Network Working Group M. Shimaoka Request for Comments: DRAFT SECOM N. Hastings NIST R. Nielsen Booz Allen Hamilton January 2006 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 aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on July 12, 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 (CP) at a Shimaoka, et al. [Page 1] INTERNET DRAFT January 2006 specified trust level. A multi-domain PKI is established by combining more than one single-domain PKI. A multi-domain PKI can be categorized as either a multi-trust point model based on the trust list model; or single-trust point model based on the Cross- Certification model. Table of Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 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 PKIs . . . . . . . . . . . . . . . . . . 17 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 Shimaoka, et al. [Page 2] INTERNET DRAFT January 2006 9.1 Normative References . . . . . . . . . . . . . . . . . . . . 30 9.2 Informative References . . . . . . . . . . . . . . . . . . . 30 10 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31 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 trust relationships fall into the operation based one or the certificate based one. In the multi-domain PKI they are called the multi trust point model and the single trust point model. 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 the definition of PKI domain and its trust relationships for the 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 | | Shimaoka, et al. [Page 3] INTERNET DRAFT January 2006 | | Trust | | | +-----+ | Relationship | +-----+ | | | PCA |<===========================>| PCA | | | +-----+ | | +-----+ | | ^ | | ^ | | | CA-CA Trust | | | CA-CA Trust | | | Relationship | | | Relationship | | v | | v | | +----+ | | +----+ | | | CA | | | | CA | | | +----+ | | +----+ | +------------------+ +-------------------+ Figure 1 - Structure of multi-domain PKI 2 Requirements and Assumptions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. 2.1 Abbreviations ARL: Authority Revocation List CA: Certification Authority CP: Certification Policy CPS: Certification Practice Statement CRL: Certificate Revocation List DN: Distinguished Name EE: End Entity PCA: Principal Certificate Authority PKI: Public Key Infrastructure RP: Relying Party 2.2 Terminology CA-CA Trust Relationship ### TBD ### Shimaoka, et al. [Page 4] INTERNET DRAFT January 2006 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 broader 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. Domain Policy Domain Policy is a common certificate policy (Object Identifier) that is shared in a PKI domain. Each CAs in the PKI domain MUST be operated under the domain policy at least. Each CAs is able to have another policy for itself in addition to the domain policy. In such a case, the CAs MUST comply with both policies. The policy OID of the domain policy is used to distinguish the PKI domain from another. EE-CA Trust Relationship ### TBD ### 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 relying 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]. Intermediate Certificate Certificates in a certification path except the trust anchor certificate and the certificate being validated. Irresponsible EE Relying party who is not issued a certificate from a certain CA, and is irresponsible for that CA. Multi-domain PKI A set of PKI domains which interoperate each other. Shimaoka, et al. [Page 5] INTERNET DRAFT January 2006 PKI In this document, this wording is more limited than the definition in RFC 2828. PKI in this document means a minimal system operated under unique policy. PKI domain PKI domain can consist of a set of CAs which are possibly operated by different organizations under shared CP even though each of them may have additional CPs that differ. 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. 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. 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 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. Relying Party (RP) Entity who trusts a trust anchor and may not be issued the certificates. Responsible EE Relying party who is issued a certificate from a certain CA, and is responsible for that CA. Shimaoka, et al. [Page 6] INTERNET DRAFT January 2006 Subordination 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. Subscriber This is an equivalent term with the responsible EE. 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. 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 a unified domain model, a unificate CA SHOULD be used as defined later in this section. Trust Anchor Starting point of a certification path specified by a relying party. Relying party SHOULD specify the CA which has self-signed certificate as a trust anchor. In this document, Top CA is used as a trust anchor only for Hierarchical PKI. There may not be Top CA in the other trust models. 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. Trust Relationship This word is used for two purposes. One is the "CA-CA trust relationship" which is used for the relationship between CAs. Another one is the "EE-CA trust relationship" which is used for the relationship between CA and relying parties. Validation policy Shimaoka, et al. [Page 7] INTERNET DRAFT January 2006 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. 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. 