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PROPOSED STANDARD
Errata Exist
Network Working Group                                          S. Tuecke
Request for Comments: 3820                                           ANL
Category: Standards Track                                       V. Welch
                                                                    NCSA
                                                               D. Engert
                                                                     ANL
                                                             L. Pearlman
                                                                 USC/ISI
                                                             M. Thompson
                                                                    LBNL
                                                               June 2004


            Internet X.509 Public Key Infrastructure (PKI)
                       Proxy Certificate Profile

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document forms a certificate profile for Proxy Certificates,
   based on X.509 Public Key Infrastructure (PKI) certificates as
   defined in RFC 3280, for use in the Internet.  The term Proxy
   Certificate is used to describe a certificate that is derived from,
   and signed by, a normal X.509 Public Key End Entity Certificate or by
   another Proxy Certificate for the purpose of providing restricted
   proxying and delegation within a PKI based authentication system.














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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview of Approach . . . . . . . . . . . . . . . . . . . . .  4
       2.1.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  4
       2.2.  Background . . . . . . . . . . . . . . . . . . . . . . .  5
       2.3.  Motivation for Proxying. . . . . . . . . . . . . . . . .  5
       2.4.  Motivation for Restricted Proxies. . . . . . . . . . . .  7
       2.5.  Motivation for Unique Proxy Name . . . . . . . . . . . .  8
       2.6.  Description Of Approach. . . . . . . . . . . . . . . . .  9
       2.7.  Features Of This Approach. . . . . . . . . . . . . . . . 10
   3.  Certificate and Certificate Extensions Profile . . . . . . . . 12
       3.1.  Issuer . . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.2.  Issuer Alternative Name. . . . . . . . . . . . . . . . . 12
       3.3.  Serial Number. . . . . . . . . . . . . . . . . . . . . . 12
       3.4.  Subject. . . . . . . . . . . . . . . . . . . . . . . . . 13
       3.5.  Subject Alternative Name . . . . . . . . . . . . . . . . 13
       3.6.  Key Usage and Extended Key Usage . . . . . . . . . . . . 13
       3.7.  Basic Constraints. . . . . . . . . . . . . . . . . . . . 14
       3.8.  The ProxyCertInfo Extension. . . . . . . . . . . . . . . 14
   4.  Proxy Certificate Path Validation. . . . . . . . . . . . . . . 17
       4.1.  Basic Proxy Certificate Path Validation. . . . . . . . . 19
       4.2.  Using the Path Validation Algorithm. . . . . . . . . . . 23
   5.  Commentary . . . . . . . . . . . . . . . . . . . . . . . . . . 24
       5.1.  Relationship to Attribute Certificates . . . . . . . . . 24
       5.2.  Kerberos 5 Tickets . . . . . . . . . . . . . . . . . . . 28
       5.3.  Examples of usage of Proxy Restrictions. . . . . . . . . 28
       5.4.  Delegation Tracing . . . . . . . . . . . . . . . . . . . 29
   6.  Security Considerations. . . . . . . . . . . . . . . . . . . . 30
       6.1.  Compromise of a Proxy Certificate. . . . . . . . . . . . 30
       6.2.  Restricting Proxy Certificates . . . . . . . . . . . . . 31
       6.3.  Relying Party Trust of Proxy Certificates. . . . . . . . 31
       6.4.  Protecting Against Denial of Service with Key Generation 32
       6.5.  Use of Proxy Certificates in a Central Repository. . . . 32
   7.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 33
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
       8.1.  Normative References . . . . . . . . . . . . . . . . . . 33
       8.2.  Informative References . . . . . . . . . . . . . . . . . 33
   9.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 34
   Appendix A. 1988 ASN.1 Module. . . . . . . . . . . . . . . . . . . 35
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
   Full Copyright Notice. . . . . . . . . . . . . . . . . . . . . . . 37









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1.  Introduction

   Use of a proxy credential [i7] is a common technique used in security
   systems to allow entity A to grant to another entity B the right for
   B to be authorized with others as if it were A.  In other words,
   entity B is acting as a proxy on behalf of entity A.  This document
   forms a certificate profile for Proxy Certificates, based on the RFC
   3280, "Internet X.509 Public Key Infrastructure Certificate and CRL
   Profile" [n2].

   In addition to simple, unrestricted proxying, this profile defines:

   *  A framework for carrying policies in Proxy Certificates that
      allows proxying to be limited (perhaps completely disallowed)
      through either restrictions or enumeration of rights.

   *  Proxy Certificates with unique names, derived from the name of the
      end entity certificate name.  This allows the Proxy Certificates
      to be used in conjunction with attribute assertion approaches such
      as Attribute Certificates [i3] and have their own rights
      independent of their issuer.

   Section 2 provides a non-normative overview of the approach.  It
   begins by defining terminology, motivating Proxy Certificates, and
   giving a brief overview of the approach.  It then introduces the
   notion of a Proxy Issuer, as distinct from a Certificate Authority,
   to describe how end entity signing of a Proxy Certificate is
   different from end entity signing of another end entity certificate,
   and therefore why this approach does not violate the end entity
   signing restrictions contained in the X.509 keyCertSign field of the
   keyUsage extension.  It then continues with discussions of how
   subject names are used by this proxying approach, and features of
   this approach.

   Section 3 defines requirements on information content in Proxy
   Certificates.  This profile addresses two fields in the basic
   certificate as well as five certificate extensions.  The certificate
   fields are the subject and issuer fields.  The certificate extensions
   are subject alternative name, issuer alternative name, key usage,
   basic constraints, and extended key usage.  A new certificate
   extension, Proxy Certificate Information, is introduced.

   Section 4 defines path validation rules for Proxy Certificates.

   Section 5 provides non-normative commentary on Proxy Certificates.

   Section 6 discusses security considerations relating to Proxy
   Certificates.



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   References, listed in Section 8, are sorted into normative and
   information references.  Normative references, listed in Section 8.1,
   are in the form [nXX].  Informative references, listed in Section
   8.2, are in the form [iXX].

   Section 9 contains acknowledgements.

   Following Section 9, contains the Appendix, the contact information
   for the authors, the intellectual property information, and the
   copyright information for this document.

   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 BCP 14, RFC 2119 [n1].

2.  Overview of Approach

   This section provides non-normative commentary on Proxy Certificates.

   The goal of this specification is to develop a X.509 Proxy
   Certificate profile and to facilitate their use within Internet
   applications for those communities wishing to make use of restricted
   proxying and delegation within an X.509 Public Key Infrastructure
   (PKI) authentication based system.

   This section provides relevant background, motivation, an overview of
   the approach, and related work.

2.1.  Terminology

   This document uses the following terms:

   *  CA: A "Certification Authority", as defined by X.509 [n2]

   *  EEC: An "End Entity Certificate", as defined by X.509.  That is,
      it is an X.509 Public Key Certificate issued to an end entity,
      such as a user or a service, by a CA.

   *  PKC: An end entity "Public Key Certificate".  This is synonymous
      with an EEC.

   *  PC: A "Proxy Certificate", the profile of which is defined by this
      document.








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   *  PI: A "Proxy Issuer" is an entity with an End Entity Certificate
      or Proxy Certificate that issues a Proxy Certificate.  The Proxy
      Certificate is signed using the private key associated with the
      public key in the Proxy Issuer's certificate.

   *  AC: An "Attribute Certificate", as defined by "An Internet
      Attribute Certificate Profile for Authorization" [i3].

   *  AA: An "Attribute Authority", as defined in [i3].

2.2.  Background

   Computational and Data "Grids" have emerged as a common approach to
   constructing dynamic, inter-domain, distributed computing
   environments.  As explained in [i5], large research and development
   efforts starting around 1995 have focused on the question of what
   protocols, services, and APIs are required for effective, coordinated
   use of resources in these Grid environments.

