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 6, 2015                           MIT Kerberos Consortium
                                                         October 3, 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.  This document updates RFC

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Motivations . . . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Assigned numbers  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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

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   (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 a service ticket
   that the KDC previously issued, as evidence that the service is
   authorized to impersonate the client principal named in that ticket.
   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
   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 has the same
   functional and security properties as the ad-checksum element of AD-
   KDC-ISSUED; the svc-verifier allows the service to verify the
   integrity of the AD-CAMMAC 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 for the
   issued ticket; this operation 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-

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   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.


      AD-CAMMAC                   ::= SEQUENCE {
            elements              [0] AuthorizationData,
            kdc-verifier          [1] Verifier-MAC OPTIONAL,
            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


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      Contains an RFC 3961 [RFC3961] Checksum (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.  The key usage for computing the MAC (Checksum) is 64.

      A Verifier-MAC where the key is a long-term key 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
      key.  The kvno and enctype SHOULD be specified to disambiguate
      which of the long-term keys of the trusted service is used.

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5.  Usage

   Application servers and KDCs MAY ignore the AD-CAMMAC container and
   the authorization data elements it contains.  For compatibility with
   older Kerberos implementations, a KDC issuing an AD-CAMMAC SHOULD
   enclose it in an AD-IF-RELEVANT container unless the KDC knows that
   the application server is likely to recognize it.

6.  Assigned numbers

   The ad-type number for AD-CAMMAC is 96.

   The key usage number for the Verifier-MAC checksum is 64.

7.  IANA Considerations

   [ RFC Editor: please remove this section prior to publication. ]

   There are no IANA considerations in this document.  Any numbers
   assigned in this document are not in IANA-controlled number spaces.

8.  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, AD-SIGNEDPATH,

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   attempts to bind to other authorization data in a ticket, and it is
   very difficult for two such authorization data types to coexist.

   To minimize ticket size when embedding CAMMACs in Kerberos tickets, a
   KDC MAY omit the kdc-verifier from the CAMMAC when it is not needed.
   In this situation, the KDC cannot always determine whether the CAMMAC
   contents are intact.  The KDC MUST NOT create a new CAMMAC from an
   existing one unless the existing CAMMAC has a valid kdc-verifier,
   with two exceptions.

   Only KDCs for the local realm have knowledge of the local TGS key, so
   the outer encryption of a local TGT is sufficient to protect the
   CAMMAC of a local TGT from tampering, assuming that all of the KDCs
   in the local realm consistently filter out CAMMAC authorization data
   submitted by clients.  The KDC MAY create a new CAMMAC from an
   existing CAMMAC lacking a kdc-verifier if that CAMMAC is contained
   within a local TGT and all of the local realm KDCs are configured to
   filter out CAMMAC authorization data submitted by clients.

   An application service might not use the S4U2Proxy extension, or the
   realm policy might disallow the use of S4U2Proxy by that service.  In
   such situations where there is no flow of authorization data from the
   service to the KDC, the application service could modify the CAMMAC
   contents, but such modifications would have no effect on other
   services.  Because of the lack of security impact, the KDC MAY create
   a new CAMMAC from an existing CAMMAC lacking a kdc-verifier if it is
   inserting the new CAMMAC into a service ticket for the same service
   principal as the ticket that contained the existing CAMMAC, but MUST
   NOT place a kdc-verifier in the new CAMMAC.

   The kdc-verifier in CAMMAC does not bind the service principal name
   to the CAMMAC contents, because the service principal name is not
   part of the EncTicketPart.  An entity that has access to the keys of
   two different service principals can decrypt a ticket for one service
   and encrypt it in the key of the other service, altering the svc-
   verifier to match.  Both the kdc-verifier and the svc-verifier would
   still validate, but the KDC never issued this fabricated ticket.  The
   impact of this manipulation is minor if the CAMMAC contents only
   communicate attributes related to the client.  If an application
   requires an authenticated binding between the service principal name
   and the CAMMAC or ticket contents, the KDC MUST include in the CAMMAC
   some authorization data element that names the service principal.

9.  Acknowledgements

   Shawn Emery, Sam Hartman, Greg Hudson, Ben Kaduk, Zhanna Tsitkov, and
   Kai Zheng provided helpful technical and editorial feedback on
   earlier versions of this document.

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10.  References

10.1.  Normative References

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

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

   [MS-SFU]   Microsoft, "[MS-SFU]: Kerberos Protocol Extensions:
              Service for User and Constrained Delegation Protocol",
              January 2013,

Authors' Addresses

   Simo Sorce (editor)
   Red Hat


   Tom Yu (editor)
   MIT Kerberos Consortium


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   Thomas Hardjono (editor)
   MIT Kerberos Consortium


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