Subject: Using TLS with Kerberos 5

From: Simon Josefsson <[email-masked]>
Date: Thu, 11 Nov 2004 01:06:53 +0100

Here is a description of a protocol extension I'm using.  If people
are interested in seeing this pursued within IETF, let me know.  For
more information, see <http://josefsson.org/krb5starttls/>.

Thanks,
Simon

Network Working Group                                       S. Josefsson
Internet-Draft                                         November 11, 2004
Expires: May 12, 2005

          Using Transport Layer Security (TLS) with Kerberos 5
                 draft-josefsson-kerberos5-starttls-00

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Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document specify how the Transport Layer Security (TLS) protocol
   is used in conjunction with the Kerberos 5 protocol.

   $Id: draft-josefsson-kerberos5-starttls.xml,v 1.13 2004/11/10
   23:48:56 jas Exp $

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Table of Contents

   1.  Introduction and Background  . . . . . . . . . . . . . . . . .  3
   2.  Extension Mechanism for TCP/IP transport . . . . . . . . . . .  4
   3.  Kerberos 5 STARTTLS Extension  . . . . . . . . . . . . . . . .  4
     3.1   STARTTLS requested by client (extension 1) . . . . . . . .  4
     3.2   STARTTLS request accepted by server (extension 2)  . . . .  5
     3.3   Proceeding after successful TLS negotiation  . . . . . . .  5
     3.4   Proceeding after failed TLS negotiation  . . . . . . . . .  5
     3.5   STARTTLS aware KDC Discovery . . . . . . . . . . . . . . .  5
     3.6   Initial Authentication via TLS . . . . . . . . . . . . . .  5
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.1   Normative References . . . . . . . . . . . . . . . . . . . .  6
   5.2   Informative References . . . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  7
       Intellectual Property and Copyright Statements . . . . . . . .  8

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1.  Introduction and Background

   This document describe how Shishi, a Kerberos 5 [1] implementation,
   upgrade communication between clients and Key Distribution Centers
   (KDCs) to use the Transport Layer Security (TLS) [2] protocol.

   The TLS protocol offer integrity and privacy protected exchanges that
   can be authentication using X.509 certificates, OpenPGP keys [6], and
   user name and passwords via SRP [5].

   An inconclusive list of the motivation for using TLS with Kerberos 5
   is given below.

   o  Explicit server authentication of the KDC to the client.  In
      traditional Kerberos 5, authentication of the KDC is proved as a
      side effect that the KDC knows your encryption key (i.e., your
      password).

   o  Flexible authentication against KDC.  Kerberos 5 assume the user
      knows a key (usually in the form of a password).  Sometimes
      external factors make this hard to fulfill.  In some situations,
      users are equipped with smart cards with a RSA authentication key.
      In others, users have a OpenPGP client on their desktop, with a
      public OpenPGP key known to the server.  In some situations, the
      policy may be that password authentication may only be done
      through SRP.

   o  Kerberos exchanges are privacy protected.  Part of many Kerberos
      packets are transfered without privacy protection (i.e.,
      encryption).  That part contains information, such as the client
      principal name, the server principal name, the encryption types
      supported by the client, the lifetime of tickets, etc.  Revealing
      such information is, in some threat models, considered a problem.

   o  Prevents downgrade attacks affecting encryption types.  The
      encryption type of the ticket in KDC-REQ are sent in the clear in
      Kerberos 5.  This allows an attacker to replace the encryption
      type with a compromised mechanisms, e.g.  56-bit DES.  Since
      clients in general cannot know the encryption types other servers
      support, it is difficult for the client to detect if there was a
      man-in-the-middle or if the remote server simply did not support a
      stronger mechanism.  Clients could chose to refuse 56-bit DES
      altogether, but in some environments this leads to operational
      difficulties.

   o  The TLS protocol has been studied by many parties.  In some threat
      models, the designer prefer to reduce the number of protocols that
      can hurt the overall system security if they are compromised.

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [4].

2.  Extension Mechanism for TCP/IP transport

   Kerberos 5 require Key Distribution Centers (KDCs) to accept requests
   over TCP.  Each request and response is prefixed by 4 octets,
   encoding an integer in network byte order, that indicate the length
   of the packet.  The high bit of the 4 octet length field was reserved
   for future expansion.  Servers that do not understand how to
   interpret a set high bit are required to return a KRB-ERROR with the
   KRB_ERR_FIELD_TOOLONG error code, and to close the TCP stream.

   We will use the reserved bit to provide an extension mechanism.  When
   the reserved high bit is set, the remaining 31 bits of the 4 octets
   are treated as an extensible typed hole, and thus form a 31 bit
   integer enumerating various extensions.  Each of the values indicate
   a specific extended operation mode, two of which are used and defined
   here, and the rest are left for others to use.

   If the KDC do not understand a requested extension, it MUST return a
   KRB-ERROR with a KRB_ERR_FIELD_TOOLONG value (prefixed by the 4 octet
   length integer, with the high bit clear, as usual) and close the TCP
   stream.

