draft-josefsson-kerberos5-starttls-07.txt   [plain text]

Network Working Group                                       S. Josefsson
Internet-Draft                                                    SJD AB
Intended status: Informational                             July 31, 2009
Expires: February 1, 2010

   Using Kerberos V5 over the Transport Layer Security (TLS) protocol

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
   from IETF Documents or IETF Contributions published or made publicly
   available before November 10, 2008.  The person(s) controlling the
   copyright in some of this material may not have granted the IETF
   Trust the right to allow modifications of such material outside the
   IETF Standards Process.  Without obtaining an adequate license from
   the person(s) controlling the copyright in such materials, this
   document may not be modified outside the IETF Standards Process, and
   derivative works of it may not be created outside the IETF Standards
   Process, except to format it for publication as an RFC or to
   translate it into languages other than English.

   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-

   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

   This Internet-Draft will expire on February 1, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal

Josefsson               Expires February 1, 2010                [Page 1]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Josefsson               Expires February 1, 2010                [Page 2]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009


   This document specify how the Kerberos V5 protocol can be transported
   over the Transport Layer Security (TLS) protocol, to provide
   additional security features.

Table of Contents

   1.  Introduction and Background  . . . . . . . . . . . . . . . . .  4
   2.  Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . .  6
   3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . .  8
   5.  Server Certificates  . . . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14

Josefsson               Expires February 1, 2010                [Page 3]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

1.  Introduction and Background

   This document describe how a Kerberos V5 [RFC4120] implementation may
   upgrade communication between clients and Key Distribution Centers
   (KDCs) to use the Transport Layer Security (TLS) [RFC5246] protocol.

   The TLS protocol offer integrity and privacy protected exchanges that
   can be authentication using X.509 certificates, OpenPGP keys
   [RFC5081], and user name and passwords via SRP [RFC5054].

   There are several reasons to use Kerberos V5 over TLS.

   o  Prevents downgrade attacks affecting, e.g., encryption types and
      pre-auth data negotiation.  The encryption type field in KDC-REQ,
      and the METHOD-DATA field with the requested pre-auth types from
      the server in KDC_ERR_PREAUTH_REQUIRED errors in KDC-REP, are sent
      without integrity or privacy protection in Kerberos 5.  This
      allows an active attacker to replace the encryption type with a
      compromised encryption type, e.g., 56-bit DES, or request that
      clients should use a broken pre-auth type.  Since clients in
      general cannot know the encryption types other servers support, or
      the pre-auth types servers prefer or require, 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 encryption type or
      preferred another pre-auth type.

   o  Kerberos exchanges are privacy protected.  Part of many Kerberos
      packets are transferred 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  Additional authentication against the KDC.  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.

   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.

   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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

Josefsson               Expires February 1, 2010                [Page 4]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

   document are to be interpreted as described in RFC 2119 [RFC2119].

Josefsson               Expires February 1, 2010                [Page 5]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

2.  Kerberos V5 STARTTLS Extension

   The STARTTLS extension uses the Kerberos V5 TCP extension mechanism
   [RFC5021].  The extension uses bit #TBD in the extension bitmask.

   The protocol is as follows.  After the server has sent the 4-octet
   value 0x00000000 to indicate support of this extension, the stream
   will be controlled by the TLS protocol and its framing.  The TLS
   protocol is initiated by the client.

   Typically, the client initiate the TLS handshake protocol by sending
   a client hello, and the server responds, and the handshake continues
   until it either succeed or fails.

   If for any reason the handshake fails, the STARTTLS protocol will
   also fail, and the TLS error is used as the error indication.  In
   this case, no further messages can be exchanged over the same TCP

   If the handshake succeeds, the Kerberos V5 authentication protocol is
   performed within the protected TLS channel, like a normal TCP
   Kerberos V5 exchange.  In particular, this means that every Kerberos
   V5 packet will be prefixed by a 4-octet length field, that indicate
   the length of the Kerberos V5 packet.

   When no further Kerberos V5 messages needs to be transferred in the
   TLS session, the TLS session MUST be shut down properly using the
   close_notify alert.  When the TLS session is shut down, the TCP
   connection cannot be re-used to send any further data and MUST be

Josefsson               Expires February 1, 2010                [Page 6]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

3.  Examples

   A complete packet flow for a successful AS-REQ/REP exchange protected
   by this mechanism will be as follows.  The "STARTTLS-bit" is a
   4-octet value with only the bit allocated for this extension set.

       Client                                               Server

        [ Kerberos V5 TCP extension mechanism negotiation starts ]

       [0x70000000 & STARTTLS-bit]  -------->

                            [ TLS negotiation starts ]

       ClientHello                  -------->
                                    <--------      ServerHelloDone
       Finished                     -------->
                                    <--------             Finished

                       [ Kerberos V5 negotiation starts ]

       4 octet length field
       Kerberos V5 AS-REQ           -------->
                                                4 octet length field
                                                Kerberos V5 AS-REP

       * Indicates optional or situation-dependent messages that are not
         always sent.

