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: November 8, 2014                        MIT Kerberos Consortium
                                                             May 7, 2014

  Kerberos Authorization Data Container Authenticated by Multiple MACs


   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

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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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Motivations

   The Kerberos protocol allows clients to submit arbitrary
   authorization data for a KDC to insert into a Kerberos ticket.  These
   client-requested authorization data allow the client to express
   authorization restrictions that the application service will
   interpret.  With few exceptions, the KDC can safely copy these
   client-requested authorization data to the issued ticket without
   necessarily inspecting, interpreting, or filtering their contents.

   The AD-KDC-ISSUED authorization data container specified in RFC 4120
   [RFC4120] is a means for KDCs to include positive or permissive
   (rather than restrictive) authorization data in service tickets in a
   way that the service named in a ticket can verify that the KDC has
   issued the contained authorization data.  This capability takes
   advantage of a shared symmetric key between the KDC and the service
   to assure the service that the KDC did not merely copy client-
   requested authorization data to the ticket without inspecting them.

   The AD-KDC-ISSUED container works well for situations where the flow
   of authorization data is from the KDC to the service.  However,
   protocol extensions such as Constrained Delegation (S4U2Proxy
   [MS-SFU]) require that a service present to the KDC an "evidence"
   service ticket that the service received from a client.  In the
   S4U2Proxy extension, the KDC uses the evidence ticket as the basis
   for issuing a derivative ticket that the service can then use to
   impersonate the client.  The authorization data contained within the
   evidence ticket constitute a flow of authorization data from the
   application service to the KDC.  The properties of the AD-KDC-ISSUED

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   container are insufficient for this use case because the service
   knows the symmetric key for the checksum in the AD-KDC-ISSUED
   container.  Therefore, the KDC has no way to detect whether the
   service has tampered with the contents of the AD-KDC-ISSUED container
   within the evidence ticket.

   The new AD-CAMMAC authorization data container specified in this
   document improves upon AD-KDC-ISSUED by including additional verifier
   elements.  The svc-verifier element of the CAMMAC is equivalent to
   the ad-checksum element of AD-KDC-ISSUED and allows the service to
   verify the integrity of the contents as it already could with the AD-
   KDC-ISSUED container.  The kdc-verifier and other-verifiers elements
   are new to AD-CAMMAC and provide its enhanced capabilities.

   The kdc-verifier element of the AD-CAMMAC container allows a KDC to
   verify the integrity of authorization data that it previously
   inserted into a ticket, by using a key that only the KDC knows.  The
   KDC thus avoids recomputing all of the authorization data, an
   operation that might not always be possible when that data includes
   ephemeral information such as the strength or type of authentication
   method used to obtain the original ticket.

   The verifiers in the other-verifiers element of the AD-CAMMAC
   container are not required, but can be useful when a lesser-
   privileged service receives a ticket from a client and needs to
   extract the CAMMAC to demonstrate to a higher-privileged "trusted
   service" on the same host that it is legitimately acting on behalf of
   that client.  The trusted service can use a verifier in the other-
   verifiers element to validate the contents of the CAMMAC without
   further communication with the KDC.

4.  Encoding

   The Kerberos protocol is defined in [RFC4120] using Abstract Syntax
   Notation One (ASN.1) [X.680] and using the ASN.1 Distinguished
   Encoding Rules (DER) [X.690].  For consistency, this specification
   also uses ASN.1 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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      AD-CAMMAC                   ::= SEQUENCE {
            elements              [0] AuthorizationData,
            kdc-verifier          [1] Verifier-MAC,
            svc-verifier          [2] Verifier-MAC OPTIONAL,
            other-verifiers       [3] SEQUENCE (SIZE (1..MAX))
                                      OF Verifier OPTIONAL

      Verifier             ::= CHOICE {
            mac            Verifier-MAC,

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


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

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

      Contains a MAC computed over the ASN.1 DER 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.  For the kdc-verifier and
      the svc-verifier, the identifier, kvno and enctype fields are
      often obvious from context and MAY be omitted.  For the kdc-
      verifier, the MAC is computed differently than for the svc-
      verifier and the other-verifiers, as described later.

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      A Verifier-MAC where the key is that of the local Ticket-Granting
      Service (TGS).  The checksum type is the required checksum type
      for the enctype of the TGS key.  In contrast to the other
      Verifier-MAC elements, the KDC computes the MAC in the kdc-
      verifier over the ASN.1 DER encoding of the EncTicketPart of the
      surrounding ticket, but where the AuthorizationData value in the
      EncTicketPart contains the AuthorizationData value contained in
      the CAMMAC instead of the AuthorizationData value that would
      otherwise be present in the ticket.  This altered Verifier-MAC
      computation binds the kdc-verifier to the other contents of the
      ticket, assuring the KDC that a malicious service has not
      substituted a mismatched CAMMAC received from another ticket.

      A Verifier-MAC where the key is the same long-term service key
      that the KDC uses to encrypt the surrounding ticket.  The checksum
      type is the required checksum type for the enctype of the service
      key used to encrypt the ticket.  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.

      A sequence of additional verifiers.  In each additional Verifier-
      MAC, the key is a 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 KDC SHOULD determine the "trusted service" principal name by
      replacing the service identifier component of the sname of the
      surrounding ticket with "host".  The checksum is computed using a
      long-term key of the identified principal, and the checksum type
      is the required checksum type for the enctype of that long-term
      the key.  The kvno and enctype SHOULD be specified to disambiguate
      which of the long-term keys of the trusted service is used.  The
      key usage is TBD.

5.  Assigned numbers


6.  IANA Considerations


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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 scheme used for the surrounding 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.  The extent of this duplication would depend on
   the security properties required by the application protocol.

   The method for computing the kdc-verifier does not bind it to any
   authorization data within the ticket but outside of the CAMMAC.  At
   least one (non-standard) authorization data type attempts to bind to
   other authorization data in a ticket, and it is very difficult to
   have two such authorization data types coexist.

8.  Open Issues

   Allow an optional KDC-verified element, kdc-verifier-unbound, that is
   not bound to the ticket contents?  This would allow a KDC to provide
   the service of verifying an extracted CAMMAC without needing a copy
   of the ticket ciphertext.

9.  Acknowledgements

   Ben Kaduk and Zhanna Tsitkov provided helpful technical and editorial
   feedback on earlier versions of this document.

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

              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,

   [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


   Tom Yu (editor)
   MIT Kerberos Consortium


   Thomas Hardjono (editor)
   MIT Kerberos Consortium


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