< draft-tso-telnet-enc-des-ofb   rfc2953.txt 
Network Working Group T. Ts'o, Editor
Internet-Draft VA Linux Systems Network Working Group T. Ts'o
draft-tso-telnet-enc-des-ofb-04.txt September 1999 Request for Comments: 2953 VA Linux Systems
Category: Informational September 2000
Telnet Encryption: DES 64 bit Output Feedback Telnet Encryption: DES 64 bit Output Feedback
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This memo provides information for the Internet community. It does
all provisions of Section 10 of RFC2026. Internet-Drafts are working not specify an Internet standard of any kind. Distribution of this
documents of the Internet Engineering Task Force (IETF), its areas, memo is unlimited.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", Copyright (C) The Internet Society (2000). All Rights Reserved.
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
0. Abstract Abstract
This document specifies how to use the DES encryption algorithm in This document specifies how to use the data encryption standard (DES)
output feedback mode with the telnet encryption option. encryption algorithm in output feedback mode with the telnet
encryption option.
1. Command Names and Codes 1. Command Names and Codes
Encryption Type Encryption Type
DES_OFB64 2 DES_OFB64 2
Suboption Commands Suboption Commands
OFB64_IV 1 OFB64_IV 1
OFB64_IV_OK 2 OFB64_IV_OK 2
OFB64_IV_BAD 3 OFB64_IV_BAD 3
2. Command Meanings 2. Command Meanings
IAC SB ENCRYPT IS DES_OFB64 OFB64_IV <initial vector> IAC SE IAC SB ENCRYPT IS DES_OFB64 OFB64_IV <initial vector> IAC SE
The sender of this command generates a random 8 byte initial vec- The sender of this command generates a random 8 byte initial
tor, and sends it to the other side of the connection using the vector, and sends it to the other side of the connection using the
OFB64_IV command. The initial vector is sent in clear text. Only OFB64_IV command. The initial vector is sent in clear text. Only
the side of the connection that is WILL ENCRYPT may send the the side of the connection that is WILL ENCRYPT may send the
OFB64_IV command OFB64_IV command
IAC SB ENCRYPT REPLY DES_OFB64 OFB64_IV_OK IAC SE IAC SB ENCRYPT REPLY DES_OFB64 OFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY DES_OFB64 OFB64_IV_BAD IAC SE IAC SB ENCRYPT REPLY DES_OFB64 OFB64_IV_BAD IAC SE
The sender of these commands either accepts or rejects the initial
The sender of these commands either accepts or rejects the initial vector received in a OFB64_IV command. Only the side of the
vector received in a OFB64_IV command. Only the side of the con- connection that is DO ENCRYPT may send the OFB64_IV_OK and
nection that is DO ENCRYPT may send the OFB64_IV_OK and OFB64_IV_BAD commands. The OFB64_IV_OK command MUST be sent for
OFB64_IV_BAD commands. The OFB64_IV_OK command MUST be sent for backwards compatibility with existing implementations; there really
backwards compatibility with existing implementations; there real- isn't any reason why a sender would need to send the OFB64_IV_BAD
ly isn't any reason why a sender would need to send the command except in the case of a protocol violation where the IV
OFB64_IV_BAD command except in the case of a protocol violation sent was not of the correct length (i.e., 8 bytes).
where the IV sent was not of the correct length (i.e., 8 bytes).
3. Implementation Rules 3. Implementation Rules
Once a OFB64_IV_OK command has been received, the WILL ENCRYPT side Once a OFB64_IV_OK command has been received, the WILL ENCRYPT side
of the connection should do keyid negotiation using the ENC_KEYID of the connection should do keyid negotiation using the ENC_KEYID
command. Once the keyid negotiation has successfully identified a command. Once the keyid negotiation has successfully identified a
common keyid, then START and END commands may be sent by the side of common keyid, then START and END commands may be sent by the side of
the connection that is WILL ENCRYPT. Data will be encrypted using the connection that is WILL ENCRYPT. Data will be encrypted using
the DES 64 bit Output Feedback algorithm. the DES 64 bit Output Feedback algorithm.
If encryption (decryption) is turned off and back on again, and the If encryption (decryption) is turned off and back on again, and the
same keyid is used when re-starting the encryption (decryption), the same keyid is used when re-starting the encryption (decryption), the
intervening clear text must not change the state of the encryption intervening clear text must not change the state of the encryption
(decryption) machine. (decryption) machine.
If a START command is sent (received) with a different keyid, the en- If a START command is sent (received) with a different keyid, the
cryption (decryption) machine must be re-initialized immediately fol- encryption (decryption) machine must be re-initialized immediately
lowing the end of the START command with the new key and the initial following the end of the START command with the new key and the
vector sent (received) in the last OFB64_IV command. initial vector sent (received) in the last OFB64_IV command.
If a new OFB64_IV command is sent (received), and encryption (decryp- If a new OFB64_IV command is sent (received), and encryption
tion) is enabled, the encryption (decryption) machine must be re-ini- (decryption) is enabled, the encryption (decryption) machine must be
tialized immediately following the end of the OFB64_IV command with re-initialized immediately following the end of the OFB64_IV command
the new initial vector, and the keyid sent (received) in the last with the new initial vector, and the keyid sent (received) in the
START command. last START command.
If encryption (decryption) is not enabled when a OFB64_IV command is If encryption (decryption) is not enabled when a OFB64_IV command is
sent (received), the encryption (decryption) machine must be re-ini- sent (received), the encryption (decryption) machine must be re-
tialized after the next START command, with the keyid sent (received) initialized after the next START command, with the keyid sent
in that START command, and the initial vector sent (received) in this (received) in that START command, and the initial vector sent
OFB64_IV command. (received) in this OFB64_IV command.