   In 1997, the Globus Project (www.globus.org) introduced the Grid
   Security Infrastructure (GSI) [i4].  This library provides for public
   key based authentication and message protection, based on standard
   X.509 certificates and public key infrastructure, the SSL/TLS
   protocol [i2], and delegation using proxy certificates similar to
   those profiled in this document.  GSI has been used, in turn, to
   build numerous middleware libraries and applications, which have been
   deployed in large-scale production and experimental Grids [i1].  GSI
   has emerged as the dominant security solution used by Grid efforts
   worldwide.

   This experience with GSI has proven the viability of restricted
   proxying as a basis for authorization within Grids, and has further
   proven the viability of using X.509 Proxy Certificates, as defined in
   this document, as the basis for that proxying.  This document is one
   part of an effort to migrate this experience with GSI into standards,
   and in the process clean up the approach and better reconcile it with
   existing and recent standards.

2.3.  Motivation for Proxying

   A motivating example will assist in understanding the role proxying
   can play in building Internet based applications.

   Steve is an engineer who wants to use a reliable file transfer
   service to manage the movement of a number of large files around
   between various hosts on his company's Intranet-based Grid.  From his
   laptop he wants to submit a number of transfer requests to the
   service and have the files transferred while he is doing other



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   things, including being offline.  The transfer service may queue the
   requests for some time (e.g., until after hours or a period of low
   resource usage) before initiating the transfers.  The transfer
   service will then, for each file, connect to each of the source and
   destination hosts, and instruct them to initiate a data connection
   directly from the source to the destination in order to transfer the
   file.  Steve will leave an agent running on his laptop that will
   periodically check on progress of the transfer by contacting the
   transfer service.  Of course, he wants all of this to happen securely
   on his company's resources, which requires that he initiate all of
   this using his PKI smartcard.

   This scenario requires authentication and delegation in a variety of
   places:

   *  Steve needs to be able to mutually authenticate with the reliable
      file transfer service to submit the transfer request.

   *  Since the storage hosts know nothing about the file transfer
      service, the file transfer service needs to be delegated the
      rights to mutually authenticate with the various storage hosts
      involved directly in the file transfer, in order to initiate the
      file transfer.

   *  The source and destination hosts of a particular transfer must be
      able to mutual authenticate with each other, to ensure the file is
      being transferred to and from the proper parties.

   *  The agent running on Steve's laptop must mutually authenticate
      with the file transfer service in order to check the result of the
      transfers.

   Proxying is a viable approach to solving two (related) problems in
   this scenario:

   *  Single sign-on: Steve wants to enter his smartcard password (or
      pin) once, and then run a program that will submit all the file
      transfer requests to the transfer service, and then periodically
      check on the status of the transfer.  This program needs to be
      given the rights to be able to perform all of these operations
      securely, without requiring repeated access to the smartcard or
      Steve's password.

   *  Delegation: Various remote processes in this scenario need to
      perform secure operations on Steve's behalf, and therefore must be
      delegated the necessary rights.  For example, the file transfer





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      service needs to be able to authenticate on Steve's behalf with
      the source and destination hosts, and must in turn delegate rights
      to those hosts so that they can authenticate with each other.

   Proxying can be used to secure all of these interactions:

   *  Proxying allows for the private key stored on the smartcard to be
      accessed just once, in order to create the necessary proxy
      credential, which allows the client/agent program to be authorized
      as Steve when submitting the requests to the transfer service.
      Access to the smartcard and Steve's password is not required after
      the initial creation of the proxy credential.

   *  The client program on the laptop can delegate to the file transfer
      service the right to act on Steve's behalf.  This, in turn, allows
      the service to authenticate to the storage hosts and inherit
      Steve's privileges in order to start the file transfers.

   *  When the transfer service authenticates to hosts to start the file
      transfer, the service can delegate to the hosts the right to act
      on Steve's behalf so that each pair of hosts involved in a file
      transfer can mutually authenticate to ensure the file is securely
      transferred.

   *  When the agent on the laptop reconnects to the file transfer
      service to check on the status of the transfer, it can perform
      mutual authentication.  The laptop may use a newly generated proxy
      credential, which is just created anew using the smartcard.

   This scenario, and others similar to it, is being built today within
   the Grid community.  The Grid Security Infrastructure's single sign-
   on and delegation capabilities, built on X.509 Proxy Certificates,
   are being employed to provide authentication services to these
   applications.

2.4.  Motivation for Restricted Proxies

   One concern that arises is what happens if a machine that has been
   delegated the right to inherit Steve's privileges has been
   compromised?  For example, in the above scenario, what if the machine
   running the file transfer service is compromised, such that the
   attacker can gain access to the credential that Steve delegated to
   that service?  Can the attacker now do everything that Steve is
   allowed to do?

   A solution to this problem is to allow for restrictions to be placed
   on the proxy by means of policies on the proxy certificates. For
   example, the machine running the reliable file transfer service in



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   the above example might only be given Steve's right for the purpose
   of reading the source files and writing the destination files.
   Therefore, if that file transfer service is compromised, the attacker
   cannot modify source files, cannot create or modify other files to
   which Steve has access, cannot start jobs on behalf of Steve, etc.
   All that an attacker would be able to do is read the specific files
   to which the file transfer service has been delegated read access,
   and write bogus files in place of those that the file transfer
   service has been delegated write access. Further, by limiting the
   lifetime of the credential that is delegated to the file transfer
   service, the effects of a compromise can be further mitigated.

   Other potential uses for restricted proxy credentials are discussed
   in [i7].

2.5.  Motivation for Unique Proxy Name

   The dynamic creation of entities (e.g., processes and services) is an
   essential part of Grid computing.  These entities will require rights
   in order to securely perform their function.  While it is possible to
   obtain rights solely through proxying as described in previous
   sections, this has limitations.  For example what if an entity should
   have rights that are granted not just from the proxy issuer but from
   a third party as well?  While it is possible in this case for the
   entity to obtain and hold two proxy certifications, in practice it is
   simpler for subsequent credentials to take the form of attribute
   certificates.

   It is also desirable for these entities to have a unique identity so
   that they can be explicitly discussed in policy statements.  For
   example, a user initiating a third-party FTP transfer could grant
   each FTP server a PC with a unique identity and inform each server of
   the identity of the other, then when the two servers connected they
   could authenticate themselves and know they are connected to the
   proper party.

   In order for a party to have rights of it's own it requires a unique
   identity.  Possible options for obtaining an unique identity are:

   1) Obtain an identity from a traditional Certification Authority
      (CA).

   2) Obtain a new identity independently - for example by using the
      generated public key and a self-signed certificate.

   3) Derive the new identity from an existing identity.





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   In this document we describe an approach to option #3, because:

      *  It is reasonably light-weight, as it can be done without
         interacting with a third party.  This is important when
         creating identities dynamically.

      *  As described in the previous section, a common use for PCs is
         for restricted proxying, so deriving their identity from the
         identity of the EEC makes this straightforward.  Nonetheless
         there are circumstances where the creator does not wish to
         delegate all or any of its rights to a new entity.  Since the
         name is unique, this is easily accomplished by #3 as well, by
         allowing the application of a policy to limit proxying.

2.6.  Description Of Approach

   This document defines an X.509 "Proxy Certificate" or "PC" as a means
   of providing for restricted proxying within an (extended) X.509 PKI
   based authentication system.

   A Proxy Certificate is an X.509 public key certificate with the
   following properties:

   1) It is signed by either an X.509 End Entity Certificate (EEC), or
      by another PC.  This EEC or PC is referred to as the Proxy Issuer
      (PI).

   2) It can sign only another PC.  It cannot sign an EEC.

   3) It has its own public and private key pair, distinct from any
      other EEC or PC.

   4) It has an identity derived from the identity of the EEC that
      signed the PC.  When a PC is used for authentication, in may
      inherit rights of the EEC that signed the PC, subject to the
      restrictions that are placed on that PC by the EEC.

   5) Although its identity is derived from the EEC's identity, it is
      also unique.  This allows this identity to be used for
      authorization as an independent identity from the identity of the
      issuing EEC, for example in conjunction with attribute assertions
      as defined in [i3].

