





Network Working Group                                   M. Shimaoka, Ed.
Request for Comments: 5217                                         SECOM
Category: Informational                                      N. Hastings
                                                                    NIST
                                                              R. Nielsen
                                                     Booz Allen Hamilton
                                                               July 2008


 Memorandum for Multi-Domain Public Key Infrastructure Interoperability

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   The objective of this document is to establish a terminology
   framework and to suggest the operational requirements of Public Key
   Infrastructure (PKI) domain for interoperability of multi-domain
   Public Key Infrastructure, where each PKI domain is operated under a
   distinct policy.  This document describes the relationships between
   Certification Authorities (CAs), provides the definition and
   requirements for PKI domains, and discusses typical models of multi-
   domain PKI.
























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Objective  . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Document Outline . . . . . . . . . . . . . . . . . . . . .  3
   2.  Public Key Infrastructure (PKI) Basics . . . . . . . . . . . .  3
     2.1.  Basic Terms  . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Relationships between Certification Authorities  . . . . .  4
       2.2.1.  Hierarchical CA Relationships  . . . . . . . . . . . .  5
       2.2.2.  Peer-to-Peer CA Relationships  . . . . . . . . . . . .  6
     2.3.  Public Key Infrastructure (PKI) Architectures  . . . . . .  7
       2.3.1.  Single CA Architecture . . . . . . . . . . . . . . . .  7
       2.3.2.  Multiple CA Architectures  . . . . . . . . . . . . . .  8
     2.4.  Relationships between PKIs and Relying Parties . . . . . . 12
   3.  PKI Domain . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.1.  PKI Domain Properties  . . . . . . . . . . . . . . . . . . 13
     3.2.  Requirements for Establishing and Participating in PKI
           Domains  . . . . . . . . . . . . . . . . . . . . . . . . . 13
       3.2.1.  PKI Requirements . . . . . . . . . . . . . . . . . . . 13
       3.2.2.  PKI Domain Documentation . . . . . . . . . . . . . . . 14
       3.2.3.  PKI Domain Membership Notification . . . . . . . . . . 15
       3.2.4.  Considerations for PKIs and PKI Domains with
               Multiple Policies  . . . . . . . . . . . . . . . . . . 16
     3.3.  PKI Domain Models  . . . . . . . . . . . . . . . . . . . . 16
       3.3.1.  Unifying Trust Point (Unifying Domain) Model . . . . . 16
       3.3.2.  Independent Trust Point Models . . . . . . . . . . . . 17
     3.4.  Operational Considerations . . . . . . . . . . . . . . . . 21
   4.  Trust Models External to PKI Relationships . . . . . . . . . . 22
     4.1.  Trust List Models  . . . . . . . . . . . . . . . . . . . . 22
       4.1.1.  Local Trust List Model . . . . . . . . . . . . . . . . 22
       4.1.2.  Trust Authority Model  . . . . . . . . . . . . . . . . 23
     4.2.  Trust List Considerations  . . . . . . . . . . . . . . . . 24
       4.2.1.  Considerations for a PKI . . . . . . . . . . . . . . . 24
       4.2.2.  Considerations for Relying Parties and Trust
               Authorities  . . . . . . . . . . . . . . . . . . . . . 24
       4.2.3.  Additional Considerations for Trust Authorities  . . . 25
   5.  Abbreviations  . . . . . . . . . . . . . . . . . . . . . . . . 25
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
     6.1.  PKI Domain Models  . . . . . . . . . . . . . . . . . . . . 25
     6.2.  Trust List Models  . . . . . . . . . . . . . . . . . . . . 26
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 27
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 27








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1.  Introduction

1.1.  Objective

   The objective of this document is to establish a terminology
   framework and to provide the operational requirements, which can be
   used by different Public Key Infrastructure (PKI) authorities who are
   considering establishing trust relationships with each other.  The
   document defines different types of possible trust relationships,
   identifies design and implementation considerations that PKIs should
   implement to facilitate trust relationships across PKIs, and
   identifies issues that should be considered when implementing trust
   relationships.  This document defines terminology and
   interoperability requirements for multi-domain PKIs from one
   perspective.  A PKI domain can achieve multi-domain PKI
   interoperability by complying with the requirements in this document.
   However, there are other ways to define and realize multi-domain PKI
   interoperability.

1.2.  Document Outline

   Section 2 introduces the PKI basics, which provide a background for
   multi-domain PKI.  Section 3 provides the definitions and
   requirements of 'PKI domain' and describes the typical models of
   multi-domain PKI.  Section 4 considers the Trust List Models
   depending on relying party-CA relationships (not CA-CA trust
   relationships, as they are not a focus of this document).  Section 5
   identifies abbreviations used in the document.

2.  Public Key Infrastructure (PKI) Basics

2.1.  Basic Terms

   The following terms are used throughout this document.  Where
   possible, definitions found in RFC 4949 [RFC4949] have been used.

   Certificate:  A digitally signed data structure that attests to the
      binding of a system entity's identity to a public key value (based
      on the definition of public key certificate in RFC 4949
      [RFC4949]).

   Certificate Policy:  A named set of rules that indicates the
      applicability of a certificate to a particular community and/or
      class of application with common security requirements (X.509
      [CCITT.X509.2000]).  Note that to avoid confusion, this document
      uses the terminology "Certificate Policy Document" to refer to the
      document that defines the rules and "Policy Object Identifier
      (OID)" to specify a particular rule set.



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   Certificate Policy Document:  A document that defines the rules for
      the issuance and management of certificates and identifies Policy
      Object Identifiers (OIDs) for these rules.  A Certificate Policy
      Document may define more than one Policy OID.

