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NETWORK WORKING GROUP                                            B. Tung
Internet-Draft                                                 C. Neuman
Expires: June 6, 2005                 USC Information Sciences Institute
                                                                  L. Zhu
                                                                  M. Hur
                                                   Microsoft Corporation
                                                            S. Medvinsky
                                                          Motorola, Inc.
                                                        December 6, 2004


     Public Key Cryptography for Initial Authentication in Kerberos
                    draft-ietf-cat-kerberos-pk-init

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on June 6, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes protocol extensions (hereafter called PKINIT)
   to the Kerberos protocol specification.  These extensions provide a



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   method for integrating public key cryptography into the initial
   authentication exchange, by passing digital certificates and
   associated authenticators in preauthentication data fields.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Extensions . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1   Definitions, Requirements, and Constants . . . . . . . . .  5
       3.1.1   Required Algorithms  . . . . . . . . . . . . . . . . .  5
       3.1.2   Defined Message and Encryption Types . . . . . . . . .  6
       3.1.3   Algorithm Identifiers  . . . . . . . . . . . . . . . .  7
     3.2   PKINIT Preauthentication Syntax and Use  . . . . . . . . .  7
       3.2.1   Client Request . . . . . . . . . . . . . . . . . . . .  8
       3.2.2   Validation of Client Request . . . . . . . . . . . . . 10
       3.2.3   KDC Reply  . . . . . . . . . . . . . . . . . . . . . . 12
       3.2.4   Validation of KDC Reply  . . . . . . . . . . . . . . . 17
     3.3   KDC Indication of PKINIT Support . . . . . . . . . . . . . 17
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 22
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23
   A.  PKINIT ASN.1 Module  . . . . . . . . . . . . . . . . . . . . . 24
       Intellectual Property and Copyright Statements . . . . . . . . 28

























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1.  Introduction

   A client typically authenticates itself to a service in Kerberos
   using three distinct though related exchanges.  First, the client
   requests a ticket-granting ticket (TGT) from the Kerberos
   authentication server (AS).  Then, it uses the TGT to request a
   service ticket from the Kerberos ticket-granting server (TGS).
   Usually, the AS and TGS are integrated in a single device known as a
   Kerberos Key Distribution Center, or KDC.  Finally, the client uses
   the service ticket to authenticate itself to the service.

   The advantage afforded by the TGT is that the client need explicitly
   request a ticket and expose his credentials only once.  The TGT and
   its associated session key can then be used for any subsequent
   requests.  One result of this is that all further authentication is
   independent of the method by which the initial authentication was
   performed.  Consequently, initial authentication provides a
   convenient place to integrate public-key cryptography into Kerberos
   authentication.

   As defined, Kerberos authentication exchanges use symmetric-key
   cryptography, in part for performance.  One cost of using
   symmetric-key cryptography is that the keys must be shared, so that
   before a client can authenticate itself, he must already be
   registered with the KDC.

   Conversely, public-key cryptography (in conjunction with an
   established Public Key Infrastructure) permits authentication without
   prior registration with a KDC.  Adding it to Kerberos allows the
   widespread use of Kerberized applications by clients without
   requiring them to register first with a KDC--a requirement that has
   no inherent security benefit.

   As noted above, a convenient and efficient place to introduce
   public-key cryptography into Kerberos is in the initial
   authentication exchange.  This document describes the methods and
   data formats for integrating public-key cryptography into Kerberos
   initial authentication.













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2.  Conventions Used in This Document

   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 [RFC2119].

   In this document, we will refer to both the AS and the TGS as the
   KDC.











































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3.  Extensions

   This section describes extensions to [CLAR] for supporting the use of
   public-key cryptography in the initial request for a ticket.

   Briefly, this document defines the following extensions to [CLAR]:

   1.  The client indicates the use of public-key authentication by
      including a special preauthenticator in the initial request.  This
      preauthenticator contains the client's public-key data and a
      signature.

   2.  The KDC tests the client's request against its policy and trusted
      Certification Authorities (CAs).

   3.  If the request passes the verification tests, the KDC replies as
      usual, but the reply is encrypted using either:

      a.  a symmetric encryption key, signed using the KDC's signature
         key and encrypted using the client's encryption key; or

      b.  a key generated through a Diffie-Hellman exchange with the
         client, signed using the KDC's signature key.

      Any keying material required by the client to obtain the
      Encryption key is returned in a preauthentication field
      accompanying the usual reply.

