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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5217

Network Working Group                                        M. Shimaoka
Request for Comments: DRAFT                                        SECOM
<draft-shimaoka-multidomain-pki-04.txt>                      N. Hastings
                                                                    NIST
                                                            January 2005


                      Memorandum for multi-domain
            Public Key Infrastructure (PKI) Interoperability


Status of this Memo

   By submitting this Internet-Draft, each author certifies that any
   applicable patent or other IPR claims of which he or she is aware
   have been disclosed, or will be disclosed, and any of which he or she
   is become aware will be disclosed, in accordance with RFC 3668.  By
   submitting this Internet-Draft, each author accepts the provisions of
   Section 3 of RFC 3667.

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   This Internet-Draft will expire on July 10, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.






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Abstract

   This memo is intended to describe the foundation necessary to the
   deployment of a multi-domain PKI. The scope of this memo is to
   establish and clarify the trust relationships and interoperability
   between multiple PKI domains.  A Certification Authority (CA) is able
   to extend a certification path by establishing trust with other CAs.
   Both single- and multi-domain PKIs are established by such trust
   relationships between CAs.  Typical and primitive PKI model is a
   single-domain PKI that shares the same certificate policy at a
   specified trust level.  A multi-domain PKI is established by
   combining more than one single-domain PKI.  A multi-domain PKI can be
   categorized as either a multi-trust point model based on the trust
   list model; or single-trust point model based on the Cross-
   Certification model.

Table of Contents

   1  Introduction      . . . . . . . . . . . . . . . . . . . . . . . .  2
   2  Requirements and Assumptions    . . . . . . . . . . . . . . . . .  3
   2.1  Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.2  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.3  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3  Trust Relationship  . . . . . . . . . . . . . . . . . . . . . . .  6
   3.1  Operation based Trust Relationship  . . . . . . . . . . . . . .  6
   3.1.1  User Trust List model . . . . . . . . . . . . . . . . . . . .  7
   3.1.2  Authority Trust List model  . . . . . . . . . . . . . . . . .  8
   3.2  Certificate based Trust Relationship  . . . . . . . . . . . . .  8
   3.2.1  Unilateral Cross-Certification  . . . . . . . . . . . . . . .  9
   3.2.2  Mutual Cross-Certification  . . . . . . . . . . . . . . . . . 11
   3.3  Subordination (Hierarchy) . . . . . . . . . . . . . . . . . . . 12
   4  PKI Domain  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.1  Requirements for PKI domain . . . . . . . . . . . . . . . . . . 14
   4.2  Risk Analysis of non interoperable PKI domain . . . . . . . . . 14
   4.3  Requirements for multi-domain PKI interoperability  . . . . . . 14
   5  Single-domain PKI . . . . . . . . . . . . . . . . . . . . . . . . 15
   5.1  Single PKI model  . . . . . . . . . . . . . . . . . . . . . . . 15
   5.2  Hierarchy PKI model . . . . . . . . . . . . . . . . . . . . . . 16
   5.3  Mesh PKI model  . . . . . . . . . . . . . . . . . . . . . . . . 17
   6  multi-domain PKI  . . . . . . . . . . . . . . . . . . . . . . . . 18
   6.1  Multi Trust point model . . . . . . . . . . . . . . . . . . . . 18
   6.1.1  Based on User Trust List    . . . . . . . . . . . . . . . . . 19
   6.1.2  Based on Authority Trust List . . . . . . . . . . . . . . . . 19
   6.2  Single Trust Point model  . . . . . . . . . . . . . . . . . . . 19
   6.2.2  Unified Domain model  . . . . . . . . . . . . . . . . . . . . 20
   6.2.3  Bridge model  . . . . . . . . . . . . . . . . . . . . . . . . 21
   7  Operational Considerations  . . . . . .  .  . . . . . . . . . . . 23
   7.1  Directory . . . . . . . . . . . . . . . . . . . . . . . . . . . 23



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   7.2  Cross-Certification . . . . . . . . . . . . . . . . . . . . . . 24
   8  Security Considerations . . . . . . . .  .  . . . . . . . . . . . 23
   8.1  Certificate and CRL Profile . . . . . . . . . . . . . . . . . . 23
   8.2  Path Validation . . . . . . . . . . . . . . . . . . . . . . . . 24
   8.3  Asymmetric problem  . . . . . . . . . . . . . . . . . . . . . . 25
   8.3.1  Hybrid trust model  . . . . . . . . . . . . . . . . . . . . . 25
   8.3.2  Asymmetric policy mapping . . . . . . . . . . . . . . . . . . 25
   9  References  . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   9.1  Normative References  . . . . . . . . . . . . . . . . . . . . . 26
   9.2  Informative References  . . . . . . . . . . . . . . . . . . . . 26
   10  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 27
   11 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 27
   12  Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 27

1  Introduction

   PKI is extendable to realize various architectures, through the way
   in which CAs establish trust relationships with each other.  When a
   CA wishes to establish a trust relationship with another CA, the CAs
   MUST compare the security requirements defined in their certificate
   policies since certificate policies vary greatly across CAs.  Those
   CAs should choose an appropriate trust relationship which satisfies
   both security requirements, as a result of that comparison.  To
   establish appropriate trust relationships, full understanding of the
   relationship between the establishment method and comparison is
   required.  In addition, all established trust relationships fall into
   one of two categories: a single- or multi-domain PKI.  In order to
   establish trust relationships between CAs, technology, such as
   protocol specifications and data formats, alone is insufficient.  The
   existing protocol specifications and data formats do not define the
   PKI architectures and boundary of the PKI domains, designing them are
   left up to the individual PKIs.  Therefore, an understanding of the
   CAs' PKI architectures and domains are required to determine the
   appropriateness of establishing the trust relationship.  This
   document clarifies these definitions for multi-domain PKI
   interoperability.

   Section 2 describes the terminology necessary to consider multi-
   domain PKI.  Section 3 categorizes the trust relationships between
   CAs as Trust List, Cross-Certification, and Subordination.  Section 4
   defines a PKI domain and requirements for multi-domain
   interoperability.  Section 5 defines major models necessary to
   establish single-domain PKIs.  Section 6 profiles multi-domain PKIs
   as multi-trust point model and single-trust point model.  Multi-trust
   point model is based on trust list model.  Single-trust point model
   is based on the cross-certification model, and is categorized as peer
   model, unified domain model and hub model.  Finally, section 7
   describes considerations focused on Certificate and Certificate



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   Revocation List (CRL) profiles, Repositories, and path validation.

     +------------------+               +-------------------+
     |    PKI domain    |               |     PKI domain    |
     |                  | Domain-Domain |                   |
     |                  |    Trust      |                   |
     | +-----+          | Relationship  |  +-----+          |
     | | PCA |<===========================>| PCA |          |
     | +-----+          |               |  +-----+          |
     |   ^              |               |    ^              |
     |   | CA-CA Trust  |               |    | CA-CA Trust  |
     |   | Relationship |               |    | Relationship |
     |   v              |               |    v              |
     | +----+           |               |  +----+           |
     | | CA |           |               |  | CA |           |
     | +----+           |               |  +----+           |
     +------------------+               +-------------------+

                Figure 1 - Structure of multi-domain PKI

2  Requirements and Assumptions

   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.