   6) It contains a new X.509 extension to identify it as a PC and to
      place policies on the use of the PC.  This new extension, along
      with other X.509 fields and extensions, are used to enable proper
      path validation and use of the PC.




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   The process of creating a PC is as follows:

   1) A new public and private key pair is generated.

   2) That key pair is used to create a request for a Proxy Certificate
      that conforms to the profile described in this document.

   3) A Proxy Certificate, signed by the private key of the EEC or by
      another PC, is created in response to the request.  During this
      process, the PC request is verified to ensure that the requested
      PC is valid (e.g., it is not an EEC, the PC fields are
      appropriately set, etc).

   When a PC is created as part of a delegation from entity A to entity
   B, this process is modified by performing steps #1 and #2 within
   entity B, then passing the PC request from entity B to entity A over
   an authenticated, integrity checked channel, then entity A performs
   step #3 and passes the PC back to entity B.

   Path validation of a PC is very similar to normal path validation,
   with a few additional checks to ensure, for example, proper PC
   signing constraints.

2.7.  Features Of This Approach

   Using Proxy Certificates to perform delegation has several features
   that make it attractive:

   *  Ease of integration

      o  Because a PC requires only a minimal change to path validation,
         it is very easy to incorporate support for Proxy Certificates
         into existing X.509 based software.  For example, SSL/TLS
         requires no protocol changes to support authentication using a
         PC.  Further, an SSL/TLS implementation requires only minor
         changes to support PC path validation, and to retrieve the
         authenticated subject of the signing EEC instead of the subject
         of the PC for authorization purposes.

      o  Many existing authorization systems use the X.509 subject name
         as the basis for access control.  Proxy Certificates can be
         used with such authorization systems without modification,
         since such a PC inherits its name and rights from the EEC that
         signed it and the EEC name can be used in place of the PC name
         for authorization decisions.






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   *  Ease of use

      o  Using PC for single sign-on helps make X.509 PKI authentication
         easier to use, by allowing users to "login" once and then
         perform various operations securely.

      o  For many users, properly managing their own EEC private key is
         a nuisance at best, and a security risk at worst.  One option
         easily enabled with a PC is to manage the EEC private keys and
         certificates in a centrally managed repository. When a user
         needs a PKI credential, the user can login to the repository
         using name/password, one time password, etc.  Then the
         repository can delegate a PC to the user with proxy rights, but
         continue to protect the EEC private key in the repository.

   *  Protection of private keys

      o  By using the remote delegation approach outlined above, entity
         A can delegate a PC to entity B, without entity B ever seeing
         the private key of entity A, and without entity A ever seeing
         the private key of the newly delegated PC held by entity B.  In
         other words, private keys never need to be shared or
         communicated by the entities participating in a delegation of a
         PC.

      o  When implementing single sign-on, using a PC helps protect the
         private key of the EEC, because it minimizes the exposure and
         use of that private key.  For example, when an EEC private key
         is password protected on disk, the password and unencrypted
         private key need only be available during the creation of the
         PC.  That PC can then be used for the remainder of its valid
         lifetime, without requiring access to the EEC password or
         private key.  Similarly, when the EEC private key lives on a
         smartcard, the smartcard need only be present in the machine
         during the creation of the PC.

   *  Limiting consequences of a compromised key

      o  When creating a PC, the PI can limit the validity period of the
         PC, the depth of the PC path that can be created by that PC,
         and key usage of the PC and its descendents.  Further, fine-
         grained policies can be carried by a PC to even further
         restrict the operations that can be performed using the PC.
         These restrictions permit the PI to limit damage that could be
         done by the bearer of the PC, either accidentally or
         maliciously.





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      o  A compromised PC private key does NOT compromise the EEC
         private key.  This makes a short term, or an otherwise
         restricted PC attractive for day-to-day use, since a
         compromised PC does not require the user to go through the
         usually cumbersome and time consuming process of having the EEC
         with a new private key reissued by the CA.

   See Section 5 below for more discussion on how Proxy Certificates
   relate to Attribute Certificates.

3.  Certificate and Certificate Extensions Profile

   This section defines the usage of X.509 certificate fields and
   extensions in Proxy Certificates, and defines one new extension for
   Proxy Certificate Information.

   All Proxy Certificates MUST include the Proxy Certificate Information
   (ProxyCertInfo) extension defined in this section and the extension
   MUST be critical.

3.1.  Issuer

   The Proxy Issuer of a Proxy Certificate MUST be either an End Entity
   Certificate, or another Proxy Certificate.

   The Proxy Issuer MUST NOT have an empty subject field.

   The issuer field of a Proxy Certificate MUST contain the subject
   field of its Proxy Issuer.

   If the Proxy Issuer certificate has the KeyUsage extension, the
   Digital Signature bit MUST be asserted.

3.2.  Issuer Alternative Name

   The issuerAltName extension MUST NOT be present in a Proxy
   Certificate.

3.3.  Serial Number

   The serial number of a Proxy Certificate (PC) SHOULD be unique
   amongst all Proxy Certificates issued by a particular Proxy Issuer.
   However, a Proxy Issuer MAY use an approach to assigning serial
   numbers that merely ensures a high probability of uniqueness.







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   For example, a Proxy Issuer MAY use a sequentially assigned integer
   or a UUID to assign a unique serial number to a PC it issues.  Or a
   Proxy Issuer MAY use a SHA-1 hash of the PC public key to assign a
   serial number with a high probability of uniqueness.

3.4.  Subject

   The subject field of a Proxy Certificate MUST be the issuer field
   (that is the subject of the Proxy Issuer) appended with a single
   Common Name component.

   The value of the Common Name SHOULD be unique to each Proxy
   Certificate bearer amongst all Proxy Certificates with the same
   issuer.

   If a Proxy Issuer issues two proxy certificates to the same bearer,
   the Proxy Issuer MAY choose to use the same Common Name for both.
   Examples of this include Proxy Certificates for different uses (e.g.,
   signing vs encryption) or the re-issuance of an expired Proxy
   Certificate.

   The Proxy Issuer MAY use an approach to assigning Common Name values
   that merely ensures a high probability of uniqueness.  This value MAY
   be the same value used for the serial number.

   The result of this approach is that all subject names of Proxy
   Certificates are derived from the name of the issuing EEC (it will be
   the first part of the subject name appended with one or more CN
   components) and are unique to each bearer.

3.5.  Subject Alternative Name

   The subjectAltName extension MUST NOT be present in a Proxy
   Certificate.

3.6.  Key Usage and Extended Key Usage

   If the Proxy Issuer certificate has a Key Usage extension, the
   Digital Signature bit MUST be asserted.

   This document places no constraints on the presence or contents of
   the key usage and extended key usage extension.  However, section 4.2
   explains what functions should be allowed a proxy certificate by a
   relying party.







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3.7.  Basic Constraints

   The cA field in the basic constraints extension MUST NOT be TRUE.

3.8.  The ProxyCertInfo Extension

   A new extension, ProxyCertInfo, is defined in this subsection.
   Presence of the ProxyCertInfo extension indicates that a certificate
   is a Proxy Certificate and whether or not the issuer of the
   certificate has placed any restrictions on its use.

   id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
            dod(6) internet(1) security(5) mechanisms(5) pkix(7) }

   id-pe OBJECT IDENTIFIER ::= { id-pkix 1 }

   id-pe-proxyCertInfo OBJECT IDENTIFIER ::= { id-pe 14 }

   ProxyCertInfo ::= SEQUENCE {
        pCPathLenConstraint   INTEGER (0..MAX) OPTIONAL,
        proxyPolicy           ProxyPolicy }


   ProxyPolicy ::= SEQUENCE {
        policyLanguage        OBJECT IDENTIFIER,
        policy          OCTET STRING OPTIONAL }

   If a certificate is a Proxy Certificate, then the proxyCertInfo
   extension MUST be present, and this extension MUST be marked as
   critical.

   If a certificate is not a Proxy Certificate, then the proxyCertInfo
   extension MUST be absent.

