Network Working Group                                           T. Ts'oRequest for Comments: 2946                             VA Linux SystemsCategory: Standards Track                                September 2000                     Telnet Data Encryption OptionStatus 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 describes a the telnet encryption option as a generic   method of providing data confidentiality services for the telnet data   stream.  While this document summarizes currently utilized encryption   types and codes, it does not define a specific encryption algorithm.   Separate documents are to be published defining implementations of   this option for each encryption algorithm.1.  Command Names and Codes   ENCRYPT         38       Encryption Commands       IS               0       SUPPORT          1       REPLY            2       START            3       END              4       REQUEST-START    5       REQUEST-END      6       ENC_KEYID        7       DEC_KEYID        8       Encryption Types       NULL             0       DES_CFB64        1       DES_OFB64        2Ts'o                        Standards Track                     [Page 1]RFC 2946             Telnet Data Encryption Option        September 2000       DES3_CFB64       3       DES3_OFB64       4       CAST5_40_CFB64   8       CAST5_40_OFB64   9       CAST128_CFB64   10       CAST128_OFB64   11       Following historical practice, future encryption type numbers       will be assigned by the IANA under a First Come First Served       policy as outlined by RFC 2434 [3].  Despite the fact that       authentication type numbers are allocated out of an 8-bit number       space (as are most values in the telnet specification) it is not       anticipated that the number space is or will become in danger of       being exhausted.  However, if this should become an issue, when       over 50% of the number space becomes allocated, the IANA shall       refer allocation requests to either the IESG or a designated       expert for approval.2.  Command Meanings   IAC WILL ENCRYPT      The sender of this command is willing to send encrypted data.   IAC WONT ENCRYPT      The sender of this command refuses to send encrypted data.   IAC DO ENCRYPT      The sender of this command is willing to receive encrypted data.   IAC DONT ENCRYPT      The sender of this command refuses to accept encrypted data.   IAC SB ENCRYPT SUPPORT encryption-type-list IAC SE      The sender of this command is stating which types of encryption it      will support.  Only the side of the connection that is DO ENCRYPT      may send the SUPPORT command.  The current types of encryption are      listed in the current version of the Assigned Numbers document      [1].      The encryption-type-list may only include types which can actually      be supported during the current session.  If ENCRYPT is negotiated      in conjunction with AUTH the SUPPORT message MUST NOT be sent      until after the session key has been determined.  Otherwise,Ts'o                        Standards Track                     [Page 2]RFC 2946             Telnet Data Encryption Option        September 2000      it is impossible to know if the selected encryption type can be      properly initialized based upon the type and length of the key      that is available."   IAC SB ENCRYPT IS encryption-type ... IAC SE      The sender of this command is stating which type of encryption to      use, and any initial data that is needed.  Only the side of the      connection that is WILL ENCRYPT may send the IS command to      initialize the encryption-type scheme.   IAC SB ENCRYPT REPLY encryption-type ... IAC SE      The sender of this command is continuing the initial data exchange      in order to initialize the encryption-type scheme.  Only the side      of the connection that is DO ENCRYPT may send the REPLY command.   IAC SB ENCRYPT START keyid IAC SE      The sender of this command is stating that all data following the      command in the data stream will be be encrypted via the previously      negotiated method of data encryption.  Only the side of the      connection that is WILL ENCRYPT may send the START command.      The keyid is a variable length field.  It is used by various      encryption mechanisms to identify which encryption key is to be      used, when multiple encryption keys might be known on either side      of the connection.  The keyid field is encoded with the most      significant byte first, and a keyid value of zero is reserved to      indicate the default encryption key (this would typically be an      encryption key derived during authentication, with the      AUTHENTICATION option).  The keyid field must be at least one byte      long.  The only valid values for "keyid" will be those that have      been received in a DEC_KEYID command.   IAC SB ENCRYPT END IAC SE      The sender of this command is stating that all data following the      command in the data stream will not be encrypted.  Only the side      of the connection that is WILL ENCRYPT may send the END   IAC SB ENCRYPT REQUEST-START keyid IAC SE      The sender of this command requests that the remote side begin      encryption of the telnet data stream.  Only the side of the      connection that is DO ENCRYPT may send the REQUEST-START command.      The keyid is only advisory, and my be omitted.Ts'o                        Standards Track                     [Page 3]RFC 2946             Telnet Data Encryption Option        September 2000   IAC SB ENCRYPT REQUEST-END IAC SE      The sender of this command requests that the remote side stop      encryption of the telnet data stream.  Only the side of the      connection that is DO ENCRYPT may send the REQUEST-END command.   IAC SB ENCRYPT ENC_KEYID keyid IAC SE      The sender of this requests that the remote side verify that      "keyid" maps to a valid key; or verifies that the "keyid" received      in a DEC_KEYID command is valid.  If keyid is omitted, it implies      that there are no more known keyids, and that the attempt to find      a common keyid has failed.  Only the side of the connection that      is WILL ENCRYPT may send the ENC_KEYID command.   IAC SB ENCRYPT DEC_KEYID keyid IAC SE      The sender of this requests that the remote side verify that      "keyid" maps to a valid key on the remote side; or verifies that      the "keyid" received in a ENC_KEYID command is valid.  If keyid is      omitted, it implies that there are no more known keyids, and that      the attempt to find a common keyid has failed.  Only the side of      the connection that is DO ENCRYPT may send the DEC_KEYID command.3.  Default Specification   The default specification for this option is      WONT ENCRYPT      DONT ENCRYPT   meaning there will not be any encryption of the Telnet data stream.4.  Motivation   The Telnet protocol has no form of protection from some intervening   gateway looking at IP packets as they travel through the network.   This is especially dangerous when passwords are sent as clear text   over the network.  This option provides a method for encrypting the   data stream.5.  Implementation Rules   Once the Encryption option is in effect, all data in the negotiated   direction, including TELNET options, is encrypted.  Encryption begins   with the octet of data immediately following the "IAC SB ENCRYPT   START encryption-type IAC SE" command.  Encryption ends after the   "IAC SB ENCRYPT END IAC SE" command.Ts'o                        Standards Track                     [Page 4]