Эта записка предоставляет информацию для интернет-сообщества. Оно делает
не указывать какой-либо интернет-стандарт. Распределение этого
Памятка не ограничена.

Абстрактные

В этом документе содержатся рекомендации и рекомендации для разработчиков
создание сертификационных путей открытого ключа X.509 в рамках их
Приложения. Следуя указаниям и рекомендациям, определенным
в этом документе разработчик приложения, скорее всего,
надежное приложение с поддержкой сертификатов X.509, которое может создавать действительные
сертификационных путей в широком диапазоне сред PKI.

=====================================

Network Working Group M. Cooper
Request for Comments: 4158 Orion Security Solutions
Category: Informational Y. Dzambasow
A&N Associates
P. Hesse
Gemini Security Solutions
S. Joseph
Van Dyke Technologies
R. Nicholas
BAE Systems
September 2005

Internet X.509 Public Key Infrastructure:
Certification Path Building

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

This document provides guidance and recommendations to developers
building X.509 public-key certification paths within their
applications. By following the guidance and recommendations defined
in this document, an application developer is more likely to develop
a robust X.509 certificate-enabled application that can build valid
certification paths across a wide range of PKI environments.

Table of Contents

1. Introduction ....................................................3
1.1. Motivation .................................................4
1.2. Purpose ....................................................4
1.3. Terminology ................................................5
1.4. Notation ...................................................8
1.5. Overview of PKI Structures .................................8
1.5.1. Hierarchical Structures .............................8
1.5.2. Mesh Structures ....................................10
1.5.3. Bi-Lateral Cross-Certified Structures ..............11
1.5.4. Bridge Structures ..................................13
1.6. Bridge Structures and Certification Path Processing .......14

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2. Certification Path Building ....................................15
2.1. Introduction to Certification Path Building ...............15
2.2. Criteria for Path Building ................................16
2.3. Path-Building Algorithms ..................................17
2.4. How to Build a Certification Path .........................21
2.4.1. Certificate Repetition .............................23
2.4.2. Introduction to Path-Building Optimization .........24
2.5. Building Certification Paths for Revocation Signer
Certificates ..............................................30
2.6. Suggested Path-Building Software Components ...............31
2.7. Inputs to the Path-Building Module ........................33
2.7.1. Required Inputs ....................................33
2.7.2. Optional Inputs ....................................34
3. Optimizing Path Building .......................................35
3.1. Optimized Path Building ...................................35
3.2. Sorting vs. Elimination ...................................38
3.3. Representing the Decision Tree ............................41
3.3.1. Node Representation for CA Entities ................41
3.3.2. Using Nodes to Iterate Over All Paths ..............42
3.4. Implementing Path-Building Optimization ...................45
3.5. Selected Methods for Sorting Certificates .................46
3.5.1. basicConstraints Is Present and cA Equals True .....47
3.5.2. Recognized Signature Algorithms ....................48
3.5.3. keyUsage Is Correct ................................48
3.5.4. Time (T) Falls within the Certificate Validity .....48
3.5.5. Certificate Was Previously Validated ...............49
3.5.6. Previously Verified Signatures .....................49
3.5.7. Path Length Constraints ............................50
3.5.8. Name Constraints ...................................50
3.5.9. Certificate Is Not Revoked .........................51
3.5.10. Issuer Found in the Path Cache ....................52
3.5.11. Issuer Found in the Application Protocol ..........52
3.5.12. Matching Key Identifiers (KIDs) ...................52
3.5.13. Policy Processing .................................53
3.5.14. Policies Intersect the Sought Policy Set ..........54
3.5.15. Endpoint Distinguished Name (DN) Matching .........55
3.5.16. Relative Distinguished Name (RDN) Matching ........55
3.5.17. Certificates are Retrieved from
cACertificate Directory Attribute .................56
3.5.18. Consistent Public Key and Signature Algorithms ....56
3.5.19. Similar Issuer and Subject Names ..................57
3.5.20. Certificates in the Certification Cache ...........57
3.5.21. Current CRL Found in Local Cache ..................58
3.6. Certificate Sorting Methods for Revocation Signer
Certification Paths .......................................58
3.6.1. Identical Trust Anchors ............................58
3.6.2. Endpoint Distinguished Name (DN) Matching ..........59
3.6.3. Relative Distinguished Name (RDN) Matching .........59

