Network Working Group T. Ts'o
Request for Comments: 2946 VA Linux Systems
Category: Standards Track September 2000
Telnet Data Encryption Option
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes a the telnet encryption option as a generic
method of providing data confidentiality services for the telnet data
stream. While this document summarizes currently utilized encryption
types and codes, it does not define a specific encryption algorithm.
Separate documents are to be published defining implementations of
this option for each encryption algorithm.
1. Command Names and Codes
ENCRYPT 38
Encryption Commands
IS 0
SUPPORT 1
REPLY 2
START 3
END 4
REQUEST-START 5
REQUEST-END 6
ENC_KEYID 7
DEC_KEYID 8
Encryption Types
NULL 0
DES_CFB64 1
DES_OFB64 2
DES3_CFB64 3
DES3_OFB64 4
CAST5_40_CFB64 8
CAST5_40_OFB64 9
CAST128_CFB64 10
CAST128_OFB64 11
Following historical practice, future encryption type numbers
will be assigned by the IANA under a First Come First Served
policy as outlined by RFC2434 [3]. Despite the fact that
authentication type numbers are allocated out of an 8-bit number
space (as are most values in the telnet specification) it is not
anticipated that the number space is or will become in danger of
being exhausted. However, if this should become an issue, when
over 50% of the number space becomes allocated, the IANA shall
refer allocation requests to either the IESG or a designated
eXPert for approval.
2. Command Meanings
IAC WILL ENCRYPT
The sender of this command is willing to send encrypted data.
IAC WONT ENCRYPT
The sender of this command refuses to send encrypted data.
IAC DO ENCRYPT
The sender of this command is willing to receive encrypted data.
IAC DONT ENCRYPT
The sender of this command refuses to accept encrypted data.
IAC SB ENCRYPT SUPPORT encryption-type-list IAC SE
The sender of this command is stating which types of encryption it
will support. Only the side of the connection that is DO ENCRYPT
may send the SUPPORT command. The current types of encryption are
listed in the current version of the Assigned Numbers document
[1].
The encryption-type-list may only include types which can actually
be supported during the current session. If ENCRYPT is negotiated
in conjunction with AUTH the SUPPORT message MUST NOT be sent
until after the session key has been determined. Otherwise,
it is impossible to know if the selected encryption type can be
properly initialized based upon the type and length of the key
that is available."
IAC SB ENCRYPT IS encryption-type ... IAC SE
The sender of this command is stating which type of encryption to
use, and any initial data that is needed. Only the side of the
connection that is WILL ENCRYPT may send the IS command to
initialize the encryption-type scheme.
IAC SB ENCRYPT REPLY encryption-type ... IAC SE
The sender of this command is continuing the initial data exchange
in order to initialize the encryption-type scheme. Only the side
of the connection that is DO ENCRYPT may send the REPLY command.
IAC SB ENCRYPT START keyid IAC SE
The sender of this command is stating that all data following the
command in the data stream will be be encrypted via the previously
negotiated method of data encryption. Only the side of the
connection that is WILL ENCRYPT may send the START command.
The keyid is a variable length field. It is used by various
encryption 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 default encryption key (this would typically be an
encryption key derived during authentication, with the
AUTHENTICATION option). The keyid field must be at least one byte
long. The only valid values for "keyid" will be those that have
been received in a DEC_KEYID command.
IAC SB ENCRYPT END IAC SE
The sender of this command is stating that all data following the
command in the data stream will not be encrypted. Only the side
of the connection that is WILL ENCRYPT may send the END
IAC SB ENCRYPT REQUEST-START keyid IAC SE
The sender of this command requests that the remote side begin
encryption of the telnet data stream. Only the side of the
connection that is DO ENCRYPT may send the REQUEST-START command.
The keyid is only advisory, and my be omitted.
IAC SB ENCRYPT REQUEST-END IAC SE
The sender of this command requests that the remote side stop
encryption of the telnet data stream. Only the side of the
connection that is DO ENCRYPT may send the REQUEST-END command.
IAC SB ENCRYPT ENC_KEYID keyid IAC SE
The sender of this requests that the remote side verify that
"keyid" maps to a valid key; or verifies that the "keyid" received
in a DEC_KEYID command is valid. If keyid is omitted, it implies
that there are no more known keyids, and that the attempt to find
a common keyid has failed. Only the side of the connection that
is WILL ENCRYPT may send the ENC_KEYID command.
IAC SB ENCRYPT DEC_KEYID keyid IAC SE
The sender of this requests that the remote side verify that
"keyid" maps to a valid key on the remote side; or verifies that
the "keyid" received in a ENC_KEYID command is valid. If keyid is
omitted, it implies that there are no more known keyids, and that
the attempt to find a common keyid has failed. Only the side of
the connection that is DO ENCRYPT may send the DEC_KEYID command.
3. Default Specification
The default specification for this option is
WONT ENCRYPT
DONT ENCRYPT
meaning there will not be any encryption of the Telnet data stream.
4. Motivation
The Telnet protocol has no form of protection from some intervening
gateway looking at IP packets as they travel through the network.
This is especially dangerous when passWords are sent as clear text
over the network. This option provides a method for encrypting the
data stream.
5. Implementation Rules
Once the Encryption option is in effect, all data in the negotiated
direction, including TELNET options, is encrypted. Encryption begins
with the octet of data immediately following the "IAC SB ENCRYPT
START encryption-type IAC SE" command. Encryption ends after the
"IAC SB ENCRYPT END IAC SE" command.
