rfc2951.txt

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Network Working Group                                         R. Housley
Request for Comments: 2951                                    T. Horting
Category: Informational                                           P. Yee
                                                                  SPYRUS
                                                          September 2000


              TELNET Authentication Using KEA and SKIPJACK

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document defines a method to authenticate TELNET using the Key
   Exchange Algorithm (KEA), and encryption of the TELNET stream using
   SKIPJACK.  Two encryption modes are specified; one provides data
   integrity and the other does not.  The method relies on the TELNET
   Authentication Option.

1. Command Names and Codes

   AUTHENTICATION           37

     Authentication Commands:

       IS                       0
       SEND                     1
       REPLY                    2
       NAME                     3

     Authentication Types:

       KEA_SJ                  12
       KEA_SJ_INTEG            13

     Modifiers:

       AUTH_WHO_MASK            1
       AUTH_CLIENT_TO_SERVER    0
       AUTH_SERVER_TO CLIENT    1



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RFC 2951       TELNET Authentication Using KEA & SKIPJACK September 2000


       AUTH_HOW_MASK            2
       AUTH_HOW_ONE_WAY         0
       AUTH_HOW_MUTUAL          2

       ENCRYPT_MASK            20
       ENCRYPT_OFF              0
       ENCRYPT_USING_TELOPT     4
       ENCRYPT_AFTER_EXCHANGE  16
       ENCRYPT_RESERVED        20

       INI_CRED_FWD_MASK        8
       INI_CRED_FWD_OFF         0
       INI_CRED_FWD_ON          8

     Sub-option Commands:

       KEA_CERTA_RA             1
       KEA_CERTB_RB_IVB_NONCEB  2
       KEA_IVA_RESPONSEB_NONCEA 3
       KEA_RESPONSEA            4

2. TELNET Security Extensions

   TELNET, as a protocol, has no concept of security.  Without
   negotiated options, it merely passes characters back and forth
   between the NVTs represented by the two TELNET processes.  In its
   most common usage as a protocol for remote terminal access (TCP port
   23), TELNET normally connects to a server that requires user-level
   authentication through a user name and password in the clear.  The
   server does not authenticate itself to the user.

   The TELNET Authentication Option provides for:

     *  User authentication -- replacing or augmenting the normal host
        password mechanism;
     *  Server authentication -- normally done in conjunction with user
        authentication;
     *  Session parameter negotiation -- in particular, encryption key
        and attributes;
     *  Session protection -- primarily encryption of the data and
        embedded command stream, but the encryption algorithm may also
        provide data integrity.

   In order to support these security services, the two TELNET entities
   must first negotiate their willingness to support the TELNET
   Authentication Option.  Upon agreeing to support this option, the
   parties are then able to perform sub-option negotiations to determine




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RFC 2951       TELNET Authentication Using KEA & SKIPJACK September 2000


   the authentication protocol to be used, and possibly the remote user
   name to be used for authorization checking.  Encryption is negotiated
   along with the type of the authentication.

   Authentication and parameter negotiation occur within an unbounded
   series of exchanges.  The server proposes a preference-ordered list
   of authentication types (mechanisms) that it supports.  In addition
   to listing the mechanisms it supports, the server qualifies each
   mechanism with a modifier that specifies whether encryption of data
   is desired.  The client selects one mechanism from the list and
   responds to the server indicating its choice and the first set of
   authentication data needed for the selected authentication type.  The
   client may ignore a request to encrypt data and so indicate, but the
   server may also terminate the connection if the client refuses
   encryption.  The server and the client then proceed through whatever
   number of iterations is required to arrive at the requested
   authentication.

   Encryption is started immediately after the Authentication Option is
   completed.

3. Use of Key Exchange Algorithm (KEA)

   This paper specifies the method in which KEA is used to achieve
   TELNET Authentication.  KEA (in conjunction with SKIPJACK) [4]
   provides authentication and confidentiality.  Integrity may also be
   provided.

   TELNET entities may use KEA to provide mutual authentication and
   support for the setup of data encryption keys.  A simple token format
   and set of exchanges delivers these services.

   NonceA and NonceB used in this exchange are 64-bit bit strings.  The
   client generates NonceA, and the server generates NonceB.  The nonce
   value is selected randomly.  The nonce is sent in a big endian form.
   The encryption of the nonce will be done with the same mechanism that
   the session will use, detailed in the next section.

   Ra and Rb used in this exchange are 1024 bit strings and are defined
   by the KEA Algorithm [4].

   The IVa and IVb are 24 byte Initialization Vectors.  They are
   composed of "THIS IS NOT LEAF" followed by 8 random bytes.








