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Network Working Group                                        M. Shimaoka
Request for Comments: DRAFT                                        SECOM
<draft-shimaoka-multidomain-pki-05.txt>                      N. Hastings
                                                                    NIST
                                                               July 2005


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


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on Jan 10, 2006.

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



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   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      . . . . . . . . . . . . . . . . . . . . . . . .  3
   2  Requirements and Assumptions    . . . . . . . . . . . . . . . . .  4
   2.1  Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.2  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.3  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3  Trust Relationship  . . . . . . . . . . . . . . . . . . . . . . .  8
   3.1  Operation based Trust Relationship  . . . . . . . . . . . . . .  9
   3.1.1  User Trust List model . . . . . . . . . . . . . . . . . . . . 10
   3.1.2  Authority Trust List model  . . . . . . . . . . . . . . . . . 10
   3.2  Certificate based Trust Relationship  . . . . . . . . . . . . . 11
   3.2.1  Unilateral Cross-Certification  . . . . . . . . . . . . . . . 12
   3.2.2  Mutual Cross-Certification  . . . . . . . . . . . . . . . . . 13
   3.3  Subordination (Hierarchy) . . . . . . . . . . . . . . . . . . . 14
   4  PKI Domain  . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   4.1  Requirements for PKI domain . . . . . . . . . . . . . . . . . . 16
   4.2  Risk Analysis of non-interoperable PKI domains  . . . . . . . . 16
   4.3  Trust Relationship Disclosure Requirements for multi-domain PKIs17
   5  Single-domain PKI . . . . . . . . . . . . . . . . . . . . . . . . 18
   5.1  Single CA PKI model . . . . . . . . . . . . . . . . . . . . . . 18
   5.2  Hierarchy PKI model . . . . . . . . . . . . . . . . . . . . . . 19
   5.3  Mesh PKI model  . . . . . . . . . . . . . . . . . . . . . . . . 19
   6  multi-domain PKI  . . . . . . . . . . . . . . . . . . . . . . . . 21
   6.1  Multi Trust point model . . . . . . . . . . . . . . . . . . . . 21
   6.1.1  Based on User Trust List  . . . . . . . . . . . . . . . . . . 22
   6.1.2  Based on Authority Trust List . . . . . . . . . . . . . . . . 22
   6.2  Single Trust Point model  . . . . . . . . . . . . . . . . . . . 22
   6.2.1  Unified Domain model  . . . . . . . . . . . . . . . . . . . . 22
   6.2.2  Bridge model  . . . . . . . . . . . . . . . . . . . . . . . . 23
   7  Operational Considerations  . . . . . .  .  . . . . . . . . . . . 26
   7.1  Directory . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   7.2  Cross-Certification . . . . . . . . . . . . . . . . . . . . . . 27
   7.3  Providing Directory Information Across PKI-domains  . . . . . . 28
   8  Security Considerations . . . . . . . .  .  . . . . . . . . . . . 28
   8.1  Certificate and CRL Profile . . . . . . . . . . . . . . . . . . 28
   8.2  Path Validation . . . . . . . . . . . . . . . . . . . . . . . . 29
   8.3  Asymmetric problem  . . . . . . . . . . . . . . . . . . . . . . 29
   8.3.1  Hybrid trust model  . . . . . . . . . . . . . . . . . . . . . 29
   8.3.2  Asymmetric policy mapping . . . . . . . . . . . . . . . . . . 29
   9  References  . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   9.1  Normative References  . . . . . . . . . . . . . . . . . . . . . 30
   9.2  Informative References  . . . . . . . . . . . . . . . . . . . . 30
   10  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 31



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   11 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 31
   12  Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 32

1  Introduction

   PKIs are 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, a complete understanding
   of the relationship between the establishment method and a comparison
   of security requirements in each certificate policies 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 and do leave those
   decisions up to the designers of specific 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
   Revocation List (CRL) profiles, Repositories, and path validation.

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



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     |   | 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

2.2  Terminology

   End Entity (EE)

     The preferred definition of EE is based on the X.509 4th edition
     definition instead of that found in RFC 2828, since it includes
     relaying parties and not just subjects of certificates as EEs.
     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].



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   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, the trust anchors and validation policies 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 some responsibilities and rights based on a contract or
     agreement with the trust anchor.  For example, the responsible EE
     is issued its certificate under the contract or agreement with the
     trust anchor, and should specify a certain validation policy in the
     path validation.

