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Telnet Encryption: CAST-128 64 bit Cipher Feedback (RFC2950)

IP.com Disclosure Number: IPCOM000005143D
Original Publication Date: 2000-Sep-01
Included in the Prior Art Database: 2001-Aug-16
Document File: 6 page(s) / 9K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

J. Altman: AUTHOR

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.

This text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 37% of the total text.

Network Working Group J. Altman Request for Comments: 2950 Columbia University Category: Standards Track September 2000

Telnet Encryption: 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.

Altman Standards Track [Page 1]

RFC 2950 CAST-128 64 bit Cipher Feedback September 2000

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 th...