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. Since there should be different policies for every trust anchors whether operated by the same body or not, there SHOULD be different validation policies for every trust anchors. Considerations Shimaoka, et al. [Page 8] INTERNET DRAFT January 2006 A relying party using the trust list MAY trust multiple trust anchors, but finding out a revocation of each trust anchor is more difficult than finding out it for one. Trust List +--------------------------------------------------------------+ | Trusted CA | | | | +---------------+ +---------------+ +----------------------+ | | | PKI 1 | | PKI 2 | | PKI 3 | | | | | | | | | | | | +-----+ | | +-----+ | | +-----+ | | | | +---| PCA | | | | PCA | | | | PCA |<--+ | | | | | +-----+ | | +-----+ | | +-----+ | | | | | | | | | | | | ^ | | | +-----|------|-----------|----------------|-------|------------+ | | | | | | | | | | | | | | | | | | | | v | | | | | | | | | | +----+ | | | | | | | | | | | CA |---+ | | | | | | | | | | +----+ | | | | | | | | | | | ^ | | | | | | | | v | | v | | | | | | | | | +----+ | | +----+ | | | | | | | | | | CA |---+ | | | CA |---+ | | | | | | | | +----+ | | | +----+ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | 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 Shimaoka, et al. [Page 9] INTERNET DRAFT January 2006 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 relationship. There is a risk that the EE trusts unknown PKI domain irresponsibly. If the EE trusts unknown PKI domains irresponsibly, then the issuer CA cannot apply its CP to the EE. A trust anchor MAY not apply its validation policy to the EE. Considerations EE MUST update its own user trust list when the status of CA which is included in the trust list changes, such as revocation or updating. 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 Shimaoka, et al. [Page 10] INTERNET DRAFT January 2006 A subject CA in the cross-certification MUST have a self-signed certificate. Characteristics Cross-Certification is a more formal expression of the trust relationship than the trust list model, because the trust relationship is represented by a certificate, (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. Shimaoka, et al. [Page 11] INTERNET DRAFT January 2006 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 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.(See 3.2.3) 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 publish 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 as well as 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 illustrates helping RPs who use the 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 | | | | | | +----+ | Shimaoka, et al. [Page 12] INTERNET DRAFT January 2006 | | | | | | CA |---+ | | | | | | +----+ | | | | | | | ^ | | | | v | | v | | | | | +----+ | | +----+ | | | | | | CA |---+ | | | CA |---+ | | | | +----+ | | | +----+ | | | | | | | | | | | | | | | | | | | | | | v v | | v v v | | +----+ +----+ | | +----+ +----+ +----+ | | | EE | | EE | | | | EE | | EE | | EE | | | +----+ +----+ | | +----+ +----+ +----+ | +---------------+ +----------------------+ Figure 3 - Unilateral Cross-Certification 3.2.2 Mutual cross-certification Definition The model in which two self-signed CAs issue cross-certificates to each other. Characteristics Both CAs cross-certify with each other mutually. 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 and place these in the directory system. +---------------+ +----------------------+ | PKI 1 | | PKI 2 | | | cross-certified | | Shimaoka, et al. [Page 13] INTERNET DRAFT January 2006 | +-----+ | PKI 1 and PKI 2 | +-----+ | | | PCA |<-------------------------->| PCA |<--+ | | +-----+ | | +-----+ | | | | | | ^ | | | | | | | v | | | | | | +----+ | | | | | | | CA |---+ | | | | | | +----+ | | | | | | | ^ | | | | v | | v | | | | | +----+ | | +----+ | | | | | | CA |---+ | | | CA |---+ | | | | +----+ | | | +----+ | | | | | | | | | | | | | | | | | | | | | | v v | | v v v | | +----+ +----+ | | +----+ +----+ +----+ | | | EE | | EE | | | | EE | | EE | | EE | | | +----+ +----+ | | +----+ +----+ +----+ | +---------------+ +----------------------+ Figure 4 - Mutual Cross-Certification 3.3 Subordination (Hierarchy) Subordination is a special subset of 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 always has 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 subordinate CAs and their EEs by trusting only the root CA. Subordination is different from unilateral cross- certification, in that the subordination model MUST NOT allow a subordinate CA to be issued a certificate by more than one issuer CAs. A subordinate CA MAY NOT necessarily require an accreditation, such as WebTrust or license under the e-signature law. The accreditation is rather required only for the superior CA Shimaoka, et al. [Page 14] INTERNET DRAFT January 2006 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 only to CAs that operate under its accredited policies and procedures. In this case, accreditation means that the subordinate CA can inherit the benefit of the trustworthiness of the superior CA. An existence of the subordinate CA is dependent on the superior CA. The subordinate CA is dependent on the superior CA for its existing. 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 domain, 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 CPs. The CPs 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. Shimaoka, et al. [Page 15] INTERNET DRAFT January 2006 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 certificate under including a valid CP of the domain policy. 