2.1  Abbreviations

   PKI:  Public Key Infrastructure

   CA:   Certification Authority

   EE:   End Entity

   CRL:  Certificate Revocation List

   ARL:  Authority Revocation List

   PCA: Principal Certificate Authority

   RP: Relying Party

   CP: Certificate Policy

   CPS: Certification Practice Statement

   DN: Distinguished Name




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2.2  Terminology

   Subscriber

     Holder of the verified certificate.

   End Entity (EE)

     The definition of EE prefers X.509 4th edition to RFC 2828, since
     X.509 defines more clearly about relying party.  That is, a
     certificate subject that uses its private key for purposes other
     than signing certificates or an entity that is a relying party
     [sic].

   Relying Party (RP)

     Entity who verifies the certificate.  RP MUST have one or more
     trust anchors and MAY have a set of validation policies.  In
     single-domain PKI, these MAY be omitted implicitly.

   Responsible EE

     Responsible EE directly trusts the CA that issued a self-signed
     certificate used as a trust anchor and has been issued a
     certificate by the CA under its subscriber agreement.  Responsible
     EE assumes a responsibility based on a contract with the trust
     anchor, and has some rights from the trust anchor.  For example,
     the responsible EE is issued its certificate by the contract with
     the trust anchor, and must specify a certain validation policy in
     the path validation.

   Irresponsible EE

     Against a responsible EE, Irresponsible EE trusts directly the
     trust anchor but has no contract with the trust anchor.  In other
     words, irresponsible EE trusts the trust anchor one-sidedly.

   Subscriber

     An entity that is the subject identified within a public key
     certificate issued by a certificate authority of a PKI.

   Subscriber Agreement

     A document used to describe the rights, obligations, and
     responsibilities of a subscriber to a PKI. This may take the form
     of a contract.




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   Intermediate Certificate

     Certificates in a certification path except the trust anchor and
     target certificate.

   PKI domain

     The word "PKI domain" is used to represent and identify a set of
     PKIs which share the same trust level.  The scale of the PKI domain
     MAY vary by the required trust level.  For example, a PKI domain
     which requires the high trust level may be quite narrow and a PKI
     domain which requires the low trust level may be broad.  PKI domain
     is established by the specific trust relationship which is achieved
     by the agreement on the certificate policy representing the shared
     trust level.

   Domain Policy

     A common certificate policy or agreed upon certificate policy that
     are shared in a PKI domain.  When the PKIs in a PKI domain can
     share the multiple certificate policies of different levels, there
     can be multiple domain policies within the PKI domain.  The Object
     Identifier(s) belonging to a PKI domain is used to distinguish that
     PKI domain from another.  A PKI domain having no certificate policy
     MAY not be identified by the relying party in another PKI domain.

   Top CA

     Only CA that is a root in Hierarchy PKI model.  Top CA MUST issue a
     self-signed certificate.  Top CA SHOULD be used for Hierarchy PKI
     model.  For unified domain model, unificate CA SHOULD be used as
     defined later in this section.

   PKI domain

     PKI domain is a set of PKIs for identifying the PKI operated on the
     same certificate policy.  Such certificate policy is called a
     "domain policy".  PKI domain MUST have one or more principal CAs
     and SHOULD have one or more domain policy.

   Domain Policy

     A common certificate policy (Object Identifier) that is shared in a
     PKI domain.  There can be multiple domain policies in a PKI domain.
     The Object Identifier(s) belonging to a PKI domain is used to
     distinguish that PKI domain from another.  A PKI domain having no
     certificate policy MAY not be identified by the relying party in
     another PKI domain.



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   Trust Anchor

     Starting point of a certification path to a subscriber certificate,
     which is specified by a relying party.  If a relying party has to
     perform a validation of the trust anchor, it SHOULD be verified by
     some trustworthy out-of-band procedure, and is not within the scope
     of this memo.  In addition, trust anchor SHOULD be a CA issuing a
     self-signed certificate for an operational reason, which is capable
     of verifying easily the binding of the private key and the public
     key.

   Posterior PKI domain

     This word is used to describe the relative trust relationship of
     adjoined PKI domain in the certification path, in combination with
     the word "prior PKI domain".  This is a posterior (next) PKI domain
     in the certification path from the trust anchor to the target
     certificate.  That is, the posterior PKI domain is trusted from the
     prior PKI domain.

   Prior PKI domain

     This word is used to describe the relative trust relationship of
     adjoined PKI domain in the certification path, in combination with
     the word "posterior PKI domain".  This is a prior (previous) PKI
     domain in the certification path from the trust anchor to the
     target certificate.  That is, the prior PKI domain trusts the
     posterior PKI domain.  First prior PKI domain in the certification
     path means the PKI domain trusted directly by a relying party.

   Trust Relationship

     Generally, trust relationship in PKI means a relationship between
     each CA or between CA and EE.  In this document however, this means
     a trust relationship between CAs.  This relationship is required
     for tracing trust from a trust anchor to a subscriber.

   Validation policy

     The concept of this is defined in RFC 3379. In this document, the
     items which should be focused on are the following.
        (c) user-initial-policy-set
        (d) trust anchor information,
        (e) initial-policy-mapping-inhibit
        (f) initial-explicit-policy
        (g) initial-any-policy-inhibit





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     These are parts of input for the path validation, which is defined
     in RFC 3280 section 6.1.1.  The reason why this document focuses on
     these items is that these values may be different depending on each
     trust anchor.

   Principal CA

     CA which has a self-signed certificate and is trusted from the
     other PKI domain.  The reason why a principal CA has a self-signed
     certificate is the principal CA must be independent from all other
     certification including inside of its PKI domain.

   Unificate CA

     CA which has a self-signed certificate and issues unilateral cross-
     certificates to each principal CA of other posterior PKI domains.
     Unificate CA is specified as a trust anchor for the PKI domains
     that are cross-certified with it.

   Trust List

     Trust list is a list of one or more trust anchors, which MAY be a
     set of the trust anchor certificates in general.  Otherwise, it MAY
     be a set of public keys or Distinguished Names.  Trust list is used
     for specifying a trust anchor by a relying party.

   Subordination

     Subordination is a mechanism to authorize the existence of a
     subject CA.  The authorization of the existence means issuing a
     certificate called esubordinate (CA) certificatef to a CA who has
     no certificate.

   Cross-Certification

     Cross-certification is a mechanism to recognize the existence of a
     subject CA.  The recognition of the existence means issuing a
     certificate called ecross-certificatef to a CA who has another
     certificate already.  In this document, this wording is more
     limited than the definition in RFC 2828.  That is, cross-
     certification in RFC 2828 means mutual cross-certification, but
     cross-certification in this document allows unilateral cross-
     certification.

