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rfc:rfc2951

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

Housley, et al. Informational [Page 1] 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

Housley, et al. Informational [Page 2] 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.

Housley, et al. Informational [Page 3] 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.

Housley, et al. Informational [Page 4] 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 –>

Housley, et al. Informational [Page 5] 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.
     Cn is the nth character in the TELNET stream.
     Hn = SHA-1( Hn-1||Cn ), where Hn is the hash value
          associated with the nth character in the stream.
     ICVn is set to the three most significant bytes of Hn.
     Transmit Encrypt( Cn||ICVn ).
 The ciphertext that is transmitted is the SKIPJACK CFB-32 encryption
 of ( Cn||ICVn ).  The receiving end of the TELNET link reverses the
 process, first decrypting the ciphertext, separating Cn and ICVn,
 recalculating Hn, recalculating ICVn, and then comparing the received
 ICVn with the recalculated ICVn.  Integrity is indicated if the
 comparison succeeds, and Cn can then be processed normally as part of
 the TELNET stream.  Failure of the comparison indicates some loss of
 integrity, whether due to active manipulation or loss of
 cryptographic synchronization.  In either case, the only recourse is
 to drop the TELNET connection and start over.
 For SKIPJACK with stream integrity, the two-octet authentication type
 pair is KEA_SJ_INTEG AUTH_CLIENT_TO_SERVER | AUTH_HOW_MUTUAL |
 ENCRYPT_AFTER_EXCHANGE | INI_CRED_FWD_OFF.  This indicates that the
 KEA SKIPJACK with integrity mechanism will be used for mutual
 authentication and TELNET stream encryption.  Figure 2 illustrates
 the authentication mechanism of KEA SKIPJACK with stream integrity.

Housley, et al. Informational [Page 6] 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_INTEG 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_INTEG
                                      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_INTEG AUTH_CLIENT_TO_SERVER |

   AUTH_HOW_MUTUAL |
   ENCRYPT_AFTER_EXCHANGE |
   INI_CRED_FWD_OFF

KEA_IVA_RESPONSEB_NONCEA IVa||Encrypt( (NonceB XOR 0x0D12)||NonceA ) IAC SE –>

Housley, et al. Informational [Page 7] RFC 2951 TELNET Authentication Using KEA & SKIPJACK September 2000

Client (Party A) Server (Party B)

<client begins encryption>

                                  <-- IAC SB AUTHENTICATION REPLY
                                      KEA_SJ_INTEG
                                      AUTH_CLIENT_TO_SERVER |
                                          AUTH_HOW_MUTUAL |
                                          ENCRYPT_AFTER_EXCHANGE |
                                          INI_CRED_FWD_OFF
                                      KEA_RESPONSEA
                                      Encrypt( NonceA XOR 0x0D12 )
                                      IAC SE
                                      <server begins encryption>

———————————————————————

                            Figure 2

4.0. Security Considerations

 This entire memo is about security mechanisms.  For KEA to provide
 the authentication discussed, the implementation must protect the
 private key from disclosure.  Likewise, the SKIPJACK keys must be
 protected from disclosure.
 Implementations must randomly generate KEA private keys,
 initialization vectors (IVs), and nonces.  The use of inadequate
 pseudo-random number generators (PRNGs) to generate cryptographic
 keys can result in little or no security.  An attacker may find it
 much easier to reproduce the PRNG environment that produced the keys,
 searching the resulting small set of possibilities, rather than brute
 force searching the whole key space.  The generation of quality
 random numbers is difficult.  RFC 1750 [8] offers important guidance
 in this area, and Appendix 3 of FIPS Pub 186 [9] provides one quality
 PRNG technique.
 By linking the enabling of encryption as a side effect of successful
 authentication, protection is provided against an active attacker.
 If encryption were enabled as a separate negotiation, it would
 provide a window of vulnerability from when the authentication
 completes, up to and including the negotiation to turn on encryption.
 The only safe way to restart encryption, if it is turned off, is to
 repeat the entire authentication process.

Housley, et al. Informational [Page 8] RFC 2951 TELNET Authentication Using KEA & SKIPJACK September 2000

5. IANA Considerations

 The authentication types KEA_SJ and KEA_SJ_INTEG and their associated
 suboption values are registered with IANA.  Any suboption values used
 to extend the protocol as described in this document must be
 registered with IANA before use.  IANA is instructed not to issue new
 suboption values without submission of documentation of their use.

6.0. Acknowledgements

 We would like to thank William Nace for support during implementation
 of this specification.

7.0. References

 [1] Postel, J. and J. Reynolds, "TELNET Protocol Specification", ASTD
     8, RFC 854, May 1983.
 [2] Ts'o, T. and J. Altman, "Telnet Authentication Option", RFC 2941,
     September 2000.
 [3] Secure Hash Standard. FIPS Pub 180-1. April 17, 1995.
 [4] "SKIPJACK and KEA Algorithm Specification", Version 2.0, May 29,
     1998. Available from http://csrc.nist.gov/encryption/skipjack-
     kea.htm
 [5] Postel, J. and J. Reynolds, "TELNET Option Specifications", STD
     8, RFC 855, May 1983.
 [6] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet X.509
     Public Key Infrastructure: X.509 Certificate and CRL Profile",
     RFC 2459, January 1999.
 [7] Housley, R. and W. Polk, "Internet X.509 Public Key
     Infrastructure - Representation of Key Exchange Algorithm (KEA)
     Keys in Internet X.509 Public Key Infrastructure Certificates",
     RFC 2528, March 1999.
 [8] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
     Recommendations for Security", RFC 1750, December 1994.
 [9) National Institute of Standards and Technology.  FIPS Pub 186:
     Digital Signature Standard.  19 May 1994.

Housley, et al. Informational [Page 9] RFC 2951 TELNET Authentication Using KEA & SKIPJACK September 2000

8.0. Authors' Addresses

 Russell Housley
 SPYRUS
 381 Elden Street, Suite 1120
 Herndon, VA 20170
 USA
 EMail: housley@spyrus.com
 Todd Horting
 SPYRUS
 381 Elden Street, Suite 1120
 Herndon, VA 20170
 USA
 EMail: thorting@spyrus.com
 Peter Yee
 SPYRUS
 5303 Betsy Ross Drive
 Santa Clara, CA 95054
 USA
 EMail: yee@spyrus.com

Housley, et al. Informational [Page 10] RFC 2951 TELNET Authentication Using KEA & SKIPJACK September 2000

9. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Housley, et al. Informational [Page 11]

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