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draft-galvin-telnet-authenc
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Network Working Group J. Galvin
INTERNET-DRAFT S. Murphy
draft-galvin-telnet-authenc-00.txt D. Balenson
TIS
July 1995
Telnet Authentication and Encryption Option
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 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 learn the current status of any Internet Draft, please check the
1id-abstracts.txt listing contained in one of the Internet Drafts
Shadow Directories on ds.internic.net (US East Coast), venera.isi.edu
(US West Coast), munnari.oz.au (Pacific Rim), or nic.nordu.net
(Europe).
Abstract
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 the clear through the network. This document specifies the
AUTH_ENCRYPT option, whose purpose is two-fold: to provide a
framework for the passing of authentication information through the
TELNET session and to provide a mechanism to enable encryption of the
data stream as a side effect of successful authentication.
Acknowledgements
This document represents a revision of RFC1416 [1] that has been
enhanced to include the optional encryption of the data stream. The
work of Dave Borman (Editor), Steve Alexander (Telnet working group
chair), and the Telnet working group in the preparation of that
document is gratefully acknowledged. Ted Ts'o deserves special
mention for keeping track of the details of this enhancement and
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providing them to us so that this document could be prepared.
1. Introduction
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 the clear through the network. If the connection goes
through untrusted networks, there is the possibility that an intruder
may eavesdrop on the packets as they go by and thus obtain the
password.
This document specifies the AUTH_ENCRYPT option, whose purpose is
two-fold: to provide a framework for the passing of authentication
information through the TELNET session and to provide a mechanism to
enable encryption of the data stream as a side effect of successful
authentication. This means that:
1) the user's password will not be sent unencrypted across the
network,
2) if the front end telnet process has the appropriate authentication
information it can automatically send it instead of the user typing
any password, and
3) once authentication has succeeded the data stream can be encrypted
to provide protection against active attacks.
It is intended that the AUTH_ENCRYPT option be general enough that it
can be used to pass information for any authentication and encryption
type.
It is expected that any implementation that supports the Telnet
AUTH_ENCRYPT option will support all of this specification.
2. Command Names and Codes
This section lists the codes for the Telnet authentication and
encryption option, commands, and modifiers, as well as the initially
defined authentication and encryption types. The codes for the
authentication and encryption types are officially assigned and
maintained by IANA. The values are published regularly in STD2,
which is currently RFC1700 [2].
AUTH_ENCRYPT XX
Authentication and Encryption Commands
IS 0
SEND 1
REPLY 2
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NAME 3
END_ENCRYPT 4
REQUEST_END_ENCRYPT 5
Authentication and Encryption Types
NULL 0
KERBEROS_V4 1
KERBEROS_V5 2
SPX 3
RSA 6
LOKI 10
GSSAPI XX
Modifiers
AUTH_WHO_MASK 1
AUTH_CLIENT_TO_SERVER 0
AUTH_SERVER_TO_CLIENT 1
AUTH_HOW_MASK 2
AUTH_HOW_ONE_WAY 0
AUTH_HOW_MUTUAL 2
ENCRYPT_MASK 4
ENCRYPT_OFF 0
ENCRYPT_ON 4
INI_CRED_FWD_MASK 8
INI_CRED_FWD_OFF 0
INI_CRED_FWD_ON 8
3. 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 AUTH_ENCRYPT
The client side of the connection sends this command to indicate
that it is willing to send and receive authentication and
encryption information.
IAC DO AUTH_ENCRYPT
The server side of the connection sends this command to indicate
that it is willing to send and receive authentication and
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encryption information.
IAC WONT AUTH_ENCRYPT
The client side of the connection sends this command to indicate
that it refuses to send or receive authentication or encryption
information. The server side sends this command if it receives a
DO AUTH_ENCRYPT command and it refuses to send or receive
authentication or encryption information.
IAC DONT AUTH_ENCRYPT
The server side of the connection sends this command to indicate
that it refuses to send or receive authentication or encryption
information. The client side sends this command if it receives a
WILL AUTH_ENCRYPT command and it refuses to send or receive
authentication or encryption information.