   The ProxyCertInfo extension consists of one required and two optional
   fields, which are described in detail in the following subsections.

3.8.1.  pCPathLenConstraint

   The pCPathLenConstraint field, if present, specifies the maximum
   depth of the path of Proxy Certificates that can be signed by this
   Proxy Certificate.  A pCPathLenConstraint of 0 means that this
   certificate MUST NOT be used to sign a Proxy Certificate.  If the
   pCPathLenConstraint field is not present then the maximum proxy path
   length is unlimited.  End entity certificates have unlimited maximum
   proxy path lengths.





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3.8.2.  proxyPolicy

   The proxyPolicy field specifies a policy on the use of this
   certificate for the purposes of authorization.  Within the
   proxyPolicy, the policy field is an expression of policy, and the
   policyLanguage field indicates the language in which the policy is
   expressed.

   The proxyPolicy field in the proxyCertInfo extension does not define
   a policy language to be used for proxy restrictions; rather, it
   places the burden on those parties using that extension to define an
   appropriate language, and to acquire an OID for that language (or to
   select an appropriate previously-defined language/OID).  Because it
   is essential for the PI that issues a certificate with a proxyPolicy
   field and the relying party that interprets that field to agree on
   its meaning, the policy language OID must correspond to a policy
   language (including semantics), not just a policy grammar.

   The policyLanguage field has two values of special importance,
   defined in Appendix A, that MUST be understood by all parties
   accepting Proxy Certificates:

   *  id-ppl-inheritAll indicates that this is an unrestricted proxy
      that inherits all rights from the issuing PI.  An unrestricted
      proxy is a statement that the Proxy Issuer wishes to delegate all
      of its authority to the bearer (i.e., to anyone who has that proxy
      certificate and can prove possession of the associated private
      key).  For purposes of authorization, this an unrestricted proxy
      effectively impersonates the issuing PI.

   *  id-ppl-independent indicates that this is an independent proxy
      that inherits no rights from the issuing PI.  This PC MUST be
      treated as an independent identity by relying parties.  The only
      rights this PC has are those granted explicitly to it.

   For either of the policyLanguage values listed above, the policy
   field MUST NOT be present.

   Other values for the policyLanguage field indicates that this is a
   restricted proxy certification and have some other policy limiting
   its ability to do proxying.  In this case the policy field MAY be
   present and it MUST contain information expressing the policy.  If
   the policy field is not present the policy MUST be implicit in the
   value of the policyLanguage field itself.  Authors of additional
   policy languages are encouraged to publicly document their policy
   language and list it in the IANA registry (see Section 7).





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   Proxy policies are used to limit the amount of authority delegated,
   for example to assert that the proxy certificate may be used only to
   make requests to a specific server, or only to authorize specific
   operations on specific resources.  This document is agnostic to the
   policies that can be placed in the policy field.

   Proxy policies impose additional requirements on the relying party,
   because only the relying party is in a position to ensure that those
   policies are enforced.  When making an authorization decision based
   on a proxy certificate based on rights that proxy certificate
   inherited from its issuer, it is the relying party's responsibility
   to verify that the requested authority is compatible with all
   policies in the PC's certificate path.  In other words, the relying
   party MUST verify that the following three conditions are all met:

   1) The relying party MUST know how to interpret the proxy policy and
      the request is allowed under that policy.

   2) If the Proxy Issuer is an EEC then the relying party's local
      policies MUST authorize the request for the entity named in the
      EEC.

   3) If the Proxy Issuer is another PC, then one of the following MUST
      be true:

      a. The relying party's local policies authorize the Proxy Issuer
         to perform the request.

      b. The Proxy Issuer inherits the right to perform the request from
         its issuer by means of its proxy policy.  This must be verified
         by verifying these three conditions on the Proxy Issuer in a
         recursive manner.

   If these conditions are not met, the relying party MUST either deny
   authorization, or ignore the PC and the whole certificate chain
   including the EEC entirely when making its authorization decision
   (i.e., make the same decision that it would have made had the PC and
   it's certificate chain never been presented).

   The relying party MAY impose additional restrictions as to which
   proxy certificates it accepts.  For example, a relying party MAY
   choose to reject all proxy certificates, or MAY choose to accept
   proxy certificates only for certain operations, etc.

   Note that since a proxy certificate has a unique identity it MAY also
   have rights granted to it by means other than inheritance from it's
   issuer via its proxy policy.  The rights granted to the bearer of a
   PC are the union of the rights granted to the PC identity and the



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   inherited rights.  The inherited rights consist of the intersection
   of the rights granted to the PI identity intersected with the proxy
   policy in the PC.

   For example, imagine that Steve is authorized to read and write files
   A and B on a file server, and that he uses his EEC to create a PC
   that includes the policy that it can be used only to read or write
   files A and C.  Then a trusted attribute authority grants an
   Attribute Certificate granting the PC the right to read file D. This
   would make the rights of the PC equal to the union of the rights
   granted to the PC identity (right to read file D) with the
   intersection of the rights granted to Steve, the PI, (right to read
   files A and B) with the policy in the PC (can only read files A and
   C).  This would mean the PC would have the following rights:

   *  Right to read file A: Steve has this right and he issued the PC
      and his policy grants this right to the PC.

   *  Right to read file D: This right is granted explicitly to the PC
      by a trusted authority.

   The PC would NOT have the following rights:

   *  Right to read file B: Although Steve has this right, it is
      excluded by his policy on the PC.

   *  Right to read file C: Although Steve's policy grants this right,
      he does not have this right himself.

   In many cases, the relying party will not have enough information to
   evaluate the above criteria at the time that the certificate path is
   validated.  For example, if a certificate is used to authenticate a
   connection to some server, that certificate is typically validated
   during that authentication step, before any requests have been made
   of the server.  In that case, the relying party MUST either have some
   authorization mechanism in place that will check the proxy policies,
   or reject any certificate that contains proxy policies (or that has a
   parent certificate that contains proxy policies).

4.  Proxy Certificate Path Validation

   Proxy Certification path processing verifies the binding between the
   proxy certificate distinguished name and proxy certificate public
   key.  The binding is limited by constraints which are specified in
   the certificates which comprise the path and inputs which are
   specified by the relying party.





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   This section describes an algorithm for validating proxy
   certification paths.  Conforming implementations of this
   specification are not required to implement this algorithm, but MUST
   provide functionality equivalent to the external behavior resulting
   from this procedure.  Any algorithm may be used by a particular
   implementation so long as it derives the correct result.

   The algorithm presented in this section validates the proxy
   certificate with respect to the current date and time.  A conformant
   implementation MAY also support validation with respect to some point
   in the past.  Note that mechanisms are not available for validating a
   proxy certificate with respect to a time outside the certificate
   validity period.

   Valid paths begin with the end entity certificate (EEC) that has
   already been validated by public key certificate validation
   procedures in RFC 3280 [n2].  The algorithm requires the public key
   of the EEC and the EEC's subject distinguished name.

   To meet the goal of verifying the proxy certificate, the proxy
   certificate path validation process verifies, among other things,
   that a prospective certification path (a sequence of n certificates)
   satisfies the following conditions:

   (a) for all x in {1, ..., n-1}, the subject of certificate x is the
       issuer of proxy certificate x+1 and the subject distinguished
       name of certificate x+1 is a legal subject distinguished name to
       have been issued by certificate x;

   (b) certificate 1 is valid proxy certificate issued by the end entity
       certificate whose information is given as input to the proxy
       certificate path validation process;

   (c) certificate n is the proxy certificate to be validated;

   (d) for all x in {1, ..., n}, the certificate was valid at the time
       in question; and

   (e) for all certificates in the path with a pCPathLenConstraint
       field, the number of certificates in the path following that
       certificate does not exceed the length specified in that field.

   At this point there is no mechanism defined for revoking proxy
   certificates.