4.2 Risk Analysis of non-interoperable PKI domains A PKI domain that satisfies the requirements presented in 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 A PKI domain SHOULD agree to use namespaces that do not overlap. If the PKI domain namespaces overlap, a namespace conflict occurs resulting in the name constraints extension possibly not being able to 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 the relying parties use the certificate policies extension to identify the PKI domain. The PKI domain policy is necessary in path validation through the 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 explicitly include 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 CP in the path validation. Even if 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 EEs and irresponsible EEs. A responsible EE may set the flag to TRUE, but an irresponsible EE may not. Therefore, CA-X SHOULD issue a certificate which uses requireExplicitPolicy explicitly in the policyConstraints extension. If CA-X issues Shimaoka, et al. [Page 16] INTERNET DRAFT January 2006 all certificates which use requireExplicitPolicy in the policyConstraints extension, RP MUST evaluate the CP whether it has responsibility or not. - 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 the 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 domain policy OID. * initial-explicit-policy set to TRUE. * trust anchor which is the principal CA of its PKI domain. - Distribution of Certificate and Certificate Status Information A PKI domain 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 NOT trust an unnecessary PKI domain. * Posterior PKI domain X MAY publish what PKI domains it is trusted by to prior PKI domain Y, because PKI domain Y MAY consider the other certification paths to PKI domain X. In addition, a PKI domain SHOULD give the appropriate policy mappings between the prior PKI domains and the posterior PKI domains for certificate based trust relationship. 5 Single-domain PKI Shimaoka, et al. [Page 17] INTERNET DRAFT January 2006 This section describes the appropriate PKI architectures for establishing a single PKI domain. All PKIs that are going to participate in multi-domain PKI SHOULD adopt any of the following models for multi-domain PKI interoperability. 5.1 Single 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. +---------+ Shimaoka, et al. [Page 18] INTERNET DRAFT January 2006 +---| top CA |---+ | +---------+ | | | | | v v +----+ +----+ +-----| CA | +-----| CA |------+ | +----+ | +----+ | | | | v v v +----+ +----+ +----+ +--| CA |-----+ | CA |-+ +---| CA |---+ | +----+ | +----+ | | +----+ | | | | | | | | | | | | | | | | | v v v v v v v v +----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+ | EE | | EE | | EE | | EE | | EE | | EE | | EE | | EE | +----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+ Figure 6 - Hierarchy PKI model 5.3 Mesh PKI model Definition Mesh PKI consists of multiple CAs and their EEs. All CAs MUST be 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 does not have a self-signed certificate is authorized by another CA. In such a case, the relying parties may not recognize the revocation of the CA even if the issuing CA publishes the revocation information about that CA. Therefore, this document recommends that relying party SHOULD only trust other self-signed CAs. If there is no self-signed CA in that mesh, i.e. all CAs in the mesh certify with each other, the relying parties SHOULD choose a trust anchor from those CAs carefully. For Shimaoka, et al. [Page 19] INTERNET DRAFT January 2006 example, a relying party may choose a CA that is highly unlikely to be revoked. This model SHOULD be used sparingly, because of the complexity in certification path building. However, one should not assume that this model does not exist or is not 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. Shimaoka, et al. [Page 20] INTERNET DRAFT January 2006 - Intermediate certificate - Target certificate (optional) - Revocation information for all certificates For this, CAs MAY operate a repository, and SHOULD include authorityInfoAccess or cRLDistributionPoints extensions in the certificates they issue to maximize PKI interoperability. 6.1 Multi Trust point model (based on Trust List) The model in which a relying party trusts multiple PKI domains by a trust list. Considerations If the owner of the trust list adds a CA in the existing certification path, it SHOULD do so carefully since a constraint 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 trusts 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, the 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 PKIs, 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 Shimaoka, et al. [Page 21] INTERNET DRAFT January 2006 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. 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 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. 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 |---+ | +--------------+ | Shimaoka, et al. [Page 22] INTERNET DRAFT January 2006 | | | | cross-certified| | | Unificate CA | | | to PKI 2 | | +-----------|---+ +-----------|---+ +----|-----------------+ | PKI 1 | | | PKI 2 | | | | PKI 3 | | v | | v | | v | | +-----+ | | +-----+ | | +-----+ | | +---| PCA | | | | PCA | | | | PCA |<--+ | | | +-----+ | | +-----+ | | +-----+ | | | | | | | | | | ^ | | | | | | | | | | | v | | | | | | | | | | +----+ | | | | | | | | | | | CA |---+ | | | | | | | | | | +----+ | | | | | | | | | | | ^ | | | | | | | | v | | v | | | | | | | | | +----+ | | +----+ | | | | | | | | | +---| CA | | | | CA |---+ | | | | | | | | | +----+ | | +----+ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | v v | | v v | | v v v | | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ | | | EE | | EE | | | | EE | | EE | | | | EE | | EE | | EE | | | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ | +---------------+ +---------------+ +----------------------+ Figure 8 - Unified Domain model 6.2.2 Bridge model The model in which every PKI domain trusts each other through a Bridge CA by Cross-Certification. In this model, the 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. Shimaoka, et al. [Page 23] INTERNET DRAFT January 2006 - 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. - Bridge CA SHOULD be a neutral position to all PKI domains which trust through the Bridge CA. 