IAC SB AUTH_ENCRYPT SEND authentication-type-pair-list IAC SE
The server side of the connection sends this command to request
that the client side of the connection send authentication
information for one of the authentication types listed in the
"authentication-type-pair-list". The "authentication-type-pair-
list" is an ordered list of "authentication-type" pairs.
IAC SB AUTH_ENCRYPT IS authentication-type-pair authentication-data
IAC SE
The client side of the connection sends this command to indicate
the "authentication-type-pair" that was chosen for the connection
from the list provided by the server side and to send the
authentication-data necessary for it.
IAC SB AUTH_ENCRYPT REPLY authentication-type-pair authentication-
data IAC SE
The server side of the connection sends this command in response
to authentication-data in a previous IS command and to send the
authentication-data the client needs.
IAC SB AUTH_ENCRYPT NAME remote-user IAC SE
The client side of the connection sends this command to indicate
the account name (remote-user) on the server that the user wishes
to be authorized to use. Note that while authentication may
succeed, the authorization to use a particular account may fail.
Some authentication types may ignore this command. This command
supercedes the value of the USER environment variable if it is
passed from the client to the server.
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IAC SB AUTH_ENCRYPT END_ENCRYPT IAC SE
The sender of this command is stating that at this point in the
data stream, all following data will no longer be encrypted. This
command should only be sent in an encrypted data stream and should
be ignored if received in an unencrypted data stream. See the
"Implementation Rules" section for more details.
IAC SB AUTH_ENCRYPT REQUEST_END_ENCRYPT IAC SE
The sender of this command requests that the remote side stop
encryption of the telnet data stream. This command is advisory
only. This command should only be sent in an encrypted data
stream and should be ignored if received in an unencrypted data
stream. See the "Implementation Rules" section for more details.
The "authentication-type-pair" is two octets: the first is the
authentication type and the second is a modifier to the type. There
are currently four one bit fields defined in the modifier. Two of
these are processed as a pair: the AUTH_WHO_MASK bit and the
AUTH_HOW_MASK bit. There are four possible combinations of these two
bits:
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
connected to the server to which it wants to be connected.
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 itself
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 to which it wants to be connected.
AUTH_SERVER_TO_CLIENT
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AUTH_HOW_MUTUAL
The server will authenticate itself to the client and then the
client will send authentication information about the local
user to the server. If the negotiation is successful, the
client will know that it is connected to the server to which it
wants to be connected and the server will have authenticated
the user on the client side of the connection.
The third bit field in the modifier is the ENCRYPT_MASK bit. This
bit is either set to ENCRYPT_ON or ENCRYPT_OFF. Setting this bit
to ENCRYPT_ON implies that once authentication completes, the data
stream is to be encrypted in both directions using the encryption
method specified for the authentication type.
The fourth bit field in the modifier is the INI_CRED_FWD_MASK bit.
This bit is either set to INI_CRED_FWD_ON or INI_CRED_FWD_OFF.
Setting this bit to INI_CRED_FWD_ON implies that once
authentication completes, the client will immediately forward
authentication credentials to the server. This bit is set by the
client to advise the server to expect forwarded credentials from
the client.
The motivation for this advisory bit is that the server may wish
to wait until the forwarded credentials have been sent before
starting any operating system specific login procedures which may
depend on these credentials. Note that credentials forwarding may
not be supported by all authentication types. It is a protocol
error to set this bit if the underlying authentication type does
not support credentials forwarding.
The authentication-data may be omitted if there is none to be
provided for the type being negotiated.
4. Default Specification
The default specification for this option is
WONT AUTH_ENCRYPT
DONT AUTH_ENCRYPT
meaning there will not be any exchange of authentication or
encryption information.
5. Implementation Rules
WILL and DO are used only at the beginning of the connection to
obtain and grant permission for future negotiations.
