Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) Freshness Extension
RFC 8070

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Internet Engineering Task Force (IETF)                     M. Short, Ed.
Request for Comments: 8070                                      S. Moore
Category: Standards Track                                      P. Miller
ISSN: 2070-1721                                    Microsoft Corporation
                                                           February 2017

Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
                          Freshness Extension

Abstract

   This document describes how to further extend the Public Key
   Cryptography for Initial Authentication in Kerberos (PKINIT)
   extension (defined in RFC 4556) to exchange an opaque data blob that
   a Key Distribution Center (KDC) can validate to ensure that the
   client is currently in possession of the private key during a PKINIT
   Authentication Service (AS) exchange.

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 7841.

   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/rfc8070.

Copyright Notice

   Copyright (c) 2017 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.

Short, et al.                Standards Track                    [Page 1]
RFC 8070                    PKINIT Freshness               February 2017

Table of Contents

   1. Introduction ....................................................2
      1.1. Kerberos Message Flow Using KRB_AS_REQ without
           Pre-authentication .........................................3
      1.2. Requirements Language ......................................3
   2. Message Exchanges ...............................................4
      2.1. Generation of KRB_AS_REQ Message ...........................4
      2.2. Generation of KRB_ERROR Message ............................4
      2.3. Generation of KRB_AS_REQ Message ...........................4
      2.4. Receipt of KRB_AS_REQ Message ..............................5
      2.5. Receipt of Second KRB_ERROR Message ........................5
   3. PreAuthentication Data Types ....................................5
   4. Extended PKAuthenticator ........................................6
   5. IANA Considerations .............................................6
   6. Security Considerations .........................................7
   7. Interoperability Considerations .................................7
   8. Normative References ............................................8
   Acknowledgements ...................................................8
   Authors' Addresses .................................................9

1.  Introduction

   The Kerberos PKINIT extension [RFC4556] defines two schemes for using
   asymmetric cryptography in a Kerberos pre-authenticator.  One uses
   Diffie-Hellman key exchange and the other depends on public key
   encryption.  The public key encryption scheme is less commonly used
   for two reasons:

   o  Elliptic Curve Cryptography (ECC) Support for PKINIT [RFC5349]
      only specified Elliptic Curve Diffie-Hellman (ECDH) key agreement,
      so it cannot be used for public key encryption.

   o  Public key encryption requires certificates with an encryption
      key, which is not deployed on many existing smart cards.

   In the Diffie-Hellman exchange, the client uses its private key only
   to sign the AuthPack structure (specified in Section 3.2.1 of
   [RFC4556]), which is performed before any traffic is sent to the KDC.
   Thus, a client can generate requests with future times in the
   PKAuthenticator, and then send those requests at those future times.
   Unless the time is outside the validity period of the client's
   certificate, the KDC will validate the PKAuthenticator and return a
   Ticket-Granting Ticket (TGT) the client can use without possessing
   the private key.

Short, et al.                Standards Track                    [Page 2]
RFC 8070                    PKINIT Freshness               February 2017