DES3 64 bit Output Feedback(1)

　　Network Working Group J. Altman

　　Request for Comments: 2948 Columbia University

　　Category: Standards Track September 2000

　　TelnetEncryption: DES3 64 bit Output 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 output feedback mode with the　telnet encryption option.

　　1. Command Names and Codes

　　Encryption Type

　　DES3_OFB64 4

　　Suboption Commands

　　OFB64_IV 1

　　OFB64_IV_OK 2

　　OFB64_IV_BAD 3

　　2. Command Meanings

　　IAC SB ENCRYPT IS DES3_OFB64 OFB64_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　OFB64_IV command. The initial vector is sent in clear text. Only　the side of the connection that is WILL ENCRYPT may send the　OFB64_IV command.

　　IAC SB ENCRYPT REPLY DES3_OFB64 OFB64_IV_OK IAC SE

　　IAC SB ENCRYPT REPLY DES3_OFB64 OFB64_IV_BAD IAC SE

　　The sender of these commands either accepts or rejects the initial vector received in a OFB64_IV command. Only the side of the　connection that is DO ENCRYPT may send the OFB64_IV_OK and　OFB64_IV_BAD commands. The OFB64_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　OFB64_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 OFB64_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 Output 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 OFB64_IV command.　If a new OFB64_IV command is sent (received), and encryption　(decryption) is enabled, the encryption (decryption) machine must be　re-initialized immediately following the end of the OFB64_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 OFB64_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 OFB64_IV command.

　　4. Algorithm

　　DES3 64 bit Output 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)

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

　　On = Dn ^ Vn

　　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_OFB64 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).