A Generalized Framework for Kerberos Pre-Authentication
RFC 6113

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Internet Engineering Task Force (IETF)                        S. Hartman
Request for Comments: 6113                             Painless Security
Updates: 4120                                                     L. Zhu
Category: Standards Track                          Microsoft Corporation
ISSN: 2070-1721                                               April 2011

        A Generalized Framework for Kerberos Pre-Authentication

Abstract

   Kerberos is a protocol for verifying the identity of principals
   (e.g., a workstation user or a network server) on an open network.
   The Kerberos protocol provides a facility called pre-authentication.
   Pre-authentication mechanisms can use this facility to extend the
   Kerberos protocol and prove the identity of a principal.

   This document describes a more formal model for this facility.  The
   model describes what state in the Kerberos request a pre-
   authentication mechanism is likely to change.  It also describes how
   multiple pre-authentication mechanisms used in the same request will
   interact.

   This document also provides common tools needed by multiple pre-
   authentication mechanisms.  One of these tools is a secure channel
   between the client and the key distribution center with a reply key
   strengthening mechanism; this secure channel can be used to protect
   the authentication exchange and thus eliminate offline dictionary
   attacks.  With these tools, it is relatively straightforward to chain
   multiple authentication mechanisms, utilize a different key
   management system, or support a new key agreement algorithm.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc6113.

Hartman & Zhu                Standards Track                    [Page 1]
RFC 6113               Kerberos Preauth Framework             April 2011

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.

Hartman & Zhu                Standards Track                    [Page 2]
RFC 6113               Kerberos Preauth Framework             April 2011

Table of Contents

   1. Introduction ....................................................4
      1.1. Conventions and Terminology Used in This Document ..........5
      1.2. Conformance Requirements ...................................5
   2. Model for Pre-Authentication ....................................6
      2.1. Information Managed by the Pre-Authentication Model ........7
      2.2. Initial Pre-Authentication Required Error ..................9
      2.3. Client to KDC .............................................10
      2.4. KDC to Client .............................................11
   3. Pre-Authentication Facilities ..................................12
      3.1. Client Authentication Facility ............................13
      3.2. Strengthening Reply Key Facility ..........................13
      3.3. Replace Reply Key Facility ................................14
      3.4. KDC Authentication Facility ...............................15
   4. Requirements for Pre-Authentication Mechanisms .................15
      4.1. Protecting Requests/Responses .............................16
   5. Tools for Use in Pre-Authentication Mechanisms .................17
      5.1. Combining Keys ............................................17
      5.2. Managing States for the KDC ...............................19
      5.3. Pre-Authentication Set ....................................20
      5.4. Definition of Kerberos FAST Padata ........................23
           5.4.1. FAST Armors ........................................24
           5.4.2. FAST Request .......................................26
           5.4.3. FAST Response ......................................30
           5.4.4. Authenticated Kerberos Error Messages Using