Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) Freshness Extension
draft-ietf-kitten-pkinit-freshness-00

Kitten Working Group                                       M. Short, Ed.
Internet-Draft                                                  S. Moore
Intended status: Standards Track                               P. Miller
Expires: July 26, 2015                             Microsoft Corporation
                                                        January 22, 2015

Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
                          Freshness Extension
                 draft-ietf-kitten-pkinit-freshness-00

Abstract

   This document describes how to further extend the Public Key
   Cryptography for Initial Authentication in Kerberos (PKINIT)
   extension [RFC4556] to exchange an opaque data blob which a KDC can
   validate to ensure that the client is currently in possession of the
   private key during a PKInit AS exchange.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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 . . . . . . . . . . . . . . . . . . . . . .   3
     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 . . . . . . . . . . . . . .   4
     2.5.  Receipt of second KRB_ERROR Message . . . . . . . . . . .   5
   3.  PreAuthentication Data Types  . . . . . . . . . . . . . . . .   5
   4.  PA-AS-FRESHNESS-REQUEST . . . . . . . . . . . . . . . . . . .   5
   5.  PA-AS-FRESHNESS . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Extended PKAuthenticator  . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   10. Interoperability Considerations . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The Kerberos PKINIT extension [RFC4556] defines two schemes for using
   asymmetric cryptography in a Kerberos preauthenticator.  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.

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   Unless the time is outside the validity period of the client's
   certificate, the KDC will validate the PKAuthenticator and return a
   TGT the client can use without possessing the private key.

   As a result, a client performing PKINIT with the Diffie-Hellman key
   exchange does not prove current possession of the private key being
   used for authentication.  It proves only prior use of that key.
   Ensuring that the client has current possession of the private key
   requires that the signed PKAuthenticator data include information
   that the client could not have predicted.

1.1.  Kerberos message flow using KRB_AS_REQ without pre-authentication

   Today some password-based AS exchanges [RFC4120] depend on the client
   sending a KRB_AS_REQ without pre-authentication to trigger the KDC to
   provide the Kerberos client with information needed to complete an AS
   exchange such as the supported encryption types and salt values (see
   the message flow below):

   KDC                     Client

                 <----     AS-REQ without pre-authentication
   KRB-ERROR     ---->

                 <----     AS-REQ
   AS-REP        ---->

                 <----     TGS-REQ
   TGS-REP       ---->

                                 Figure 1

   We can use this mechanism in PKInit for KDCs to provide data which
   the client returns as part of the KRB_AS_REQ to ensure that the
   PA_PK_AS_REQ [RFC4556] was not pregenerated.

1.2.  Requirements Language

   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.  Message Exchanges

   The following summarizes the message flow with extensions to
   [RFC4120] and [RFC4556] required to support a KDC provided freshness
   token during the initial request for a ticket:

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   1.  The client generates a KRB_AS_REQ as specified in Section 2.9.3
       [RFC4120] without an authenticator which includes the freshness
       token request to the KDC.

   2.  The KDC generates a KRB_ERROR as specified in Section 3.1.3 of
       [RFC4120] providing a freshness token.

   3.  The client receives the error as specified in Section 3.1.4 of
       [RFC4120] and includes the freshness token as part of the
       KRB_AS_REQ as specified in [RFC4120] and [RFC4556].

   4.  The KDC receives and validates the KRB_AS_REQ as specified in
       Section 3.2.2 [RFC4556] then additionally validates the freshness
       token.

   5.  The KDC and client continue as specified in [RFC4120] and
       [RFC4556].

2.1.  Generation of KRB_AS_REQ Message

   The client indicates support of freshness tokens by adding a PA-AS-
   FRESHNESS-REQUEST padata type.

2.2.  Generation of KRB_ERROR Message

   The KDC will respond by adding a PA-AS-FRESHNESS padata type with the
   freshness token to the METHOD-DATA object.

2.3.  Generation of KRB_AS_REQ Message

   After the client receives the KRB-ERROR message containing a
   freshness token, it extracts the PA-AS-FRESHNESS padata value as an
   opaque data blob.  The PA-AS-FRESHNESS padata value SHALL then be
   added as an opaque blob in the freshnessToken field when the client
   generates the PKAuthenticator for the PA_PK_AS_REQ message.  This
   ensures that the freshness token value will be included in the signed
   data portion of the KRB_AS_REQ value.