   Policy Object Identifier (Policy OID):  An identifier applied to a
      set of rules governing the issuance and management of
      certificates.  Policy OIDs are defined in the Certificate Policy
      Documents.

   Certification Authority (CA):  An entity that issues certificates
      (especially X.509 certificates) and vouches for the binding
      between the data items in a certificate (RFC 4949 [RFC4949]).

   End Entity (EE):  A system entity that is the subject of a
      certificate and that is using, or is permitted and able to use,
      the matching private key only for a purpose or purposes other than
      signing a certificate; i.e., an entity that is not a CA (RFC 4949
      [RFC4949]).

   Relying party:  A system entity that depends on the validity of
      information (such as another entity's public key value) provided
      by a certificate (from the RFC 4949 [RFC4949] definition of
      certificate user).

2.2.  Relationships between Certification Authorities

   CAs establish trust relationships by issuing certificates to other
   CAs.  CA relationships are divided into 'certification hierarchy'
   [RFC4949] and 'cross-certification' [RFC4949].

   In a certification hierarchy, there are two types of CAs: 'superior
   CA' and 'subordinate CA', as described in RFC 4949 [RFC4949].

   Superior CA:  A CA that is an issuer of a subordinate CA certificate.

   A cross-certification can be either unilateral or bilateral.

   Unilateral cross-certification:  Cross-certification of one CA (CA1)
      by another CA (CA2) but no cross-certification of CA2 by CA1.

   Bilateral cross-certification:  Cross-certification of one CA (CA1)
      by another CA (CA2) and cross-certification of CA2 by CA1.








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2.2.1.  Hierarchical CA Relationships

   In a hierarchical relationship, as shown in Figure 1, one CA assumes
   a parent relationship to the other CA.

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

                  Figure 1: Hierarchical CA Relationship

   There are two types of hierarchical relationships, depending on
   whether a subordinate CA certificate or a unilateral cross-
   certificate is used.  In the case where one (superior) CA issues a
   subordinate CA certificate to another, the CA at the top of the
   hierarchy, which must itself have a self-signed certificate, is
   called a root CA.  In the case where one CA issues unilateral cross-
   certificates to other CAs, the CA issuing unilateral cross-
   certificates is called a Unifying CA.  Unifying CAs use only
   unilateral cross-certificates.

   NOTE: In this document, the definition of root CA is according to the
   second definition (context for hierarchical PKI) of 'root CA' in RFC
   4949 [RFC4949].  This document uses the terminology 'trust anchor CA'
   for the first definition (context for PKI) of 'root CA' in RFC 4949.

   Root CA:  A CA that is at the top of a hierarchy, and itself should
      not issue certificates to end entities (except those required for
      its own operation) but issues subordinate CA certificates to one
      or more CAs.

   Subordinate CA:  A CA whose public key certificate is issued by
      another superior CA, and itself must not be used as a trust anchor
      CA.

   Unifying CA:  A CA that is at the top of a hierarchy, and itself
      should not issue certificates to end entities (except those
      required for its own operation) but establishes unilateral cross-
      certification with other CAs.  A Unifying CA must permit CAs to
      which it issues cross-certificates to have self-signed
      certificates.





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2.2.2.  Peer-to-Peer CA Relationships

   In a peer relationship, no parent-child relationship is created.  To
   establish peer relationships, only cross-certificates are used.  Peer
   relationships can be either unilateral or bilateral, as shown in
   Figure 2.

                                              Bilateral
                    Unilateral           Cross-Certification
                Cross-Certification      +----+      +----+
                +----+      +----+       |    | ---> |    |
                | CA | ---> | CA |       | CA |      | CA |
                +----+      +----+       |    | <--- |    |
                                         +----+      +----+

                  Figure 2: Peer-to-Peer CA Relationships

   In the case where a CA exists only to manage cross-certificates, that
   CA is called a Bridge CA.  CAs can establish unilateral or bilateral
   cross-certification with a Bridge CA, as shown in Figure 3.

   Bridge CA:  A CA that, itself, does not issue certificates to end
      entities (except those required for its own operation) but
      establishes unilateral or bilateral cross-certification with other
      CAs.

                                  Bilateral
                             Cross-Certification
                  +----+ ----------+    +--------- +----+
                  | CA |           |    |          | CA |
                  +----+ <-------+ |    | +------> +----+
                                 | v    v |
                               +-----------+
                               | Bridge CA |
                               +-----------+
                  +----+         |       |         +----+
                  | CA | <-------+       +-------> | CA |
                  +----+         Unilateral        +----+
                            Cross-Certification

                            Figure 3: Bridge CA










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2.3.  Public Key Infrastructure (PKI) Architectures

   Public Key Infrastructure (PKI):  A system of CAs that perform some
      set of certificate management, archive management, key management,
      and token management functions for a community of users in an
      application of asymmetric cryptography and share trust
      relationships, operate under the same Certificate Policy Document
      specifying a shared set of Policy OID(s), and are either operated
      by a single organization or under the direction of a single
      organization.

   In addition, a PKI that intends to enter into trust relationships
   with other PKIs must designate a Principal CA (PCA) that will manage
   all trust relationships.  This Principal CA should also be the trust
   anchor CA for relying parties of that PKI.

   Principal CA (PCA):  A CA that should have a self-signed certificate
      is designated as the CA that will issue cross-certificates to
      Principal CAs in other PKIs, and may be the subject of cross-
      certificates issued by Principal CAs in other PKIs.

   In discussing different possible architectures for PKI, the concept
   of a certification path is necessary.  A certification path is built
   based on trust relationships between CAs.