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 policyConstraints extension. - Cross-certificate issued to Bridge CA SHOULD include the requireExplicitPolicy with a value that is greater than zero in the policyConstraints extension. - Cross-certificate issued by Bridge CA SHOULD NOT include any constraints to keep its neutral position. - 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 a 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, the 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 the 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 the 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. Shimaoka, et al. [Page 24] INTERNET DRAFT January 2006 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 +-----------+ PKI 3 with BCA +------->| Bridge CA |<------+ | +-----------+ | | ^ | | cross-certified | | | PKI 2 with BCA | | | | | +-----------|---+ +-----------|---+ +----|-----------------+ | PKI 1 | | | PKI 2 | | | | PKI 3 | | v | | v | | v | | +-----+ | | +-----+ | | +-----+ | | +---| PCA | | | | PCA | | | | PCA |<--+ | | | +-----+ | | +-----+ | | +-----+ | | | | | | | | | | ^ | | | | | | | | | | | v | | | | | | | | | | +----+ | | | | | | | | | | | CA |---+ | | | | | | | | | | +----+ | | | | | | | | | | | ^ | | | | | | | | v | | v | | | | | | | | | +----+ | | +----+ | | | | | | | | | +---| CA | | | | CA |---+ | | | | | | | | | +----+ | | +----+ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | v v | | v v | | v v v | | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ | | | EE | | EE | | | | EE | | EE | | | | EE | | EE | | EE | | | +----+ +----+ | | +----+ +----+ | | +----+ +----+ +----+ | +---------------+ +---------------+ +----------------------+ 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 Shimaoka, et al. [Page 25] INTERNET DRAFT January 2006 When CA-X cross-certifies CA-Y unilaterally, both CAs SHOULD operate their directory server in the following way. CA-X SHOULD generate the following crossCertificatePair and store it in its own directory entry. issuedToThisCA := NULL issuedByThisCA := cross-certificate for CA-Y issued by CA-X CA-Y MAY generate the 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. Shimaoka, et al. [Page 26] INTERNET DRAFT January 2006 7.2 Cross-Certification When updating the Cross-Certificate: There is a standard method for what to do when a cross- certificate is updated by modifying some of its contents, e.g., policy identifier. When issuer CA-X re-issues a cross-certificate to subject CA-Y before the issued cross-certificate expires, both CA-X and CA-Y MUST each update their own crossCertificatePair corresponding to the cross-certificate, and MUST publish 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 that the 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 the 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 the 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 Shimaoka, et al. [Page 27] INTERNET DRAFT January 2006 information is updated or changed. Directory infrastructures composed of stand alone directories provide certificate and CRL information from a set (list) of directories the relying parties are aware of. If a directory is queried but cannot satisfy the request, it MAY provide referrals to another directory that might be able to provide the requested information. To help promote interoperability, the PKI domains SHOULD provide access to the PKI domain's directory infrastructure via LDAP or HTTP and information to access (e.g. IP address or FQDN) at least one of the PKI domain's 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 a 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 cRLDistributionPoints 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 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 the PKI domain name spaces. 8.2 Path Validation Shimaoka, et al. [Page 28] INTERNET DRAFT January 2006 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 its validation policies, including the trust anchor. 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 creates 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 create 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 the same PKI actually Shimaoka, et al. [Page 29] INTERNET DRAFT January 2006 describe different policies, differing policy identifiers mapped unexpectedly in the same entity represent a critical security issue. To prevent such a security hole, a loop certification path, one where the same DN appears twice and non-continuously on one certification path MUST NOT be allowed. 9 References 9.1 Normative References [RFC 3280] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet 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] ITU-T Recommendation X.509 (2005 E): Information Technology - Open Systems Interconnection - The Directory: Authentication Framework, August 2005. 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. Shimaoka, et al. [Page 30] INTERNET DRAFT January 2006 Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S. and Nicholas, R., "Internet X.509 Public Key Infrastructure: Certification Path Building", RFC 4158, September 2005. 10 Acknowledgements This document is based on some valuable documents and many experiences with PKI interoperability experiments. The authors gratefully acknowledge the contributions of members of various multi- domain PKI interoperability experiments, in particular: Kenji Nakada, Kiyoshi Watanabe, Sang Hwan Park, Ryu Inada, Hiroyuki Yoshida and Yasushi Matsumoto. The authors 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 consists 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 Rebecca Nielsen Booz Allen Hamilton 8283 Greensboro Drive McLean, VA 22102 USA Email: nielsen_rebecca@bah.com 12 Full Copyright Statement Shimaoka, et al. [Page 31] INTERNET DRAFT January 2006 Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Shimaoka, et al. [Page 32]