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Versions: 00 01                                                         
INTERNET-DRAFT                                    Matthew Hur
Transport Layer Security Working Group            Joseph Salowey
draft-ietf-tls-kerb-01.txt                        Cisco Systems
Obsoletes: RFC 2712                               Ari Medvinsky
November 8, 2001 (Expires May 8, 2001)            Liberate

        Kerberos Cipher Suites in Transport Layer Security (TLS)

0. Status Of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  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

     The list of Internet-Draft Shadow Directories can be accessed at

1. Abstract

   RFC 2712 [KERBTLS] introduced mechanisms for supporting Kerberos
   [KERB] authentication within the TLS protocol [TLS].  This document
   extends RFC 2712 to support delegation of Kerberos credentials.  In
   this way, a TLS server may obtain a Kerberos service ticket on behalf
   of the TLS client.  Thus, a single client identity may be used for
   authentication within a multi-tier architecture.  This draft also
   proposes a mechanism for a TLS server to indicate Kerberos-specific
   information to the client within the certificate request message in
   the initial exchange.

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.  RFC 2712 specified how TLS could be
   extended to support organizations with heterogeneous security
   deployments that include authentication systems based on symmetric
   cryptography.  Kerberos, originally developed at MIT, is based on an
   open standard and is the most widely deployed symmetric key
   authentication system.  Just as other documents specify hybrid
   asymmetric/symmetric key protocols [PKINIT] [PKCROSS] [PKTAPP], this
   document specifies how TLS may incorporate both symmetric and
   asymmetric key crypto systems.

   This document describes the use of Kerberos authentication within
   the TLS framework.  This achieves mutual authentication and the
   establishment of a master secret using Kerberos credentials.
   Additionally, this document specifies support for delegation of
   Kerberos credentials, which enables end to end authentication within
   an n-tier architecture.  The proposed changes are minimal and, in
   fact, no different from adding a new public key algorithm to the TLS

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 [TLS].

   | 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 RFC 2246.

   The TLS security context is negotiated in the client and server hello
   messages.  For example: TLS_RSA_WITH_RC4_128_MD5 means the initial
   authentication will be done using the RSA public key algorithm, RC4
   will be used with a 128 bit session key, and MACs will be based on
   the MD5 algorithm.  Thus, to facilitate the Kerberos authentication
   option, we must start by defining Kerberos cipher suites including
   (but not limited to):

   CipherSuite     TLS_KRB5_WITH_3DES_EDE_CBC_SHA       = { 0x00,0x70 };
   CipherSuite     TLS_KRB5_WITH_3DES_EDE_CBC_MD5       = { 0x00,0x71 };
   CipherSuite     TLS_KRB5_WITH_RC4_128_SHA            = { 0x00,0x72 };
   CipherSuite     TLS_KRB5_WITH_RC4_128_MD5            = { 0x00,0x73 };
   CipherSuite     TLS_KRB5_WITH_DES_CBC_SHA            = { 0x00,0x74 };
   CipherSuite     TLS_KRB5_WITH_DES_CBC_MD5            = { 0x00,0x75 };
   CipherSuite     TLS_KRB5_WITH_AES_128_CBC_SHA        = { 0x00,0x76 };
   CipherSuite     TLS_KRB5_WITH_AES_256_CBC_SHA        = { 0x00,0x77 };
   CipherSuite     TLS_KRB5_WITH_NULL_SHA               = { 0x00,0x78 };
   CipherSuite     TLS_KRB5_WITH_NULL_MD5               = { 0x00,0x79 };

   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 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.  Note
   that these messages are specified as optional in the TLS protocol;
   therefore, omitting them is permissible.

   The Kerberos option affects three of the TLS messages: the
   CertificateRequest, the client Certificate, and the
   ClientKeyExchange.  However, only the client Certificate and the
   ClientKeyExchange are required.

3.1. Usage of the CertificateRequest Message

   If the server accepts a Kerberos-based ciphersuite, then it MUST send
   the CertificateRequest message to the client.  This message conveys
   Kerberos-specific characteristics such as realm name or attributes
   such as forwarded ticket.

   RFC 2246 defines the CertificateRequest message as follows:
   |                                                                   |
   |   enum {                                                          |
   |       rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), |
   |       (255)                                                       |
   |   } ClientCertificateType;                                        |
   |                                                                   |
   |   opaque DistinguishedName<1..2^16-1>;                            |
   |                                                                   |
   |   struct { ClientCertificateType certificate_types<1..2^8-1>;     |
   |            DistinguishedName certificate_authorities<3..2^16-1>;  |
   |   } CertificateRequest;                                           |
   |                                                                   |
   FIGURE 2: CertificateRequest message from RFC 2246

