_V4 1
KERBEROS_V5 2
SPX 3
RSA 6
LOKI 10
Modifiers
AUTH_WHO_MASK 1
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AUTH_CLIENT_TO_SERVER 0
AUTH_SERVER_TO_CLIENT 1
AUTH_HOW_MASK 2
AUTH_HOW_ONE_WAY 0
AUTH_HOW_MUTUAL 2
AUTH_ENCRYPT_MASK4 XXX
AUTH_ENCRYPT_NO0 XXX
AUTH_ENCRYPT_YES4 XXX
2. Command Meanings
This document makes reference to a "server" and a "client". For the
purposes of this document, the "server" is the side of the connection
that did the passive TCP open (TCP LISTEN state), and the "client" is
the side of the connection that did the active open.
IAC WILL AUTHENTICATION
The client side of the connection sends this command to indicate
that it is willing to send and receive authentication information.
IAC DO AUTHENTICATION
The servers side of the connection sends this command to indicate
that it is willing to send and receive authentication information.
IAC WONT AUTHENTICATION
The client side of the connection sends this command to indicate
that it refuses to send or receive authentication information; the
server side sends this command if it receives a DO AUTHENTICATION
command.
IAC DONT AUTHENTICATION
The server side of the connection sends this command to indicate
that it refuses to send or receive authentication information; the
client side sends this command if it receives a WILL AUTHENTICA-
TION command.
IAC SB AUTHENTICATION AUTH SEND authentication-type-pair-list IAC SE
The sender of this command (the server) requests that the remote
side send authentication information for one of the authentication
types listed in "authentication-type-pair-list". The
"authentication-type-pair-list" is an ordered list of
"authentication-type" pairs. Only the server side (DO AUTHENTICA-
TION) is allowed to send this.
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IAC SB AUTHENTICATION ENCRYPT SEND encryption-type-pair-list IAC SE
The sender of this command is stating what types of encryption it
will support. Only the server side (DO AUTHENTICATION) is allowed
to send this. The current types of encryption are listed in the
current version of the Assigned Numbers document[1].
IAC SB AUTHENTICATION AUTH IS authentication-type-pair <auth data>
IAC SE
The sender of this command (the client) is sending the authentica-
tion information for authentication type "authentication-type-
pair". Only the client side (WILL AUTHENTICATION) is allowed to
send this.
IAC SB AUTHENTICATION ENCRYPT IS encryption-type ... IAC SE
The sender of this command is stating what type of encryption to
use, and any initial data that is needed Only the client side of
the connection (WILL AUTHENTICATION) may send the IS command. to
initialize the encryption-type scheme.
IAC SB AUTHENTICATION AUTH REPLY authentication-type-pair <auth data>
IAC SE
The sender of this command (the server) is sending a reply to the
the authentication information received in a previous IS command.
Only the server side (DO AUTHENTICATION) is allowed to send this.
IAC SB AUTHENTICATION ENCRYPT REPLY encryption-type ... IAC SE
The sender of this command is continuing the initial data exchange
that is needed to initialize the encryption-type scheme. Only the
server side of the connection (DO AUTHENTICATION) may send the IS
command.
IAC SB AUTHENTICATION NAME remote-user IAC SE
This optional command is sent to specify the account name on the
remote host that the user wishes to be authorized to use. Note
that authentication may succeed, and the authorization to use a
particular account may still fail. Some authentication mechanisms
may ignore this command.
The "authentication-type-pair" is two octets, the first is the au-
thentication type, and the second is a modifier to the type. There
are currently two one bit fields defined in the modifier, the
AUTH_WHO_MASK bit and the AUTH_HOW_MASK bit, so there are four possi-
ble combinations:
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AUTH_CLIENT_TO_SERVER
AUTH_HOW_ONE_WAY
The client will send authentication information about the local
user to the server. If the negotiation is successful, the
server will have authenticated the user on the client side of
the connection.
AUTH_SERVER_TO_CLIENT
AUTH_HOW_ONE_WAY
The server will authenticate itself to the client. If the
negotiation is successful, the client will know that it is con-
nected to the server that it wants to be connected to.
AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL
The client will send authentication information about the local
user to the server, and then the server will authenticate it-
self to the client. If the negotiation is successful, the
server will have authenticated the user on the client side of
the connection, and the client will know that it is connected
to the server that it wants to be connected to.
AUTH_SERVER_TO_CLIENT
AUTH_HOW_MUTUAL
The server will authenticate itself to the client, and then the
client will authenticate itself to the server. If the negotia-
tion is successful, the client will know that it is connected
to the server that it wants to be connected to, and the server
will know that the client is who it claims to be.