4. Algorithm 4. Algorithm
Given that V[i] is the initial 64 bit vector, V[n] is the nth 64 bit Given that V[i] is the initial 64 bit vector, V[n] is the nth 64 bit
vector, D[n] is the nth chunk of 64 bits of data to encrypt (de- vector, D[n] is the nth chunk of 64 bits of data to encrypt
crypt), and O[n] is the nth chunk of 64 bits of encrypted (decrypted) (decrypt), and O[n] is the nth chunk of 64 bits of encrypted
data, then: (decrypted) data, then:
V[0] = DES(V[i], key) V[0] = DES(V[i], key)
V[n+1] = DES(V[n], key) V[n+1] = DES(V[n], key)
O[n] = D[n] <exclusive or> V[n] O[n] = D[n] <exclusive or> V[n]
5. Integration with the AUTHENTICATION telnet option 5. Integration with the AUTHENTICATION telnet option
As noted in the telnet ENCRYPTION option specifications, a keyid val- As noted in the telnet ENCRYPTION option specifications, a keyid
ue of zero indicates the default encryption key, as might be derived value of zero indicates the default encryption key, as might be
from the telnet AUTHENTICATION option. If the default encryption key derived from the telnet AUTHENTICATION option. If the default
negotiated as a result of the telnet AUTHENTICATION option contains encryption key negotiated as a result of the telnet AUTHENTICATION
less than 8 bytes, then the DES_OFB64 option may not be offered or option contains less than 8 bytes, then the DES_OFB64 option may not
used as a valid telnet encryption option. If the encryption key ne- be offered or used as a valid telnet encryption option. If the
gotiated as a result of the telnet AUTHENTICATION option is greater encryption key negotiated as a result of the telnet AUTHENTICATION
than 16 bytes the first 8 bytes of the key should be used as keyid 0 option is greater than 16 bytes the first 8 bytes of the key should
for data sent from the telnet server to the telnet client, and the be used as keyid 0 for data sent from the telnet server to the telnet
second 8 bytes of the key should be used as keyid 0 for data sent by client, and the second 8 bytes of the key should be used as keyid 0
the telnet client to the telnet server. Otherwise, the first 8 bytes for data sent by the telnet client to the telnet server. Otherwise,
of the encryption key is used as keyid zero for the telnet ENCRYPTION the first 8 bytes of the encryption key is used as keyid zero for the
option in both directions (with the client as WILL ENCRYPT and the telnet ENCRYPTION option in both directions (with the client as WILL
server as WILL ENCRYPT). ENCRYPT and the server as WILL ENCRYPT).
In all cases, if the key negotiated by the telnet AUTHENTICATION op- In all cases, if the key negotiated by the telnet AUTHENTICATION
tion was not a DES key, the key used by the DES_CFB64 must have its option was not a DES key, the key used by the DES_CFB64 must have its
parity corrected after it is detrmined using the above algorithm. parity corrected after it is determined using the above algorithm.
Note that the above algorithm assumes that it is safe to use a non- Note that the above algorithm assumes that it is safe to use a non-
DES key (or part of a non-DES key) as a DES key. This is not neces- DES key (or part of a non-DES key) as a DES key. This is not
sarily true of all cipher systems, but we specify this behaviour as necessarily true of all cipher systems, but we specify this behaviour
the default since it is true for most authentication systems in popu- as the default since it is true for most authentication systems in
lar use today, and for compatibility with existing implementations. popular use today, and for compatibility with existing
New telnet AUTHENTICATION mechanisms may specify althernative methods implementations. New telnet AUTHENTICATION mechanisms may specify
for determining the keys to be used for this cipher suite in their alternative methods for determining the keys to be used for this
specification, if the session key negotiated by that authentication cipher suite in their specification, if the session key negotiated by
mechanism is not a DES key and and where this algorithm may not be that authentication mechanism is not a DES key and and where this
safely used. algorithm may not be safely used.
6. Security Considerations
6. Security considerations
Encryption using Output Feedback does not ensure data integrity; an Encryption using Output Feedback does not ensure data integrity; an
active attacker may be able to substitute text, if he can predict the active attacker may be able to substitute text, if he can predict the
clear-text that was being transmitted. For this reason, the Cipher clear-text that was being transmitted. For this reason, the Cipher
Feedback encryption type should be used instead, since it provides Feedback encryption type should be used instead, since it provides
limited detectability to data modification. Neither provides true limited detectability to data modification. Neither provides true
data integrity, however. data integrity, however.
The tradeoff here is that adding a message authentication code (MAC) The tradeoff here is that adding a message authentication code (MAC)
will significantly increase the number of bytes needed to send a sin- will significantly increase the number of bytes needed to send a
gle character in the telnet protocol, which will impact performance single character in the telnet protocol, which will impact
on slow (i.e. dialup) links. performance on slow (i.e. dialup) links.
7. Acknowledgments 7. Acknowledgments
This document was originally written by Dave Borman of Cray Research This document was originally written by Dave Borman of Cray Research
with the assistance of the IETF Telnet Working Group. with the assistance of the IETF Telnet Working Group.
Author's Address Author's Address
Theodore Ts'o, Editor Theodore Ts'o, Editor
VA Linux Systems VA Linux Systems
43 Pleasant St. 43 Pleasant St.
Medford, MA 02155 Medford, MA 02155
Phone: (781) 391-3464 Phone: (781) 391-3464
EMail: [email protected]
EMail: [email protected] Full Copyright Statement
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Funding for the RFC Editor function is currently provided by the
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