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Internet Engineering Task Force                            S. Sorce, Ed.
Internet-Draft                                                   Red Hat
Updates: 4120 (if approved)                                   T. Yu, Ed.
Intended status: Standards Track                        T. Hardjono, Ed.
Expires: April 4, 2013                           MIT Kerberos Consortium
                                                                Oct 2012


  Kerberos Authorization Data Container Authenticated by Multiple MACs
                      draft-ietf-krb-wg-cammac-03

Abstract

   Abstract: This document specifies a Kerberos Authorization Data
   container that supersedes AD-KDC-ISSUED.  It allows for multiple
   Message Authentication Codes (MACs) or signatures to authenticate the
   contained Authorization Data elements.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on April 4, 2013.

Copyright Notice

   Copyright (c) 2012 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



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
   3.  Validation  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  AD-CAMMAC . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Assigned numbers  . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Additional Stuff . . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
































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1.  Introduction

   This document specifies a new Authorization Data container for
   Kerberos, called AD-CAMMAC (Container Authenticated by Multiple
   MACs), that supersedes AD-KDC-ISSUED.  The container allows both the
   receiving application service and the Key Distribution Center (KDC)
   itself to verify the authenticity of the contained authorization
   data.  The AD-CAMMAC container can also include additional verifiers
   that "trusted services" can use to verify the contained authorization
   data.


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].


3.  Validation

   Kerberos ticket authorization data are highly sensitive and must be
   validated to insure that no tampering has occurred.  Although
   authorization data are in the encrypted part of a Kerberos ticket and
   therefore have their integrity protected by the ticket encryption,
   clients can request that KDCs insert potentially arbitrary
   authorization data into tickets on their behalf.  The Kerberos
   protocol specifications allow this client behavior because the
   originally envisioned usage of authorization data was to serve as
   restrictions on the client's privileges.  Services that need to
   interpret specific authorization data as granting increased
   privileges need some way to ensure that the KDC originated those
   authorization data.

   In order to validate any information, the receiving application
   service needs to be able to cryptographically verify the data.  This
   is done by introducing a new AuthorizationData element called AD-
   CAMMAC that contains enough information to bind the contents to a
   principal in a way that a receiving application service can verify
   autonomously without further contact with the KDC.

   The following information is needed:

   o  The KDC MAC

   o  The Service MAC





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   o  Optional Trusted Service MAC

   The KDC MAC is required to allow the KDC to validate the data without
   needing to recompute the contents at every Ticket Granting Service
   (TGS) request.

   The Service MAC is required so that the Service can verify that the
   authorization data has been validated by the KDC.

   The Trusted Service MAC is useful to verify the authenticity of the
   contents on the same host, when the data is received by a less
   trusted service and passed to a more trusted service on the same host
   without the need for additional round trips to the KDC.

   The ad-type for AD-CAMMAC is (TBD).


4.  Encoding

   The Kerberos protocol is defined in [RFC4120] using Abstract Syntax
   Notation One (ASN.1) [X.680][X.690].  As such, this specification
   also uses the ASN.1 syntax for specifying both the abstract layout of
   the AD-CAMMAC attributes, as well as its encoding.

4.1.  AD-CAMMAC


     KerberosV5CAMMAC DEFINITIONS EXPLICIT TAGS ::= BEGIN

     AD-CAMMAC                   ::= SEQUENCE {
           elements              [0] AuthorizationData,
           kdc-verifier          [1] Verifier-MAC,
           svc-verifier          [2] Verifier-MAC OPTIONAL,
           other-verifiers       [3] SEQUENCE OF Verifier
     }

     Verifier             ::= CHOICE {
           mac            Verifier-MAC,
           ...
     }

     Verifier-MAC         ::= SEQUENCE {
           identifier     [0] PrincipalName OPTIONAL,
           kvno           [1] UInt32,
           enctype        [2] Int32,
           mac            [3] Checksum
     }




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     END

     elements
        A sequence of authorization data elements issued by the
        KDC. These elements are the authorization data that the verifier
        fields authenticate.

