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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 24, 2014                          MIT Kerberos Consortium
                                                        October 21, 2013


  Kerberos Authorization Data Container Authenticated by Multiple MACs
                      draft-ietf-krb-wg-cammac-06

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 24, 2014.

Copyright Notice

   Copyright (c) 2013 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.  Motivations . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  AD-CAMMAC . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Assigned numbers  . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   8.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     10.1. Normative References  . . . . . . . . . . . . . . . . . . . 8
     10.2. Informative References  . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Additional Stuff . . . . . . . . . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9































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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.  This new 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.  Motivations

   The new AD-CAMMAC authorization data container specified in this
   document is an improvement upon AD-KDC-ISSUED because it provides
   assurance to the KDC that the service named in the ticket did not
   tamper with the contained authorization data.  By adding MACs
   verifiable by the KDC and trusted services, AD-CAMMAC enables several
   new use cases for the Kerberos protocol that AD-KDC-ISSUED does not
   accommodate.

   The existing AD-KDC-ISSUED authorization data container allows a
   service to verify that the KDC has issued the contained authorization
   data.  However, because the symmetric key for the MAC is known to
   both the KDC and the service, the KDC cannot generally detect whether
   the service has forged the contents of an AD-KDC-ISSUED container in
   an existing ticket.  The new kdc-verifier MAC in the AD-CAMMAC
   container, because it uses a key known only to the KDC, allows the
   KDC to verify the integrity of the contents of that container.

   For example, the new AD-CAMMAC container can protect authorization
   data when using the Constrained Delegation (S4U2Proxy [MS-SFU])
   protocol extension.  This extension allows a service to use a ticket
   to itself as evidence that it received a user request and
   consequently ask the KDC to issue a new ticket on behalf of the user
   to perform operations against another service.

   If the KDC had issued a AD-KDC-ISSUED container in the S4U2Proxy
   evidence ticket instead of AD-CAMMAC, it would have no way to
   subsequently verify whether the service had tampered with the



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   contents of that container.  The service would know the key for the
   MAC for the AD-KDC-ISSUED container in the evidence ticket, and could
   therefore forge its contents.

   The kdc-verifier MAC in the AD-CAMMAC container allows a KDC to
   verify the integrity of the contained authorization data without
   having to compute all of the authorization data, an operation that
   might not always be possible when the data contains ephemeral
   information such as the strength or type of authentication method
   used to obtain the original ticket.

   A lesser-privileged service on a host may receive an authentication
   from a client, and might then ask a higher-privileged service
   ("trusted service") on the same host to act on behalf of the client.
   To demonstrate that the client has authenticated to it, the lesser-
   privileged service can extract the AD-CAMMAC container from the
   ticket and submit it to the trusted service.  The trusted service can
   either ask a specialized service (not yet specified) on the KDC to
   validate the AD-CAMMAC container, or use verify the optional
   additional verifiers (the other-verifiers field) that are part of the
   AD-CAMMAC.


4.  Encoding

   The Kerberos protocol is defined in [RFC4120] using Abstract Syntax
   Notation One (ASN.1) [X.680][X.690].  For consistency, this
   specification also uses the ASN.1 syntax for specifying the layout of
   AD-CAMMAC.  The ad-data of the AD-CAMMAC authorization data element
   is the ASN.1 DER encoding of the AD-CAMMAC ASN.1 type specified
   below.




















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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
      }

      AD-CAMMAC-BINDING    ::= OCTET STRING

      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.






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   AD-CAMMAC-BINDING:
      An optional AuthorizationData element that binds the CAMMAC
      contents to the enclosing ticket.  This AuthorizationData element
      has ad-type number TBD, and if it appears in the AD-CAMMAC, it
      MUST be the first member of the elements field of the AD-CAMMAC.
      The contents of the AD-CAMMAC-BINDING element are a local matter
      for the KDC implementation.  A KDC can use this element to
      checksum portions of the ticket outside of the CAMMAC, to ensure
      that a service has not tampered with them.  This can be useful if
      the KDC implements a capability resembling the Windows Constrained
      Delegation (S4U2Proxy) [MS-SFU] extension.

   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


6.  IANA Considerations

   TBD.






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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.

   Some protocol extensions such as S4U2Proxy allow the KDC to issue a
   new ticket based on an evidence ticket provided by the service.  If
   the evidence ticket contains authorization data that needs to be
   preserved in the new ticket, then the KDC MUST revalidate it.

   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 necessarily 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
   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,
   but including an AD-CAMMAC-BINDING element could be sufficient.


8.  Open Issues

   Consider making other-verifiers "[3] SEQUENCE (SIZE (1..MAX)) OF
   VERIFIER OPTIONAL" to make the common case encoding smaller.

   Enclose in AD-IF-RELEVANT?


9.  Acknowledgements

   TBD.


10.  References






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10.1.  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,
              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.

10.2.  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.

   [MS-SFU]   Microsoft, "[MS-SFU]: Kerberos Protocol Extensions:
              Service for User and Constrained Delegation Protocol",
              January 2013,
              <http://msdn.microsoft.com/en-us/library/cc246071.aspx>.

   [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.






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Appendix A.  Additional Stuff

   This becomes an Appendix.


Authors' Addresses

   Simo Sorce (editor)
   Red Hat

   Email: ssorce@redhat.com


   Tom Yu (editor)
   MIT Kerberos Consortium

   Email: tlyu@mit.edu


   Thomas Hardjono (editor)
   MIT Kerberos Consortium

   Email: hardjono@mit.edu




























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