2.3  Assumptions

   In this document, each PKI MUST have a repository for supporting the
   path validation, but this document does not specify whether the



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   repository is web server or directory server.

3  Trust Relationship

   This section describes major trust relationships for multiple PKI(CA)
   interconnections.  All PKIs that are going to participate in multi-
   domain PKI SHOULD use these trust relationships for multi-domain PKI
   interoperability.

3.1 Operation based Trust Relationship

   Definition

     Trust List is defined in terminology section 2.2.

   Requirements

     CAs on the same trust list SHOULD NOT cross-certify each other.
     All relying parties in this model MUST have a trust list.  There
     SHOULD be different validation policies for every trust anchor.

   Considerations

     A relying party using the trust list MAY trust multiple trust
     anchors, but finding out a revocation of each trust anchor is more
     difficult than finding out it for one.

                               Trust List
     +--------------------------------------------------------------+
     |                         Trusted CA                           |
     |                                                              |
     | +---------------+ +---------------+ +----------------------+ |
     | |     PKI 1     | |     PKI 2     | |         PKI 3        | |
     | |               | |               | |                      | |
     | |       +-----+ | | +-----+       | | +-----+              | |
     | |   +---| PCA | | | | PCA |       | | | PCA |<--+          | |
     | |   |   +-----+ | | +-----+       | | +-----+   |          | |
     | |   |      |    | |    |          | |   ^       |          | |
     +-----|------|-----------|----------------|-------|------------+
       |   |      |    | |    |          | |   |       |          |
       |   |      |    | |    |          | |   |       v          |
       |   |      |    | |    |          | |   |     +----+       |
       |   |      |    | |    |          | |   |     | CA |---+   |
       |   |      |    | |    |          | |   |     +----+   |   |
       |   |      |    | |    |          | |   |      ^ |     |   |
       |   |      |    | |    v          | |   v      | |     |   |
       |   |      |    | | +----+        | | +----+   | |     |   |
       |   |      |    | | | CA |---+    | | | CA |---+ |     |   |



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       |   |      |    | | +----+   |    | | +----+     |     |   |
       |   |      |    | |   |      |    | |   |        |     |   |
       |   |      |    | |   |      |    | |   |        |     |   |
       |   v      v    | |   v      v    | |   v        v     v   |
       | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ |
       | | EE | | EE | | | | EE | | EE | | | | EE | | EE | | EE | |
       | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ |
       +---------------+ +---------------+ +----------------------+

                      Figure 2 - Trust List model

3.1.1  User Trust List model

   Definition

     The model in which a trust list is managed by End Entities (EEs).
     Each EE is able to have its own user trust list.


   Characteristics

     EE is able to manage its own user trust list.  EE is able to add or
     delete a trust anchor from its own user trust list.  This is an
     easier and typical method for making a trust relationship with
     another PKI.

     Except for EE itself, no one is able to control the trust
     relationship.  There is a risk that EE trusts unknown PKI domain
     irresponsibility.  If EE trusts unknown PKI domains irresponsibly,
     then its issuer CA cannot apply its certificate policy to the EE.
     A trust anchor MAY not apply its validation policy to the EE.

   Considerations

     To consider how to update the user trust list, when a CA
     certificate in the user trust list is updated.

3.1.2  Authority Trust List model

   Definition

     The model in which a trust list is managed by the trust anchor of
     relying party.  The trust anchor MAY issue multiple trust lists for
     some purposes or parties.  EEs trusting the same trust anchor may
     share the authority trust list given by the trust anchor.

   Characteristics




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     EE does not have control over any trust relationships from its
     trust anchor.  Trust anchor SHOULD control an appropriate trust
     relationship with other CAs keeping the same security level.

   Considerations

     Since there is no standard for the use of this model, management
     methods for authority trust list are not established.  In
     generally, this model MAY not achieve sufficient interoperability.

3.2  Certificate based Trust Relationship

   Certificate based trust relationship is realized by the cross-
   certification unilaterally or mutually.

   Definition

     Cross-certification is defined in terminology section 2.2.

   Requirement

     A subject CA in the cross-certification MUST have a certificate
     other than the cross-certificate.

   Characteristics

     Cross-Certification is a more formal expression of the trust
     relationship than the trust list model, because the trust
     relationship is represented by a certificate and (authority)
     revocation list and is recorded to an audit log.  Cross-
     certification is able to manage the trust relationship without
     changing the trust list of EEs.  Because all subject CAs have a
     self-signed certificate, revoking a cross-certificate does not
     always mean also compromising the subject CA.

     A PKI which issues a cross-certificate SHOULD have a repository and
     CA SHOULD publish the cross-certificate to a relying party.  The
     formal way to publish the cross-certificate is to use the
     crossCertificatePair attribute with the directory server like a
     LDAP.  At least, a PKI which issues a cross-certificate MUST
     provide a mechanism for obtaining the cross-certificate to a
     relying party, like a caIssuers in accessMethod of the AIA
     extension.

   Considerations

     A subject CA which has not a self-signed certificate has been
     authorized by another CA.  This means the issuer CA of the cross-



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     certificate may not be able to recognize the existence of the
     subject CA.  Therefore, this document strongly recommends that a
     subject CA SHOULD have a self-signed certificate.

     Especially for the inter-domain cross-certification, this document
     recommends that issuer CA SHOULD accept and agree on the way the
     subject CA is operated.

     For path construction

        Because the key identifier of each CA MAY be calculated
        differently, subject CA SHOULD issue a cross-certification
        request that contains subjectKeyIdentifier in extensionRequest,
        with a value that MUST be identical to the subjectKeyIdentifier
        in the self-signed certificate.  Then, issuer CA SHOULD issue a
        cross-certificate with the subjectKeyIdentifier set to the same
        value in the corresponding cross-certification request.

     For PKI issuing Revocation List

        Issuing CAs MAY issue Authority Revocation Lists (ARLs), or
        SHOULD at least issue fullCRLs.  However, ARL with an
        issuingDistributionPoint extension MAY NOT be processed by some
        applications.

3.2.1  Unilateral cross-certification

   Definition

     The model in which a CA issues a cross-certificate unilaterally to
     another CA which has a self-signed certificate.

   Characteristics

     This certification is used like subordination, but is able to
     establish a more flexible trust relationship than subordination;
     even if the cross-certificate is revoked, subject CA MAY be able to
     continue its operation.

     If the PKI uses a directory system, the CA MUST populate a
     crossCertificatePair, even when the cross-certification is
     unilateral, to avoid being categorized as subordination.

   Considerations

     Subordination is a special case of unilateral cross-certification.
     Note that unilateral cross-certification is easily established
     without an agreement from the subject CA because a cross-



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     certificate can be issued from the public key of the subject CA.

     In the example of figure 3 below, RPs who use the PCA of PKI 1 as
     the trust anchor can trust EEs of PKI 2 not only EEs of PKI 1.  And
     RPs who use the PCA of PKI 2 as the trust anchor cannot trust EEs
     of PKI 1.  This configuration means helping RPs who use PCA of PKI
     1 to interoperate with EEs of PKI 2, but not vice versa.

