Internet Engineering Task Force                                 S. Sorce
Internet-Draft                                                   Red Hat
Intended status: Standards Track                                   T. Yu
Expires: December 6, 2011                                    T. Hardjono
                                                 MIT Kerberos Consortium
                                                               June 2011


                   A Generalized PAC for Kerberos V5
                    draft-sorce-krbwg-general-pac-02

Abstract

   This draft proposes a generalized authorization structure for the
   Kerberos V5 protocol.  Such an authorization structure would allow
   for greater interoperability among directory services and other
   related Kerberos services across differing realms.

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 December 6, 2011.

Copyright Notice

   Copyright (c) 2011 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.  Use-Case: Cross-Realm Directory Services . . . . . . . . . . .  3
   4.  A Generalized Authorization Structure for Kerberos V5  . . . .  4
     4.1.  Attributes . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.2.  PAD-Realm  . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.3.  PAD-Principal  . . . . . . . . . . . . . . . . . . . . . .  5
     4.4.  PAD-DNS-Domain . . . . . . . . . . . . . . . . . . . . . .  5
     4.5.  PAD-Short-Domain . . . . . . . . . . . . . . . . . . . . .  5
     4.6.  PAD-Domain-UUID  . . . . . . . . . . . . . . . . . . . . .  6
     4.7.  PAD-Posix-Username . . . . . . . . . . . . . . . . . . . .  6
     4.8.  PAD-Posix-UID  . . . . . . . . . . . . . . . . . . . . . .  6
     4.9.  PAD-Posix-GID  . . . . . . . . . . . . . . . . . . . . . .  6
     4.10. PAD-Posix-Gecos  . . . . . . . . . . . . . . . . . . . . .  6
     4.11. PAD-Posix-Homedir  . . . . . . . . . . . . . . . . . . . .  6
     4.12. PAD-Posix-Shell  . . . . . . . . . . . . . . . . . . . . .  6
     4.13. PAD-Fullname . . . . . . . . . . . . . . . . . . . . . . .  7
     4.14. PAD-AlternateNames . . . . . . . . . . . . . . . . . . . .  7
     4.15. PAD-Groups . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.16. PAD Mapped Attributes  . . . . . . . . . . . . . . . . . .  8
   5.  Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  PAD Format . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Data Structures and Extensions . . . . . . . . . . . . . . . . 10
     6.1.  SignedPricipalAuthorizationData  . . . . . . . . . . . . . 10
     6.2.  GSS-API Authenticator Extension  . . . . . . . . . . . . . 12
   7.  Assigned numbers . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     11.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Additional Stuff  . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15












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1.  Introduction

   There is an increasing need today for Kerberos to support the
   delivery and processing of authorization information pertaining to
   the principals seeking access to the servers.  Kerberos today is used
   extensively for authentication to directory services within the
   Enterprise.  In many cases, a directory service is implemented as a
   distributed database system organized across multiple realms.  As
   such, when a client in one realm seeks access to a directory service
   component located within a different realm, information regarding
   both the identity of the client and the permissions associated with
   that client must be communicated across the realms.  Currently there
   does not exist a common and standardized structure in Kerberos (V5)
   for conveying access control or authorization information.

   This draft proposes a general authorization structure for Kerberos
   that identifies a base set of common data elements or fields within
   the authorization structure, as well as the format of that structure.
   We refer to this data strcuture as the Principal Authorization Data
   (PAD) structure in order to distinguish it from existing structures,
   such as the Privilege Attribute Certificate defined by Microsoft in
   [MS-PAC].


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.  Use-Case: Cross-Realm Directory Services

   In this section we discuss one of the primary use-case scenarios for
   the Principal Authorization Data (PAD) structure within Kerberos V5.
   In this use-case a client principal is seeking to access a service in
   a different realm.  Since the remote service does not have
   authorization information regarding the client, it needs to obtain it
   either from querying the directory service in its own realm or the
   directory service located in the client's realm.  It is here that a
   common PAD structure becomes necessary and invaluable in order to
   achieve a high-degree of interoperability between directory services
   in distinct realms.

   In this use-case a client principal C1 in realm R1 is seeking access
   to services (or servers) located in a different realm R2.  In
   accessing local service S1 in realm R1 the client must first be
   authenticated by KDC1 in that realm.  A directory service (e.g.



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   LDAP) called D1 is used in realm R1 to perform authorization of the
   client, after the client has been authenticated by KDC1.

   When the client prinicipal later seeks to access services or
   resources S2 in realm R2, following the usual Kerberos flow the
   client must first obtain a cross-realm TGT from KDC1 (in realm R1)
   and then present it to KDC2 (in realm R2) in order to obtain a
   service-ticket for S2.  However, one immediate issue is the fact that
   service S2 does not have authorization information regarding the
   permissions or privileges of client C1 in realm R1.  The service S2
   could query its own directory service D2 to obtain authorization
   information pertaining to client C1.  In the absence of such
   information in D2, the service S2 could then perform a cross-realm
   query to the directory services D1 operating in realm R1.