Josefsson               Expires February 1, 2010                [Page 7]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

4.  STARTTLS aware KDC Discovery

   Section 7.2.3 of Kerberos V5 [RFC4120] describe how Domain Name
   System (DNS) SRV records [RFC2782] can be used to find the address of
   an KDC.  We define a new Proto of "tls" to indicate that the
   particular KDC is intended to support this STARTTLS extension.  The
   Service, Realm, TTL, Class, SRV, Priority, Weight, Port and Target
   have the same meaning as in RFC 4120.

   For example:

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

Josefsson               Expires February 1, 2010                [Page 8]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

5.  Server Certificates

   The TLS protocol may be used in a mode that provides server
   authentication using, for example, X.509 and OpenPGP.

   The Kerberos V5 STARTTLS protocol do not require clients to verify
   the server certificate.  The goal is that support for TLS in Kerberos
   V5 clients should be as easy to implement and deploy as support for
   UDP/TCP.  Use of TLS, even without server certificate validation,
   protects against some attacks that Kerberos V5 over UDP/TCP do not.
   Requiring server certificates to be used at all times would enable
   attacks in those situations.

   Many client environments do not have secure long-term storage, which
   is required to validate certificates.  This makes it impossible to
   use server certificate validation on a large number of client

   When clients have the ability, they need to be able to validate the
   server certificate.  For this reason, if a KDC presents a X.509
   server certificate over TLS, it MUST contain an otherName Subject
   Alternative Name (SAN) identified using a type-id of id-krb5starttls-
   san.  The intention is to bind the server certificate to the Kerberos
   realm for the purpose of using Kerberos V5 STARTTLS.  The value field
   of the otherName should contain the realm as the "Realm" ASN.1 type.

          id-krb5starttls-san OBJECT IDENTIFIER ::=
            { iso(1) identified-organization(3) dod(6) internet(1)
              private(4) enterprise(1) gnu(11591)
              shishi(6) krb5starttls-san(1) }

   To validate a server certificate, the client MAY use local
   configuration (e.g., a list that map realm names to a copy of the
   server's certificate) and compare that with the authentication
   information provided from the server via TLS.  For illustration, the
   server certificate could be a X.509 certificate or an OpenPGP key.
   In this mode, the client need no processing related to id-

   When the server presents a X.509 server certificate, clients MAY use
   "Certification Path Validation" as described in [RFC5280] to validate
   the KDC server certificate.  In addition, unless the client can
   otherwise verify that the server certificate is bound to the KDC of
   the target realm, the client MUST verify that the server certificate
   contains the id-krb5starttls-san SAN and that the value is identical
   to the intended Kerberos realm.

Josefsson               Expires February 1, 2010                [Page 9]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

6.  IANA Considerations

   The IANA is requested to allocate a bit in the "Kerberos TCP
   Extensions" registry for the extension described in this document, as
   per [RFC5021].

Josefsson               Expires February 1, 2010               [Page 10]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

7.  Acknowledgements

   Jeffrey Hutzelman and Sam Hartman provided comments that improved the
   protocol and document.

Josefsson               Expires February 1, 2010               [Page 11]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

8.  Security Considerations

   The security considerations in Kerberos V5, TLS, and the Kerberos V5
   TCP extension mechanism are inherited.

   Note that TLS does not protect against Man-In-The-Middle (MITM)
   attacks unless clients verify the KDC's credentials (X.509
   certificate, OpenPGP key, etc) correctly.

   If server authentication is used, some information about the server
   (such as its name) is visible to passive attackers.

   To protect against the inherent downgrade attack in the extension
   framework, implementations SHOULD offer a policy mode that requires
   this extension to always be successfully negotiated, for a particular
   realm, or generally.  For interoperability with implementations that
   do not support this extension, the policy mode SHOULD be disabled by

Josefsson               Expires February 1, 2010               [Page 12]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5021]  Josefsson, S., "Extended Kerberos Version 5 Key
              Distribution Center (KDC) Exchanges over TCP", RFC 5021,
              August 2007.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

9.2.  Informative References

   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, November 2007.

   [RFC5081]  Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
              Layer Security (TLS) Authentication", RFC 5081,
              November 2007.

Josefsson               Expires February 1, 2010               [Page 13]
Internet-Draft       Protecting Kerberos V5 with TLS           July 2009

Author's Address

   Simon Josefsson
   Simon Josefsson Datakonsult AB
   Hagagatan 24
   Stockholm  113 47

   Email: simon@josefsson.org
   URI:   http://josefsson.org/

Josefsson               Expires February 1, 2010               [Page 14]