IAC SB ENCRYPT START keyid IAC SE
The sender of this command is stating that at this point in the data
stream, all following data will be encrypted, via the previously
negotiated method of data encryption. Only the client side of the
connection (WILL AUTHENTICATION) may send the START command.
The keyid is a variable length field. It may be used by various en-
cryption mechanisms to identify which encryption key is to be used,
when multiple encryption keys might be known on either side of the
connection. The keyid field is encoded with the most significant
byte first, and a keyid value of zero is reserved to indicate the de-
fault encryption key derived during the authentication process. The
keyid field must be at least one byte long.
3. Default Specification
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The default specification for this option is
WONT AUTHENTICATION
DONT AUTHENTICATION
meaning there will not be any exchange of authentication information.
4. Motivation
One of the deficiences of the Telnet protocol is that in order to log
into remote systems, users have to type their passwords, which are
passed in clear text through the network. If the connections goes
through untrusted networks, there is the possibility that passwords
will be compromised by someone watching the packets as they go by.
The purpose of the AUTHENTICATION option is to provide a framework
for the passing of authentication information through the TELNET ses-
sion. This means that: 1) the users password will not be sent in
clear text across the network, and 2) if the front end telnet process
has the appropriate authentication information, it can automatically
send the information, and the user will not have to type any pass-
word.
It is intended that the AUTHENTICATION option be general enough that
it can be used to pass information for any authentication system.
Additionally, other sensitive information be communicated via the
TELNET data stream. By negotiating the use of data encryption during
the authentication process, the user can ensure that session data
cannot be intercepted as it traverses the network.
5. 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 in-
truder could attempt to negotiate the use of an authentication system
which is either weak, or already compromised by the intruder.
The use of encryption is intended to provide protection against pas-
sive attacks, not against active attacks. In other words, the en-
cryption can be used to provide protection from someone who is just
watching the IP packets as they pass through the network, but may not
from someone who is able to modify packets in flight. This is not to
say that the use of encryption doesn't provide any protection against
an active attacker, but that additional code and steps would have to
be done in order to provide compelete protection from an active at-
tacker.
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6. Implementation Rules
WILL and DO are used only at the beginning of the connection to ob-
tain and grant permission for future negotiations.
The authentication is only negotiated in one directions; the server
must send the "DO", and the client must send the "WILL". This res-
triction is due to the nature of authentication; there are three pos-
sible cases; server authenticates client, client authenticates
server, and server and client authenticate each other. By only nego-
tiating the option in one direction, and then determining which of
the three cases is being used via the suboption, potential ambiguity
is removed. If the server receives a "DO", it must respond with a
"WONT". If the client receives a "WILL", it must respond with a
"DONT".
Once the two hosts have exchanged a DO and a WILL, the server is free
to request authentication information. In the request, a list of
supported authentication types is sent. Only the server may send re-
quests ("IAC SB AUTHENTICATION SEND authentication-type-pair-list IAC
SE"). Only the client may transmit authentication information via
the "IAC SB AUTHENTICATION IS authentication-type ... IAC SE" com-
mand. Only the server may send replys ("IAC SB AUTHENTICATION REPLY
authentication-type ... IAC SE"). As many IS and REPLY suboptions
may be exchanged as are needed for the particular authentication
scheme chosen.
If the client does not support any of the authentication types listed
in the authentication-type-pair-list, a type of NULL should be used
to indicate this in the IS reply. Note that in this case, the server
may choose to close the connection.
The order of the authentication types MUST be ordered to indicate a
preference for different authentication types, the first type being
the most preferred, and the last type the least preferred.
The following is an example of use of the option:
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 Kerberos authentication, but is
willing to do just one-way Kerberos authentication. The client
will now respond with the name of the user that it wants to log
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in as, and the Kerberos ticket. ]
IAC SB AUTHENTICATION NAME "joe"
IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V4 CLIENT|MUTUAL AUTH 4
7 1 67 82 65 89 46 67 7 9 77 0
48 24 49 244 109 240 50 208 43
35 25 116 104 44 167 21 201 224
229 145 20 2 244 213 220 33 134
148 4 251 249 233 229 152 77 2
109 130 231 33 146 190 248 1 9
31 95 94 15 120 224 0 225 76 205
70 136 245 190 199 147 155 13
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 REPLY
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
It is expected that any implementation that supports the Telnet AU-
THENTICATION option will support all of this specification.
7. References
[1] Reynolds, Joyce, and Postel, Jon, "Assigned Numbers", RFC 1340,
ISI, July 1992
Author's Address
David A. Borman, Editor
Cray Research, Inc.
655F Lone Oak Drive
Eagan, MN 55123
Phone: (612) 452-6650
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Mailing List: telnet-ietf@CRAY.COM
EMail: dab@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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