   However, this cross-realm query from S2 to D1 is not only
   inefficient, but it also implies knowledge of multiple eterogenous
   systems by all actors.  Two different realms may rely on completely
   different infrastructures for user information storage, ranging from
   different LDAP implementations with different schema conventions to
   NIS, SQL databases, flat files, and so on.  Every service in the
   realm R2 would have to know what information system is in use in R1,
   how to reach it, how to read and eventually how to map data from it.
   Moreover security related aspects on the authentication of S2 by the
   directory D1, the authorization of S2 to make such a query, the
   protection of responses from D1 to S2, and so on, would have to be
   addressed.

   This use-case illustrates the need for a common PAD structure to
   address this cross-realm authorization problem.  In particular, the
   PAD structure for the cross-realm access to remote services needs to
   be contained or carried within cross-realm TGTs and service-tickets.
   Such a PAD structure needs to carry enough authorization information
   such that a decision can be made by service D2 in realm R2 regarding
   the access request originating from the client principal C1 within
   realm R1.


4.  A Generalized Authorization Structure for Kerberos V5

4.1.  Attributes

   The following attributes are defined in this document:

   o  PAD-Realm

   o  PAD-Principal




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   o  PAD-DNS-Domain

   o  PAD-Short-Domain

   o  PAD-Domain-UUID

   o  PAD-Posix-Username

   o  PAD-Posix-UID

   o  PAD-Posix-GID

   o  PAD-Posix-Gecos

   o  PAD-Posix-Homedir

   o  PAD-Posix-Shell

   o  PAD-Fullname

   o  PAD-AlternateNames

   o  PAD-Groups

   These are each defined and discussed further below

4.2.  PAD-Realm

   The full Realm Name of the Realm the principal belongs to.

4.3.  PAD-Principal

   The name of the principal.  Joined with the PAD-Realm component it
   MUST match the full principal name of the owner of the ticket.

4.4.  PAD-DNS-Domain

   The DNS Domain name associated to the Realm.

4.5.  PAD-Short-Domain

   A short domain name that uniquely identifies, within the set of
   trusted realms, the domain the principal belongs to.  The short
   Domain name is useful for representation purposes in the OS.







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4.6.  PAD-Domain-UUID

   A UUID that universally identifies the domain the following local
   identifiers belongs to.  This is used to differentiate between local
   identifiers belonging to different domains/realms.

   The UUID size can be dependent on the specific Domain type and
   imlementation.  However it SHOULD be not less than 96bit in size so
   that chances of conflicts are relatively low.

   If the UUID is shorter than 128bit it can be used as a 128bit UUID by
   prepending enough bits all set to zero.

4.7.  PAD-Posix-Username

   This is the user name that correspond to the kerberos principal, this
   is the name that SHOULD be used by the OS to represent the user.  The
   OS may decide to prefix or posfix this name with the PAD-Domain or
   PAD-Realm names in case of name conflicts with local accounts.

4.8.  PAD-Posix-UID

   This is the UID Number associated to the user.  This number is local
   to the domain identified by PAD-Domain-UUID.

4.9.  PAD-Posix-GID

   This is the Primary GID Number associated to the user.  This number
   is local to the domain identified by PAD-Domain-UUID.

4.10.  PAD-Posix-Gecos

   The Gecos field for the User associated to the Principal if
   available.  Can be omitted.  If not available PAD-Fullname can be
   used instead.

4.11.  PAD-Posix-Homedir

   The home directory path relative to the local system, if available.
   If not available local defined defaults apply.

4.12.  PAD-Posix-Shell

   The default shell for the user, defined as the path of the binary
   relative to the local filesystem, if available.  If not available
   local defined defaults apply.





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4.13.  PAD-Fullname

   The full name of the user if available.

4.14.  PAD-AlternateNames

   Alternate names can be used by application to identify a user by
   means that differ from the user principal.  Names are in string form
   and utf8 encoded [UTF-8].  In order to allow applications to
   recognize the name type without guesswork, alternate names are
   prefixed with a string followed by the colon ':' character and the
   name, without any space or other separation character.  The following
   Alternate names are currently recognized: EMAIL, OS, OPENID, OAUTH It
   is allowed to include multiple alternate names of the same type.  The
   order in which they are provided represent the priority within the
   same name type, if applications need to choose between names.

   (TODO: need discussion on whether these needs labeled prefixes or
   explicit attributes for each alternate representation etc...)

4.15.  PAD-Groups

   This is a structured attribute and defines the groups the principal
   is member of.

