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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 6251

Network Working Group                                       S. Josefsson
Internet-Draft                                                    SJD AB
Intended status: Informational                          January 22, 2010
Expires: July 26, 2010

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


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

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   This Internet-Draft is submitted to IETF in full conformance with the
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   to this document.  Code Components extracted from this document must
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Table of Contents

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

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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 Secure Remote Password
   (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  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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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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.  The client requests the extension by
   setting the STARTTLS bit in the TCP extension mechanism bitmask.
   (How to deal with extension negotiation failures at this point is
   described in [RFC5021].)  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

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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, and
   | is the binary OR operation.

       Client                                               Server

        [ Kerberos V5 TCP extension mechanism negotiation starts ]

       0x80000000 | 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.

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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.

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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.
   (For example, passive network sniffing between the user and the KDC
   to track which Kerberos services are used by the user.)  To require
   server certificates to be validated at all times would lead to
   disabling of TLS when clients are unable to validate server
   certificates, and this may have worse security properties than using
   TLS and not validate the server certificate would have.

   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 MUST 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 maps the Kerberos realm 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

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   contains the id-krb5starttls-san SAN and that the value is identical
   to the intended Kerberos realm.

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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].

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7.  Acknowledgements

   Miguel A. Garcia, Jeffrey Hutzelman, Sam Hartman, and Magnus Nystroem
   (in alphabetical order) provided comments that improved the protocol
   and document.

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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

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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.

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Author's Address

   Simon Josefsson
   Simon Josefsson Datakonsult AB
   Hagagatan 24
   Stockholm  113 47

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

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