2.4.  Receipt of KRB_AS_REQ Message

   After validating the PA_PK_AS_REQ message normally, the KDC will
   validate the PA-AS-FRESHNESS padata value in an implementation
   specific way.  If the freshness token is not valid, the KDC MUST
   return KDC_ERR_PREAUTH_FAILED with PA-AS-FRESHNESS.  Since the
   freshness tokens are validated by KDCs in the same realm,
   standardizing the contents of the freshness token is not a concern
   for interoperability.

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2.5.  Receipt of second KRB_ERROR Message

   Clients SHOULD retry in the cases when receiving a
   KDC_ERR_PREAUTH_FAILED KRB_ERROR message which includes a freshness
   token where there is a possibility that there was too much delay
   between the client receiving the freshness token and sending the
   PA_PK_AS_REQ message.

3.  PreAuthentication Data Types

          The following are the new PreAuthentication data types:

              +-------------------------+-------------------+
              |   Padata and Data Type  | Padata-type Value |
              +-------------------------+-------------------+
              | PA_AS_FRESHNESS_REQUEST |        TBD        |
              |     PA_AS_FRESHNESS     |        TBD        |
              +-------------------------+-------------------+

4.  PA-AS-FRESHNESS-REQUEST

   The PA-AS-FRESHNESS-REQUEST indicates support for freshness tokens:

   PA-AS-FRESHNESS-REQUEST ::= NULL

5.  PA-AS-FRESHNESS

   The PA-AS-FRESHNESS structure specifies a freshness token:

   PA-AS-FRESHNESS ::= OCTET STRING

6.  Extended PKAuthenticator

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   The PKAuthenticator structure specified in Section 3.2.1 [RFC4556] is
   extended to include a new freshnessToken as follows:

   PKAuthenticator ::= SEQUENCE {
      cusec        [0] INTEGER (0..999999),
      ctime        [1] KerberosTime,
                -- cusec and ctime are used as in [RFC4120], for
                -- replay prevention.
      nonce        [2] INTEGER (0..4294967295),
                -- Chosen randomly;  this nonce does not need to
                -- match with the nonce in the KDC-REQ-BODY.
      paChecksum   [3] OCTET STRING OPTIONAL,
                -- MUST be present.
                -- Contains the SHA1 checksum, performed over
                -- KDC-REQ-BODY.
      ...,
      freshnessToken     [4] PA-AS-FRESHNESS OPTIONAL,
                -- MUST be present if sent by KDC
      ...
   }

7.  Acknowledgements

   Henry B.  Hotz, Nico Williams, Sam Hartman, Tom Yu, Martin Rex, and
   Douglas E.  Engert were key contributors to the discover of the
   freshness issue in PKINIT.

   Greg Hudson, Nathan Ide, Benjamin Kaduk, Magnus Nystrom Nico Williams
   and Tom Yu reviewed the document and provided suggestions for
   improvements.

8.  IANA Considerations

   IANA is requested to assign numbers for PA_AS_FRESHNESS listed in the
     Kerberos Parameters registry Pre-authentication and Typed Data as
                                 follows:

              +------+-------------------------+------------+
              | Type |          Value          | Reference  |
              +------+-------------------------+------------+
              | TBD  | PA_AS_FRESHNESS_REQUEST | [This RFC] |
              | TBD  |     PA_AS_FRESHNESS     | [This RFC] |
              +------+-------------------------+------------+

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9.  Security Considerations

   The freshness token SHOULD include signing, encrypting or sealing
   data from the KDC to determine authenticity and prevent tampering.
   Kerberos error messages are not integrity protected unless
   authenticated using Kerberos FAST [RFC6113].  Even if FAST is
   required to provide integrity protection, a different KDC would not
   be able to validate freshness tokens without some kind of shared
   database.

10.  Interoperability Considerations

   Since the client treats the KDC provided data blob as opaque,
   changing the contents will not impact existing clients.  Thus
   extensions to the freshness token do not impact client
   interoperability.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
              Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.

   [RFC5349]  Zhu, L., Jaganathan, K., and K. Lauter, "Elliptic Curve
              Cryptography (ECC) Support for Public Key Cryptography for
              Initial Authentication in Kerberos (PKINIT)", RFC 5349,
              September 2008.

11.2.  Informative References

   [RFC6113]  Hartman, S. and L. Zhu, "A Generalized Framework for
              Kerberos Pre-Authentication", RFC 6113, April 2011.

Authors' Addresses

   Michiko Short (editor)
   Microsoft Corporation
   USA

   Email: michikos@microsoft.com

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   Seth Moore
   Microsoft Corporation
   USA

   Email: sethmo@microsoft.com

   Paul Miller
   Microsoft Corporation
   USA

   Email: paumil@microsoft.com

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