   Irresponsible EE

     Unlike a responsible EE, an 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 without
     having rights from the trust anchor.

   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.

   Intermediate Certificate

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

   PKI domain

     The word "PKI domain" is used to represent and identify a set of
     PKIs which share the same trust level.  PKI domain MUST have one or
     more principal CAs and SHOULD have one or more domain policy.
     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



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     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 or agreed upon certificate policy that is shared in a PKI
     domain.  When the PKIs in a PKI domain can share 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.

   Trust Anchor

     Starting point of a certification path 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 which is beyond 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 term is used to describe the relative trust relationship of
     adjoined PKI domains in the certification path and is used in
     combination with the term "prior PKI domain".  The next trusted PKI
     domain(s) in the certification path from the trust anchor to the
     target certificate is called the posterior PKI domain.  That is,
     the posterior PKI domain is trusted from the prior PKI domain.

   Prior PKI domain

     This term is used to describe the relative trust relationship of
     adjoined PKI domains in the certification path and is used in
     combination with the term "posterior PKI domain".  The previous PKI
     domain in the certification path from the trust anchor to the
     target certificate is called the prior PKI domain.  That is, the



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     prior PKI domain trusts the posterior PKI domain.  First prior PKI
     domain in the certification path is 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
     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




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     Subordination is a mechanism to authorize the existence of a
     subject CA.  The authorization of the existence means issuing a
     certificate called subordinate (CA) certificate 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 cross-certificate 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
   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,
     except the case that those trust anchors are operated by the same
     body.

   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.



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                               Trust List
     +--------------------------------------------------------------+
     |                         Trusted CA                           |
     |                                                              |
     | +---------------+ +---------------+ +----------------------+ |
     | |     PKI 1     | |     PKI 2     | |         PKI 3        | |
     | |               | |               | |                      | |
     | |       +-----+ | | +-----+       | | +-----+              | |
     | |   +---| PCA | | | | PCA |       | | | PCA |<--+          | |
     | |   |   +-----+ | | +-----+       | | +-----+   |          | |
     | |   |      |    | |    |          | |   ^       |          | |
     +-----|------|-----------|----------------|-------|------------+
       |   |      |    | |    |          | |   |       |          |
       |   |      |    | |    |          | |   |       v          |
       |   |      |    | |    |          | |   |     +----+       |
       |   |      |    | |    |          | |   |     | CA |---+   |
       |   |      |    | |    |          | |   |     +----+   |   |
       |   |      |    | |    |          | |   |      ^ |     |   |
       |   |      |    | |    v          | |   v      | |     |   |
       |   |      |    | | +----+        | | +----+   | |     |   |
       |   |      |    | | | CA |---+    | | | CA |---+ |     |   |
       |   |      |    | | +----+   |    | | +----+     |     |   |
       |   |      |    | |   |      |    | |   |        |     |   |
       |   |      |    | |   |      |    | |   |        |     |   |
       |   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 easier
     than cross-certification and is typical method for making a trust
     relationship with another PKI.

     Except for EE itself, no one is able to control the trust



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     relationship.  There is a risk that EE trusts unknown PKI domain
     irresponsibility.  If EE trusts unknown PKI domains irresponsibly,
     then the 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 authority,
     which manages the trust anchors that are to be used by a relying
     party.  The trust authority MAY issue multiple trust lists for some
     purposes or parties.  EEs trusting the same trust authority may
     share the authority trust list given by the trust authority.

   Characteristics

     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 self-signed
     certificate.

   Characteristics




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     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, (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.
     The issuing CA SHOULD publish the cross-certificate in the
     repository for relying parties.  In general, the cross-certificate
     is populated to the crossCertificatePair attribute of a repository
     such as a LDAP or X.500 directory server.  At minimum, 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

     When a subject CA does not have a self-signed certificate, such
     subject CA is established by another CA issuing the cross-
     certificate to the subject CA.  This, however, means the issuer CA
     of the cross-certificate may not be able to recognize the existence
     of the subject CA because the issuer CA may not have a formal
     agreement or contract to the establishing 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 full CRLs.  However, ARL with an
        issuingDistributionPoint extension MAY NOT be processed by some



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        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-
     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.  But
     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 |---+ |     |   |



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        | +----+   |    |                 | +----+     |     |   |
        |   |      |    |                 |   |        |     |   |
        |   |      |    |                 |   |        |     |   |
        |   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
        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 agree and accept 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         |



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        |    |          |                 |    |    +----+       |
        |    |          |                 |    |    | 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
     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.  Although a full third party accreditation of the
     subordinate CA is not required, it is the responsibility of the
     superior or root CA to ensure it issues CA certificates to CAs that
     operate under its accredited policies and procedures.  In this
     case, accreditation means that the subordinate CA can succeed the



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     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
     relationship will change from the subordination model into the
     unified domain model as described in section 6.2.2.

