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

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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.

Copyright Notice

   Copyright (c) 2011 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Josefsson                     Informational                     [Page 1]
RFC 6251             Protecting Kerberos V5 with TLS            May 2011

   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.

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.

Josefsson                     Informational                     [Page 2]
RFC 6251             Protecting Kerberos V5 with TLS            May 2011

      Since clients in general cannot know the encryption types other
      servers support, or the pre-auth types servers prefer or require,