   Certification Path:  An ordered sequence of certificates where the
      subject of each certificate in the path is the issuer of the next
      certificate in the path.  A certification path begins with a trust
      anchor certificate and ends with an end entity certificate.

2.3.1.  Single CA Architecture

   Definition:  A simple PKI consists of a single CA with a self-signed
      certificate that issues certificates to End Entities (EEs), as
      shown in Figure 4.

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

                     Figure 4: Simple PKI Architecture




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   Trust anchor CA:  The trust anchor CA must be the CA that has a self-
      signed certificate.

   Principal CA:  Since this PKI architecture has one CA, the Principal
      CA must be that CA.

2.3.2.  Multiple CA Architectures

2.3.2.1.  Hierarchical PKI Architecture

   Definition:  A hierarchical PKI consists of a single root CA and one
      or more subordinate CAs that issue certificates to EEs.  A
      hierarchical PKI may have intermediate CAs, which are subordinate
      CAs that themselves have subordinate CAs.  The root CA must
      distribute a trust anchor (public key and associated data), but
      the format and protocol are irrelevant for this specification.
      And all subordinate CAs must have subordinate CA certificates, as
      shown in Figure 5.

   Trust anchor CA:  The trust anchor CA must be the root CA.

   Principal CA:  The Principal CA must be the root CA.

                            +---------+
                            | Root CA |
                            +---------+
                                 |
                    +------------+------------+
                    v                         v
                  +----+                    +----+
                  | CA |                    | CA |
                  +----+                    +----+
                    |                         |
             +------+------+         +--------+-------+
             v      v      v         v                v
           +----+ +----+ +----+    +----+           +----+
           | EE | | EE | | EE |    | CA |           | CA |
           +----+ +----+ +----+    +----+           +----+
                                     |                |
                                 +---+--+      +------+------+
                                 v      v      v      v      v
                               +----+ +----+ +----+ +----+ +----+
                               | EE | | EE | | EE | | EE | | EE |
                               +----+ +----+ +----+ +----+ +----+

                  Figure 5: Hierarchical PKI Architecture





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2.3.2.2.  Mesh PKI Architectures

   Definition:  A mesh PKI consists of multiple CAs with self-signed
      certificates that issue certificates to EEs and issue cross-
      certificates to each other.  A mesh PKI may be a full mesh, where
      all CAs issue cross-certificates to all other CAs, as shown in
      Figure 6.  A mesh PKI may also be a partial mesh, where all CAs do
      not issue cross-certificates to all other CAs.  In a partial mesh
      PKI, certification paths may not exist from all CAs to all other
      CAs, as shown in Figure 7.

                     +--------- +-----+ <--------+
                     |          | CA1 |          |
                     | +------> +-----+ -------+ |
                     | |           |           | |
                     | |       +---+--+        | |
                     | |       v      v        | |
                     | |     +----+ +----+     | |
                     | |     | EE | | EE |     | |
                     | |     +----+ +----+     | |
                     v |                       v |
                   +-----+ ----------------> +-----+
                   | CA2 |                   | CA3 |
                   +-----+ <---------------- +-----+
                      |                         |
                  +---+--+               +------+------+
                  v      v               v      v      v
                +----+ +----+          +----+ +----+ +----+
                | EE | | EE |          | EE | | EE | | EE |
                +----+ +----+          +----+ +----+ +----+

                   Figure 6: Full Mesh PKI Architecture



















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                     +--------- +-----+
                     |          | CA1 | --------+
                     | +------> +-----+         |
                     | |           |            |
                     | |       +---+--+         |
                     | |       v      v         |
                     | |     +----+ +----+      |
                     | |     | EE | | EE |      |
                     | |     +----+ +----+      |
                     v |                        v
                   +-----+                   +-----+
                   | CA2 | ----------------> | CA3 |
                   +-----+                   +-----+
                      |                         |
                  +---+--+               +------+------+
                  v      v               v      v      v
                +----+ +----+          +----+ +----+ +----+
                | EE | | EE |          | EE | | EE | | EE |
                +----+ +----+          +----+ +----+ +----+

                  Figure 7: Partial Mesh PKI Architecture

   Trust anchor CA:  The trust anchor CA for an end entity is usually
      the CA that issued the end entity's certificate.  The trust anchor
      CA for an end entity that is not issued a certificate from the
      mesh PKI may be any CA in the PKI.  In a partial mesh, selection
      of the trust anchor may result in no certification path from the
      trust anchor to one or more CAs in the mesh.  For example, in
      Figure 7 above, the selection of CA1 or CA2 as the trust anchor CA
      will result in paths from all end entities in the figure.
      However, the selection of CA3 as the trust anchor CA will result
      in certification paths only for those EEs whose certificates were
      issued by CA3.  No certification path exists to CA1 or CA2.

   Principal CA:  The Principal CA may be any CA within the mesh PKI.
      However, the mesh PKI must have only one Principal CA, and a
      certification path should exist from the Principal CA to all other
      CAs within the mesh PKI.

   Considerations:  This model should be used sparingly, especially the
      partial mesh model, because of the complexity of determining trust
      anchors and building certification paths.  A full mesh PKI may be
      useful for certification path building because paths of length one
      exist from all CAs to all other CAs in the mesh.







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2.3.2.3.  Hybrid PKI Architectures

   Definition:  A hybrid PKI is a PKI that uses a combination of the
      pure hierarchical model using subordinate CA certificates and the
      pure mesh model using cross-certificates.