        +---------------+                 +----------------------+
        |     PKI 1     |                 |         PKI 2        |
        |               | cross-certified |                      |
        | +-----+       | PKI 1 to PKI 2  |  +-----+             |
        | | PCA |--------------------------->| PCA |<--+         |
        | +-----+       |                 |  +-----+   |         |
        |    |          |                 |    ^       |         |
        |    |          |                 |    |       v         |
        |    |          |                 |    |    +----+       |
        |    |          |                 |    |    | CA |---+   |
        |    |          |                 |    |    +----+   |   |
        |    |          |                 |    |     ^ |     |   |
        |    v          |                 |    v     | |     |   |
        | +----+        |                 | +----+   | |     |   |
        | | CA |---+    |                 | | CA |---+ |     |   |
        | +----+   |    |                 | +----+     |     |   |
        |   |      |    |                 |   |        |     |   |
        |   |      |    |                 |   |        |     |   |
        |   v      v    |                 |   v        v     v   |
        | +----+ +----+ |                 | +----+ +----+ +----+ |
        | | EE | | EE | |                 | | EE | | EE | | EE | |
        | +----+ +----+ |                 | +----+ +----+ +----+ |
        +---------------+                 +----------------------+

               Figure 3 - Unilateral Cross-Certification

3.2.2  Mutual cross-certification

   Definition

     The model in which a self-signed CA issues a cross-certificate to
     the other self-signed CA, and vice versa.

   Characteristics

     Both CAs cross-certify with each other mutually.

     For PKI using directory system

        Both CAs MUST generate a crossCertificatePair that consists of



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        the cross-certificate it issued to the other CA and the
        corresponding cross-certificate that it was issued by the other
        CA.  When either CA updates a cross-certificate, each CA MUST
        re-generate their crossCertificatePair synchronously.

        If re-generating asynchronously, the result of the path
        validation may differ by the method of path building, such as
        forward path building and reverse path building.

   Considerations

     Both CAs MUST accept upon more information in order to issue a
     cross-certificate (e.g., validity, keyUsage, and constraints) and
     MUST exchange the information.

        +---------------+                 +----------------------+
        |     PKI 1     |                 |         PKI 2        |
        |               | cross-certified |                      |
        | +-----+       | PKI 1 and PKI 2 |  +-----+             |
        | | PCA |<-------------------------->| PCA |<--+         |
        | +-----+       |                 |  +-----+   |         |
        |    |          |                 |    ^       |         |
        |    |          |                 |    |       v         |
        |    |          |                 |    |    +----+       |
        |    |          |                 |    |    | CA |---+   |
        |    |          |                 |    |    +----+   |   |
        |    |          |                 |    |     ^ |     |   |
        |    v          |                 |    v     | |     |   |
        | +----+        |                 | +----+   | |     |   |
        | | CA |---+    |                 | | CA |---+ |     |   |
        | +----+   |    |                 | +----+     |     |   |
        |   |      |    |                 |   |        |     |   |
        |   |      |    |                 |   |        |     |   |
        |   v      v    |                 |   v        v     v   |
        | +----+ +----+ |                 | +----+ +----+ +----+ |
        | | EE | | EE | |                 | | EE | | EE | | EE | |
        | +----+ +----+ |                 | +----+ +----+ +----+ |
        +---------------+                 +----------------------+

                 Figure 4 - Mutual Cross-Certification

3.3  Subordination (Hierarchy)

   Subordination is a special unilateral cross-certification.

   Definition

     The model in which a CA issues a certificate to a CA which has no



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     self-signed certificate.  The model in which a PKI has always only
     one root CA.

   Requirements

     A subordinate CA MUST have only one superior CA and be managed by
     the superior CA strictly.  A subordinate CA MUST never issue its
     self-signed certificate.

   Characteristics

     EEs can trust all following subordinate CA and its EEs by trusting
     only the root CA.  Subordination is different from unilateral
     cross-certification, in that this model MUST NOT allow a
     subordinate CA to be issued a certificate by more than one issuer
     CAs.  A subordinate CA MAY NOT require an accreditation
     necessarily, such as WebTrust or license under the e-signature law.
     The accreditation is rather required only for the superior CA or
     the root CA.  In this case, accreditation means that the
     subordinate CA can succeed the benefit on the trustworthiness of
     the superior CA.  An existence of the subordinate CA is dependent
     on the superior CA.  A subordinate CA is able to inherit some
     policies and constraints from its superior CA.  Because a
     subordinate CA has an explicit trust relationship with its superior
     CA, the subordinate CA is able to be trusted easily by all EEs who
     trust the superior CA.

     Subordinate CAs MUST NOT cross-certify with another PKI domains,
     but MAY just allow a subordination within the same PKI domain.
     When a subordinate CA certificate is revoked by a superior CA, all
     certificates issued by the subordinate CA are also invalid.

   Considerations

     A subordinate CA MUST NOT override the constraints given by the
     superior CA.  Subordination MUST be used only in single-domain PKI,
     not multi-domain PKI.

     The violation with issuing a self-signed certificate to a
     subordinate CA may be considered as the following two cases.  If
     the subordinate CA issues a self-signed certificate, and if RPs
     change their trust anchor from the original root CA to the former
     subordinate CA, the entities which the RPs can trust will shrink to
     only under the former subordinate CA.

     If the subordinate CA issues a self-signed certificate, but if RPs
     do not change their trust anchor (original root CA), the entities
     which the RPs can trust will still be same but the trust



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     relationship will change from the subordination model into the
     unified domain model.

4  PKI Domain
4.1  Requirements for PKI domain

   PKIs in a PKI domain SHOULD share one or more certificate policy, and
   the PKI domain MUST have a principal CA.  This shared policy is
   called the "domain policy".  The domain policy SHOULD be described in
   the policyIdentifier of the certificate policies extension for each
   certificate.

   All CAs in a PKI domain MUST be operated under ACPS that conforms to
   the domain policy.  All CAs in a PKI domain MUST be able to issue a
   certificate including a valid policy.

4.2  Risk Analysis of non-interoperable PKI domain

   A PKI domain that satisfies the requirements presented in the
   sections ??? of this document MAY be used in the multi-trust point
   model.  However, such requirements are insufficient for the single-
   trust point model.  To use a PKI domain under the single-trust point
   model, more requirements are necessary.

   Therefore, such a PKI domain SHOULD NOT be used in single-trust point
   model.  If such a PKI domain makes the single-trust point model, the
   following problems will be considered:

     - A lack of the PKI Domain identification method for the third
     party

        All certificates in the PKI domain MAY NOT have the
        identification information of the PKI domain.  Distinguished
        Name cannot be used as the identity for the PKI domain, because
        no one administers the name space.