   4.  The client obtains the encryption key, decrypts the reply, and
      then proceeds as usual.

   Section 3.1 of this document defines the necessary message formats.
   Section 3.2 describes their syntax and use in greater detail.

3.1  Definitions, Requirements, and Constants

3.1.1  Required Algorithms

   All PKINIT implementations MUST support the following algorithms:

   o  AS reply key: AES256-CTS-HMAC-SHA1-96 etype [KCRYPTO].

   o  Signature algorithm: SHA-1 digest and RSA.

   o  Reply key delivery method: RSA or ephemeral-ephemeral
      Diffie-Hellman.





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3.1.2  Defined Message and Encryption Types

   PKINIT makes use of the following new preauthentication types:

       PA-PK-AS-REQ                             16
       PA-PK-AS-REP                             17

   PKINIT also makes use of the following new authorization data type:

       AD-INITIAL-VERIFIED-CAS                   9

   PKINIT introduces the following new error codes:


       KDC_ERR_CLIENT_NOT_TRUSTED                62
       KDC_ERR_KDC_NOT_TRUSTED                   63
       KDC_ERR_INVALID_SIG                       64
       KDC_ERR_KEY_SIZE                          65
       KDC_ERR_CERTIFICATE_MISMATCH              66
       KDC_ERR_CANT_VERIFY_CERTIFICATE           70
       KDC_ERR_INVALID_CERTIFICATE               71
       KDC_ERR_REVOKED_CERTIFICATE               72
       KDC_ERR_REVOCATION_STATUS_UNKNOWN         73
       KDC_ERR_CLIENT_NAME_MISMATCH              75

   PKINIT uses the following typed data types for errors:

       TD-TRUSTED-CERTIFIERS                    104
       TD-CERTIFICATE-INDEX                     105
       TD-DH-PARAMETERS                         109

   PKINIT defines the following encryption types, for use in the
   KRB_AS_REQ message (to indicate acceptance of the corresponding
   encryption OIDs in PKINIT):

       dsaWithSHA1-CmsOID                         9
       md5WithRSAEncryption-CmsOID               10
       sha1WithRSAEncryption-CmsOID              11
       rc2CBC-EnvOID                             12
       rsaEncryption-EnvOID   (PKCS1 v1.5)       13
       rsaES-OAEP-EnvOID      (PKCS1 v2.0)       14
       des-ede3-cbc-EnvOID                       15

   The above encryption types are used by the client only within the
   KDC-REQ-BODY to indicate which CMS [RFC2630] algorithms it supports.
   Their use within Kerberos EncryptedData structures is not specified
   by this document.




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   The ASN.1 module for all structures defined in this document (plus
   IMPORT statements for all imported structures) are given in Appendix
   A.

   All structures defined in this document MUST be encoded using
   Distinguished Encoding Rules (DER) [X690].  All imported data
   structures must be encoded according to the rules specified in
   Kerberos [CLAR] or CMS [RFC2630] as appropriate.

   Interoperability note: Some implementations may not be able to decode
   CMS objects encoded with BER but not DER; specifically, they may not
   be able to decode infinite length encodings.  To maximize
   interoperability, implementers SHOULD encode CMS objects used in
   PKINIT with DER.

3.1.3  Algorithm Identifiers

   PKINIT does not define, but does make use of, the following algorithm
   identifiers.

   PKINIT uses the following algorithm identifier for Diffie-Hellman key
   agreement [FIPS74]:

        dhpublicnumber

   PKINIT uses the following signature algorithm identifiers [RFC3279]:

       sha-1WithRSAEncryption (RSA with SHA1)
       md5WithRSAEncryption   (RSA with MD5)
       id-dsa-with-sha1       (DSA with SHA1)

   PKINIT uses the following encryption algorithm identifiers [RFC2437]
   for encrypting the temporary key with a public key:

       rsaEncryption          (PKCS1 v1.5)
       id-RSAES-OAEP          (PKCS1 v2.0)

   PKINIT uses the following algorithm identifiers [RFC2630] for
   encrypting the reply key with the temporary key:

       des-ede3-cbc           (three-key 3DES, CBC mode)
       rc2-cbc                (RC2, CBC mode)
       aes256_CBC             (AES-256, CBC mode)


3.2  PKINIT Preauthentication Syntax and Use

   This section defines the syntax and use of the various



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   preauthentication fields employed by PKINIT.

3.2.1  Client Request

   The initial authentication request (KRB_AS_REQ) is sent as per

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