   The following table specify the meaning of the 31 lower bits in the 4
   octet field, when the high bit is set:

   0               RESERVED.
   1               STARTTLS requested by client.
   2               STARTTLS request accepted by server.
   3...2147483647  AVAILABLE for registration (via [email-masked]).
   2147483648      RESERVED.

3.  Kerberos 5 STARTTLS Extension

3.1  STARTTLS requested by client (extension 1)

   When this message is sent by the client, the client is requesting the
   server to start TLS negotiation on the TCP stream.  The client MUST
   NOT start TLS negotiation immediately.  Instead, the client wait for
   either a KRB-ERROR (sent normally, prefixed by a 4 octet length
   integer) indicating the server do not understand the set high bit, or
   4 octets which is to be interpreted as an integer in network byte
   order, where the high bit is set and the remaining 31 bit are
   interpreted as an integer specifying ``STARTTLS request accepted by

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   server'' (extension 2).  In the first case, the client infer that the
   server do not understand (or wish to support) STARTTLS, and can
   re-try using normal TCP, if unprotected Kerberos 5 exchanges are
   acceptable to the client policy.  In the latter case, it should
   invoke TLS negotiation on the stream.  If any other data is received,
   the client MUST close the TCP stream.

3.2  STARTTLS request accepted by server (extension 2)

   This message should be sent by the server when it has received the
   extension 1 message.  The message is an acknowledgment of the
   client's request to initiate STARTTLS on the channel.  The server
   MUST then invoke a TLS negotiation.

3.3  Proceeding after successful TLS negotiation

   If the TLS negotiation ended successfully, possibly also considering
   client or server policies, the exchange within the TLS protected
   stream is performed like normal UDP Kerberos 5 exchanges, i.e., there
   is no TCP 4 octet length field before each packet.  Instead each
   Kerberos packet MUST be sent within one TLS record, so the
   application can use the TLS record length as the Kerberos 5 packet
   length.

3.4  Proceeding after failed TLS negotiation

   If the TLS negotiation fails, possibly due to client or server policy
   (e.g., inadequate support of encryption types in TLS, or lack of
   client or server authentication) the entity that detect the failure
   MUST disconnected the connection.  It is expected that any error
   messages that explain the error condition is transfered by TLS.

3.5  STARTTLS aware KDC Discovery

   Section 7.2.3 of Kerberos 5 [1] describe how Domain Name System (DNS)
   SRV records [3] can be used to find the address of an KDC.  To locate
   a KDC that support the STARTTLS extension, we use the "_tls" domain.
   For example:

   _kerberos._tls._tcp.EXAMPLE.COM.   IN      SRV     0 0 88 kdc1.example.com.
   _kerberos._tls._tcp.EXAMPLE.COM.   IN      SRV     1 0 88 kdc2.example.com.

3.6  Initial Authentication via TLS

   The server MAY consider the authentication performed by the TLS
   exchange as sufficient to issue Kerberos 5 tickets to the client,
   without requiring, e.g., pre-authentication.  However, it is not an

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   error to require or use pre-authentication as well.

   The client may also indicate that it wishes to use TLS both for
   authentication and data protection by using the NULL encryption type
   in its request.  The server can decide from its local policy whether
   or not issuing tickets based solely on TLS authentication, and
   whether NULL encryption within TLS, is acceptable or not.

4.  Security Considerations

   Because the initial token is not protected, it is possible for an
   active attacker to make it appear to the client that the server do
   not support this extension.  It is up to client configuration to
   disallow non-TLS connections, if that vulnerability is deemed
   unacceptable.  For interoperability, we suggest the default behaviour
   should be to allow automatic fall back to TCP or UDP.

   The security considerations of both TLS and Kerberos 5 are inherited.
   Using TLS for authentication and/or data protection together with
   Kerberos alter the authentication logic fundamentally.  Thus, it may
   be that even if the TLS and Kerberos 5 protocols and implementations
   were secure, the combination of TLS and Kerberos 5 described here
   could be insecure.

   No channel bindings are provided in the Kerberos messages.  It is an
   open question whether, and how, this could be solved.  One idea for
   solving this may be to specify a new encryption algorithm in Kerberos
   5 that is similar to the NULL encryption algorithm, but also include
   the TLS session identifier.

5.  References

5.1  Normative References

   [1]  Neuman, C., "The Kerberos Network Authentication Service (V5)",
        draft-ietf-krb-wg-kerberos-clarifications-07 (work in progress),
        September 2004.

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

   [3]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
        specifying the location of services (DNS SRV)", RFC 2782,
        February 2000.

5.2  Informative References

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement

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        Levels", BCP 14, RFC 2119, March 1997.

   [5]  Taylor, D., "Using SRP for TLS Authentication",
        draft-ietf-tls-srp-08 (work in progress), August 2004.

   [6]  Mavroyanopoulos, N., "Using OpenPGP keys for TLS
        authentication", draft-ietf-tls-openpgp-keys-05 (work in
        progress), April 2004.

Author's Address

   Simon Josefsson

   EMail: [email-masked]

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