   This specification defines a new ClientCertificateType for a Kerberos
   certificate.  This enables a client to respond to the
   CertificateRequest message when using Kerberos ciphersuites.  The
   Kerberos ClientCertificateType MUST NOT be included in
   certificate_types for non-Kerberos ciphersuites. Thus the following
   change for ClientCertificateType is required (Figure 3).

   |                                                                   |
   |   enum {                                                          |
   |       rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), |
   |       kerberos(5), (255)                                          |
   |   } ClientCertificateType;                                        |
   |                                                                   |
   FIGURE 3: New Kerberos ClientCertificateType

   In the case of a public key based authentication algorithm, the
   opaque DistinguishedName field is derived from [X509], and it
   contains the name of an acceptable certification authority (This is
   as specified in [TLS]).  In the case of a Kerberos
   ClientCertificateType, the DistinguishedName field is defined to
   represent Kerberos information (KerbInfo) as shown in Figure 4.  The
   srv_tgt attribute type is used by the server to send a TGT that the
   client presents to the KDC in the case of user-to-user
   authentication.  The KDC uses the session key from this ticket to
   encrypt a service ticket for the server.  In this case the attr_data
   must be of non-zero length and contain the server's TGT.

   |                                                                   |
   |   enum                                                            |
   |   {                                                               |
   |      srv_tkt(1), fwd_tgt(2), (255)                                |
   |   } KerbInfoType;                                                 |
   |                                                                   |
   |   enum                                                            |
   |   {                                                               |
   |      initial_tkt_required(1), srv_tgt(2), (255)                   |
   |   } AttrType; /* This may be extended to include attributes    */ |
   |               /*  such as forwardable or renewable for example */ |
   |                                                                   |
   |   struct                                                          |
   |   {                                                               |
   |      AttrType       attr_type;                                    |
   |      opaque         attr_data <0..2^16-1>;                        |
   |   } AttrInfoType                                                  |
   |                                                                   |
   |   struct                                                          |
   |   {                                                               |
   |      uint32         length; /* length of this struct */           |
   |      KerbInfoType   type;                                         |
   |      opaque         sname <0..2^16-1>;                            |
   |      opaque         srealm <0..2^16-1>;                           |
   |      opaque         cname <0..2^16-1>;                            |
   |      opaque         crealm <0..2^16-1>;                           |
   |      AttrInfoType   attr_info <0..2^16-1>; /* sequence of      */ |
   |                                            /* attributes       */ |
   |      uint32         etypes <0..2^16-1>; /* list of supported   */ |
   |                                         /* Kerberos etypes     */ |
   |                                         /* for authentication  */ |
   |   } TktInfo;                                                      |
   |                                                                   |
   |   struct                                                          |
   |   {                                                               |
   |      TktInfo       tkt_info <1..2^20-1>; /* MUST have at least */ |
   |                                          /* 1 TktInfo structs */  |
   |   } KerbInfo                                                      |
   |                                                                   |
   FIGURE 4: Kerberos Information for CertificateRequest Message

3.2. Usage of the Client Certificate Message

   As specified by [TLS], when the client receives the
   CertificateRequest message, it MUST respond with the client
   Certificate message.  As stated above, this specification defines a
   Kerberos certificate type.  The format for the Kerberos certificate
   is specified in figure 5 below.  This structure consists of a
   Kerberos AP-REQ message that is used for authenticating the client to
   he server.  It optionally contains a series of Kerberos KRB-CRED
   messages to convey delegated credentials.

   Note that the client may determine the type of credentials to send to
   the server, based on local policy.  Part of the input to a client's
   decision may come from the Kerberos KDC.  For example, The client may
   convey a delegated ticket based on the ok-as-delegate ticket flag set
   in the service ticket.  Also, the session key used to protect a
   forwarded credential, MUST be of equal or greater strength than the
   key used to protect the ticket when originally sent to the client
   (typically, this key is the client principal key, shared with the
   Kerberos KDC).

   |                                                                   |
   |  opaque KrbCred <1..2^16-1>; /* Kerberos-defined KRB-CRED */      |
   |                                                                   |
   |  struct                                                           |
   |  {                                                                |
   |      opaque    ap_req <1..2^16-1>;                                |
   |      KrbCred   krb_cred <0..2^20-1>;                              |
   |  } KerberosCert;                                                  |
   |                                                                   |
   FIGURE 5: Kerberos Certificate Type

3.3. Usage of the ClientKeyExchange Message

   The Kerberos option must be added to the ClientKeyExchange message as
   shown in Figure 6.

   |                                                                   |
   |  struct                                                           |
   |  {                                                                |
   |      select (KeyExchangeAlgorithm)                                |
   |      {                                                            |
   |          case krb:             KerbEncryptedPreMasterSecret;      |
   |          case rsa:             EncryptedPreMasterSecret;          |
   |          case diffie_hellman:  ClientDiffieHellmanPublic;         |
   |      } Exchange_keys;                                             |
   |  } ClientKeyExchange;                                             |
   |                                                                   |
   | KerbEncryptedPreMasterSecret contains the PreMasterSecret         |
   | encrypted within a Kerberos-defined EncryptedData structure.      |
   | The encryption key is sealed in the ticket sent in the Client     |
   | Certificate message.                                              |
   |                                                                   |
   FIGURE 6: 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

   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.