                    +-----+ <----- +-----+
                    | CA2 |        | CA1 |
                    +-----+ -----> +-----+
                       |              |
                   +---+--+       +---+--+-------+
                   v      v       v      v       v
                +----+ +----+   +----+ +----+ +-----+
                | EE | | EE |   | EE | | EE | | CA3 |
                +----+ +----+   +----+ +----+ +-----+
                                                 |
                                          +------+------+
                                          v      v      v
                                        +----+ +----+ +----+
                                        | EE | | EE | | EE |
                                        +----+ +----+ +----+

                      Figure 8: Hybrid PKI Architecture

   Trust anchor CA:  The trust anchor CA for a hybrid PKI may be any CA
      with self-issued certificates in the hybrid PKI.  However, because
      of the potential complexity of a hybrid PKI, the PKI should
      provide guidance regarding the selection of the trust anchor to
      relying parties because a relying party may fail to build an
      appropriate certification path to a subscriber if they choose an
      inappropriate trust anchor.

   Principal CA:  The Principal CA may be any CA within the hybrid PKI
      and should have a self-signed certificate for cross-certification
      with other PKI domains.  However, the hybrid PKI must have only
      one Principal CA and a certification path must exist from the
      Principal CA to every CA within the PKI.

   Considerations:  This model should be used sparingly because of the
      complexity of determining trust anchors and building certification
      paths.  However, hybrid PKIs may occur as a result of the
      evolution of a PKI over time, such as CAs within an organization
      joining together to become a single PKI.








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2.4.  Relationships between PKIs and Relying Parties

   Relying Parties establish trust relationships by trust anchor to a
   PKI.  Relying Parties may use a Trust List for establishing trust
   relationships to one or more PKIs.  A Trust List is a set of one or
   more trust anchors for trusting one or more PKIs.

   There are two types of maintenance models of Trust List, Local Trust
   List Model and Trust Authority Model.  The two models are described
   in detail in Section 4.1.

3.  PKI Domain

   Two or more PKIs may choose to enter into trust relationships with
   each other.  For these relationships, each PKI retains its own set of
   Certificate Policy OIDs and its own Principal CA.  In addition to
   making a business decision to consider a trust relationship, each PKI
   determines the level of trust of each external PKI by reviewing
   external PKI Certificate Policy Document(s) and any other PKI
   governance documentation through a process known as policy mapping.
   Trust relationships are technically formalized through the issuance
   of cross-certificates.  Such a collection of two or more PKIs is
   known as a PKI domain.

   PKI domain:  A set of two or more PKIs that have chosen to enter into
      trust relationships with each other through the use of cross-
      certificates.  Each PKI that has entered into the PKI domain is
      considered a member of that PKI domain.

      NOTE:  This definition specifies a PKI domain recursively in terms
         of its constituent domains and associated trust relationships;
         this is different to the definition in RFC 4949 [RFC4949] that
         gives PKI domain as a synonym for CA domain and defines it in
         terms of a CA and its subject entities.

   Domain Policy Object Identifier:  A domain Policy Object Identifier
      (OID) is a Policy OID that is shared across a PKI domain.  Each CA
      in the PKI domain must be operated under the domain Policy OID.
      Each CA may also have its own Policy OID(s) in addition to the
      domain Policy OID.  In such a case, the CA must comply with both
      policies.  The domain Policy OID is used to identify the PKI
      domain.

   Policy Mapping:  A process by which members of a PKI domain evaluate
      the Certificate Policies (CPs) and other governance documentation
      of other potential PKI domain members to determine the level of
      trust that each PKI in the PKI domain places on certificates
      issued by each other PKI in the PKI domain.



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3.1.  PKI Domain Properties

   o  A PKI domain may operate a Bridge CA or a Unifying CA that defines
      members of the domain by issuing cross-certificates to those
      members.

   o  A single PKI may simultaneously belong to two or more PKI domains.

   o  A PKI domain may contain PKI domains within its own membership.

   o  Two or more PKI domains may enter into a trust relationship with
      each other, creating a new PKI domain.  They may choose to retain
      the existing PKI domains in addition to the new PKI domain or
      collapse the existing PKI domains into the new PKI domain.

   o  A member of a PKI domain may choose to participate in the PKI
      domain but restrict or deny trust in one or more other member PKIs
      of that same PKI domain.

3.2.  Requirements for Establishing and Participating in PKI Domains

   The establishment of trust relationships has a direct impact on the
   trust model of relying parties.  As a result, consideration must be
   taken in the creation and maintenance of PKI domains to prevent
   creating inadvertent trust relationships.

3.2.1.  PKI Requirements

   In order for a PKI to participate in one or more PKI domains, that
   PKI must have the following:

   o  A Certificate Policy Document documenting the requirements for
      operation of that PKI.  The Certificate Policy Document should be
      in RFC 3647 [RFC3647] format.

   o  One or more Policy OIDs defined in the Certificate Policy Document
      that are also asserted in all certificates issued by that PKI.

   o  A defined Principal CA.

   PKI domains may also impose additional technical, documentation, or
   policy requirements for membership in the PKI domain.

   When participating in a PKI domain, the domain Policy OID(s) must be
   asserted at least in cross-certificates issued by a participating
   PKI.  After the participation, the PKI can assert the domain Policy
   OID(s) in certificates issued by that PKI, or may map the domain




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   Policy OID(s) to the Policy OID(s) asserted in certificates issued by
   that PKI.

