Using Kerberos Version 5 over the Transport Layer Security (TLS) Protocol
RFC 6251

Internet Engineering Task Force (IETF)                      S. Josefsson
Request for Comments: 6251                                        SJD AB
Category: Informational                                         May 2011
ISSN: 2070-1721

                        Using Kerberos Version 5
            over the Transport Layer Security (TLS) Protocol

Abstract

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6251.

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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   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
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction and Background .....................................2
   2. Kerberos V5 STARTTLS Extension ..................................3
   3. Examples ........................................................4
   4. STARTTLS-Aware KDC Discovery ....................................5
   5. Server Certificates .............................................6
   6. IANA Considerations .............................................7
   7. Acknowledgements ................................................7
   8. Security Considerations .........................................7
   9. References ......................................................8
      9.1. Normative References .......................................8
      9.2. Informative References .....................................8

1.  Introduction and Background

   This document describes 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 offers integrity- and privacy-protected exchanges
   that can be authenticated using X.509 certificates, OpenPGP keys
   [RFC6091], and usernames and passwords via Secure Remote Password
   (SRP) [RFC5054].

   There are several reasons to use Kerberos V5 over TLS.

   o  It 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 V5.  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.

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      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.  Parts 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  It provides additional authentication against the KDC.  In some
      situations, users are equipped with smart cards with an RSA
      authentication key.  In others, users have an OpenPGP client on
      their desktop, with a public OpenPGP key known to the server.

   o  It provides explicit server authentication of the KDC to the
      client.  In traditional Kerberos V5, authentication of the KDC is
      proved as a side effect that the KDC knows your encryption key
      (i.e., your password).

   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 [RFC2119].

2.  Kerberos V5 STARTTLS Extension

   The STARTTLS extension uses the Kerberos V5 TCP extension mechanism
   [RFC5021].  The extension uses bit 0 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 initiates the TLS handshake protocol by sending
   a client hello, the server responds, and the handshake continues
   until it either succeeds 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
   session.

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   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 indicates
   the length of the Kerberos V5 packet.

   When no further Kerberos V5 messages need 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
   closed.

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.

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       Client                                               Server

        [ Kerberos V5 TCP extension mechanism negotiation starts ]

       0x80000000 | STARTTLS-bit    -------->
                                                       0x00000000
                                    <--------

                            [ TLS negotiation starts ]

       ClientHello                  -------->
                                                       ServerHello
                                                      Certificate*
                                                ServerKeyExchange*
                                               CertificateRequest*
                                    <--------      ServerHelloDone
       Certificate*
       ClientKeyExchange
       CertificateVerify*
       [ChangeCipherSpec]
       Finished                     -------->
                                                [ChangeCipherSpec]
                                    <--------             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

4.  STARTTLS-Aware KDC Discovery

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

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   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.

5.  Server Certificates

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

   A goal for the protocol described in this memo is that it should be
   as easy to implement and deploy on clients as support for UDP/TCP.
   Since many client environments do not have access to long-term
   storage, or to long-term storage that is sufficiently secure to
   enable validation of server certificates, the Kerberos V5 STARTTLS
   protocol does not require clients to verify server certificates.  If
   server certification had been required, then environments with
   constrained clients such as those mentioned would be forced to
   disable TLS; this would arguably be worse than TLS without server
   certificate validation, as the use of TLS, even without server
   certificate validation, protects against some attacks that Kerberos
   V5 over UDP/TCP does not.  For example, even without server
   certificate validation, TLS does protect against passive network
   sniffing aimed at tracking Kerberos service usage by a given client.

   However, note that the use of TLS without server certificate
   verification opens up a range of active attacks such as man in the
   middle.

   When clients have the ability, they MUST validate the server
   certificate.  For this reason, if a KDC presents an 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

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   server certificate could be an X.509 certificate or an OpenPGP key.
   In this mode, the client needs no processing related to id-
   krb5starttls-san.

   When the server presents an 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.

6.  IANA Considerations

   Per [RFC5021], the IANA has allocated a bit (value 0) in the
   "Kerberos TCP Extensions" registry for Krb5 over TLS, the extension
   described in this document.

7.  Acknowledgements

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

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 attacks
   unless clients verify the KDC's credentials (X.509 certificate,
   OpenPGP key, etc.) correctly.  Although certificate validation adds
   an extra layer of protection, that is not considered strictly
   necessary to improve the security profile of Kerberos V5 as outlined
   in this document.

   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
   default.

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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.

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

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

   [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.

   [RFC6091]  Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
              for Transport Layer Security (TLS) Authentication",
              RFC 6091, February 2011.

Author's Address

   Simon Josefsson
   Simon Josefsson Datakonsult AB
   Hagagatan 24
   Stockholm  113 47
   Sweden

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

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