Internet-Draft Massachusetts Institute of Technology draft-tso-telnet-enc-des-ofb-00.txt February 1998 Telnet Encryption: DES 64 bit Output Feedback Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 1. Command Names and Codes Encryption Type DES_OFB64 2 Suboption Commands OFB64_IV 1 OFB64_IV_OK 2 OFB64_IV_BAD 3 OFB64_CHALLENGE 4 OFB64_RESPONSE 5 2. Command Meanings IAC SB ENCRYPT IS DES_OFB64 OFB64_IV <initial vector> IAC SE The sender of this command generates a random 8 byte initial vec- tor, and sends it to the other side of the connection using the Expires August 1998 [Page 1]
Internet-Draft DES 64 bit Output Feedback February 1998 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 DES_OFB64 OFB64_IV_OK IAC SE IAC SB ENCRYPT REPLY DES_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 con- nection that is DO ENCRYPT may send the OFB64_IV_OK and OFB64_IV_BAD commands. 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 DES 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 en- cryption (decryption) machine must be re-initialized immediately fol- lowing 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 (decryp- tion) 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 (re- ceived) in that START command, and the initial vector sent (received) in this OFB64_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 (de- crypt), and O[n] is the nth chunk of 64 bits of encrypted (decrypted) data, then: Expires August 1998 [Page 2]
Internet-Draft DES 64 bit Output Feedback February 1998 V[0] = DES(V[i], key) V[n+1] = DES(V[n], key) O[n] = D[n] <exclusive or> V[n] 5. Security considerations Encryption using Output Feedback does not ensure data integrity; an active attacker may be able to substitute text, if he can predict the clear-text that was being transmitted. This option was originally drafted back when CPU speeds where not necessarily fast enough to do allow use of CFB. Since then, CPU's have gotten much faster. Given the inherent weaknesses in Output Feedback mode, perhaps it should be deprecated in favor of CFB modes? 6. Acknowledgements 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 Massachusetts Institute of Technology MIT Room E40-343 77 Massachusetts Ave. Cambridge, MA 02139 Phone: (617) 253-8091 EMail: tytso@mit.edu Expires August 1998 [Page 3]