3.2.2.  PKI Domain Documentation

   PKI domains must be formally defined and documented.  This
   documentation may vary greatly depending on the PKI domain.  However,
   it must:

   o  Establish the existence of the PKI domain;

   o  Define the authority for maintaining the PKI domain;

         Examples of PKI domain Authorities are (1) Representatives from
         two PKIs that agree to form a simple PKI domain, (2) A single
         entity that may or may not be related to any of the PKIs in the
         PKI domain, (3) A governance board made up of representatives
         from each PKI domain member.

   o  Define how the PKI domain is governed;

   o  Define the purpose and community of interest of the PKI domain;
      and

         Examples of PKI domain intents are (1) allow relying parties of
         one PKI to trust certificates issued by another PKI, (2) allow
         PKIs that support similar subscriber communities of interest to
         interact with each other, and (3) allow relying parties to
         trust certificates issued by a number of PKIs that all meet a
         set of requirements.

   o  Unless the PKI domain has a predetermined membership, describe the
      requirements and methods for joining the PKI domain, such as
      FPKIMETHOD [FPKIMETHOD].

   Examples of governance documents that PKI domains may choose to use
   are:

   o  Statement of intent between two or more parties;

   o  Memorandum of Agreement between two or more parties;

   o  Certificate Policy Document for the PKI domain;

   o  Charter for the PKI domain; or

   o  Methodology for PKI domain membership.




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3.2.3.  PKI Domain Membership Notification

   A cross-certificate from the Principal CA of one PKI to the Principal
   CA of another PKI indicates a mapping between one or more policies of
   the first PKI and one or more policies of the second PKI.  When a
   relying party is determining if a certificate can be validated, it
   builds a certification path from the certificate being presented to a
   trust anchor.  To prevent creating inadvertent trust relationships
   across PKI domains when a single PKI is a member of two or more
   disparate PKI domains, each PKI domain must be cognizant of what PKI
   domains in which its member PKIs participate.  Figure 9 illustrates
   this concept.

                              +-----------------------------+
                              |                PKI domain 2 |
               +----------------------------+               |
               |              |             |               |
               | +------+ <------ +------+ <------ +------+ |
               | | PKI1 |     |   | PKI2 |  |      | PKI3 | |
               | +------+ ------> +------+ ------> +------+ |
               |              |             |               |
               |              +-----------------------------+
               | PKI domain 1               |
               +----------------------------+

              Figure 9: Participation in Multiple PKI Domains

   As shown in Figure 9, PKI2 is a member of both PKI domain 1 and PKI
   domain 2.  Since a certification path exists from PKI1 to PKI2, and
   from PKI2 to PKI3, a certification path also exists from PKI1 to
   PKI3.  However, PKI1 does not share domain membership with PKI3, so
   the certification path validation from PKI1 to PKI3 with a validation
   policy for PKI domain 1 must not succeed.  To ensure correct
   certification path validation and policy mapping, the cross-
   certificates issued by both PKI1 and PKI3 to PKI2 must contain
   constraints such as policy mapping or name constraints disallowing
   the validation of certification paths outside their respective
   domains.

   To fully prevent inadvertent trust, any PKI that is a member of one
   or more PKI domains must inform all those PKI domains of its
   membership in all other PKI domains.  In addition, that PKI must
   inform all those PKI domains of which it is a member, any time its
   membership status changes with regards to any other PKI domain.  If a
   PKI domain is informed of the change in status of one of its member
   PKIs with regards to other PKI domains, that PKI domain must review
   the constraints in any cross-certificate issued to that PKI.  If the
   change in membership would result in a change to the allowed or



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   disallowed certification paths, the PKI domain must ensure that all
   such cross-certificates are revoked and re-issued with correct
   constraints.

3.2.4.  Considerations for PKIs and PKI Domains with Multiple Policies

   In some cases, a single PKI may issue certificates at more than one
   assurance level.  If so, the Certificate Policy Document must define
   separate Policy OIDs for each assurance level, and must define the
   differences between certificates of different assurance levels.

   A PKI domain may also support more than one assurance level.  If so,
   the PKI domain must also define separate Policy OIDs for each
   assurance level, and must define the differences in requirements for
   each level.

   When PKIs and PKI domains choose to establish trust relationships,
   these trust relationships may exist for only one defined assurance
   level, may have a one-to-one relationship between PKI assurance
   levels and PKI domain assurance levels, or may have many-to-one or
   one-to-many relationships between assurance levels.  These
   relationships must be defined in cross-certificates issued between
   PKIs in the PKI domain.

3.3.  PKI Domain Models

   Two or more PKI domains may choose to enter into trust relationships
   with each other.  In that case, they may form a larger PKI domain by
   establishing a new Unifying or Bridge CA or by issuing cross-
   certificates between their Principal CAs.

3.3.1.  Unifying Trust Point (Unifying Domain) Model

   In the Unifying Trust Point Model, a PKI domain is created by
   establishing a joint, superior CA that issues unilateral cross-
   certificates to each PKI domain, as shown in Figure 10.  Such a
   joint, superior CA is defined as a Unifying CA, and the Principal CAs
   in each PKI domain have the hierarchical CA relationship with that
   Unifying CA.  In this model, any relying party from any of the PKI
   domains must specify the Unifying CA as its trust anchor CA in order
   to validate a subscriber in the other PKI domains.  If the relying
   party does not desire to validate subscribers in other PKI domains,
   the relying party may continue to use the Principal CA from the old
   PKI domain as its trust anchor CA.

   This model may be used for merging multiple PKI domains into a single
   PKI domain with less change to existing PKI domains, or may be used
   to combine multiple PKI domains into one PKI domain for relying



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   parties.  The unilateral cross-certificate issued by the Unifying CA
   to the Principal CAs in each PKI domain may include any policy
   mapping.