     - Case in which a PKI domain is not trusted by another PKI domain

        When a relying party specifies a certificate policy as one of
        the validation policies, the certification path validation MAY
        fail, because the policy of the relying party is incapable of
        mapping to an appropriate certificate policy.

     If a PKI domain interconnects to another PKI domain, In addition to
     the above requirements in section 4.1, the following consideration
     is necessary.

     - Conflict of a name space



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        The name constraints extension MAY not perform the constraint
        that PKI intended, if no one manages a name space.

     - Policy management

        When validating a certification path crossing the PKI domains,
        relying party MAY identify the PKI domains by referring the
        certificate policies extensions.  If the domain policy is not
        described in the certificate policies extension, the path
        validation MAY fail.  Especially the domain policy is necessary
        in the path validation through the PKI that use some constraints
        or policy mapping.

     - Authority constrained

        A CA that wants to assert constraints for the certification path
        MUST include explicitly the extensions for the constraints in
        the certificates that the CA issues, since that CA assumes the
        validation policy used by a relying party which MAY NOT be under
        the CAfs control.

        For example; Assume the CA-X expects its RPs to evaluate an
        appropriate certificate policy in the path validation.  Even if
        the CA-X expects its RP to set the initial-explicit-policy flag
        to TRUE, there is no guarantee that RP sets the flag to TRUE
        because there are responsible EE and irresponsible EE.  A
        responsible EE may set the flag to TRUE, but an irresponsible EE
        may not set it.  Therefore, CA-X SHOULD issue a certificate
        which uses requireExplicitPolicy explicitly in the
        policyConstraints extension.  If CA-X issues all certificates
        which use requireExplicitPolicy in the policyConstraints
        extension, RP MUST evaluate the certificate policy whether it
        has responsibility or not.

4.3  Requirements for multi-domain PKI interoperability

   In multi-domain PKI, there MAY be a PKI domain that assumes requiring
   the explicit policy.  To validate correctly such certification path,
   the following requirements are necessary for the PKI domains:

     - All CAs in a PKI domain that has explicit domain policy as
     policyIdentifier SHOULD be able to issue a certificate which is
     verifiable with the following validation policy:
        * user-initial-policy-set which includes its own domainPolicyId.
        * initial-explicit-policy set to TRUE.
        * trust anchor which is the principal CA of its PKI domain.

     - Each PKI domain SHOULD show the trust relationship with other PKI



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     domains as follows:

        * Posterior PKI domain X SHOULD show its PKI architecture to the
        prior PKI domain Y, because the trust relationship from the PKI
        domain Y to the PKI domain X may depend on such PKI
        architecture.
        * Posterior PKI domain X should show all PKI domains that it
        trusts to the prior PKI domain Y, because the prior PKI domain Y
        MUST consider not to trust an unnecessary PKI domain.
        * Posterior PKI domain X MAY publish what kind of all PKI domain
        it is trusted by to the prior PKI domain Y, because the PKI
        domain Y may consider the other certification path to the PKI
        domain X.

   In addition, the following requirements MAY be necessary for the
   certificate based trust relationship.

     SHOULD give an appropriate policy mapping between the prior PKI
     domain and the posterior PKI domain for certificate based trust
     relationship.

5  Single-domain PKI

   This section describes appropriate PKI architectures for establishing
   a single PKI domain.  All PKIs that are going to participate in
   multi-domain PKI SHOULD adopt any of the following models for multi-
   domain PKI interoperability.

5.1  Single PKI model

   This is the simplest PKI model. All PKI models are composed of this.
   It is similar to a special case of a hierarchical PKI which has no
   subordinate CAs.

   Definition

     Single PKI consists of a single self-signed CA and its EEs.  All
     EEs MUST be issued their certificates by the only CA.

   Trust anchor

     The trust anchor MUST be the self-signed certificate of the CA.

                                +----+
                            +---| CA |---+
                            |   +----+   |
                            |      |     |
                            |      |     |



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                            v      v     v
                         +----+ +----+ +----+
                         | EE | | EE | | EE |
                         +----+ +----+ +----+

                      Figure 5 - Single PKI model

5.2  Hierarchy PKI model

   This is a typical architecture of PKI.

   Definition

     Hierarchy PKI consists of a single root CA, a number of subordinate
     CAs, and EEs.  Only the root CA MUST issue a self-signed
     certificate.  All subordinate CAs MUST have only one superior CA.

   Trust anchor

     Trust anchor MUST be the root CA.  All EEs SHOULD trust only the
     root CA.

                            +---------+
                        +---| top CA  |---+
                        |   +---------+   |
                        |                 |
                        |                 |
                        v                 v
                     +----+            +----+
               +-----| CA |      +-----| CA |------+
               |     +----+      |     +----+      |
               |                 |                 |
               v                 v                 v
            +----+            +----+            +----+
         +--| CA |-----+      | CA |-+      +---| CA |---+
         |  +----+     |      +----+ |      |   +----+   |
         |     |       |       |     |      |    |       |
         |     |       |       |     |      |    |       |
         v     v       v       v     v      v    v       v
      +----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
      | EE | | EE | | EE | | EE | | EE | | EE | | EE | | EE |
      +----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+

                     Figure 6 - Hierarchy PKI model

5.3  Mesh PKI model

   Definition



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     Mesh PKI consists of multiple CAs and their EEs.  All CAs MUST
     cross-certified with more than one CA unilaterally.  Some CAs MAY
     cross-certify mutually.

   Trust anchor

     The trust anchor for a relying party who is issued a certificate
     from a CA in the mesh PKI SHOULD be a CA who issued it a
     certificate.  The trust anchor for the other relying party who is
     not issued a certificate from the mesh PKI MAY be any CA in the
     mesh PKI.

   Considerations

     A trust anchor which has not a self-signed certificate means that
     trust anchor is authorized by another CA.  In such a case, this
     document recommends that relying party SHOULD trust the CA which
     authorizes that trust anchor.  If there is no self-signed CA in
     that mesh, such as what all CAs in the mesh certify with each
     other, relying party SHOULD choose a trust anchor from those CAs
     carefully.

     This model SHOULD be used sparingly, because of the complexity in
     certification path building.  However, one should not assume that
     this model does not exist or is implemented.  Full Mesh, which is
     that all CAs in the PKI mutually cross-certify each other directly,
     PKI MAY be useful conversely for the certification path building,
     because it is able to reach to the prior PKI domain with one path.

             cross certified  +-------+  cross certified
            +---------------->|  CA   |<----------------+
            |                 +-------+                 |
            |                  |     |                  |
            |                  |     |                  |
            |                  v     v                  |
            |               +----+ +----+               |
            |               | EE | | EE |               |
            |               +----+ +----+               |
            v                                           v
         +------+                                   +------+
         |  CA  |<--------------------------------->|  CA  |-----+
         +------+          cross certified          +------+     |
          |     |                                    |    |      |
          |     |                                    |    |      |
          v     v                                    v    v      v
      +----+ +----+                              +----+ +----+ +----+
      | EE | | EE |                              | EE | | EE | | EE |
      +----+ +----+                              +----+ +----+ +----+



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                       Figure 7 - Mesh PKI model

6  multi-domain PKI

   Each PKI domain establishes a trust relationship with more than one
   PKI domain.

