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: January 15, April 24, 2014                          MIT Kerberos Consortium
                                                           July 14,
                                                        October 21, 2013

  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

   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

   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 January 15, 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
   ( 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
   described in the Simplified BSD License.

Table of Contents

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

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  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.  Validation

   Kerberos ticket  Motivations

   The new AD-CAMMAC authorization data are highly sensitive and must be
   validated container specified in this
   document is an improvement upon AD-KDC-ISSUED because it provides
   assurance to insure the KDC that no tampering has occurred.  Although
   authorization data are the service named in the encrypted part of a Kerberos ticket and
   therefore have their integrity protected by did not
   tamper with the ticket encryption,
   clients can request that KDCs insert potentially arbitrary contained authorization data into tickets on their behalf.  The Kerberos
   protocol specifications allow this client behavior because data.  By adding MACs
   verifiable by the
   originally envisioned usage of authorization data was to serve as
   restrictions on KDC and trusted services, AD-CAMMAC enables several
   new use cases for the client's privileges.  Services Kerberos protocol that need to
   interpret specific AD-KDC-ISSUED does not

   The existing AD-KDC-ISSUED authorization data as granting increased
   privileges need some way container allows a
   service to ensure verify that the KDC originated those has issued the contained authorization

   In order  However, because the symmetric key for the MAC is known to validate any information,
   both the KDC and the service, the KDC cannot generally detect whether
   the receiving application service needs 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 be able the KDC, allows the
   KDC to cryptographically verify the data. 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
   is done by introducing extension allows a new AuthorizationData element called AD-
   CAMMAC service to use a ticket
   to itself as evidence that contains enough information it received a user request and
   consequently ask the KDC to bind issue a new ticket on behalf of the contents user
   to perform operations against another service.

   If the KDC had issued a
   principal AD-KDC-ISSUED container in a the S4U2Proxy
   evidence ticket instead of AD-CAMMAC, it would have no way that a receiving application service can to
   subsequently verify
   autonomously without further contact whether the service had tampered with the KDC.

   The following information is needed:

   o  The KDC MAC

   contents of that container.  The Service MAC
   o  Optional Trusted Service 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 kdc-verifier MAC is required to allow in the AD-CAMMAC container allows a KDC to validate
   verify the integrity of the contained authorization data without
   having to recompute the contents at every Ticket Granting Service
   (TGS) request.

   The Service MAC is required so that compute all of the Service can verify authorization data, an operation that
   might not always be possible when the
   authorization data has been validated by the KDC.

   The Trusted Service MAC is useful to verify contains ephemeral
   information such as the authenticity strength or type of authentication method
   used to obtain the
   contents original ticket.

   A lesser-privileged service on the same host, when the data is received by a less
   trusted service host may receive an authentication
   from a client, and passed to might then ask a more trusted higher-privileged service
   ("trusted service") on the same host
   without to act on behalf of the need for additional round trips client.
   To demonstrate that the client has authenticated to it, the KDC. lesser-
   privileged service can extract the AD-CAMMAC container from the
   ticket and submit it to the trusted service.  The ad-type for trusted service can
   either ask a specialized service (not yet specified) on the KDC to
   validate the AD-CAMMAC is (TBD). container, or use verify the optional
   additional verifiers (the other-verifiers field) that are part of the

4.  Encoding

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



      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



      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 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.

      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.

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

      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

      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

5.  Assigned numbers


6.  IANA Considerations


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. 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



10.  References

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.

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