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

   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

   Kerberos fits seamlessly into TLS, 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 as follows:

        - 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.

   As specified above, in the CertificateRequest message, the server may
   indicate the appropriate principal name and realm.

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

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
   Kerberos working group of the IETF).

   The unauthenticated CertificateRequest message, specified above,
   enables the server to request a particular client principal name as
   well as a particular service principal name.  In the event that a
   service principal name is specified, there is a risk that the client
   may be tricked into requesting a ticket for a rogue server.
   Furthermore, if delegation is requested, the client may be tricked
   into forwarding its TGT to a rogue server.  In order to assure that a
   service ticket is obtained for the correct server, the client should
   rely on a combination of its own local policy, local configuration
   information, and information supplied by the KDC.  The client may
   choose to use only the naming convention specified in section 4.  The
   client may rely on the KDC performing name cannonicalization (this is
   a matter that is adressed in revisions to RFC 1510).

   The client must apply its local policy to determine whether or not to
   forward its credentials.  As previously stated, the client should
   incorporate information from the KDC, in particular the ok-as-
   delegate ticket flag, in making such a policy decision.

   The forwarded credential MUST be protected in a key that is at least
   the same strength as the principal key that originally protected the

   A forwarded TGT presents more vulnerabilities in the event of a rogue
   server or the compromise of the session key.  An attacker would be
   able to impersonate the client to obtain new service tickets.  Such
   an attack may be mitigated by the use of restrictions, such as those
   described in [Neuman].

   It has been shown that 56-bit DES keys are relatively easy to
   compromise [DESCRACK]; therefore, use of 56-bit DES is discouraged.

7. Acknowledgements

   We would like to thank the following people for their input for this
    Clifford Neuma - ISI
    John Brezak and David Mowers - Microsoft

8. References

   [KERBTLS]  A. Medvinsky and M. Hur, "Addition of Kerberos Cipher
              Suites to Transport Layer Security (TLS)", RFC 2712,
              October 1999.

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

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

   [PKINIT]   B. Tung, C. Neuman, M. Hur, A. Medvinsky, S. Medvinsky,
              J. Wray, J. Trostle.  Public Key Cryptography for Initial
              Authentication in Kerberos.

   [PKTAPP]   A. Medvinsky, M. Hur, S. Medvinsky, C. Neuman.
              Public Key Utilizing Tickets for Application
              Servers (PKTAPP).  draft-ietf-cat-kerberos-pk-tapp-03.txt

   [PKCROSS]  M. Hur, B. Tung, T. Ryutov, C. Neuman, G. Tsudik,
              A. Medvinsky, B. Sommerfeld.  Public Key Cryptography for
              Cross-Realm Authentication in Kerberos.

   [X509]     ITU-T (formerly CCITT) Information technology - Open
              Systems Interconnection - The Directory: Authentication
              Framework Recommendation X.509 ISO/IEC 9594-8

   [NEUMAN]   B.C. Neuman, "Proxy-Based Authorization and Accounting for
              Distributed Systems".  Proceedings of the 13th
              International Conference on Distributed Computing Systems,
              May 1993

   [DESCRACK] Electronic Frontier Foundation, "Cracking DES: Secrets of
              Encryption Research, Wiretap Politics, and Chip Design".
              May 1998, Electronic Frontier Foundation.

9. Authors' Addresses

   Matthew Hur
   Cisco Systems
   2901 Third Avenue
   Seattle, WA 98121
   Phone: +1 206 256 3197
   EMail: mhur@cisco.com

   Joseph Salowey
   Cisco Systems
   2901 Third Avenue
   Seattle, WA 98121
   Phone: +1 206 256 3380
   EMail: jsalowey@cisco.com

   Ari Medvinsky
   2 Circle Star Way
   San Carlos, CA 94070-6200
   Phone: +1 650 701 4000
   EMail: ari@liberate.com

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

   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

11. Appendix

Changes from RFC 2712

   Added new cipher suites with NULL confidentiality:

   Added new cipher suites to support AES:

   40 bit ciphers have been removed, and AES ciphers have been added.

   All of the ciphersuites have been renumbered to avoid conflicts with
   exisiting implementations of RFC 2712.

   RFC 2712 utilized only the ClientKeyExchange message for conveying
   the Kerberos credentials and encrypted premaster-secret.  This
   specification moves the Kerberos credentials to the client
   certificate message, and it allows the client to pass delegated
   credentials as well.  Additionally, this specification allows the
   server to specify Kerberos-specific information (realm, delegation
   required, etc.) in the  CertificateRequest message.