Telnet Encryption: DES 64 bit Cipher Feedback

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2000). All Rights Reserved.

Abstract

This document specifies how to use the DES encryption algorithm in cipher feedback mode with the telnet encryption option.

1. Command Names and Codes

   Encryption Type
   
      DES_CFB64        1
   
   Suboption Commands
   
      CFB64_IV         1
      CFB64_IV_OK      2
      CFB64_IV_BAD     3

2. Command Meanings

IAC SB ENCRYPT IS DES_CFB64 CFB64_IV <initial vector> IAC SE

The sender of this command generates a random 8 byte initial vector, and sends it to the other side of the connection using the CFB64_IV command. The initial vector is sent in clear text. Only the side of the connection that is WILL ENCRYPT may send the CFB64_IV command.

IAC SB ENCRYPT REPLY DES_CFB64 CFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY DES_CFB64 CFB64_IV_BAD IAC SE

The sender of these commands either accepts or rejects the initial vector received in a CFB64_IV command. Only the side of the connection that is DO ENCRYPT may send the CFB64_IV_OK and CFB64_IV_BAD commands. The CFB64_IV_OK command MUST be sent for backwards compatibility with existing implementations; there really isn't any reason why a sender would need to send the CFB64_IV_BAD command except in the case of a protocol violation where the IV sent was not of the correct length (i.e., 8 bytes).

3. Implementation Rules

Once a CFB64_IV_OK command has been received, the WILL ENCRYPT side of the connection should do keyid negotiation using the ENC_KEYID command. Once the keyid negotiation has successfully identified a 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 DES 64 bit Cipher Feedback algorithm.

If encryption (decryption) is turned off and back on again, and the same keyid is used when re-starting the encryption (decryption), the intervening clear text must not change the state of the encryption (decryption) machine.

If a START command is sent (received) with a different keyid, the encryption (decryption) machine must be re-initialized immediately following the end of the START command with the new key and the initial vector sent (received) in the last CFB64_IV command.

If a new CFB64_IV command is sent (received), and encryption (decryption) is enabled, the encryption (decryption) machine must be re-initialized immediately following the end of the CFB64_IV command with the new initial vector, and the keyid sent (received) in the last START command.

If encryption (decryption) is not enabled when a CFB64_IV command is sent (received), the encryption (decryption) machine must be re- initialized after the next START command, with the keyid sent (received) in that START command, and the initial vector sent (received) in this CFB64_IV command.

4. Algorithm

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 (decrypt), and O[n] is the nth chunk of 64 bits of encrypted (decrypted) data, then:

      V[0] = DES(V[i], key)
      O[n] = D[n] <exclusive or> V[n]
      V[n+1] = DES(O[n], key)

5. Integration with the AUTHENTICATION telnet option

As noted in the telnet ENCRYPTION option specifications, a keyid value of zero indicates the default encryption key, as might be derived from the telnet AUTHENTICATION option. If the default encryption key negotiated as a result of the telnet AUTHENTICATION option contains less than 8 bytes, then the DES_CFB64 option must not be offered or used as a valid telnet encryption option. If the encryption key negotiated as a result of the telnet AUTHENTICATION option is greater than 16 bytes the first 8 bytes of the key should be used as keyid 0 for data sent from the telnet client to the telnet server, and the second 8 bytes of the key should be used as keyid 0 for data sent by the telnet server to the telnet client. Otherwise, the first 8 bytes of the encryption key is used as keyid zero for the telnet ENCRYPTION option in both directions (with the client as WILL ENCRYPT and the server as WILL ENCRYPT).

In all cases, if the key negotiated by the telnet AUTHENTICATION option was not a DES key, the key used by the DES_CFB64 must have its parity corrected after it is determined using the above algorithm.

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 necessarily true of all cipher systems, but we specify this behaviour as the default since it is true for most authentication systems in popular use today, and for compatibility with existing implementations. New telnet AUTHENTICATION mechanisms may specify alternative methods for determining the keys to be used for this cipher suite in their specification, if the session key negotiated by that authentication mechanism is not a DES key and and where this algorithm may not be safely used.

6. Security Considerations

Encryption using Cipher Feedback does not ensure data integrity; the active attacker has a limited ability to modify text, if he can predict the clear-text that was being transmitted. The limitations faced by the attacker (that only 8 bytes can be modified at a time, and the following 8-byte block of data will be corrupted, thus making detection likely) are significant, but it is possible that an active attacker still might be able to exploit this weakness.

The tradeoff here is that adding a message authentication code (MAC) will significantly increase the number of bytes needed to send a single character in the telnet protocol, which will impact performance on slow (i.e. dialup) links.

7. Acknowledgments

This document was originally written by Dave Borman of Cray Research with the assistance of the IETF Telnet Working Group.

Author's Address

Theodore Ts'o, Editor
VA Linux Systems
43 Pleasant St.
Medford, MA 02155

   Phone: (781) 391-3464
   EMail: tytso@mit.edu

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