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The authentication is only negotiated in one direction; the server
MUST send the "DO" and the client MUST send the "WILL". This
restriction is due to the nature of authentication; there are three
possible cases; server authenticates client, client authenticates
server, and server and client authenticate each other. By only
negotiating the option in one direction and 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
requests ("IAC SB AUTH_ENCRYPT SEND authentication-type-pair-list IAC
SE"). Only the client MAY transmit authentication information via
the "IAC SB AUTH_ENCRYPT IS authentication-type ... IAC SE" command.
Only the server MAY send replies ("IAC SB AUTH_ENCRYPT REPLY
authentication-type ... IAC SE"). As many IS and REPLY suboptions
MAY be exchanged as are needed for the particular authentication type
chosen.
When determining a match from the authentication-type-pair-list
received from the server, the client MAY ignore the AUTH_ENCRYPT_MASK
bit. If the AUTH_ENCRYPT_MASK bit was ENCRYPT_OFF, then the client
MUST respond with ENCRYPT_OFF. If the AUTH_ENCRYPT_MASK bit was on,
then the client MAY respond with either ENCRYPT_ON or ENCRYPT_OFF.
In the latter case the client is stating that it will do
authentication but it does not want to encrypt the data stream.
If the client does not support any of the authentication types listed
in the authentication-type-pair-list, it SHOULD indicate this in the
IS reply with a type of NULL. Note, if the client turns off the
ENCRYPT_ON bit or responds with a type of NULL, the server MAY choose
to close the connection.
Encryption from the server to the client begins with the first byte
immediately following the "IAC SB AUTH_ENCRYPT REPLY ... IAC SE"
command that signifies that the server has successfully completed the
authentication process. Encryption from the client to the server
begins with the first byte immediately following the "IAC SB
AUTH_ENCRYPT RESPONSE ... IAC SE" command that signifies that the
client has successfully completed the authentication process. Both
of these will be specified in the document for the specific
authentication and encryption type. All data, including TELNET
options, are encrypted.
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 being the least preferred.
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Special consideration applies to the use of END_ENCRYPT and
REQUEST_END_ENCRYPT. A scenario during which one may want to turn
off encryption is communication from the server to the client, which
has the bulk of the data; leaving the communication from the client
to the server encrypted ensures that typed passwords are not readable
by eavesdropping. To do this the client SHOULD send a
REQUEST_END_ENCRYPT command to the server, who SHOULD then send an
END_ENCRYPT command and stop encrypting the output data stream. At
this point, an active attacker could insert a REQUEST_END_ENCRYPT
command in the data stream from the server to the client to try and
get the client to stop encrypting its input stream to the server.
So, a REQUEST_END_ENCRYPT command SHOULD always be honored if
received within an encrypted data stream but SHOULD be ignored if
received over an unencrypted data stream. If it is desirable to
disable all encryption, a REQUEST_END_ENCRYPT SHOULD be sent prior to
the END_ENCRYPT to get the other side to stop encrypting first.
6. User Interface Rules
Normally protocol specifications do not address user interface
issues. However, due to the fact that the user should be able to
indicate the information necessary to achieve a successful
authentication and encryption negotiation and the user should know
whether the authentication and encryption succeeded, some guidance
must be given to implementors to assure a minimum level of user
control.
The user MUST be able to specify whether or not authentication is to
be used and whether or not encryption is to used if the
authentication succeeds. There SHOULD be at least four settings:
REQUIRE, PROMPT, WARN, and DISABLE.
Setting the authentication switch to REQUIRE means that if the
authentication fails, then an appropriate error message MUST be
displayed and the TELNET connection MUST be terminated.
Setting the authentication switch to PROMPT means that if the
authentication fails, then an appropriate error message MUST be
displayed and the user MUST be prompted for confirmation before
continuing the TELNET session.
Setting the authentication switch to WARN means that if the
authentication fails, then an appropriate error message MUST be
displayed before continuing the TELNET session.
Setting the authentication switch to DISABLE means that
authentication MUST NOT be attempted.
The encryption switch SHOULD have an independent set of the same
settings as the authentication switch. However, its settings MUST
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only be used when authentication succeeds.
The default setting for both switches SHOULD be WARN. Both of these
switchs MAY be implemented as a single switch, though having them
separate gives more control to the user.