              Cross-certified                   Cross-certified
               Unifying CA                       Unifying CA
              to PKI domain 1 +--------------+  to PKI domain 3
                    +---------|  Unifying CA |---+
                    |         +--------------+   |
                    |                 |          |
                    |  Cross-certified|          |
                    |   Unifying CA   |          |
                    |  to PKI domain 2|          |
        +-----------|---+ +-----------|---+ +----|-----------------+
        |    PKI    |   | |    PKI    |   | |    |    PKI          |
        |  domain 1 |   | |  domain 2 |   | |    |  domain 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 10: Unifying Trust Point (Unifying Domain) Model

3.3.2.  Independent Trust Point Models

   In Independent Trust Point Models, relying parties continue to use
   only the trust anchor of their PKI domain.  A relying party in the
   individual trust point model can continue to use the trust anchor of
   its PKI domain.

3.3.2.1.  Direct Cross-Certification Model

   In this model, each PKI domain trusts each other by issuing a cross-
   certificate directly between each Principal CA, as shown in



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   Figure 11.  This model may be used for shortening a certification
   path or establishing a trust relationship expeditiously.

   Considerations:  A PKI domain in this model needs to take into
      account that the other PKI domain may cross-certify with any other
      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.  A PKI domain that relies on the
      validation policy of the relying party about such constraints
      cannot guarantee that the constraints will be recognized and
      followed.


        +---------------+                 +------------------------+
        |    PKI        | cross-certified |         PKI            |
        |  domain 1     |    each other   |       domain 2         |
        |      +-----+ --------------------> +-----+ ----+         |
        |      | PCA |  |                 |  | PCA |     |         |
        |      +-----+ <-------------------- +-----+ <-+ |         |
        |         |     |                 |     ^      | v         |
        |         |     |                 |     |    +----+        |
        |         |     |                 |     |    | CA |---+    |
        |         |     |                 |     |    +----+   |    |
        |         v     |                 |     v     ^ |     |    |
        |       +----+  |                 |   +----+  | |     |    |
        |   +---| CA |  |                 |   | CA |--+ |     |    |
        |   |   +----+  |                 |   +----+    |     |    |
        |   |      |    |                 |     |       |     |    |
        |   v      v    |                 |     v       v     v    |
        | +----+ +----+ |                 |   +----+ +----+ +----+ |
        | | EE | | EE | |                 |   | EE | | EE | | EE | |
        | +----+ +----+ |                 |   +----+ +----+ +----+ |
        +---------------+                 +------------------------+

                Figure 11: Direct Cross-Certification Model

3.3.2.2.  Bridge Model

   In this model, every PKI domain trusts each other through a Bridge CA
   by cross-certification, as shown in Figure 12.  The trust
   relationship is not established between a subscriber domain and a
   relying party domain directly, but established from the Principal CA
   of the relying party's PKI domain via a Bridge CA.  This model is
   useful in reducing the number of cross-certifications required for a
   PKI domain to interoperate with other PKI domains.





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   Requirements for Bridge model:

   o  The Bridge CA must not be used as the trust anchor CA in any PKI
      domain.

   o  The Bridge CA should issue cross-certificates with other PKI
      domains mutually or may issue cross-certificates unilaterally.

   o  The Bridge CA must not issue End Entity (EE) certificates except
      when it is necessary for the CA's operation.

   o  The Bridge CA must use its own domain Policy OID, not other PKI
      domain Policy OID(s), for the policy mapping.

   o  The Bridge CA should be a neutral position to all PKI domains,
      which trust through the Bridge CA.  For example, in Figure 12, in
      the case that a relying party who trusts the PCA of PKI domain 1
      as its trust anchor CA builds the certification path to a
      subscriber in PKI domain 3:

         Cross-Certificate from PKI domain 1 to the Bridge CA:

            issuerDomainPolicy ::= domain Policy OID of PKI domain 1

            subjectDomainPolicy := domain Policy OID of the Bridge CA

         Cross-Certificate from the Bridge CA to PKI domain 3:

            issuerDomainPolicy ::= domain Policy OID of the Bridge CA

            subjectDomainPolicy ::= domain Policy OID of PKI domain 3

   o  Cross-certificates issued by the Bridge CA and cross-certificate
      issued to the Bridge CA should include the requireExplicitPolicy
      with a value that is greater than zero in the policyConstraints
      extension because a relying party may not set the initial-
      explicit-policy to TRUE.

   o  PKI domains cross-certified with the Bridge CA should not cross-
      certify directly to other PKI domains cross-certified with the
      same Bridge CA.

   o  The Bridge CA should clarify the method for the policy mapping of
      cross-certification to keep its transparency.







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   Considerations:  The Bridge CA should be operated by an independent
      third party agreed upon by the PKI domains or a consortium
      consisting of representatives from the PKI domain members.  The
      Bridge CA should do policy mapping in a well-documented and
      agreed-upon manner with all PKI domains.  When applying the name
      constraints, the Bridge CA needs to avoid creating conflicts
      between the name spaces of the cross-certified PKI domains.  The
      PKI domains that perform cross-certification with the Bridge CA
      should confirm the following:

      *  Does the Bridge CA perform the policy mapping via its own
         domain Policy OID?

      *  Does the Bridge CA clarify the method of policy mapping in the
         cross-certification?

      *  Is the Bridge CA able to accept the domain policy that the PKI
         domain desires?

         +  If the domain policy is mapped to one with a lower security
            level, the PKI domain should not accept it.  Otherwise, the
            PKI domain must carefully consider the risks involved with
            accepting certificates with a lower security level.




























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          cross-certified                      cross-certified
        PKI domain 1 with BCA               PKI domain 3 with BCA
                  +---------> +-----------+ -----+
                  |           | Bridge CA |      |
                  | +-------- +-----------+ <--+ |
                  | |                 ^ |      | |
                  | | cross-certified | |      | |
                  | |   PKI domain 2  | |      | |
                  | |     with BCA    | |      | |
        +---------|-|---+ +-----------|-|-+ +--|-|-----------------+
        |  PKI    | |   | |   PKI     | | | |  | |    PKI          |
        |domain 1 | v   | | domain 2  | v | |  | v  domain 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 12: Bridge Model

3.4.  Operational 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 affect 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 cannot guarantee the constraints will be
   recognized and followed.





