INTERNET-DRAFT                                        Ari Medvinsky
Transport Layer Security Working Group                  Matthew Hur
draft-ietf-tls-kerb-cipher-suites-00.txt      CyberSafe Corporation
                                            Nov. 96 (Expires May-97)

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

0. Status Of this Memo

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1. Abstract

This document proposes the addition of new cipher suites to the TLS
protocol (SSL 3.0) 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

2. Introduction

Flexibility is one of the main strengths of the TLS (SSL 3.0) 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 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 (SSL v3.0).  Throughout this document, we
refer to the basic SSL handshake shown in Figure 1.  For a review of
the SSL handshake see [1].

 CLIENT                                             SERVER
 ------                                             ------

ClientHello        -------------------------------->
                                                    Certificate *
change cipher spec
                                                    change cipher spec
    |                                               Finished
    |                                                   |
    |                                                   |
Application Data   <------------------------------->Application Data

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

The TLS security context is negotiated in the client and server hello
messages.  For example: SSL_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):

CipherSuite      SSL_KRB5_WITH_DES_CBC_SHA
CipherSuite      SSL_KRB5_EXPORT_WITH_RC4_40_MD5
CipherSuite      SSL_KRB5_WITH_RC4_128_MD5
CipherSuite      SSL_KRB5_WITH_RC4_128_SHA
CipherSuite      SSL_KRB5_EXPORT_WITH_RC2_CBC_40_MD5

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.

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

} ClientKeyExchange;

    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 is encrypted under the Kerberos session key and
sent to the TLS server along with the Kerberos credentials (see Figure 2).
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:
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

4. Discussion

4.1 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:
     - The "TLS" component represents the service name.
     - "MachineName" is the particular instance of the service.
     - The Kerberos "Realm" is the domain name of the machine.

Open issue:
To allow some TLS-enabled services to run in a different protection
domain, a port number may optionally be part of the service principal
name; for example, TLS/MachineName/port@Realm.
One solution for negotiating the servce port number is as follows:
The client requests the ticket for a specific port.  If the principal
name for that port is not registered, then the client requests the
generic ticket for TLS on the host.

4.2 Passing Kerberos Tickets

Clifford Neuman suggested the following approach as a topic for
Conceptually, the client's Kerberos ticket may be viewed as a custom
certificate, recognized only by the TLS server.  Therefore, the
client's certificate message may be used for passing the client's
Kerberos credentials to the TLS server.

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. Acknowledgements

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

7. References

[1] Alan O. Freier, Philip Karlton and Paul C. Kocher.
The SSL Protocol, Version 3.0 - IETF Draft.

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

Authors' Addresses

Ari Medvinsky <>
Matthew Hur   <>

CyberSafe Corporation
1605 NW Sammamish Raod
Suite 310
Issaquah, WA 98027-5378
Phone: (206) 391-6000
Fax:   (206) 391-0508