CAST-128 64 bit Cipher Feedback(1)

　　Network Working Group J. Altman

　　Request for Comments: 2950 Columbia University

　　Category: Standards Track September 2000

　　TelnetEncryption: CAST-128 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 CAST-128 encryption algorithm　in cipher feedback mode with the telnet encryption option. Two key　sizes are defined: 40 bit and 128 bit.

　　1. Command Names and Codes

　　Encryption Type

　　CAST5_40_CFB64 8

　　CAST128_CFB64 10

　　Suboption Commands

　　CFB64_IV 1

　　CFB64_IV_OK 2

　　CFB64_IV_BAD 3

　　2. Command Meanings

　　IAC SB ENCRYPT IS CAST5_40_CFB64 CFB64_IV IAC SE

　　IAC SB ENCRYPT IS CAST128_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 CAST5_40_CFB64 CFB64_IV_OK IAC SE

　　IAC SB ENCRYPT REPLY CAST128_CFB64 CFB64_IV_OK IAC SE

　　IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_BAD IAC SE

　　IAC SB ENCRYPT REPLY CAST128_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 CAST128 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

　　CAST 64 bit Cipher Feedback

　　key --->+------+

　　+->| CAST |--+

　　| +------+ |

　　| 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 = CAST(iV, key)

　　On = Dn ^ Vn

　　V(n+1) = CAST(On, key)

　　5. Integration with the AUTHENTICATION telnet option