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4.  Trust Models External to PKI Relationships

   As opposed to PKI domain trust relationships entered into by PKIs
   themselves, trust across multiple PKIs can be created by entities
   external to the PKIs through locally configured lists of trust
   anchors.

   Trust List:  A set of one or more trust anchors used by a relying
      party to explicitly trust one or more PKIs.

   Note that Trust Lists are often created without the knowledge of the
   PKIs that are included in the list.

4.1.  Trust List Models

4.1.1.  Local Trust List Model

   A Trust List can be created and maintained by a single relying party
   for its own use.

   Local Trust List:  A Trust List installed and maintained by a single
      relying party for its own use.  NOTE: This definition is similar
      to "trust-file PKI" defined in RFC 4949 [RFC4949].  However, this
      document prefers the term "Local Trust List" contrasting with
      "Trust Authority" defined below.

   Figure 13 illustrates a Local Trust List.

      +-------------------------------------------------------------+
      |  Relying party                                              |
      | +---------------------------------------------------------+ |
      | | Trust List                                              | |
      | | +--------------+  +--------------+     +--------------+ | |
      | | | PKI 1        |  | PKI 2        | ... | PKI n        | | |
      | | | Trust anchor |  | Trust anchor |     | Trust anchor | | |
      | | +--------------+  +--------------+     +--------------+ | |
      | +---------------------------------------------------------+ |
      +-------------------------------------------------------------+

              Figure 13: Relying Party Local Trust List Model

   Creating a Local Trust List is the simplest method for relying
   parties to trust EE certificates.  Using Local Trust Lists does not
   require cross-certification between the PKI that issued the relying
   party's own certificate and the PKI that issued the EE's
   certificate,nor does it require implementing mechanisms for
   processing complex certification paths, as all CAs in a path can be
   included in the Local Trust List.  As a result, Local Trust Lists are



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   the most common model in use today.  However, because Local Trust
   Lists are created and managed independently by each relying party,
   the use of Local Trust Lists can be difficult for an enterprise to
   manage.

4.1.2.  Trust Authority Model

   Alternatively, a Trust List can be created and maintained for using
   by multiple relying parties.  In this case, the entity responsible
   for the Trust List is known as a Trust Authority.

   Trust Authority:  An entity that manages a Trust List for use by one
      or more relying parties.

   Figure 14 illustrates a Trust Authority and how it is used by Relying
   Parties.  Note that the Trust Authority replaces the PKI trust
   anchor(s) in the Local Trust List for each participating relying
   party.

      +-------------------------------------------------------------+
      |  Trust Authority                                            |
      | +---------------------------------------------------------+ |
      | | Trust List                                              | |
      | | +--------------+  +--------------+     +--------------+ | |
      | | | PKI 1        |  | PKI 2        | ... | PKI n        | | |
      | | | Trust anchor |  | Trust anchor |     | Trust anchor | | |
      | | +--------------+  +--------------+     +--------------+ | |
      | +---------------------------------------------------------+ |
      +-------------------------------------------------------------+

           +---------------------+  +---------------------+
           |   Relying party 1   |  |   Relying party 2   |
           | +-----------------+ |  | +-----------------+ | ...
           | | Trust Authority | |  | | Trust Authority | |
           | +-----------------+ |  | +-----------------+ |
           +---------------------+  +---------------------+

                     Figure 14: Trust Authority Model

   A Trust Authority may be operated by a PKI, a collection of relying
   parties that share a common set of users, an enterprise on behalf of
   all of its relying parties, or an independent entity.  Although PKIs
   generally establish trust relationships through cross-certificates, a
   PKI may choose to provide a Trust Authority to support relying
   parties that do not support processing of certification paths.  A
   collection of relying parties that share a common set of users may
   choose to maintain a single Trust Authority to simplify the
   management of Trust Lists.  An enterprise may choose to provide a



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   Trust Authority to implement enterprise policies and direct all
   Relying Parties within the enterprise to use its Trust Authority.
   Finally, an independent entity may choose to operate a Trust
   Authority as a managed service.

4.2.  Trust List Considerations

4.2.1.  Considerations for a PKI

   A PKI should publish its Certificate Policy Document so that Relying
   Parties and Trust Authorities can determine what, if any, warranties
   are provided by the PKI regarding reliance on EE certificates.

   A PKI should broadly publicize information regarding revocation or
   compromise of a trust anchor CA or Principal CA certificate through
   notice on a web page, press release, and/or other appropriate
   mechanisms so that Relying Parties and Trust Authorities can
   determine if a trust anchor CA or Principal CA certificate installed
   in a Trust List should be removed.

   A PKI should publish Certificate Revocation Lists (CRLs) or other
   information regarding the revocation status of EE certificates to a
   repository that can be accessed by any party that desires to rely on
   the EE certificates.

4.2.2.  Considerations for Relying Parties and Trust Authorities

   Relying Parties and Trust Authorities are responsible for the
   following prior to including a PKI in the Trust List:

   o  Reviewing the Certificate Policy Document of each PKI to determine
      that the PKI is operated to an acceptable level of assurance;

   o  Reviewing the Certificate Policy Document of each PKI to ensure
      any requirements imposed on Relying Parties are met;

   o  Determining if the PKI provides any warranties regarding reliance
      on EE certificates, and if these warranties are acceptable for the
      intended reliance on the EE certificates.  Reliance may be at the
      relying party's own risk; and

   o  Periodically reviewing information published by the PKI to its
      repository, including Certificate Policy Document updates or
      notice of CA revocation or compromise.