   This section describes topology models for multi-domain PKI.  To
   achieve interoperability, all PKIs in a multi-domain PKI SHOULD apply
   the following models.

   Considerations

     Multi-domain PKI MAY need policy mapping or constraints to maintain
     each domain policy.  All required information for path validation
     MUST be able to be obtained through some distribution methods.

        - Intermediate certificate
        - Target certificate (optional)
        - Revocation information for all certificates

     For this, CAs MAY operate a repository, and SHOULD include
     authorityInfoAccess or cRLDistributionPoints extensions in the
     certificates they issue to maximize PKI interoperability.

6.1  Multi Trust point model (based on Trust List)

   The model in which a relying party trusts multiple PKI domains by a
   trust list.

   Considerations

     If the owner of the trust list add a CA in the existing
     certification path, it SHOULD do carefully since a constraints in
     the certification path MAY NOT be evaluated correctly.  The reason
     is the following:

        Assumes that the certification path X->Y->Z->EE(Z) exists.  When
        cross-certificate X->Y includes pathLenConstraints=1, CA-Z
        cannot extend the certification path started from CA-X by more
        cross-certificate, and a relying party who trusts CA-Z as the
        trust anchor cannot trust beyond CA-Z.  In that case, if the
        relying party trusts CA-Y directly, the constraint is ignored.
        Thus, relying party MUST recognize a risk of trusting another CA
        directly.

     Most of the actual public PKIs establish a multi-trust point model
     without a domain policy.  When using such public PKI, this document



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     recommends:

        - user-initial-policy-set SHOULD NOT be specified, - and
        initial-explicit-policy SHOULD NOT be true.

   In general, since it is difficult for the EE to check if a CA's self-
   signed certificate has been revoked, a CA SHOULD announce it to all
   its EEs when the CA is compromised and MAY issue the CRL, and so on.
   Anyway CA SHOULD do the best.  In the multi-trust point model, a
   compromised trust anchor SHOULD be removed from the trust list, and
   the removal SHOULD be performed by the subject managing the trust
   list.

6.1.1  Based on User Trust List

   Considerations

     This is an easier and typical method for making a trust
     relationship to another PKI domain.  The relying party MUST
     understand the certificate status of the trust anchor in the trust
     list.

6.1.2  Based on Authority Trust List

   Since there is no standard or established method to achieve
   interoperability, this memo does not recommend using this model in
   multi-domain PKI.

6.2  Single Trust Point model (based on Cross-Certification)

   The model in which all PKI domains are related by Cross-
   Certification.  This cross-certification is either mutual or
   unilateral.  In this model, only one trust anchor is required by EEs.

   Considerations

     Each PKI domain MAY use policy mapping for crossing different PKI
     domains.  If a PKI domain wants to restrict a certification path,
     the PKI domain SHOULD NOT rely on the validation policy of the
     relying party, but SHOULD include the constraints in the cross-
     certificate explicitly.

     For example, when each PKI domain wants to effect the constraints
     to a certification path, it SHOULD set the requireExplicitPolicy to
     zero in the policyConstraints extension of any cross-certificates.
     A PKI domain that relies on the validation policy of the relying
     party about such constraints can not effect the constraints always.




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6.2.2  Unified Domain model (based on unilateral Cross-Certification)

   The model in which multiple PKI domains have a joint superior CA that
   issues cross-certificates to each PKI domain unilaterally.  Such a
   joint superior CA is defined as unificate CA.  This model is used as
   a method to unify or fake the multiple PKI domains to one PKI domain,
   or is used as a method for transforming from subordination.  Except
   for that there are CAs who have both self-signed certificates and
   intermediate certificates issued by the Unificate CA, this model
   looks like a subordination model since unificate CA is a trust anchor
   across the PKI domains.  Therefore, often this model is used like the
   hierarchy model in multi-domain PKI.

        cross-certified                        cross-certified
     Unificate CA to PKI 1 +--------------+  Unificate CA to PKI 3
                 +---------| Unificate CA |---+
                 |         +--------------+   |
                 |                 |          |
                 |  cross-certified|          |
                 |   Unificate CA  |          |
                 |    to PKI 2     |          |
     +-----------|---+ +-----------|---+ +----|-----------------+
     |     PKI 1 |   | |     PKI 2 |   | |    |    PKI 3        |
     |           v   | |           v   | |    v                 |
     |       +-----+ | |       +-----+ | | +-----+              |
     |   +---| PCA | | |       | PCA | | | | PCA |<--+          |
     |   |   +-----+ | |       +-----+ | | +-----+   |          |
     |   |      |    | |          |    | |   ^       |          |
     |   |      |    | |          |    | |   |       v          |
     |   |      |    | |          |    | |   |     +----+       |
     |   |      |    | |          |    | |   |     | CA |---+   |
     |   |      |    | |          |    | |   |     +----+   |   |
     |   |      |    | |          |    | |   |      ^ |     |   |
     |   |      |    | |          v    | |   v      | |     |   |
     |   |      |    | |       +----+  | | +----+   | |     |   |
     |   |      |    | |   +---| CA |  | | | CA |---+ |     |   |
     |   |      |    | |   |   +----+  | | +----+     |     |   |
     |   |      |    | |   |      |    | |   |        |     |   |
     |   |      |    | |   |      |    | |   |        |     |   |
     |   v      v    | |   v      v    | |   v        v     v   |
     | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ |
     | | EE | | EE | | | | EE | | EE | | | | EE | | EE | | EE | |
     | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ |
     +---------------+ +---------------+ +----------------------+

                    Figure 8 - Unified Domain model

6.2.3  Bridge model



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   The model in which every PKI domain trust each other through a Bridge
   CA by Cross-Certification.  In this model, trust relationship is not
   established between a subscriber domain and a relying party domain
   directly, but established through the Bridge CA.  This is useful in
   reducing the number of cross-certification.

   Requirements for Bridge model

     - Bridge CA MUST NOT be used as the trust anchor in any PKI domain.
     - Bridge CA SHOULD issue cross-certificates with other PKI domains
     mutually or MAY issue cross certificates unilaterally.
     - Bridge CA MUST NOT issue EE certificates except when it is
     necessary for the CA's operation.
     - Bridge CA MUST use its own domain policy in the policy mapping
     between a prior PKI domain and a posterior PKI domain.
     - The domain policy of Bridge CA MUST be a subset of the prior PKI
     domain policy that is mapped.
     - The domain policy of Bridge CA MUST be a superset of the
     posterior PKI domain policy that is mapped.

