Network Working Group T. Ts'o, Editor
Internet-Draft VA Linux Systems
draft-tso-telnet-auth-enc-02.txt July 1999
Telnet Authentication Option
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
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0. Abstract
This document describes the telnet authentication option as a generic
method for negotiating an authentication type and mode including
whether encryption should be used and if credentials should be for-
warded. While this document summarizes currently utilized commands
and types it does not define a specific authentication type. Sepa-
rate documents are to be published defining each authentication type.
1. Command Names and Codes
AUTHENTICATION 37
Authentication Commands
IS 0
SEND 1
REPLY 2
NAME 3
Authentication Types
NULL 0
KERBEROS_V4 1
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KERBEROS_V5 2
SPX 3
MINK 4
SRP 5
RSA 6
SSL 7
LOKI 10
SSA 11
KEA_SJ 12
KEA_SJ_INTEG 13
DSS 14
NTLM 15
Note that other authentication types numbers will be assigned by
the IANA.
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 20
ENCRYPT_OFF 0
ENCRYPT_USING_TELOPT 4
ENCRYPT_AFTER_EXCHANGE 16
ENCRYPT_RESERVED 20
INI_CRED_FWD_MASK 8
INI_CRED_FWD_OFF 0
INI_CRED_FWD_ON 8
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
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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 must send this command if it receives a DO AUTHENTICA-
TION 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 must send this command if it receives a WILL AUTHENTI-
CATION command.
IAC SB AUTHENTICATION 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 "authentica-
tion-type-pair-list" is an ordered list of "authentication-type"
pairs. Only the server side (DO AUTHENTICATION) is allowed to
send this.
IAC SB AUTHENTICATION 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 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 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. The au-
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thentication type may or may not include built-in encryption. For
instance, when the Kerberos 4 authentication type is negotiated en-
cryption must be negotiated with the telnet ENCRYPT option. However,
the SSL and KEA_SJ authentication types provide an encrypted channel
as part of a successful telnet AUTH option negotiation.
There are currently five one bit fields defined in the modifier. The
first two of these bits 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 ne-
gotiation 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.
The third and fifth bits in the modifier are the ENCRYPT_MASK
bits. These bits are used to determine if and how encryption
should be enabled. Of the four possible combinations only three
are currently defined:
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ENCRYPT_OFF
Encryption will not be used for this session. TELOPT EN-
CRYPT SHOULD NOT be negotiated. This mode MUST be used with
all AUTH types that do not provide a shared secret to be
used as a session key.
ENCRYPT_USING_TELOPT
Encryption will be negotiated via the use of TELOPT ENCRYPT.
Immediately after authentication has completed TELOPT EN-
CRYPT MUST be negotiated in both directions. This is re-
quired to occur before credentials forwarding; other telnet
options are negotiated; or any user data is transmitted. A
failure to successfully negotiate TELOPT ENCRYPT in either
direction MUST result in immediate session termination.
ENCRYPT_AFTER_EXCHANGE
Encryption will be activated in both directions immediately
after the successful exchange of the shared secret to be
used as the session key. The encryption algorithm to be
used MUST be implied by the AUTH 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.
This bit is set by the client to advise the server to expect for-
warded credentials from the client.
INI_CRED_FWD_OFF
The client will not be forwarding credentials to the server.
This mode must be used if the selected authentication method
does not support credentials forwarding.
INI_CRED_FWD_ON
Once authentication, and perhaps encryption, completes, the
client will immediately forward authentication credentials
to the server.
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 mechanisms. It is a proto-
col error to set this bit if the underlying authentication mecha-
nism does not support credentials forwarding.
Credentials forwarding MUST NOT be performed if
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AUTH_CLIENT_TO_SERVER|AUTH_HOW_ONE_WAY was used since the identity
of the server can not be assured. Credentials SHOULD NOT be for-
warded if the telnet connection is not protected using some en-
cryption or integrity protection services.
Note that older implementations of the telnet authentication op-
tion will not understand the ENCRYPT_MASK and INI_CRED_FWD_MASK
bits. Hence an implementation wishing to offer these bits should
offer authentication type pairs with these bits both set and not
set if backwards compatibility is required.
3. Default Specification
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 deficiencies 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 pass-
words 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, and a mechanism to enable encryption of the data stream as a
side effect of successful authentication or via subsequent use of the
telnet ENCRYPT option. This means that: 1) the users password will
not be sent in clear text across the network, 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 password. 3) once authentication has succeeded, the data
stream can be encrypted to provide protection against active attacks.
It is intended that the AUTHENTICATION option be general enough that
it can be used to pass information for any authentication and encryp-
tion system.
5. Security Implications
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
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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.
If the authentication type requires that encryption be enabled as a
separate optional negotiation (the ENCRYPT option), it will provide a
window of vulnerability from when the authentication completes, up to
and including the negotiation to turn on encryption by an active at-
tacker. An active attack is one where the underlying TCP stream can
be modified or taken over by the active attacker. If the server only
offers authentication type pairs that include the ENCRYPT_US-
ING_TELOPT set in the ENCRYPT_MASK field, this will avoid the window
of vulnerability, since both parties will agree that telnet ENCRYPT
option must be successfully negotiated immediately following the suc-
cessful completion of telnet AUTH.
Other authentication types links the enabling of encryption as a side
effect of successful authentication. This will also provide protec-
tion against the active attacker. The ENCRYPT_AFTER_EXCHANGE bit al-
lows these authentication types to negotiate encryption so that it
can be made optional.