   A PKI can choose to join or leave PKI domains in accordance with its
   Certificate Policy Document.  If the relying party or Trust Authority
   does not wish to inherit trust in other members of these PKI domains,



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   it is the responsibility of the relying party or Trust Authority to
   inhibit policy mapping.

4.2.3.  Additional Considerations for Trust Authorities

   A Trust Authority should establish a Trust Authority Policy that
   identifies the following:

   o  The intended community of Relying Parties that will use the Trust
      Authority;

   o  The process by which trust anchors are added or removed from the
      Trust List;

   o  Any warranties provided by the Trust Authority for reliance on EE
      certificates.  These warranties may be those provided by the PKIs
      themselves or may be additional warranties provided by the Trust
      Authority;

   o  Information regarding how the Trust Authority protects the
      integrity of its Trust List; and

   o  Information regarding how Relying Parties interact with the Trust
      Authority to obtain information as to whether an EE certificate is
      trusted.

5.  Abbreviations

   CA:  Certification Authority

   EE:  End Entity

   OID:  Object Identifier

   PCA:  Principal Certification Authority

   PKI:  Public Key Infrastructure

6.  Security Considerations

   This section highlights security considerations related to
   establishing PKI domains.

6.1.  PKI Domain Models

   For all PKI domain models described in Section 3.3 created through
   the issuance of cross-certificates, standard threats including
   message insertion, modification, and man-in-the-middle are not



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   applicable because all information created by CAs, including policy
   mapping and constraints, is digitally signed by the CA generating the
   cross-certificate.

   Verifying that a given certificate was issued by a member of a PKI
   domain may be a time-critical determination.  If cross-certificates
   and revocation status information cannot be obtained in a timely
   manner, a denial of service may be experienced by the end entity.  In
   situations where such verification is critical, caching of cross-
   certificates and revocation status information may be warranted.

   An additional security consideration for PKI domains is creating
   inadvertent trust relationships, which can occur if a single PKI is a
   member of multiple PKI domains.  See Section 3.2.3 for a discussion
   of creating inadvertent trust relationships and mechanisms to prevent
   it.

   Finally, members of PKI domains must participate in domain
   governance, or at a minimum, be informed anytime a PKI joins or
   leaves the domain, so that domain members can make appropriate
   decisions for maintaining their own membership in the domain or
   choosing to restrict or deny trust in the new member PKI.

6.2.  Trust List Models

   In these models, many standard attacks are not applicable since
   certificates are digitally signed.  Additional security
   considerations apply when trust is created through a Trust List.

   A variation of the modification attack is possible in Trust List
   Models.  If an attacker is able to add or remove CAs from the relying
   party or Trust Authority Trust List, the attacker can affect which
   certificates will or will not be accepted.  To prevent this attack,
   access to Trust Lists must be adequately protected against
   unauthorized modification.  This protection is especially important
   for trust anchors that are used by multiple applications, as it is a
   key vulnerability of this model.  This attack may result in
   unauthorized usage if a CA is added to a Trust List, or denial of
   service if a CA is removed from a Trust List.

   For Trust Authority models, a denial-of-service attack is also
   possible if the application cannot obtain timely information from the
   trust anchor.  Applications should specify service-level agreements
   with Trust Authority.  In addition, applications may choose to
   locally cache the list of CAs maintained by the Trust Authority as a
   backup in the event that the trust anchor's repository (e.g.,
   Lightweight Directory Access Protocol (LDAP) directory) is not
   available.



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7.  References

7.1.  Normative References

   [RFC5280]          Cooper, D., Santesson, S., Farrell, S., Boeyen,
                      S., Housley, R., and W. Polk, "Internet X.509
                      Public Key Infrastructure Certificate and
                      Certificate Revocation List (CRL) Profile",
                      RFC 5280, May 2008.

7.2.  Informative References

   [CCITT.X509.2000]  International Telephone and Telegraph Consultative
                      Committee, "Information Technology - Open Systems
                      Interconnection - The Directory: Authentication
                      Framework", CCITT Recommendation X.509,
                      March 2000.

   [FPKIMETHOD]       "US Government PKI Cross-Certification Criteria
                      and Methodology", January 2006, <http://
                      www.cio.gov/fpkia/documents/
                      crosscert_method_criteria.pdf>.

   [RFC3647]          Chokhani, S., Ford, W., Sabett, R., Merrill, C.,
                      and S. Wu, "Internet X.509 Public Key
                      Infrastructure Certificate Policy and
                      Certification Practices Framework", RFC 3647,
                      November 2003.

   [RFC4949]          Shirey, R., "Internet Security Glossary, Version
                      2", RFC 4949, August 2007.




















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Authors' Addresses

   Masaki Shimaoka (editor)
   SECOM Co., Ltd. Intelligent System Laboratory
   SECOM SC Center, 8-10-16 Shimorenjaku
   Mitaka, Tokyo  181-8528
   JP

   EMail: m-shimaoka@secom.co.jp


   Nelson Hastings
   National Institute of Standard and Technology
   100 Bureau Drive, Stop 8930
   Gaithersburg, MD  20899-8930
   US

   EMail: nelson.hastings@nist.gov


   Rebecca Nielsen
   Booz Allen Hamilton
   8283 Greensboro Drive
   McLean, VA  22102
   US

   EMail: nielsen_rebecca@bah.com
























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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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Shimaoka, et al.             Informational                     [Page 29]