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4.1.  Basic Proxy Certificate Path Validation

   This section presents the algorithm in four basic steps to mirror the
   description of public key certificate path validation in RFC 3280:
   (1) initialization, (2) basic proxy certificate processing, (3)
   preparation for the next proxy certificate, and (4) wrap-up. Steps
   (1) and (4) are performed exactly once.  Step (2) is performed for
   all proxy certificates in the path.  Step (3) is performed for all
   proxy certificates in the path except the final proxy certificate.

   Certificate path validation as described in RFC 3280 MUST have been
   done prior to using this algorithm to validate the end entity
   certificate.  This algorithm then processes the proxy certificate
   chain using the end entity certificate information produced by RFC
   3280 path validation.

4.1.1.  Inputs

   This algorithm assumes the following inputs are provided to the path
   processing logic:

   (a) information about the entity certificate already verified using
       RFC 3280 path validation.  This information includes:

      (1) the end entity name,

      (2) the working_public_key output from RFC 3280 path validation,

      (3) the working_public_key_algorithm output from RFC 3280,

      (4) and the working_public_key_parameters output from RFC 3280
          path validation.

   (b) prospective proxy certificate path of length n.

   (c) acceptable-pc-policy-language-set: A set of proxy certificate
       policy languages understood by the policy evaluation code.  The
       acceptable-pc-policy-language-set MAY contain the special value
       id-ppl-anyLanguage (as defined in Appendix A) if the path
       validation code should not check the proxy certificate policy
       languages (typically because the set of known policy languages is
       not known yet and will be checked later in the authorization
       process).

   (d) the current date and time.






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4.1.2.  Initialization

   This initialization phase establishes the following state variables
   based upon the inputs:

   (a) working_public_key_algorithm: the digital signature algorithm
       used to verify the signature of a proxy certificate. The
       working_public_key_algorithm is initialized from the input
       information provided from RFC 3280 path validation.

   (b) working_public_key: the public key used to verify the signature
       of a proxy certificate.  The working_public_key is initialized
       from the input information provided from RFC 3280 path
       validation.

   (c) working_public_key_parameters: parameters associated with the
       current public key, that may be required to verify a signature
       (depending upon the algorithm).  The
       proxy_issuer_public_key_parameters variable is initialized from
       the input information provided from RFC 3280 path validation.

   (d) working_issuer_name: the issuer distinguished name expected in
       the next proxy certificate in the chain.  The working_issuer_name
       is initialized to the distinguished name in the end entity
       certificate validated by RFC 3280 path validation.

   (e) max_path_length: this integer is initialized to n, is decremented
       for each proxy certificate in the path.  This value may also be
       reduced by the pcPathLenConstraint value of any proxy certificate
       in the chain.

   (f) proxy_policy_list: this list is empty to start and will be filled
       in with the key usage extensions, extended key usage extensions
       and proxy policies in the chain.

   Upon completion of the initialization steps, perform the basic
   certificate processing steps specified in 4.1.3.

4.1.3.  Basic Proxy Certificate Processing

   The basic path processing actions to be performed for proxy
   certificate i (for all i in [1..n]) are listed below.

   (a) Verify the basic certificate information.  The certificate MUST
       satisfy each of the following:






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      (1) The certificate was signed with the
          working_public_key_algorithm using the working_public_key and
          the working_public_key_parameters.

      (2) The certificate validity period includes the current time.

      (3) The certificate issuer name is the working_issuer_name.

      (4) The certificate subject name is the working_issuer_name with a
          CN component appended.

   (b) The proxy certificate MUST have a ProxyCertInfo extension.
       Process the extension as follows:

      (1) If the pCPathLenConstraint field is present in the
          ProxyCertInfo field and the value it contains is less than
          max_path_length, set max_path_length to its value.

      (2) If acceptable-pc-policy-language-set is not id-ppl-
          anyLanguage, the OID in the policyLanguage field MUST be
          present in acceptable-pc-policy-language-set.

   (c) The tuple containing the certificate subject name, policyPolicy,
       key usage extension (if present) and extended key usage extension
       (if present) must be appended to proxy_policy_list.

   (d) Process other certificate extensions, as described in [n2]:

      (1) Recognize and process any other critical extensions present in
          the proxy certificate.

      (2) Process any recognized non-critical extension present in the
          proxy certificate.

   If either step (a), (b) or (d) fails, the procedure terminates,
   returning a failure indication and an appropriate reason.

   If i is not equal to n, continue by performing the preparatory steps
   listed in 4.1.4.  If i is equal to n, perform the wrap-up steps
   listed in 4.1.5.

4.1.4.  Preparation for next Proxy Certificate

   (a) Verify max_path_length is greater than zero and decrement
       max_path_length.

   (b) Assign the certificate subject name to working_issuer_name.




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   (c) Assign the certificate subjectPublicKey to working_public_key.

   (d) If the subjectPublicKeyInfo field of the certificate contains an
       algorithm field with non-null parameters, assign the parameters
       to the working_public_key_parameters variable.

       If the subjectPublicKeyInfo field of the certificate contains an
       algorithm field with null parameters or parameters are omitted,
       compare the certificate subjectPublicKey algorithm to the
       working_public_key_algorithm.  If the certificate
       subjectPublicKey algorithm and the working_public_key_algorithm
       are different, set the working_public_key_parameters to null.

   (e) Assign the certificate subjectPublicKey algorithm to the
       working_public_key_algorithm variable.

   (f) If a key usage extension is present, verify that the
       digitalSignature bit is set.

   If either check (a) or (f) fails, the procedure terminates, returning
   a failure indication and an appropriate reason.

   If (a) and (f) complete successfully, increment i and perform the
   basic certificate processing specified in 4.1.3.

4.1.5.  Wrap-up Procedures

   (a) Assign the certificate subject name to working_issuer_name.

   (b) Assign the certificate subjectPublicKey to working_public_key.

   (c) If the subjectPublicKeyInfo field of the certificate contains an
       algorithm field with non-null parameters, assign the parameters
       to the proxy_issuer_public_key_parameters variable.

       If the subjectPublicKeyInfo field of the certificate contains an
       algorithm field with null parameters or parameters are omitted,
       compare the certificate subjectPublicKey algorithm to the
       proxy_issuer_public_key_algorithm.  If the certificate
       subjectPublicKey algorithm and the
       proxy_issuer_public_key_algorithm are different, set the
       proxy_issuer_public_key_parameters to null.

   (d) Assign the certificate subjectPublicKey algorithm to the
       proxy_issuer_public_key_algorithm variable.






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4.1.6.  Outputs

   If path processing succeeds, the procedure terminates, returning a
   success indication together with final value of the
   working_public_key, the working_public_key_algorithm, the
   working_public_key_parameters, and the proxy_policy_list.

4.2.  Using the Path Validation Algorithm

   Each Proxy Certificate contains a ProxyCertInfo extension, which
   always contains a policy language OID, and may also contain a policy
   OCTET STRING.  These policies serve to indicate the desire of each
   issuer in the proxy certificate chain, starting with the EEC, to
   delegate some subset of their rights to the issued proxy certificate.
   This chain of policies is returned by the algorithm to the
   application.

   The application MAY make authorization decisions based on the subject
   distinguished name of the proxy certificate or on one of the proxy
   certificates in it's issuing chain or on the EEC that serves as the
   root of the chain.  If an application chooses to use the subject
   distinguished name of a proxy certificate in the issuing chain or the
   EEC it MUST use the returned policies to restrict the rights it
   grants to the proxy certificate.  If the application does not know
   how to parse any policy in the policy chain it MUST not use, for the
   purposes of making authorization decisions, the subject distinguished
   name of any certificate in the chain prior to the certificate in
   which the unrecognized policy appears.

   Application making authorization decisions based on the contents of
   the proxy certificate key usage or extended key usage extensions MUST
   examine the list of key usage, extended key usage and proxy policies
   resulting from proxy certificate path validation and determine the
   effective key usage functions of the proxy certificate as follows:

   *  If a certificate is a proxy certificate with a proxy policy of
      id-ppl-independent or an end entity certificate, the effective key
      usage functions of that certificate is as defined by the key usage
      and extended key usage extensions in that certificate.  The key
      usage functionality of the issuer has no bearing on the effective
      key usage functionality.

