Telnet Data Encryption Option
draft-tso-telnet-encryption-04

Versions: 00 01 02 03 04 rfc2946                         Standards Track
Network Working Group                                    T. Ts'o, Editor
Internet-Draft                                          VA Linux Systems
draft-tso-telnet-encryption-04.txt                        September 1999

                     Telnet Data Encryption Option

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

0.  Abstract

   This document describes a the telnet encryption option as a generic
   method of providing data confidentiaility services of the telnet data
   stream.   While this document summarizes currently utilized encrytion
   types and codes, it does not define a specific encryption algorithm.
   Separate documents are to be published defining each encryption algo-
   rithms.

1.  Command Names and Codes

   ENCRYPT         38

       Encryption Commands
       IS               0
       SUPPORT          1
       REPLY            2

                           Expires March 2000                   [Page 1]


Internet-Draft        Telnet Data Encryption Option       September 1999

       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        2
       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 pol-
       icy as outlined by RFC 2434 [3].  Despite the fact that authenti-
       cation type numbers are allocated out of an 8-bit number space
       (as are most values in the telnet specification) it is not antic-
       ipated 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

                           Expires March 2000                   [Page 2]


Internet-Draft        Telnet Data Encryption Option       September 1999

      The sender of this command is stating what 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 un-
      til after the session has key has been determined.  Otherwise, it
      is impossible to know if the selected encryption type cab be prop-
      erly 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 what 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 ini-
      tialize the encryption-type scheme.

   IAC SB ENCRYPT REPLY encryption-type ... IAC SE

      The sender of this command is continuing the initial data exchange
      that is needed to initialize the encryption-type scheme.  Only the
      side of the connection that is DO ENCRYPT may send the REPLY com-
      mand.

   IAC SB ENCRYPT START keyid IAC SE

      The sender of this command is stating that at this point in the
      data stream, all following data will be encrypted, via the previ-
      ously 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 en-
      cryption 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 sig-
      nificant byte first, and a keyid value of zero is reserved to in-
      dicate the default encryption key (this would typically be an en-
      cryption key derived during authentication, with the AUTHENTICA-
      TION option).  The keyid field must be at least one byte long.
      The only valid values for "keyid" will be those that have been re-
      ceived in a DEC_KEYID command.

   IAC SB ENCRYPT END IAC SE

      The sender of this command is stating that at this point in the
      data stream, all following data will no longer be encrypted.  Only
      the side of the connection that is WILL ENCRYPT may send the END

                           Expires March 2000                   [Page 3]


Internet-Draft        Telnet Data Encryption Option       September 1999

      command.

   IAC SB ENCRYPT REQUEST-START keyid IAC SE

      The sender of this command requests that the remote side begin en-
      cryption of the telnet data stream.  Only the side of the connec-
      tion that is DO ENCRYPT may send the REQUEST-START command.  The
      keyid is only advisory, and my be omitted.

   IAC SB ENCRYPT REQUEST-END IAC SE

      The sender of this command requests that the remote side stop en-
      cryption of the telnet data stream.  Only the side of the connec-
      tion 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 on the remote side; 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 com-
      mand.

   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

                           Expires March 2000                   [Page 4]


Internet-Draft        Telnet Data Encryption Option       September 1999

   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.

   WILL and DO are used only at the beginning of the connection to ob-
   tain and grant permission for future negotiations.  The ENCRYPT op-
   tion must be negotiated in both directions.

   Once the two hosts have exchanged a WILL and a DO, the sender of the
   DO ENCRYPT must send a ENCRYPT SUPPORT command to let the remote side
   know what types of encryption it is willing to accept.  In the re-
   quest, a list of supported encryption schemes is sent.  Only the
   sender of the DO may send a list of supported encryption types (IAC
   SB ENCRYPT SUPPORT encryption-type-list IAC SE).  Only the sender of
   the WILL may actually transmit encrypted data.  This is initiated via
   the "IAC SB ENCRYPT START IAC SE" command, and terminated via the
   "IAC SB ENCRYPT END IAC SE" command.  If a START is received, and
   then a second START is received before receiving an END, the second
   START is ignored.

   If the sender of the DO would like the remote side to begin sending
   encrypted data, it can send the "IAC SB ENCRYPT REQUEST-START IAC SE"
   command.  If the sender of the DO would like the remote side to stop
   sending encrypted data, it can send the "IAC SB ENCRYPT REQUEST-STOP
   IAC SE" command.

   If the receiver of the SUPPORT command does not support any of the
   encryption types listed in the SUPPORT command, it should send an
   "IAC SB ENCRYPT IS NULL IAC SE" to indicate that there is not a com-
   mon encryption type.  It may also send an IAC WONT ENCRYPT command to
   turn off the ENCRYPT option.