   The first value in the structure represents the Domain-UUID and is
   optional.  If missing the Domain UUID is assumed to be the one
   defined in the PAD-Domain-UUID attribute.

   Then an array of values that define the groups as follows.  Each
   group value includes 2 subvalues:

   o  (1) GID: This is the gid number of the group.

   o  (2) Name: This is the name of the group.

   Multiple PAD-Groups attributes can be present at the same time.  A
   trusting KDC can augment the original user's set of groups by adding
   a new PAD-Groups structure that contains groups local to the trusting
   domain.  In this case the Domain-UUID is required.  The Domain UUID
   is used for gid number conflict resolution when the PAC is
   transmitted between services of different realms.

   PAD-Groups are optional attributes and the KDC, upon PAC
   revalidation, may decide to remove the original attributes that do
   not belong to the KDC security domain in order to save space or to
   censor information to avoid disclosing data to services.




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4.16.  PAD Mapped Attributes

   In POSIX, users and groups ID are not universally unique, and
   different Realms (even different machines within an authorization
   realm actually) may have overlapping and conflicting IDs.  If this is
   the case, a trusting KDC may decide to re-map IDs coming from a
   foreign Realm to help services with uid/gid mapping and avoid ID
   conflicts that can lead to serious security issues.  The original IDs
   are generally preserved.

   If multiple PAD buffers are received and one of them contains a PAD-
   Domain-UUID that is recognized by the application to be the local
   security domain identifier, then only the mapped attributes in this
   buffer SHOULD be used for authorization purposes and the values of
   the other buffers SHOULD be ignored.


5.  Encoding

   The Kerberos protocol is defined in [RFC4120] using Abstract Syntax
   Notation One (ASN.1) [X680].  As such, this specification also uses
   the ASN.1 syntax for specifying both the abstract layout of the PAD
   attributes, as well as their encodings.

5.1.  PAD Format

   The information carried in the PAD need to be augment by some control
   information and packaged in a way that makes it possible to devise
   future extensions.

   Additional information needed to validate the PAD:

   o  The expiration time (must be the same as the ticket expiration
      time).

   o  The principal name (must be the same principal that owns the
      ticket).

   o  The KDC signature (for re-validation purposes).

   o  The Service Signature (in order to trust the PAD has not been
      tampered with).

   o  Optional Host Service Key Signature (for use by trusted services
      on a host)

   o  Optional PUBKEY KDC Signature




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   This information is needed to validate the PAD and make sure it is
   not modified, outdated, or contains information for a different
   principal.

   In order to make the PAD extensible and at the same time always
   verifiable we propose that the PAD is embedded in a ASN.1 structure
   that can contain multiple optional buffers identified by numbers (how
   to assign numbers TBD).

   Buffer number 0 is an ASN.1 strcture that includes all attributes
   described in paragraph 4.  This buffer is itself optional.

   The whole structure with all its buffers is what is signed with the
   KDC and the service keys.

   The final structure to be included in AD-IF-RELEVANT container and
   looks loosely like the following diagram.




           ============================================
           |PAD:                                      |
           |------------------------------------------|
           | KDC Signature (Checksum)                 |
           |------------------------------------------|
           | Service Signature (Checksum)             |
           |------------------------------------------|
           | Trusted Service Signature (Optional)     |
           |------------------------------------------|
           | Asymmetric Key KDC Signature (Optional)  |
           |------------------------------------------|
           | /-PAD-DATA:----------------------------\ |
           | | principal name                       | |
           | | ticket expiration time               | |
           | |                                      | |
           | | Buf 0:  --(optional)-------          | |
           | |        | PAD Attributes ...|         | |
           | |        | ..                |         | |
           | |         -------------------          | |
           | | ....    ....                         | |
           | | Buf X:  --(optional)-------          | |
           | |        | ..                |         | |
           | |         -------------------          | |
           | \--------------------------------------/ |
           ============================================





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                           Figure 1: PAD Format


6.  Data Structures and Extensions

6.1.  SignedPricipalAuthorizationData













































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    AD-PAD                   ::= SEQUENCE {
          kdc-signature         [0] Checksum,
          svc-signature         [1] Checksum,
          trusted-svc-signature [2] PAD-OPT-Checksum OPTIONAL,
          pubkey-signature      [2] PAD-OPT-Checksum OPTIONAL,
          pad-data              [3] PAD-DATA
    }

    PAD-OPT-Checksum     ::= SEQUENCE {
          Identifier     [0] Name,
          Signature      [1] Checksum
    }

     kdc-signature
       A cryptographic checksum computed over the encoding of the
       pad-data field, keyed with the krbtgt key.
       Checksum type TBD.

    svc-signature
       A cryptographic checksum computed over the encoding of the
       pad-data field, keyed with the service long term key.
       Checksum type TBD.