        Cross-Certificate from prior PKI domain to Bridge CA
          issuerDomainPolicy := Prior PKI domain policy
          subjectDomainPolicy := Bridge CA domain policy

        Cross-Certificate from Bridge CA to posterior PKI domain
          issuerDomainPolicy := Bridge CA domain policy
          subjectDomainPolicy := Posterior PKI domain policy

     - Cross-Certificates issued by Bridge CA and Cross-Certificate
     issued to Bridge CA SHOULD include the requireExplicitPolicy with a
     value that is greater than zero in the policyConstaints extension.
     - Cross-certificate issued to Bridge CA SHOULD include the
     requireExplicitPolicy with a value that is greater than zero in the
     policyConstratints extension.
     - Cross-certificate issued by Bridge CA SHOULD NOT include any
     constraints to keep its transparency.
     - PKI domains cross-certified with Bridge CA SHOULD NOT cross-
     certify directly to other PKI domains cross-certified with the same
     Bridge CA.
     - Bridge CA SHOULD clarify the method for the policy mapping of
     cross-certification to keep its transparency.

   Considerations

     The Bridge CA SHOULD be operated by neutral trusted third party
     agreed upon by the PKIs or consortium consisting of the PKIs.  The
     Bridge CA SHOULD do policy mapping appropriately with all PKI
     domains.  For using the name constraints, Bridge CA SHOULD pay



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     attention to preventing a conflict of each name space of the cross-
     certified PKI domains.

     The PKI domains that perform cross-certification with Bridge CA
     SHOULD confirm the following:
        - Does the Bridge CA perform the policy mapping via its own
        domain policy?
        - Does the Bridge CA clarify the method of policy mapping in
        cross-certification?
        - Is the Bridge CA able to accept the domain policy that the
        prior PKI domain desires?
            * If the domain policy is mapped to one with a lower
            security level, the prior PKI domain SHOULD NOT accept it.
            If it accepts, the prior PKI domain MUST consider its risk
            carefully.

         cross-certified                 cross-certified
         PKI 1 with BCA   +-----------+  PKI3 with BCA
                 +------->| Bridge CA |<------+
                 |        +-----------+       |
                 |                 ^          |
                 | cross-certified |          |
                 |  PKI 2 with BCA |          |
                 |                 |          |
     +-----------|---+ +-----------|---+ +----|-----------------+
     |     PKI 1 |   | |     PKI 2 |   | |    |    PKI 3        |
     |           v   | |           v   | |    v                 |
     |       +-----+ | |       +-----+ | | +-----+              |
     |   +---| PCA | | |       | PCA | | | | PCA |<--+          |
     |   |   +-----+ | |       +-----+ | | +-----+   |          |
     |   |      |    | |          |    | |   ^       |          |
     |   |      |    | |          |    | |   |       v          |
     |   |      |    | |          |    | |   |     +----+       |
     |   |      |    | |          |    | |   |     | CA |---+   |
     |   |      |    | |          |    | |   |     +----+   |   |
     |   |      |    | |          |    | |   |      ^ |     |   |
     |   |      |    | |          v    | |   v      | |     |   |
     |   |      |    | |       +----+  | | +----+   | |     |   |
     |   |      |    | |   +---| CA |  | | | CA |---+ |     |   |
     |   |      |    | |   |   +----+  | | +----+     |     |   |
     |   |      |    | |   |      |    | |   |        |     |   |
     |   |      |    | |   |      |    | |   |        |     |   |
     |   v      v    | |   v      v    | |   v        v     v   |
     | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ |
     | | EE | | EE | | | | EE | | EE | | | | EE | | EE | | EE | |
     | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ |
     +---------------+ +---------------+ +----------------------+




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                           Figure 9 - Bridge model

7  Operational Considerations

   This chapter explains the issues one needs to consider about the
   management of cross-certificate(s) and use of a directory.

7.1 Directory


   (1) Unilateral cross-certification

     When CA-X cross-certifies CA-Y unilaterally, both CAs SHOULD
     operate their directory server in the following way.

        CA-X SHOULD generate a following crossCertificatePair and store
        it in its own directory entry.
            issuedToThisCA := NULL
            issuedByThisCA := cross-certificate for CA-Y issued by CA-X

        CA-Y MAY generate a following crossCertificatePair and store it
        in its own directory entry.
            issuedToThisCA := cross-certificate for CA-Y issued by CA-X
            issuedByThisCA := NULL

   (2) Mutual cross-certification

     Each CA MUST generate a crossCertificatePair that consists of the
     cross-certificate it issues and the cross-certificate it is issued.

        CA-X SHOULD generate the following crossCertificatePair and
        store it in its own directory entry:
            issuedToThisCA := cross-certificate for CA-X issued by CA-Y
            issuedByThisCA := cross-certificate for CA-Y issued by CA-X

        CA-Y SHOULD generate the following crossCertificatePair and
        store it in its own directory entry:
            issuedToThisCA := cross-certificate for CA-Y issued by CA-X
            issuedByThisCA := cross-certificate for CA-X issued by CA-Y

        In the mutual cross-certification model, each CA SHOULD NOT
        individually generate two crossCertificatePairs each containing
        only one cross-certificate, similar to the unilateral cross-
        certification model.

   (3) Subordination

     A superior CA MAY store a subordinate CA certificate to



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     issuedByThisCA element of crossCertificatePair attribute in its own
     entry for the reverse path building.  However, it SHOULD be only
     for compatibility with the reverse path building, since a path
     building for subordination SHOULD be the forward direction.  A
     superior CA SHOULD NOT store a subordinate CA certificate in its
     own entry for the forward path building.  A subordinate CA MAY
     store its own subordinate CA certificate to the issuedToThisCA
     element of the crossCertificatePair attribute in its own
     (subordinate CA) entry for the forward path building.  A
     subordinate CA MUST store its own subordinate CA certificate to the
     cACertificate attribute in its own entry.

7.2 Cross-Certification

   When updating the Cross-Certificate:

        There is standard method for what to do when a cross-certificate
        is updated by modifying some of its contents, e.g., policy
        identifier

        When issuer CA-X re-issues a cross-certificate to subject CA-Y
        before the issued cross-certificate expires, both CA-X and CA-Y
        MUST each update their own crossCertificatePair corresponding to
        the cross-certificate, and MUST populate it to their own
        directory system.  Until this is done, change of cross-
        certification is not reflected completely to certification path.
        In addition, CA-X MUST revoke the old cross-certificate to CA-Y
        when CA-X does not intend to enable the old cross-certificate.
        The reason why both CA MUST update each crossCertificatePair is
        relying party may use the issuedToThisCA attribute of the
        crossCertificatePair (in subject CA-Y entry of the repository)
        for tracing the certification path.

   When updating the CA keypair:

        When a CA issues a set of self-issued certificates for key
        rollover, update of the cross-certificate is able to have a
        migration period up to the expiration of the self-issued
        certificate.  However, when a CA does not issue a set of self-
        issued certificates for key rollover, update of the cross-
        certificate is required as quickly as possible.

        When a CA keypair is compromised, the CA DN SHOULD NOT be re-
        used by the same CA without issuing the self-issued certificate.
        The reason why CA DN should not be re-used is that relying party
        can not identify which is the compromised CA certificate.