   *  If a certificate is a proxy certificate with a policy other than
      id-ppl-independent, the effective key usage and extended key usage
      functionality of the proxy certificate is the intersection of the
      functionality of those extensions in the proxy certificate and the
      effective key usage functionality of the proxy issuer.




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5.  Commentary

   This section provides non-normative commentary on Proxy Certificates.

5.1.  Relationship to Attribute Certificates

   An Attribute Certificate [i3] can be used to grant to one identity,
   the holder, some attribute such as a role, clearance level, or
   alternative identity such as "charging identity" or "audit identity".
   This is accomplished by way of a trusted Attribute Authority (AA),
   which issues signed Attribute Certificates (AC), each of which binds
   an identity to a particular set of attributes. Authorization
   decisions can then be made by combining information from the
   authenticated End Entity Certificate providing the identity, with the
   signed Attribute Certificates providing binding of that identity to
   attributes.

   There is clearly some overlap between the capabilities provided by
   Proxy Certificates and Attribute Certificates.  However, the
   combination of the two approaches together provides a broader
   spectrum of solutions to authorization in X.509 based systems, than
   either solution alone.  This section seeks to clarify some of the
   overlaps, differences, and synergies between Proxy Certificate and
   Attribute Certificates.

5.1.1.  Types of Attribute Authorities

   For the purposes of this discussion, Attribute Authorities, and the
   uses of the Attribute Certificates that they produce, can be broken
   down into two broad classes:

   1) End entity AA: An End Entity Certificate may be used to sign an
      AC.  This can be used, for example, to allow an end entity to
      delegate some of its privileges to another entity.

   2) Third party AA: A separate entity, aside from the end entity
      involved in an authenticated interaction, may sign ACs in order to
      bind the authenticated identity with additional attributes, such
      as role, group, etc.  For example, when a client authenticates
      with a server, the third party AA may provide an AC that binds the
      client identity to a particular group, which the server then uses
      for authorization purposes.

   This second type of Attribute Authority, the third party AA, works
   equally well with an EEC or a PC.  For example, unrestricted Proxy
   Certificates can be used to delegate the EEC's identity to various
   other parties.  Then when one of those other parties uses the PC to
   authenticate with a service, that service will receive the EEC's



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   identity via the PC, and can apply any ACs that bind that identity to
   attributes in order to determine authorization rights. Additionally
   PC with policies could be used to selectively deny the binding of ACs
   to a particular proxy.  An AC could also be bound to a particular PC
   using the subject or issuer and serial number of the proxy
   certificate.  There would appear to be great synergies between the
   use of Proxy Certificates and Attribute Certificates produced by
   third party Attribute Authorities.

   However, the uses of Attribute Certificates that are granted by the
   first type of Attribute Authority, the end entity AA, overlap
   considerably with the uses of Proxy Certificates as described in the
   previous sections.  Such Attribute Certificates are generally used
   for delegation of rights from one end entity to others, which clearly
   overlaps with the stated purpose of Proxy Certificates, namely single
   sign-on and delegation.

5.1.2.  Delegation Using Attribute Certificates

   In the motivating example in Section 2, PCs are used to delegate
   Steve's identity to the various other jobs and entities that need to
   act on Steve's behalf.  This allows those other entities to
   authenticate as if they were Steve, for example to the mass storage
   system.

   A solution to this example could also be cast using Attribute
   Certificates that are signed by Steve's EEC, which grant to the other
   entities in this example the right to perform various operations on
   Steve's behalf.  In this example, the reliable file transfer service
   and all the hosts involved in file transfers, the starter program,
   the agent, the simulation jobs, and the post-processing job would
   each have their own EECs.  Steve's EEC would therefore issue ACs to
   bind each of those other EEC identities to attributes that grant the
   necessary privileges allow them to, for example, access the mass
   storage system.

   However, this AC based solution to delegation has some disadvantages
   as compared to the PC based solution:

   *  All protocols, authentication code, and identity based
      authorization services must be modified to understand ACs.  With
      the PC solution, protocols (e.g., TLS) likely need no
      modification, authentication code needs minimal modification
      (e.g., to perform PC aware path validation), and identity based
      authorization services need minimal modification (e.g., possibly
      to find the EEC name and to check for any proxy policies).





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   *  ACs need to be created by Steve's EEC, which bind attributes to
      each of the other identities involved in the distributed
      application (i.e., the agent, simulation jobs, and post-processing
      job the file transfer service, the hosts transferring files).
      This implies that Steve must know in advance which other
      identities may be involved in this distributed application, in
      order to generate the appropriate ACs which are signed by Steve's
      ECC.  On the other hand, the PC solution allows for much more
      flexibility, since parties can further delegate a PC without a
      priori knowledge by the originating EEC.

   There are many unexplored tradeoffs and implications in this
   discussion of delegation.  However, reasonable arguments can be made
   in favor of either an AC based solution to delegation or a PC based
   solution to delegation.  The choice of which approach should be taken
   in a given instance may depend on factors such as the software that
   it needs to be integrated into, the type of delegation required, and
   other factors.

5.1.3.  Propagation of Authorization Information

   One possible use of Proxy Certificates is to carry authorization
   information associated with a particular identity.

   The merits of placing authorization information into End Entity
   Certificates (also called a Public Key Certificate or PKC) have been
   widely debated.  For example, Section 1 of "An Internet Attribute
   Certificate Profile for Authorization" [i3] states:

      "Authorization information may be placed in a PKC extension or
      placed in a separate attribute certificate (AC).  The placement of
      authorization information in PKCs is usually undesirable for two
      reasons.  First, authorization information often does not have the
      same lifetime as the binding of the identity and the public key.
      When authorization information is placed in a PKC extension, the
      general result is the shortening of the PKC useful lifetime.
      Second, the PKC issuer is not usually authoritative for the
      authorization information.  This results in additional steps for
      the PKC issuer to obtain authorization information from the
      authoritative source.

      For these reasons, it is often better to separate authorization
      information from the PKC.  Yet, authorization information also
      needs to be bound to an identity.  An AC provides this binding; it
      is simply a digitally signed (or certified) identity and set of
      attributes."





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   Placing authorization information in a PC mitigates the first
   undesirable property cited above.  Since a PC has a lifetime that is
   mostly independent of (always shorter than) its signing EEC, a PC
   becomes a viable approach for carrying authorization information for
   the purpose of delegation.

   The second undesirable property cited above is true.  If a third
   party AA is authoritative, then using ACs issued by that third party
   AA is a natural approach to disseminating authorization information.
   However, this is true whether the identity being bound by these ACs
   comes from an EEC (PKC), or from a PC.

   There is one case, however, that the above text does not consider.
   When performing delegation, it is usually the EEC itself that is
   authoritative (not the EEC issuer, or any third party AA).  That is,
   it is up to the EEC to decide what authorization rights it is willing
   to grant to another party.  In this situation, including such
   authorization information into PCs that are generated by the EEC
   seems a reasonable approach to disseminating such information.

5.1.4.  Proxy Certificate as Attribute Certificate Holder

   In a system that employs both PCs and ACs, one can imagine the
   utility of allowing a PC to be the holder of an AC.  This would allow
   for a particular delegated instance of an identity to be given an
   attribute, rather than all delegated instances of that identity being
   given the attribute.

   However, the issue of how to specify a PC as the holder of an AC
   remains open.  An AC could be bound to a particular instance of a PC
   using the unique subject name of the PC, or it's issuer and serial
   number combination.

   Unrestricted PCs issued by that PC would then inherit those ACs and
   independent PCs would not.  PCs issued with a policy would depend on
   the policy as to whether or not they inherit the issuing PC's ACs
   (and potentially which ACs they inherit).

   While an AC can be bound to one PC by the AA, how can the AA restrict
   that PC from passing it on to a subsequently delegated PC? One
   possible solution would be to define an extension to attribute
   certificates that allows the attribute authority to state whether an
   issued AC is to apply only to the particular entity to which it is
   bound, or if it may apply to PCs issued by that entity.