7. Example
The following is an example of the use of this option for
authentication without encryption for Kerberos Version 4 [3]:
Client Server
IAC DO AUTH_ENCRYPT
IAC WILL AUTH_ENCRYPT
[ The server is now free to request authentication information.
]
IAC SB AUTH_ENCRYPT SEND
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_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
in as and the Kerberos ticket. ]
IAC SB AUTH_ENCRYPT NAME "joe"
IAC SE
IAC SB AUTH_ENCRYPT IS
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_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 AUTH_ENCRYPT REPLY
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KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
ACCEPT IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
really talking to the right server. ]
IAC SB AUTH_ENCRYPT IS
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_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 AUTH_ENCRYPT REPLY
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
RESPONSE yy yy yy yy yy yy yy yy
IAC SE
The following is an example of the use of this option for authentica-
tion with encryption for Kerberos Version 4 [3]:
Client Server
IAC DO AUTH_ENCRYPT
IAC WILL AUTH_ENCRYPT
[ The server is now free to request authentication information.
]
IAC SB AUTH_ENCRYPT SEND
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
|ENCRYPT_ON KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_ONE_WAY
|ENCRYPT_ON IAC SE
[ The server has requested mutual Kerberos authentication but is
willing to do just one-way Kerberos authentication. In both
cases it is willing to encrypt the data stream. The client
will now respond with the name of the user that it wants to log
in as and the Kerberos ticket. ]
IAC SB AUTH_ENCRYPT NAME "joe"
IAC SE
IAC SB AUTH_ENCRYPT IS
KERBEROS_V4
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AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL|ENCRYPT_ON
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 AUTH_ENCRYPT REPLY
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
|ENCRYPT_ON ACCEPT IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
really talking to the right server. ]
IAC SB AUTH_ENCRYPT IS
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL|ENCRYPT_ON
CHALLENGE xx xx xx xx xx xx xx
xx IAC SE
[ At this point, the client begins to encrypt the outgoing data
stream, and the server, after receiving this command, begins to
decrypt the incoming data stream. Lastly, the server sends
across a RESPONSE to prove that it really is the right server.
]
IAC SB AUTH_ENCRYPT REPLY
KERBEROS_V4
AUTH_CLIENT_TO_SERVER|AUTH_HOW_MUTUAL
|ENCRYPT_ON RESPONSE yy yy yy yy
yy yy yy yy IAC SE
[ At this point, the server begins to encrypt its outgoing data
stream, and the client, after receiving this command, begins to
decrypt its incoming data stream. ]
8. Security Considerations
The ability to negotiate a common authentication type between a
client and a server system is a feature of the authentication option
that should be used with caution. When the negotiation is performed
no authentication has yet occurred. Therefore, neither system knows
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whether it is communicating with the intended system. An active at-
tacker could attempt to negotiate the use of an authentication system
which is either weak or already compromised by the intruder.
By linking the enabling of encryption as a side effect of successful
authentication, protection is provided against an active attacker.
An active attack is one where the underlying TCP stream can be modi-
fied or taken over by an active attacker. If encryption were enabled
as a separate negotiation, it would provide a window of vulnerability
from when the authentication completes up to and including the nego-
tiation to turn on encryption. It is because of this there is no
command to restart encryption. The only safe way to restart encryp-
tion once it has been turned off is to repeat the entire authentica-
tion process.
9. References
[1] D. Borman, Editor. Telnet Authentication Option. RFC1416, Cray
Research, Inc., February 1993.
[2] J. Reynolds, J. Postel. Assigned Numbers. RFC1700, ISI, October
1994.
[3] D. Borman, Editor. Telnet Authentication: Kerberos Version 4.
RFC1411, Cray Research, Inc., January 1993.
10. Authors' Address
Jim Galvin <galvin@tis.com>
Sandy Murphy <murphy@tis.com>
Dave Balenson <balenson@tis.com>
Trusted Information Systems
3060 Washington Road
Glenwood, MD 21738
Phone: 301.854.6889
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