DES3 64 bit Cipher Feedback(1)

　　Network Working Group J. Altman

　　Request for Comments: 2947 Columbia University

　　Category: Standards Track September 2000

　　TelnetEncryption: DES3 64 bit Cipher Feedback

　　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 specifies how to use the Triple-DES (data encryption　standard) encryption algorithm in cipher feedback mode with the　telnet encryption option.

　　1. Command Names and Codes

　　Encryption Type

　　DES3_CFB64 3

　　Suboption Commands

　　CFB64_IV 1

　　CFB64_IV_OK 2

　　CFB64_IV_BAD 3

　　2. Command Meanings

　　IAC SB ENCRYPT IS DES3_CFB64 CFB64_IV 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 DES3_CFB64 CFB64_IV_OK IAC SE IAC SB ENCRYPT

　　REPLY DES3_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 DES3 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

　　DES3 64 bit Cipher Feedback

　　key1 key2 key3

　　| | |

　　v v v

　　+-------+ +-------+ +-------+

　　+->| DES-e |->| DES-d |->| DES-e |-- +

　　| +-------+ +-------+ +-------+ |

　　| v

　　INPUT --(-------------------------------->(+)+---> DATA

　　| |

　　+------------------------------------+

　　Given:

　　iV: Initial vector, 64 bits (8 bytes) long.

　　Dn: the nth chunk of 64 bits (8 bytes) of data to encrypt (decrypt).

　　On: the nth chunk of 64 bits (8 bytes) of encrypted (decrypted) output.

　　V0 = DES-e(DES-d(DES-e(iV, key1),key2),key3)

　　On = Dn ^ Vn

　　V(n+1) = DES-e(DES-d(DES-e(On, key1),key2),key3)

　　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 16 bytes, then the DES3_CFB64 option must　not be offered or used as a valid telnet encryption option.

　　The following rules are to be followed for creating three DES　encryption keys based upon the available encrypt key data:

　　keys_to_use = bytes of key data / DES block size (8 bytes)　where the keys are labeled "key1" through "key6" with "key1" being　the first 8 bytes; "key2" the second 8 bytes; ... and "key6" being　sixth 8 bytes (if available).

　　When two keys are available:

　　. data sent from the server is encrypted with key1, decrypted with　key2, and encrypted with key1;

　　. data sent from the client is encrypted with key2, decrypted with　key1, and encrypted with key2

　　When three keys are available:

　　. data sent from the server is encrypted with key1, decrypted with　key2, and encrypted with key3;

　　. data sent from the client is encrypted with key2, decrypted with　key3, and encrypted with key1