   One issue that an AA in this circumstance would need to be aware of
   is that the PI of the PC that the AA bound the AC to, could issue
   another PC with the same name as the original PC to a different



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   entity, effectively stealing the AC.  This implies that an AA issuing
   an AC to a PC need to not only trust the entity holding the PC, but
   the entity holding the PC's issuer as well.

5.2.  Kerberos 5 Tickets

   The Kerberos Network Authentication Protocol (RFC 1510 [i6]) is a
   widely used authentication system based on conventional (shared
   secret key) cryptography.  It provides support for single sign-on via
   creation of "Ticket Granting Tickets" or "TGT", and support for
   delegation of rights via "forwardable tickets".

   Kerberos 5 tickets have informed many of the ideas surrounding X.509
   Proxy Certificates.  For example, the local creation of a short-lived
   PC can be used to provide single sign-on in an X.509 PKI based
   system, just as creation of short-lived TGT allows for single sign-on
   in a Kerberos based system.  And just as a TGT can be forwarded
   (i.e., delegated) to another entity to allow for proxying in a
   Kerberos based system, so can a PC can be delegated to allow for
   proxying in an X.509 PKI based system.

   A major difference between a Kerberos TGT and an X.509 PC is that
   while creation and delegation of a TGT requires the involvement of a
   third party (Key Distribution Center), a PC can be unilaterally
   created without the active involvement of a third party.  That is, a
   user can directly create a PC from an EEC for single sign-on
   capability, without requiring communication with a third party.  And
   an entity with a PC can delegate the PC to another entity (i.e., by
   creating a new PC, signed by the first) without requiring
   communication with a third party.

   The method used by Kerberos implementations to protect a TGT can also
   be used to protect the private key of a PC.  For example, some Unix
   implementations of Kerberos use standard Unix file system security to
   protect a user's TGT from compromise.  Similarly, the Globus
   Toolkit's Grid Security Infrastructure implementation of Proxy
   Certificates protects a user's PC private key using this same
   approach.

5.3.  Examples of usage of Proxy Restrictions

   This section gives some examples of Proxy Certificate usage and some
   examples of how the Proxy policy can be used to restrict Proxy
   Certificates.







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5.3.1.  Example use of proxies without Restrictions

   Steve wishes to perform a third-party FTP transfer between two FTP
   servers.  Steve would use an existing PC to authenticate to both
   servers and delegate a PC to both hosts.  He would inform each host
   of the unique subject name of the PC given to the other host.  When
   the servers establish the data channel connection to each other, they
   use these delegated credentials to perform authentication and verify
   they are talking to the correct entity by checking the result of the
   authentication matches the name as provided by Steve.

5.3.2.  Example use of proxies with Restrictions

   Steve wishes to delegate to a process the right to perform a transfer
   of a file from host H1 to host H2 on his behalf.  Steve would
   delegate a PC to the process and he would use Proxy Policy to
   restrict the delegated PC to two rights - the right to read file F1
   on host H1 and the right to write file F2 on host H2.

   The process then uses this restricted PC to authenticate to servers
   H1 and H2.  The process would also delegate a PC to both servers.
   Note that these delegated PCs would inherit the restrictions of their
   parents, though this is not relevant to this example.  As in the
   example in the previous Section, each host would be provided with the
   unique name of the PC given to the other server.

   Now when the process issues the command to transfer the file F1 on H1
   and to F2 on H2, these two servers perform an authorization check
   based on the restrictions in the PC that the process used to
   authenticate with them (in addition to any local policy they have).
   Namely H1 checks that the PC gives the user the right to read F1 and
   H2 checks that the PC gives the user the right to write F2. When
   setting up the data channel the servers would again verify the names
   resulting from the authentication match the names provided by Steve
   as in the example in the previous Section.

   The extra security provided by these restrictions is that now if the
   PC delegated to the process by Steve is stolen, its use is greatly
   limited.

5.4.  Delegation Tracing

   A relying party accepting a Proxy Certificate may have an interest in
   knowing which parties issued earlier Proxy Certificates in the
   certificate chain and to whom they delegated them.  For example it
   may know that a particular service or resource is known to have been





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   compromised and if any part of a Proxy Certificate's chain was issued
   to the compromised service a relying party may wish to disregard the
   chain.

   A delegation tracing mechanism was considered by the authors as
   additional information to be carried in the ProxyCertInfo extension.
   However at this time agreement has not been reached as to what this
   information should include so it was left out of this document, and
   will instead be considered in future revisions.  The debate mainly
   centers on whether the tracing information should simply contain the
   identity of the issuer and receiver or it should also contain all the
   details of the delegated proxy and a signed statement from the
   receiver that the proxy was actually acceptable to it.

5.4.1.  Site Information in Delegation Tracing

   In some cases, it may be desirable to know the hosts involved in a
   delegation transaction (for example, a relying party may wish to
   reject proxy certificates that were created on a specific host or
   domain).  An extension could be modified to include the PA's and
   Acceptor's IP addresses; however, IP addresses are typically easy to
   spoof, and in some cases the two parties to a transaction may not
   agree on the IP addresses being used (e.g., if the Acceptor is on a
   host that uses NAT, the Acceptor and the PA may disagree about the
   Acceptor's IP address).

   Another suggestion was, in those cases where domain information is
   needed, to require that the subject names of all End Entities
   involved (the Acceptor(s) and the End Entity that appears in a PC's
   certificate path) include domain information.

6.  Security Considerations

   In this Section we discuss security considerations related to the use
   of Proxy Certificates.

6.1.  Compromise of a Proxy Certificate

   A Proxy Certificate is generally less secure than the EEC that issued
   it.  This is due to the fact that the private key of a PC is
   generally not protected as rigorously as that of the EEC.  For
   example, the private key of a PC is often protected using only file
   system security, in order to allow that PC to be used for single
   sign-on purposes.  This makes the PC more susceptible to compromise.

   However, the risk of a compromised PC is only the misuse of a single
   user's privileges.  Due to the PC path validation checks, a PC cannot
   be used to sign an EEC or PC for another user.



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   Further, a compromised PC can only be misused for the lifetime of the
   PC, and within the bound of the restriction policy carried by the PC.
   Therefore, one common way to limit the misuse of a compromised PC is
   to limit its validity period to no longer than is needed, and/or to
   include a restriction policy in the PC that limits the use of the
   (compromised) PC.

   In addition, if a PC is compromised, it does NOT compromise the EEC
   that created the PC.  This property is of great utility in protecting
   the highly valuable, and hard to replace, public key of the EEC.  In
   other words, the use of Proxy Certificates to provide single sign-on
   capabilities in an X.509 PKI environment can actually increase the
   security of the end entity certificates, because creation and use of
   the PCs for user authentication limits the exposure of the EEC
   private key to only the creation of the first level PC.

6.2.  Restricting Proxy Certificates

   The pCPathLenConstraint field of the proxyCertInfo extension can be
   used by an EEC to limit subsequent delegation of the PC.  A service
   may choose to only authorize a request if a valid PC can be delegated
   to it.  An example of such as service is a job starter, which may
   choose to reject a job start request if a valid PC cannot be
   delegated to it.  By limiting the pCPathLenConstraint, an EEC can
   ensure that a compromised PC of one job cannot be used to start
   additional jobs elsewhere.

   An EEC or PC can limit what a new PC can be used for by turning off
   bits in the Key Usage and Extended Key Usage extensions.  Once a key
   usage or extended key usage has been removed, the path validation
   algorithm ensures that it cannot be added back in a subsequent PC.
   In other words, key usage can only be decreased in PC chains.

   The EEC could use the CRL Distribution Points extension and/or OCSP
   to take on the responsibility of revoking PCs that it had issued, if
   it felt that they were being misused.

6.3.  Relying Party Trust of Proxy Certificates

   The relying party that is going to authorize some actions on the
   basis of a PC will be aware that it has been presented with a PC, and
   can determine the depth of the delegation and the time that the
   delegation took place.  It may want to use this information in
   addition to the information from the signing EEC.  Thus a highly
   secure resource might refuse to accept a PC at all, or maybe only a
   single level of delegation, etc.