4  PKI Domain
4.1  Requirements for PKI domain

   PKIs in a PKI domain SHOULD share a common "domain policy" consisting
   of one or more certificate policies.  The certificate policies of the
   domain policy SHOULD be populated in the certificate policies
   extension for each certificate.  The PKI domain MUST have at least
   one principal CA that can be trusted by other PKI domains.  The PKI
   domain MUST have a mechanism to propagate certificate status
   information to other PKI domains.  The PKI domain SHOULD agree to a
   minimum common certificate profile.

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




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4.2  Risk Analysis of non-interoperable PKI domains

   A PKI domain that satisfies the requirements presented in the section
   4.1 of this document MAY be used in the multi-trust or single-trust
   point model.  However when one PKI domain interconnects with another
   PKI domain, the following items need to be considered to reduce the
   risk of being non-interoperable:

     - Namespace Conflicts

        PKI domains SHOULD agree to use namespaces that do not overlap.
        If PKI domain namespaces overlap, a namespace conflict occurs
        resulting in the name constraints extension possibly not perform
        the specified name constraint as intended.

     - PKI Domain Policy in Certificates

        CAs of PKI domains SHOULD populate the certificate policy
        extension with the PKI domain policy.  If the PKI domain policy
        is not described in the certificate policies extension, the path
        validation MAY fail when relying parties use the certificate
        policy extension to identify PKI domain.  The PKI domain policy
        is necessary in path validation through PKI domains that use
        policy constraints or policy mapping.

     - Certificate Authority Certification Path Constraints

        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 CA's control.  By explicitly including the constraints in
        certificates, a certification path that otherwise would validate
        will fail, regardless of a relying party's path validation
        settings or configuration.

        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.



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     - End Entity Certification Path Constraints

        Some PKI domains that require an explicit policy MAY NOT assert
        the requiredExplicitPolicy constraint in certificates they
        issue.  These PKI domains assume relying parties will configure
        their validation policy appropriately.  A PKI domain requiring
        an explicit domain policy SHOULD set the following validation
        policy for its end entities:
          * 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.

     - Distribution of Certificate and Certificate Status Information

        PKI domains SHOULD make certificate and certificate revocation
        lists (CRLs) available using LDAP or HTTP.  This provides the
        ability for other certificate status protocols such as OCSP and
        SCVP to be implemented without requiring the PKI domain
        providing the certificates and CRLS to implement the protocol.
        If certificate and certificate status information is not made
        available by a PKI domain, certification paths passing through
        that PKI domain MAY not be constructed and validated.

4.3  Trust Relationship Disclosure Requirements for multi-domain PKIs

   For any multi-domain PKI model, each PKI domain SHOULD show the trust
   relationship(s) it has with other PKI 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, PKI domains SHOULD give the appropriate policy mappings
   between prior PKI domains and the posterior PKI domains 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.



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5.1  Single CA PKI model

   This is the simplest PKI model which is a special case of a
   hierarchical PKI which has no subordinate CAs.  All PKIs in this
   model are composed using this building block of a CA and its EE.

   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 |---+
                            |   +----+   |
                            |      |     |
                            |      |     |
                            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



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                     +----+            +----+
               +-----| 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

     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 the CA who issued the
     certificate to the relying party.  The trust anchor for the 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, relying
     parties may not recognize the revocation of the CA even if the
     issuing CA publish the revocation information about that CA.
     Therefore, this document recommends that relying party SHOULD trust
     other self-signed CA.  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.  For
     example, relying party may choose a CA which is highly unlikely
     that CA is revoked.

     This model SHOULD be used sparingly, because of the complexity in
     certification path building.  However, one should not assume that



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     this model does not exist or is implemented.  A Full Mesh PKI,
     which is one where all CAs in the PKI mutually cross-certify each
     other directly, MAY be useful for certification path building,
     because it is able to reach any prior PKI domain directly without
     passing through another PKI domain.