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RFC 2951       TELNET Authentication Using KEA & SKIPJACK September 2000


   CertA is the client's certificate.  CertB is the server's
   certificate.  Both certificates are X.509 certificates [6] that
   contain KEA public keys [7].  The client must validate the server's
   certificate before using the KEA public key it contains.  Likewise,
   the server must validate the client's certificate before using the
   KEA public key it contains.

   On completing these exchanges, the parties have a common SKIPJACK
   key.  Mutual authentication is provided by verification of the
   certificates used to establish the SKIPJACK encryption key and
   successful use of the derived SKIPJACK session key.  To protect
   against active attacks, encryption will take place after successful
   authentication.  There will be no way to turn off encryption and
   safely turn it back on; repeating the entire authentication is the
   only safe way to restart it.  If the user does not want to use
   encryption, he may disable encryption after the session is
   established.

3.1.  SKIPJACK Modes

   There are two distinct modes for encrypting TELNET streams; one
   provides integrity and the other does not.  Because TELNET is
   normally operated in a character-by-character mode, the SKIPJACK with
   stream integrity mechanism requires the transmission of 4 bytes for
   every TELNET data byte.  However, a simplified mode SKIPJACK without
   integrity mechanism will only require the transmission of one byte
   for every TELNET data byte.

   The cryptographic mode for SKIPJACK with stream integrity is Cipher
   Feedback on 32 bits of data (CFB-32) and the mode of SKIPJACK is
   Cipher Feedback on 8 bits of data (CFB-8).

3.1.1.  SKIPJACK without stream integrity

   The first and least complicated mode uses SKIPJACK CFB-8.  This mode
   provides no stream integrity.

   For SKIPJACK without stream integrity, the two-octet authentication
   type pair is KEA_SJ AUTH_CLIENT_TO_SERVER | AUTH_HOW_MUTUAL |
   ENCRYPT_AFTER_EXCHANGE | INI_CRED_FWD_OFF.  This indicates that the
   SKIPJACK without integrity mechanism will be used for mutual
   authentication and TELNET stream encryption.  Figure 1 illustrates
   the authentication mechanism of KEA followed by SKIPJACK without
   stream integrity.







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RFC 2951       TELNET Authentication Using KEA & SKIPJACK September 2000


---------------------------------------------------------------------
 Client (Party A)                   Server (Party B)

                                    <-- IAC DO AUTHENTICATION

 IAC WILL AUTHENTICATION        -->

                                    <-- IAC SB AUTHENTICATION SEND
                                        <list of authentication options>
                                        IAC SE

 IAC SB AUTHENTICATION
 NAME <user name>               -->

 IAC SB AUTHENTICATION IS
 KEA_SJ
 AUTH_CLIENT_TO_SERVER |
     AUTH_HOW_MUTUAL |
     ENCRYPT_AFTER_EXCHANGE |
     INI_CRED_FWD_OFF
 KEA_CERTA_RA
 CertA||Ra IAC SE               -->

                                    <-- IAC SB AUTHENTICATION REPLY
                                        KEA_SJ
                                        AUTH_CLIENT_TO_SERVER |
                                            AUTH_HOW_MUTUAL |
                                            ENCRYPT_AFTER_EXCHANGE |
                                            INI_CRED_FWD_OFF
                                        IVA_RESPONSEB_NONCEA
                                        KEA_CERTB_RB_IVB_NONCEB
                                        CertB||Rb||IVb||
                                            Encrypt( NonceB )
                                        IAC SE

 IAC SB AUTHENTICATION IS
 KEA_SJ
 AUTH_CLIENT_TO_SERVER |
     AUTH_HOW_MUTUAL |
     ENCRYPT_AFTER_EXCHANGE |
     INI_CRED_FWD_OFF
 KEA_IVA_RESPONSEB_NONCEA
 IVa||Encrypt( (NonceB XOR 0x0C12)||NonceA )
 IAC SE                         -->







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RFC 2951       TELNET Authentication Using KEA & SKIPJACK September 2000


 Client (Party A)                   Server (Party B)

 <client begins encryption>
                                    <-- IAC SB AUTHENTICATION REPLY
                                        KEA_SJ
                                        AUTH_CLIENT_TO_SERVER |
                                            AUTH_HOW_MUTUAL |
                                            ENCRYPT_AFTER_EXCHANGE |
                                            INI_CRED_FWD_OFF
                                        KEA_RESPONSEA
                                        Encrypt( NonceA XOR 0x0C12 )
                                        IAC SE

                                        <server begins encryption>
---------------------------------------------------------------------
                              Figure 1.

3.1.2.  SKIPJACK with stream integrity

   SKIPJACK with stream integrity is more complicated.  It uses the
   SHA-1 [3] one-way hash function to provide integrity of the
   encryption stream as follows:

       Set H0 to be the SHA-1 hash of a zero-length string.