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   The relying party should also be aware that since the policy
   restricting the rights of a PC is the intersection of the policy of
   all the PCs in it's certificate chain, this means any change in the
   certificate chain can effect the policy of the PC.  Since there is no
   mechanism in place to enforce unique subject names of PCs, if an
   issuer were to issue two PCs with identical names and keys, but
   different rights, this could allow the two PCs to be substituted for
   each other in path validation and effect the rights of a PC down the
   chain.  Ultimately, this means the relying party places trust in the
   entities that are acting as Proxy Issuers in the chain to behave
   properly.

6.4.  Protecting Against Denial of Service with Key Generation

   As discussed in Section 2.3, one of the motivations for Proxy
   Certificates is to allow for dynamic delegation between parties. This
   delegation potentially requires, by the party receiving the
   delegation, the generation of a new key pair which is a potentially
   computationally expensive operation.  Care should be taken by such
   parties to prevent another entity from performing a denial of service
   attack by causing them to consume large amount of resource doing key
   generation.

   A general guideline would always to perform authentication of the
   delegating party to prevent such attacks from being performed
   anonymously.  Another guideline would be to maintain some state to
   detect and prevent such attacks.

6.5.  Use of Proxy Certificates with a Central Repository

   As discussed in Section 2.7, one potential use of Proxy Certificates
   is to ease certificate management for end users by storing the EEC
   private keys and certificates in a centrally managed repository.
   When a user needs a PKI credential, the user can login to the
   repository using name/password, one time password, etc. and the
   repository would then delegate a PC to the user with proxy rights,
   but continue to protect the EEC private key in the repository.

   Care must be taken with this approach since compromise of the
   repository will potentially give the attacker access to the long-term
   private keys stored in the repository.  It is strongly suggested that
   some form of hardware module be used to store the long-term private
   keys, which will serve to help prevent their direct threat though it
   may still allow a successful attacker to use the keys while the
   repository is compromised to sign arbitrary objects (including Proxy
   Certificates).





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7.  IANA Considerations

   IANA has established a registry for policy languages.  Registration
   under IETF space is by IETF standards action as described in [i8].
   Private policy languages should be under organizational OIDs; policy
   language authors are encouraged to list such languages in the IANA
   registry, along with a pointer to a specification.

   OID                      Description
   ---                      -----------
   1.3.6.1.5.5.7.21.1       id-ppl-inheritALL
   1.3.6.1.5.5.7.21.2       id-ppl-independent

8.  References

8.1.  Normative References

   [n1]    Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

   [n2]    Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
           Public Key Infrastructure Certificate and Certificate
           Revocation List (CRL) Profile", RFC 3280, April 2002.

8.2.  Informative References

   [i1]    Butler, R., Engert, D., Foster, I., Kesselman, C., and S.
           Tuecke, "A National-Scale Authentication Infrastructure",
           IEEE Computer, vol. 33, pp. 60-66, 2000.

   [i2]    Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
           2246, January 1999.

   [i3]    Farrell, S. and R. Housley, "An Internet Attribute
           Certificate Profile for Authorization", RFC 3281, April 2002.

   [i4]    Foster, I., Kesselman, C., Tsudik, G., and S. Tuecke, "A
           Security Architecture for Computational Grids", presented at
           Proceedings of the 5th ACM Conference on Computer and
           Communications Security, 1998.

   [i5]    Foster, I., Kesselman, C., and S. Tuecke, "The Anatomy of the
           Grid: Enabling Scalable Virtual Organizations", International
           Journal of Supercomputer Applications, 2001.

   [i6]    Kohl, J. and C. Neuman, "The Kerberos Network Authentication
           Service (V5)", RFC 1510, September 1993.




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   [i7]    Neuman, B. Clifford, "Proxy-Based Authorization and
           Accounting for Distributed Systems", In Proceedings of the
           13th International Conference on Distributed Computing
           Systems, pages 283-291, May 1993.

   [i8]    Narten, T. and H. Alvestrand. "Guidelines for Writing an IANA
           Considerations Section in RFC", RFC 2434, October 1998.

9.  Acknowledgments

   We are pleased to acknowledge significant contributions to this
   document by David Chadwick, Ian Foster, Jarek Gawor, Carl Kesselman,
   Sam Meder, Jim Schaad, and Frank Siebenlist.

   We are grateful to numerous colleagues for discussions on the topics
   covered in this paper, in particular (in alphabetical order, with
   apologies to anybody we've missed): Carlisle Adams, Joe Bester, Randy
   Butler, Keith Jackson, Steve Hanna, Russ Housley, Stephen Kent, Bill
   Johnston, Marty Humphrey, Sam Lang, Ellen McDermott, Clifford Neuman,
   Gene Tsudik.

   We are also grateful to members of the Global Grid Forum (GGF) Grid
   Security Infrastructure working group (GSI-WG), and the Internet
   Engineering Task Force (IETF) Public-Key Infrastructure (X.509)
   working group (PKIX) for feedback on this document.

   This work was supported in part by the Mathematical, Information, and
   Computational Sciences Division subprogram of the Office of Advanced
   Scientific Computing Research, U.S. Department of Energy, under
   Contract W-31-109-Eng-38 and DE-AC03-76SF0098; by the Defense
   Advanced Research Projects Agency under contract N66001-96-C-8523; by
   the National Science Foundation; and by the NASA Information Power
   Grid project.


















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Appendix A. 1988 ASN.1 Module

   PKIXproxy88 { iso(1) identified-organization(3) dod(6)
       internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
       proxy-cert-extns(25) }

   DEFINITIONS EXPLICIT TAGS ::=

   BEGIN

   -- EXPORTS ALL --

   -- IMPORTS NONE --

   -- PKIX specific OIDs

   id-pkix OBJECT IDENTIFIER ::=
           { iso(1) identified-organization(3)
                dod(6) internet(1) security(5) mechanisms(5) pkix(7) }

   -- private certificate extensions
   id-pe   OBJECT IDENTIFIER ::= { id-pkix 1 }

   -- Locally defined OIDs

   -- The proxy certificate extension
   id-pe-proxyCertInfo    OBJECT IDENTIFIER ::= { id-pe 14 }

   -- Proxy certificate policy languages
   id-ppl  OBJECT IDENTIFIER ::= { id-pkix 21 }

   -- Proxy certificate policies languages defined in
   id-ppl-anyLanguage     OBJECT IDENTIFIER ::= { id-ppl 0 }
   id-ppl-inheritAll      OBJECT IDENTIFIER ::= { id-ppl 1 }
   id-ppl-independent     OBJECT IDENTIFIER ::= { id-ppl 2 }

   -- The ProxyCertInfo Extension
   ProxyCertInfoExtension  ::= SEQUENCE {
         pCPathLenConstraint     ProxyCertPathLengthConstraint
                                       OPTIONAL,
         proxyPolicy             ProxyPolicy }

   ProxyCertPathLengthConstraint  ::= INTEGER
   ProxyPolicy  ::= SEQUENCE {
         policyLanguage          OBJECT IDENTIFIER,
         policy                  OCTET STRING OPTIONAL }

   END



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Authors' Addresses

   Steven Tuecke
   Distributed Systems Laboratory
   Mathematics and Computer Science Division
   Argonne National Laboratory
   Argonne, IL 60439

   Phone: 630-252-8711
   EMail: tuecke@mcs.anl.gov


   Von Welch
   National Center for Supercomputing Applications
   University of Illinois

   EMail: vwelch@ncsa.uiuc.edu


   Doug Engert
   Argonne National Laboratory

   EMail: deengert@anl.gov


   Laura Pearlman
   University of Southern California, Information Sciences Institute

   EMail: laura@isi.edu


   Mary Thompson
   Lawrence Berkeley National Laboratory

   EMail: mrthompson@lbl.gov
















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Full Copyright Statement

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