             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 |
      +----+ +----+                              +----+ +----+ +----+

                       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



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     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:

        Assume 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
        certificates.  However, if the relying party trust CA-Y
        directly, the cross certificate constraint in X->Y is ignored
        allowing CA-Z to extend the certification path by more cross
        certificates.  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
     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.  Anyway,
   for announcement to all its EEs, the CA SHOULD do the best: phoning,
   emailing, press release, and etc.  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 a simple 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.




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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 guarantee the constraints will
     be recognized and followed.

6.2.1  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 reconfigure the multiple PKI domains to one PKI
   domain by subordinating the individual PKI domains.  Except that
   Principal CAs transformed into subordinate CAs have both self-signed
   certificates and intermediate certificates issued by the Unificate
   CA, this model looks like a subordination model with the Unificate CA
   as the trust anchor across the PKI domains.  Therefore, this model is
   often 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     |          |



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     +-----------|---+ +-----------|---+ +----|-----------------+
     |     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

6.2.2  Bridge model

   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-certifications required for a PKI domain
   to interoperate with other PKI domains.

   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.



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        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 in a well documented and agreed
     upon manner with all PKI domains.  For using the name constraints,
     Bridge CA SHOULD pay 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.
            Otherwise, the prior PKI domain MUST carefully consider the
            risks involved with accepting certificates with a lower
            security level.

         cross-certified                 cross-certified
         PKI 1 with BCA   +-----------+  PKI3 with BCA
                 +------->| Bridge CA |<------+



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                 |        +-----------+       |
                 |                 ^          |
                 | 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 | |
     | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ |
     +---------------+ +---------------+ +----------------------+

                           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



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        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
     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




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        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, the change of cross-
        certification is not reflected completely in certification
        paths.  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 CAs 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 originally issued
        self-issued certificate.


   When the keypair of subject CA is compromised:

        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.

7.3  Providing Directory Information Across PKI-domains

   The directory infrastructures used by individual PKI domains to
   distribute certificates and CRLs usually consist of either a set of
   interconnected or stand alone directories.

   An interconnected directory infrastructure connects directories via
   the use of a directory protocol such as chaining, replicating, or
   shadowing.  An interconnected directory infrastructure allows relying
   parties to reduce the number of directories they need to be aware of
   in order to obtain certificates and CRLs.  However, this technique
   MAY lead to infrastructure propagation delays as directory
   information is updated or changes.

   Directory infrastructures composed of stand alone directories
   provides certificate and CRL information from a set (list) of
   directories the relying parties is aware of.  If a directory that is
   queried but cannot satisfy the request, it MAY provide referrals to
   another directory might be able to provide the requested information.




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   To help promote interoperability, PKI domains SHOULD provide access
   to the PKI domainfs directory infrastructure via LDAP or HTTP and
   information to access (e.g. IP address or FQDN) at least one of the
   PKI domainfs directories.  EE SHOULD be able to process LDAP
   referrals in order to operate with a set of stand alone directories.

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:

     * Extensions intended for processing only by a 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 allowing for the possibility of a name space
     conflict.  If a name space conflict exists, the name constraint
     extension MAY unintentionally exclude a PKI domain.
        If a PKI domain uses the name constraints in multi-domain PKI,
        the PKI domain SHOULD pay attention for conflicts in PKI domain
        name spaces.

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 assume 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.



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8.3  Asymmetric problem

8.3.1  Hybrid trust model

   This clause considers the case in which PKI domains trust each other
   by different trust relationship models such as user trust lists and
   unilateral cross-certification trust models.

   Inter-domain trust relationships do not have to be symmetric.  Since
   inter-domain trust relationships in this document are defined as
   directional trust relationships, there is no additional requirement
   for a hybrid trust model.  What each PKI domain does is merely the
   same as a symmetric trust relationship model.  For example when 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.  This document
   does not strongly recommend using asymmetric mapping because the
   following unequivalent mapping often derives a security hole.

                    +-------+  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




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9.1  Normative References

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



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   The author are also grateful to members of the Internet Engineering
   Task Force (IETF) Public Key Infrastructure working group (PKIX), and
   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 / m-shimaoka@secom.co.jp

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

12  Full Copyright Statement

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