Network Working Group                                      A. Medvinsky
Request for Comments: 2712                                       Excite
Category: Standards Track                                        M. Hur
                                                 CyberSafe Corporation
                                                          October 1999


 Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)

Status 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 (1999).  All Rights Reserved.

IESG Note:

  The 40-bit ciphersuites defined in this memo are included only for
  the purpose of documenting the fact that those ciphersuite codes have
  already been assigned.  40-bit ciphersuites were designed to comply
  with US-centric, and now obsolete, export restrictions.  They were
  never secure, and nowadays are inadequate even for casual
  applications.  Implementation and use of the 40-bit ciphersuites
  defined in this document, and elsewhere, is strongly discouraged.

1. Abstract

  This document proposes the addition of new cipher suites to the TLS
  protocol [1] to support Kerberos-based authentication.  Kerberos
  credentials are used to achieve mutual authentication and to
  establish a master secret which is subsequently used to secure
  client-server communication.

2. Introduction

  Flexibility is one of the main strengths of the TLS protocol.
  Clients and servers can negotiate cipher suites to meet specific
  security and administrative policies.  However, to date,
  authentication in TLS is limited only to public key solutions.  As a
  result, TLS does not fully support organizations with heterogeneous
  security deployments that include authentication systems based on
  symmetric cryptography.  Kerberos, originally developed at MIT, is



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RFC 2712       Addition of Kerberos Cipher Suites to TLS   October 1999


  based on an open standard [2] and is the most widely deployed
  symmetric key authentication system.  This document proposes a new
  option for negotiating Kerberos authentication within the TLS
  framework.  This achieves mutual authentication and the establishment
  of a master secret using Kerberos credentials.  The proposed changes
  are minimal and, in fact, no different from adding a new public key
  algorithm to the TLS framework.

3. Kerberos Authentication Option In TLS

  This section describes the addition of the Kerberos authentication
  option to the TLS protocol.  Throughout this document, we refer to
  the basic SSL handshake shown in Figure 1.  For a review of the TLS
  handshake see [1].

 CLIENT                                             SERVER
 ------                                             ------
ClientHello
                   -------------------------------->
                                                    ServerHello
                                                    Certificate *
                                                    ServerKeyExchange*
                                                    CertificateRequest*
                                                    ServerHelloDone
                   <-------------------------------
Certificate*
ClientKeyExchange
CertificateVerify*
change cipher spec
Finished
    |              -------------------------------->
    |                                               change cipher spec
    |                                               Finished
    |                                                   |
    |                                                   |
Application Data   <------------------------------->Application Data

FIGURE 1: The TLS protocol.  All messages followed by a star are
          optional.  Note: This figure was taken from an IETF document
          [1].

  The TLS security context is negotiated in the client and server hello
  messages.  For example: TLS_RSA_WITH_RC4_MD5 means the initial
  authentication will be done using the RSA public key algorithm, RC4
  will be used for the session key, and MACs will be based on the MD5
  algorithm.  Thus, to facilitate the Kerberos authentication option,
  we must start by defining new cipher suites including (but not
  limited to):



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CipherSuite      TLS_KRB5_WITH_DES_CBC_SHA            = { 0x00,0x1E };
CipherSuite      TLS_KRB5_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x1F };
CipherSuite      TLS_KRB5_WITH_RC4_128_SHA            = { 0x00,0x20 };
CipherSuite      TLS_KRB5_WITH_IDEA_CBC_SHA           = { 0x00,0x21 };
CipherSuite      TLS_KRB5_WITH_DES_CBC_MD5            = { 0x00,0x22 };
CipherSuite      TLS_KRB5_WITH_3DES_EDE_CBC_MD5       = { 0x00,0x23 };
CipherSuite      TLS_KRB5_WITH_RC4_128_MD5            = { 0x00,0x24 };
CipherSuite      TLS_KRB5_WITH_IDEA_CBC_MD5           = { 0x00,0x25 };

CipherSuite      TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA  = { 0x00,0x26 };
CipherSuite      TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA  = { 0x00,0x27 };
CipherSuite      TLS_KRB5_EXPORT_WITH_RC4_40_SHA      = { 0x00,0x28 };
CipherSuite      TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5  = { 0x00,0x29 };
CipherSuite      TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5  = { 0x00,0x2A };
CipherSuite      TLS_KRB5_EXPORT_WITH_RC4_40_MD5      = { 0x00,0x2B };

  To establish a Kerberos-based security context, one or more of the
  above cipher suites must be specified in the client hello message.
  If the TLS server supports the Kerberos authentication option, the
  server hello message, sent to the client, will confirm the Kerberos
  cipher suite selected by the server.  The server's certificate, the
  client

  CertificateRequest, and the ServerKeyExchange shown in Figure 1 will
  be omitted since authentication and the establishment of a master
  secret will be done using the client's Kerberos credentials for the
  TLS server.  The client's certificate will be omitted for the same
  reason.  Note that these messages are specified as optional in the
  TLS protocol; therefore, omitting them is permissible.