WILL and DO are used only at the beginning of the connection to
oBTain and grant permission for future negotiations. The ENCRYPT
option must be negotiated in both directions.
Once the two hosts have exchanged a WILL and a DO, the sender of the
DO ENCRYPT must send a ENCRYPT SUPPORT command to let the remote side
know the types of encryption it is willing to accept. In the
request, a list of supported encryption schemes is sent. Only the
sender of the DO may send a list of supported encryption types (IAC
SB ENCRYPT SUPPORT encryption-type-list IAC SE). Only the sender of
the WILL may actually transmit encrypted data. This is initiated via
the "IAC SB ENCRYPT START IAC SE" command, and terminated via the
"IAC SB ENCRYPT END IAC SE" command. If a START is received, and
then a second START is received before receiving an END, the second
START is ignored.
If the sender of the DO would like the remote side to begin sending
encrypted data, it can send the "IAC SB ENCRYPT REQUEST-START IAC SE"
command. If the sender of the DO would like the remote side to stop
sending encrypted data, it can send the "IAC SB ENCRYPT REQUEST-STOP
IAC SE" command.
If the receiver of the SUPPORT command does not support any of the
encryption types listed in the SUPPORT command, it should send an
"IAC SB ENCRYPT IS NULL IAC SE" to indicate that there are no
encryption types in common. It may also send an IAC WONT ENCRYPT
command to turn off the ENCRYPT option.
The order of the encryption types in a SUPPORT command must be
ordered to indicate a preference for different encryption types, the
first type being the most preferred, and the last type the least
preferred.
If the ENCRYPT option has been enabled, and encrypted data is being
received, the receipt of an "IAC WONT ENCRYPT" implies the receipt of
an "IAC SB ENCRYPT END IAC SE", e.g., the Telnet data stream is no
longer encrypted.
The following example demonstrates the use of the option:
Host1 Host2
[ Host1 requests Host2 negotiate the encryption of data that
Host2 sends to Host1. Host2 agrees to negotiate the encryption
of data that it sends to Host1. ]
DO ENCRYPT
WILL ENCRYPT
[ Host1 requests that Host2 enable encryption as soon as the
initialization is completed, and informs Host2 that is supports
DES_CFB64. ]
IAC SB ENCRYPT REQUEST-START IAC
SE
IAC SB ENCRYPT SUPPORT DES_CFB64
IAC SE
[ Host2 sends the initial feed to Host1. Host1 acknowledges
receipt of the IV. ]
IAC SB ENCRYPT IS DES_CFB64
CFB64_IV 144 146 63 229 237 148
81 143 IAC SE
IAC SB ENCRYPT REPLY DES_CFB64
CFB64_IV_OK 103 207 181 71 224
55 229 98 IAC SE
[ Host2 is now free to start sending encrypted data, and since a
REQUEST-START was received, it enables encryption. ]
IAC SB ENCRYPT START IAC SE
[ All data from Host2 to Host1 is now encrypted. ]
IAC SB ENCRYPT END IAC SE
[ All data from Host2 to Host1 is now in clear text again. ]
It is expected that any implementation that supports the Telnet
ENCRYPT option will support all of this specification.
6. Security Considerations
The ENCRYPT option used in isolation provides protection against
passive attacks, but not against active attacks. In other words, it
will provide protection from someone who is just watching the IP
packets as they pass through the network. However, an attacker who
is able to modify packets in flight could prevent the ENCRYPT option
from being negotiated.
This flaw can be remedied by using the Telnet Authentication option
alongside the ENCRYPT option. Specifically, setting
ENCRYPT_USING_TELOPT in the authentication-type-pair can be used to
force that Encryption be negotiated even in the face of active
attacks.
In addition, an active attacker can interfere with attempts to start
or restart encryption. If encryption is requested by the user, and
the client is unable to negotiate enabling or re-enabling encryption,
the client must assume that it is being attacked, and MUST
immediately terminate the telnet connection.
7. Future directions for Telnet Encryption
The specification defines a method for providing data confidentiality
to the telnet data stream. Unfortunately all of the encryption
mechanism provided under this option do not provide data integrity,
because of the complexity of specifying a protocol which provided
integrity services efficiently in a stream-oriented protocol.
The TELNET START_TLS specification provides a scheme which provides
confidentiality, integrity, and compression, and future work for
telnet encryption should closely examine using this specification.
One promising approach would use the anonymous Diffie-Hellman mode of
TLS, followed by the telnet AUTHENTICATION option where the
authentication mechanism would include the client and server finished
messages computed during the TLS negotiation.
8. Acknowledgments
This document was originally written by Dave Borman of Cray Research,
with the assistance of Theodore Ts'o of MIT and the IETF Telnet
Working Group.
9. References
[1] Reynolds, J. and J. Postel, "Telnet Protocol Specification", STD
8, RFC854, May 1983.
[2] Ts'o, T. and J. Altman, "Telnet Authentication Option", RFC2941,
September 2000.
[3] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC2434, October 1998.
10. Author's Address
Theodore Ts'o, Editor
VA Linux Systems
43 Pleasant St.
Medford, MA 02155
Phone: (781) 391-3464
EMail: tytso@mit.edu
11. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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