     Verifier
        A CHOICE type that currently contains only one alternative:
        Verifier-MAC. Future extensions might add support for public-key
        signatures.

     Verifier-MAC
        Contains a MAC computed over the encoding of the
        AuthorizationData value in the elements field of the
        AD-CAMMAC. The identifier, kvno, and enctype fields help the
        recipient locate the key required for verifying the MAC.

     kdc-verifier
        A Verifier-MAC where the key is the TGS key. The checksum type
        is the mandatory checksum type for the TGS key.

     svc-verifier
        A Verifier-MAC where the key is the long-term key of the service
        for which the ticket is issued. The checksum type is the
        mandatory checksum type for the long-term key of the
        service. This field MUST be present if the service principal of
        the ticket is not the local TGS, including when the ticket is a
        cross-realm TGT.

     other-verifiers
        A sequence of additional verifiers. In each additional
        Verifier-MAC, the key is the long-term key of the principal name
        specified in the identifier field. The PrincipalName MUST be
        present and be a valid principal in the realm. KDCs MAY add one
        or more 'trusted service' verifiers. Unless otherwise
        administratively configured, the 'trusted service' SHOULD be
        found by replacing the service identifier component of the
        principal name of the svc-verifier with 'host'. The checksum
        type is the mandatory checksum type for the long-term key (which
        one?) of the principal. The key usage is TBD.



5.  Assigned numbers

   TBD




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6.  IANA Considerations

   TBD.


7.  Security Considerations

   Although authorization data are generally conveyed within the
   encrypted part of a ticket and are thereby protected by the existing
   encryption methods on the ticket, some authorization data requires
   the additional protection provided by the CAMMAC.

   Extracting a CAMMAC from a ticket for use as a credential removes it
   from the context of the ticket.  In the general case, this could turn
   it into a bearer token, with all of the associated security
   implications.  Also, the CAMMAC does not itself contain sufficient
   information to identify the client principal.  Therefore, application
   protocols that rely on extracted CAMMACs might need to duplicate a
   substantial portion of the ticket contents and include that
   duplicated information in the authorization data contained within the
   CAMMAC.

   A KDC that needs to verify the contents of a CAMMAC in a non-TGS
   service ticket MUST ensure that the CAMMAC in the ticket is the same
   one that it inserted into the ticket.  A malicious service could
   substitute legitimate CAMMACs from other tickets that it has received
   (but not fabricate completely new CAMMACs) into a service ticket.  A
   CAMMAC by itself does not contain sufficient information to
   accomplish this.


8.  Acknowledgements

   TBD.


9.  References

9.1.  Normative References

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC3962]  Raeburn, K., "Advanced Encryption Standard (AES)
              Encryption for Kerberos 5", RFC 3962, February 2005.

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



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              July 2005.

   [X.680]    ISO, "Information technology -- Abstract Syntax Notation
              One (ASN.1): Specification of basic notation -- ITU-T
              Recommendation X.680 (ISO/IEC International Standard 8824-
              1:2008)", 2008.

   [X.690]    ISO, "Information technology -- ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER) -- ITU-T Recommendation X.690 (ISO/IEC International
              Standard 8825-1:2008)", 1997.

9.2.  Informative References

   [MIT-Athena]
              Steiner, J., Neuman, B., and J. Schiller, "Kerberos: An
              Authentication Service for Open Network Systems. In
              Proceedings of the Winter 1988 Usenix Conference.
              February.", 1988.

   [RFC1510]  Kohl, J. and B. Neuman, "The Kerberos Network
              Authentication Service (V5)", RFC 1510, September 1993.

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

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.


Appendix A.  Additional Stuff

   This becomes an Appendix.


Authors' Addresses

   Simo Sorce (editor)
   Red Hat

   Email: ssorce@redhat.com








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   Tom Yu (editor)
   MIT Kerberos Consortium

   Email: tlyu@mit.edu


   Thomas Hardjono (editor)
   MIT Kerberos Consortium

   Email: hardjono@mit.edu









































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