Another opportunity for active attacks is presented when encryption
may be turned on and off without re-authentication. Once encryption
is disabled, an attacker may hijack the telnet stream, and interfere
with attempts to restart encryption. Therefore, a client SHOULD NOT
support the ability to turn off encryption. Once encryption is dis-
abled, if an attempt to re-enable encryption fails, the client MUST
terminate the telnet connection.
It is important that in both cases the authentication type pair be
integrity protected at the end of the authentication exchange. This
must be specified for each authentication type to ensure that the re-
sult of the telnet authentication option negotiation is agreed to by
both the client and the server. Some authentication type suboptions
may wish to include all of the telnet authentication negotiation ex-
changes in the integrity checksum, to fully protect the entire ex-
change.
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 direction; the server
must send the "DO", and the client must send the "WILL". This re-
striction 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 nego-
tiating the option in one direction, and then determining which of
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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 replies ("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 if the client responds
with a type of NULL, the server may choose to close the connection.
When the server has concluded that authentication cannot be nego-
tiated with the client it should send IAC DONT AUTH to the client.
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.
As long as the server is WILL AUTH it may request authentication in-
formation at any time. This is done by sending a new list of sup-
ported authentication types. Requesting authentication information
may be done as a way of verifying the validity of the client's cre-
dentials after an extended period of time or to negotiate a new ses-
sion key for use during encryption.
7. User Interface
Normally protocol specifications do not address user interface speci-
fications. However, due to the fact that the user will probably want
to be able to specify the things about authentication and encryption
and also know whether or not things succeeded, some guidance needs to
be given to implementors to provide some minimum level of user con-
trol.
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 authentica-
tion succeeds. There SHOULD be at least four settings, REQUIRE,
PROMPT, WARN and DISABLE. Setting the authentication switch to RE-
QUIRE means that if the authentication fails, then an appropriate er-
ror message must be displayed and the TELNET connection must be ter-
minated. Setting the authentication switch to PROMPT means that if
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the authentication fails, then an appropriate error message must be
displayed and the user must be prompted for confirmation before con-
tinuing the TELNET session. Setting the authentication switch to
WARN means that if the authentication fails, then an appropriate er-
ror message must be displayed before continuing the TELNET session.
Setting the authentication switch to DISABLE means that authentica-
tion will not be attempted. The encryption switch SHOULD have the
same settings as the authentication switch; however its settings are
only used when authentication succeeds. The default setting for both
switches should be WARN. Both of these switches may be implemented
as a single switch, though having them separate gives more control to
the user.
8. Example
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
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. ]
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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
The following is an example of use of the option with encryption ne-
gotiated via telnet ENCRYPT:
Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KERBEROS_V4
ENCRYPT_USING_TELOPT|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. 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 AUTHENTICATION NAME "joe"
IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V4
ENCRYPT_USING_TELOPT|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|ENCRYPT_USING_TELOPT
ACCEPT IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
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really talking to the right server. ]
IAC SB AUTHENTICATION IS
KERBEROS_V4
CLIENT|MUTUAL|ENCRYPT_USING_TELOPT
CHALLENGE xx xx xx xx xx xx xx
xx IAC SE
[ The server sends across a RESPONSE to prove that it really is
the right server. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V4
CLIENT|MUTUAL|ENCRYPT_USING_TELOPT
RESPONSE yy yy yy yy yy yy yy yy
IAC SE
[ At this point, the client and server begin to negotiate the
telnet ENCRYPT option in each direction for a secure channel.
If the option fails in either direction for any reason the
connection must be immediately terminated. ]
The following is an example of use of the option with integrated en-
cryption:
Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
IAC SE
[ The server has requested mutual KEA authentication with
SKIPJACK encryption. The client will now respond with the name
of the user that it wants to log in as and the KEA cert. ]
IAC SB AUTHENTICATION NAME "joe"
IAC SE IAC SB AUTHENTICATION IS
KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
[ The server responds with its KEA Cert. ]
IAC SB AUTHENTICATION REPLY
KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
really talking to the right server. ]
IAC SB AUTHENTICATION IS KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
'3' IV'||Encrypt(
NonceB+1||NonceA ) IAC SE
[ At this point, the client begins to encrypt the outgoing data
stream, and the server, after receiving this command, begins to
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decrypt the incoming data stream. Lastly, the server sends
across a RESPONSE to prove that it really is the right server.
]
IAC SB AUTHENTICATION REPLY
KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
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. ]
It is expected that any implementation that supports the Telnet AU-
THENTICATION option will support all of this specification.
9. Security Considerations
This memo describes a general framework for adding authentication and
encryption to the telnet protocol. The actual authentication mecha-
nism is described in the authentication suboption specifications, and
the security of the authentication option is dependent on the
strengths and weaknesses of the authentication suboption.
It should also be noted that the negotiation of the authentication
type pair is not protected, thus allowing an attacker to force the
result of the authentication to the weakest mutually acceptable
method. (For example, even if both sides of the negotiation can ac-
cept a "strong" mechanism and a "40-bit" mechanism, an attacker could
force selection of the "40-bit" mechanism.) An implementation should
therefore only accept an authentication mechanism to be negotiated if
it is willing to trust it as being secure.
10. References
[1] Reynolds, Joyce, and Postel, Jon, "Assigned Numbers", RFC 1060,
ISI, March 1990
Author's Address
Theodore Ts'o, Editor
VA Linux Systems
43 Pleasant St.
Medford, MA 02155
Phone: (781) 391-3464
EMail: tytso@valinux.com
Jeffrey Altman
Columbia University
Watson Hall Room 716
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612 West 115th Street
New York NY 10025
Phone: +1 (212) 854-1344
EMail: jaltman@columbia.edu
Mailing List: telnet-ietf@CRAY.COM
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