    Trusted-svc-signature
       A principal name and a cryptographic checksum computed over the
       encoding of the pad-data field, keyed with the long term key of
       the principal name specified in the Name field. Unless otherwise
       explicitly administratively configured, the key SHOULD be found
       by substituting the service name component of the principal name
       of the service with 'host'.
       If the service is 'host' this checksum is redundant and can be
       omitted.
       If the resulting host/<name>@REALM or the administratively
       configured service is not found in the KDC database this
       cheksum can be omitted.
       Checksum type TBD.

    pubkey-signature
       A name identifying the asymmetric key-pair used.
       A checksum computed over the encoding of the pad-data field using
       the Private Key identified in the Name field.
       If an asymmetric key is not available this checksum MUST be
       omitted.
       Signature type TBD.







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      PAD-DATA                ::=SEQUENCE {
          p-realm             [0] Realm,
          p-name              [1] PrincipalName,
          elements            [2] SEQUENCE OF AuthorizationData
      }

      p-realm, p-name
         The realm and name of the principal the authorization data
         elements apply to.

      elements
         A sequence of authorization data elements issued by the KDC.



   The AD-PAD data is intended to provide a means for a Kerberos
   principal credentials to carry authorization data that the receiving
   service can use to perform authorization decisions.

   The KDC signature is required to allow the KDC to validate the data
   withouth having to recompute the contents at every TGS request.

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

   Both the Trusted Service Checksum and the asymmetric KDC Signature
   are useful to verify the PAD authenticity on the same host, when the
   PAD is received by a less trusted service and passed to a more
   trusted service on the same host without the need for additional
   roundtrips to the KDC.

   The ad-type for AD-SIGNED-PAD is (TBD).

6.2.  GSS-API Authenticator Extension

   The Authenticator Checksum as defined in RFC 4121 limit the size of
   delegated credentials in the KRB_CRED message to a size of 64KiB.

   In order to be able to transfer larger messgaes an extension is
   defined.  This extension is used in stead of the Dlght/Deleg fields,
   and the Dlght and Deleg fileds MUST not be included when this
   extensions is appended to the authenticator.









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    The extension SHALL have the following format:

       Octet        Name      Description
      -----------------------------------------------------------------
       0..1         ExtN    A 16bit value identifying the extension.
                            Represented in little-endian order;
                            Contains the hex value XX XX (XXXX).

       2..5         Length  The length of the Extended Delegation field.
                            Represented in little-endian order;

       6..N         Data    A KRB_CRED message (N = Length + 6)



   A new flag GSS_C_EXT_DELEG_FLAG with Value X is also defined.  This
   flag is used instead of GSS_C_DELEG_FLAG when the delegated
   credentials are larger then 64KiB and cannot fit in the starndard
   Deleg field.

   Implementors SHOULD use this Extensions and this flag only if the
   KRB_CRED message is larger than 64KiB and use the standard Deleg
   field otherwise.


7.  Assigned numbers

   TBD


8.  IANA Considerations

   TBD.


9.  Security Considerations

   Although it is anticipated that the PAD structure itself will be
   carried within a ticket and thereby protected using the existing
   encryption methods on that ticket, there are a number of issues that
   have bearings on the security of the entire Kerberos realm as whole.
   Some of these issues are as follows:

   o  UID and GID Collisions: There is always the possibilty of collison
      of numbers repressing a UID and a GID.  This problem can be
      remedied to a large degree by realms using an appropriate range
      selection policy and algorithms.




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   o  When collisions are detected the KDC or, alternatively, the
      receiving Service MUST be able to remap IDs so that they do not
      conflict with locally defined IDs

   o  Transit-domain issues: The PAC must be signed by the KDC that is
      attaching it to a ticket with 2 different signatures.  The service
      signature so that the service can verify its KDC validated the
      contents.  The KDC signature, so that the OS can ask the KDC to
      confirm the PAD has not been modified by a less trusted service.
      An optional asymmetric key signature is also allowed if Keys are
      available in order to avoid additional roundtrips.  For cross-
      realm tickets the "service" signature is made with the cross-realm
      key.  When a KDC receives a PAD it is allowed to modify it in any
      way.  It can filter out information or add information (like group
      memberships defined locally).  A KDC may also decide to change
      information in different ways depending on what service it is
      targeted to.


10.  Acknowledgements

   TBD.


11.  References

11.1.  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.

11.2.  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-PAC]   Microsoft, "Microsoft MS-PAC: Privilege Attribute
              Certificate Data Structure (v20100711)", July 2010.



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   [POSIX]    The Open Group, "Portable Operating System Interface
              (POSIX.1-2008)", 2008.

   [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.

   [RFC2307]  Howard, L., "An Approach for Using LDAP as a Network
              Information Service", RFC 2307, March 1998.

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

   [X.690]    ISO, "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 8825-1:1998)", 1997.


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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