  The Kerberos option must be added to the ClientKeyExchange message as
  shown in Figure 2.



















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RFC 2712       Addition of Kerberos Cipher Suites to TLS   October 1999


struct
{
    select (KeyExchangeAlgorithm)
    {
        case krb5:            KerberosWrapper;       /* new addition */
        case rsa:             EncryptedPreMasterSecret;
        case diffie_hellman:  ClientDiffieHellmanPublic;
    } Exchange_keys;

} ClientKeyExchange;

struct
{
    opaque Ticket;
    opaque authenticator;            /* optional */
    opaque EncryptedPreMasterSecret; /* encrypted with the session key
                                        which is sealed in the ticket */
} KerberosWrapper;                   /* new addition */

        FIGURE 2: The Kerberos option in the ClientKeyExchange.

  To use the Kerberos authentication option, the TLS client must obtain
  a service ticket for the TLS server.  In TLS, the ClientKeyExchange
  message is used to pass a random 48-byte pre-master secret to the
  server.

  The client and server then use the pre-master secret to independently
  derive the master secret, which in turn is used for generating
  session keys and for MAC computations.  Thus, if the Kerberos option
  is selected, the pre-master secret structure is the same as that used
  in the RSA case; it is encrypted under the Kerberos session key and
  sent to the TLS server along with the Kerberos credentials (see
  Figure 2).  The ticket and authenticator are encoded per RFC 1510
  (ASN.1 encoding).  Once the ClientKeyExchange message is received,
  the server's secret key is used to unwrap the credentials and extract
  the pre-master secret.

  Note that a Kerberos authenticator is not required, since the master
  secret derived by the client and server is seeded with a random value
  passed in the server hello message, thus foiling replay attacks.
  However, the authenticator may still prove useful for passing
  authorization information and is thus allotted an optional field (see
  Figure 2).

  Lastly, the client and server exchange the finished messages to
  complete the handshake.  At this point we have achieved the
  following:




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RFC 2712       Addition of Kerberos Cipher Suites to TLS   October 1999


  1) A master secret, used to protect all subsequent communication, is
     securely established.

  2) Mutual client-server authentication is achieved, since the TLS
     server proves knowledge of the master secret in the finished
     message.

  Note that the Kerberos option fits in seamlessly, without adding any
  new messages.

4. Naming Conventions:

  To obtain an appropriate service ticket, the TLS client must
  determine the principal name of the TLS server.  The Kerberos service
  naming convention is used for this purpose, as follows:

    host/MachineName@Realm
     where:
       - The literal, "host", follows the Kerberos convention when not
         concerned about the protection domain on a particular machine.
       - "MachineName" is the particular instance of the service.
       - The Kerberos "Realm" is the domain name of the machine.

5. Summary

  The proposed Kerberos authentication option is added in exactly the
  same manner as a new public key algorithm would be added to TLS.
  Furthermore, it establishes the master secret in exactly the same
  manner.

6. Security Considerations

  Kerberos ciphersuites are subject to the same security considerations
  as the TLS protocol.  In addition, just as a public key
  implementation must take care to protect the private key (for example
  the PIN for a smartcard), a Kerberos implementation must take care to
  protect the long lived secret that is shared between the principal
  and the KDC.  In particular, a weak password may be subject to a
  dictionary attack.  In order to strengthen the initial authentication
  to a KDC, an implementor may choose to utilize secondary
  authentication via a token card, or one may utilize initial
  authentication to the KDC based on public key cryptography (commonly
  known as PKINIT - a product of the Common Authentication Technology
  working group of the IETF).







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RFC 2712       Addition of Kerberos Cipher Suites to TLS   October 1999


7. Acknowledgements

  We would like to thank Clifford Neuman for his invaluable comments on
  earlier versions of this document.

8. References

  [1] Dierks, T. and C. Allen, "The TLS Protocol, Version 1.0", RFC
      2246, January 1999.

  [2] Kohl J. and C. Neuman, "The Kerberos Network Authentication
      Service (V5)", RFC 1510, September 1993.

9. Authors' Addresses

  Ari Medvinsky
  Excite
  555 Broadway
  Redwood City, CA 94063

  Phone: +1 650 569 2119
  EMail: [email protected]
  http://www.excite.com


  Matthew Hur
  CyberSafe Corporation
  1605 NW Sammamish Road
  Issaquah WA 98027-5378

  Phone: +1 425 391 6000
  EMail: [email protected]
  http://www.cybersafe.com


















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10. Full Copyright Statement

  Copyright (C) The Internet Society (1999).  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.



















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