   When the keypair of subject CA is compromised:



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        When the keypair of subject CA-Y is compromised, issuer CA-X
        MUST revoke the cross-certificate for subject CA-Y, then CA-X
        SHOULD remove the crossCertificatePair attribute for CA-Y from
        its repository.

8  Security Considerations

8.1  Certificate and CRL Profile

   Defining the concrete Certificate and CRL profile for multi-domain
   PKI interoperability is not within the scope of this memo.  All
   Certificates and CRLs MUST comply with [RFC 3280].  In addition, CAs
   in multi-domain PKI SHOULD consider the following for the Certificate
   and CRL profile:

     * The extensions for processing only in local PKI domain SHOULD be
     non-critical.
     * The cRLDistributionPoint extension SHOULD be used for obtaining
     the revocation list.  distributionPoint field SHOULD include also
     the UniformResourceIdentifier.  When the CRL is separated into ARL
     and CRL, the issuingDistributionPoint extension SHOULD also be
     used.
     * The Authority Key Identifier extension and Subject Key Identifier
     extension SHOULD be used for assisting in path construction.
     * The policyIdentifier field of the Certificate Policies extension
     SHOULD be used for identifying each policy domain.
     * The Policy Mapping extension MAY be used for the validating that
     mutual domain policies are equivalent.
     * The Name Constraints extension MAY NOT be used for multi-domain
     PKI because the name space of multi-domain PKI is not managed by a
     single authority.
        If a PKI domain uses the name constraints in multi-domain PKI,
        the PKI domain SHOULD pay attention to preventing a conflict of
        each name space.

8.2  Path Validation

   Validation policy used for path validation is the intersection of
   authority-constrained parameters and user-constrained parameters.  An
   authority constrained parameter SHOULD NOT rely on the validation
   policy of a relying party, but SHOULD be included in the certificates
   explicitly.

   A Relying party MUST carefully determine their validation policies,
   including the trust anchor.

8.3  Asymmetric problem




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8.3.1  Hybrid trust model

   This clause considers the case in which PKI domains trust each other
   by a different trust relationship.

   Inter-domain trust relationships do not have to be symmetric.  The
   hybrid trust model, similar to the user trust list model and the
   unilateral cross-certification model, serves as an actual model for
   such trust relationships.  Since inter-domain trust relationships in
   this document are defined as directional trust relationships, there
   is no additional requirement for such a model.  What each PKI domain
   does is merely the same as symmetric trust relationship.  For
   example, in the case that PKI domain-X trusts PKI domain-Y by the
   user trust list model and PKI domain-Y trusts PKI domain-X by
   unilateral cross-certification, PKI domain-X merely has to comply
   with the user trust list model, and PKI domain-Y with the unilateral
   cross-certification model.

8.3.2  Asymmetric policy mapping

   This clause considers the case where a result of the policy mapping
   in mutual cross-certification model is asymmetric.

                    +-------+  cP-1.1 := cP-2.1  +-------+
                    |       |------------------->|       |
                    | PCA 1 |                    | PCA 2 |
                    |       |<-------------------|       |
                    +-------+  cP-2.1 := cP-1.2  +-------+

                           Figure 10 - Asymmetric policy mapping

   When path building allows the certification path to loop, then cP-1.1
   is mapped to cP-1.2, and such a policy mapping MAY derive an
   unforeseen security hole in the certification path.  E.g., CA-X that
   cross-certified to PCA-1 with cP-1.1 MAY be able to grow its
   certification path to another PKI domain via PCA-1 by cP-1.2.  Since
   different policy identifiers managed by same PKI actually describes
   different policies, differing policy identifiers mapped unexpectedly
   in the same entity represents a critical security issue.  To prevent
   such a security hole, a loop certification path, one where the same
   DN appears twice and non-continuously on one certification path MUST
   NOT be allowed.

9  References

9.1  Normative References

      [RFC 3280]  Housley, R., Ford, W., Polk, W. and D. Solo, "Internet



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                  X.509 Public Key Infrastructure Certificate and CRL
                  Profile", RFC 3280, April 2002.

      [RFC 2256]  Wahl, M., "A Summary of the X.500(96) User Schema for
                  use with LDAPv3", RFC 2256, Dec 1997.

      [ISO-X509]  ISO/IEC 9594-8/ITU-T Recommendation X.509, "Information
                  Technology - Open Systems Interconnection: The Directory:
                  Authentication Framework," 2001 edition.

9.2  Informative References

      Housley, R. and Polk, W., JOHN WILEY & SONS, INC., "Planning for PKI",
      Aug 2001.

      Lloyd, S., PKI Forum, "PKI Interoperability Framework", March 2001.

      Lloyd, S., PKI Forum,  "CA-CA Interoperability", March 2001.

      Shimaoka, M., Japan Network Security Association, and ISEC,
      Information Technology Promotion Agency, Japan, "Interoperability
      Issues for multi PKI domain", Jul 2002.

      Japan Network Security Association, ISEC, Information Technology
      Promotion Agency, Japan, "Implementation Problems on PKI", Feb 2003.

      Japan PKI Forum, Korea PKI Forum, PKI Forum Singapore, Chinese Taipei
      PKI Forum, "Achieving PKI Interoperability 2003", Jul 2003.

      Japan PKI Forum, Korea PKI Forum, PKI Forum Singapore, "Achieving PKI
      Interoperability", Apr 2002.

      Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S. and Nicholas, R.,
      "Internet X.509 Public Key Infrastructure: Certification Path
      Building", Work in Progress, Oct 2003.


10  Acknowledgements

   This document is based on some valuable documents and many
   experiences with PKI interoperability experiments.  The authors
   gratefully acknowledge the contributions of members of various multi-
   domain PKI interoperability experiments, in particular: Kenji Nakada,
   Kiyoshi Watanabe, Sang Hwan Park, Ryu Inada, Hiroyuki Yoshida and
   Yasushi Matsumoto.

   The author are also grateful to members of the Internet Engineering
   Task Force (IETF) Public Key Infrastructure working group (PKIX), and



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   the Technical Working Group in Interoperability Working Group, which
   is consisted of Japan PKI Forum, Korea PKI Forum, Singapore PKI Forum
   and Chinese Taipei PKI Forum (JKST-IWG) for ideas and useful
   discussions which helped us in this effort.  This work is aided by
   Information-technology Promotion Agency Information-technology
   Security Center (IPA/ISEC) and Japan Network Security Association
   (JNSA).

11  Author's Address

   Masaki SHIMAOKA
   SECOM Co., Ltd. Intelligent Systems Lab.
   SECOM SC Center, 8-10-16, Shimorenjaku
   Mitaka, Tokyo 181-8528
   JAPAN
   Email: shimaoka@secom.ne.jp

   Nelson E. Hastings
   NIST
   100 Bureau Drive, Stop 8930
   Gaithersburg, MD 20899-8930
   USA
   EMail: nelson.hastings@nist.gov

12  Full Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

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