   The order of the encryption types in a SUPPORT command must be or-
   dered to indicate a preference for different encryption types, the
   first type being the most preferred, and the last type the least pre-
   ferred.

   If the ENCRYPT option has been enabled, and encrypted data is being
   received, the receipt of an "IAC WONT ENCRYPT" implies the receipt of
   an "IAC SB ENCRYPT END IAC SE", e.g., the Telnet data stream is no
   longer encrypted.

   The following is an example of use of the option:
       Host1                            Host2

                           Expires March 2000                   [Page 5]


Internet-Draft        Telnet Data Encryption Option       September 1999

       [ Host1 requests Host2 negotiate to encrypt data that it sends to
         Host1, and Host2 verifies that it will negotiate the encryption
         of data that it sends to Host1.  ]
       DO ENCRYPT
                                        WILL ENCRYPT
       [ Host1 requests that Host2 enable encryption as soon as the
         initialization is completed, and informs Host2 that is supports
         DES_CFB64.  ]
       IAC SB ENCRYPT REQUEST-START IAC
       SE
       IAC SB ENCRYPT SUPPORT DES_CFB64
       IAC SE
       [ Host2 sends the initial feed to Host1, and Host1 acknowledges
         receipt of the IV.  ]
                                        IAC SB ENCRYPT IS DES_CFB64
                                        CFB64_IV  144 146 63 229 237 148
                                        81 143 IAC SE
       IAC SB ENCRYPT REPLY DES_CFB64
       CFB64_IV_OK  103 207 181 71 224
       55 229 98 IAC SE
       [ Host2 is now free to start sending encrypted data, and since a
         REQUEST-START was received, it enables encryption.  ]
                                        IAC SB ENCRYPT START IAC SE
       [ All data from Host2 to Host1 is now encrypted.  ]
                                        IAC SB ENCRYPT END IAC SE
       [ All data from Host2 to Host1 is now in clear text again.  ]

   It is expected that any implementation that supports the Telnet EN-
   CRYPT option will support all of this specification.

6.  Security Considerations

   The ENCRYPT option used in isolation provides protection against pas-
   sive attacks, but not against active attacks.  In other words, it
   will  provide protection from someone who is just watching the IP
   packets as they pass through the network.  However, an attacker who
   is able to modify packets in flight could prevent the ENCRYPT option
   from being negotiated.

   This flaw can be remedied by using the Telnet Authentication option
   alongside the ENCRYPT option.  Specifically, setting ENCRYPT_US-
   ING_TELOPT in the authentication-type-pair can be used to force that
   Encryption be negotiated even in the face of active attacks.

   In addition, an active attacker can interfere with attempts to start
   or restart encryption.  If encryption is requested by the user, and
   the client is unable to negotiate enabling or re-enabling encryption,
   the client must assume that it is being attacked, and MUST immediate-
   ly terminate the telnet connection.

                           Expires March 2000                   [Page 6]


Internet-Draft        Telnet Data Encryption Option       September 1999

7.  Future directions for Telnet Encryption

   The specification defines a method for providing data confidentiality
   to the telnet data stream.  Unfortunately all of the encryption mech-
   anism provided under this option do not provide data integrity, be-
   cause of the complexity of specifying a protocol which provided in-
   tegrity services efficiently in a stream-oriented protocol.

   The TELNET_OVER_TLS specification provides a scheme which provides
   confidentiality, integrity, and compression, and future work for tel-
   net encryption should closely examine using this specification.  One
   promising approach would use the anonymous Diffie-Hellman mode of
   TLS, followed by the telnet AUTHENTICATION option where the authenti-
   cation mechanism would include a signed hash of the Diffie-Hellman
   keying material negotiated by the TLS layer.

8.  Acknowledgments

   This document was originally written by Dave Borman of Cray Research,
   with the assistance of Theodore Ts'o of MIT and the IETF Telnet Work-
   ing Group.

8.  References

   [1] Reynolds, Joyce, and Postel, Jon, "Telnet Protocol Specifica-
       tion", RFC 854, May 1983.
   [2] Internet Engineering Task Force, "Telnet Authentication", draft-
       tso-telnet-auth-enc-04.txt, T. Ts'o, Editor, VA Linux Systems,
       September 1999.
   [3] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

Author's Address

   Theodore Ts'o, Editor
   VA Linux Systems
   43 Pleasant St.
   Medford, MA 02155

   Phone: (781) 391-3464

   EMail: tytso@mit.edu

                                                                [Page 7]