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3.6.4. Identical Intermediate Names .......................60
4. Forward Policy Chaining ........................................60
4.1. Simple Intersection .......................................61
4.2. Policy Mapping ............................................62
4.3. Assigning Scores for Forward Policy Chaining ..............63
5. Avoiding Path-Building Errors ..................................64
5.1. Dead Ends .................................................64
5.2. Loop Detection ............................................65
5.3. Use of Key Identifiers ....................................66
5.4. Distinguished Name Encoding ...............................66
6. Retrieval Methods ..............................................67
6.1. Directories Using LDAP ....................................67
6.2. Certificate Store Access via HTTP .........................69
6.3. Authority Information Access ..............................69
6.4. Subject Information Access ................................70
6.5. CRL Distribution Points ...................................70
6.6. Data Obtained via Application Protocol ....................71
6.7. Proprietary Mechanisms ....................................71
7. Improving Retrieval Performance ................................71
7.1. Caching ...................................................72
7.2. Retrieval Order ...........................................73
7.3. Parallel Fetching and Prefetching .........................73
8. Security Considerations ........................................74
8.1. General Considerations for Building a Certification Path ..74
8.2. Specific Considerations for Building Revocation
Signer Paths ..............................................75
9. Acknowledgements ...............................................78
10. Normative References ..........................................78
11. Informative References ........................................78

1. Introduction

[X.509] public key certificates have become an accepted method for
securely binding the identity of an individual or device to a public
key, in order to support public key cryptographic operations such as
digital signature verification and public key-based encryption.
However, prior to using the public key contained in a certificate, an
application first has to determine the authenticity of that
certificate, and specifically, the validity of all the certificates
leading to a trusted public key, called a trust anchor. Through
validating this certification path, the assertion of the binding made
between the identity and the public key in each of the certificates
can be traced back to a single trust anchor.

The process by which an application determines this authenticity of a
certificate is called certification path processing. Certification
path processing establishes a chain of trust between a trust anchor
and a certificate. This chain of trust is composed of a series of

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certificates known as a certification path. A certification path
begins with a certificate whose signature can be verified using a
trust anchor and ends with the target certificate. Path processing
entails building and validating the certification path to determine
whether a target certificate is appropriate for use in a particular
application context. See Section 3.2 of [RFC3280] for more
information on certification paths and trust.

1.1. Motivation

Many other documents (such as [RFC3280]) cover certification path
validation requirements and procedures in detail but do not discuss
certification path building because the means used to find the path
does not affect its validation. This document therefore is an effort
to provide useful guidance for developers of certification path-
building implementations.

Additionally, the need to develop complex certification paths is
increasing. Many PKIs are now using complex structures (see Section
1.5) rather than simple hierarchies. Additionally, some enterprises
are gradually moving away from trust lists filled with many trust
anchors, and toward an infrastructure with one trust anchor and many
cross-certified relationships. This document provides helpful
information for developing certification paths in these more
complicated situations.

1.2. Purpose

This document provides information and guidance for certification
path building. There are no requirements or protocol specifications
in this document. This document provides many options for performing
certification path building, as opposed to just one particular way.
This document draws upon the authors' experiences with existing
complex certification paths to offer insights and recommendations to
developers who are integrating support for [X.509] certificates into
their applications.

In addition, this document suggests using an effective general
approach to path building that involves a depth first tree traversal.
While the authors believe this approach offers the balance of
simplicity in design with very effective and infrastructure-neutral
path-building capabilities, the algorithm is no more than a suggested
approach. Other approaches (e.g., breadth first tree traversals)
exist and may be shown to be more effective under certain conditions.
Certification path validation is described in detail in both [X.509]
and [RFC3280] and is not repeated in this document.

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This document does not provide guidance for building the
certification path from an end entity certificate to a proxy
certificate as described in [RFC3820].

1.3. Terminology

Terms used throughout this document will be used in the following
ways:

Building in the Forward direction: The process of building a
certification path from the target certificate to a trust anchor.
'Forward' is the former name of the crossCertificatePair element
'issuedToThisCA'.

Building in the Reverse direction: The process of building a
certification path from a trust anchor to the target certificate.
'Reverse' is the former name of the crossCertificatePair element
'issuedByThisCA'.

Certificate: A digital binding that cannot be counterfeited between
a named entity and a public key.

Certificate Graph: A graph that represents the entire PKI (or all
cross-certified PKIs) in which all named entities are viewed as
nodes and all certificates are viewed as arcs between nodes.

Certificate Processing System: An application or device that
performs the functions of certification path building and
certification path validation.

Certification Authority (CA): An entity that issues and manages
certificates.

Certification Path: An ordered list of certificates starting with a
certificate signed by a trust anchor and ending with the target
certificate.

Certification Path Building: The process used to assemble the
certification path between the trust anchor and the target
certificate.

Certification Path Validation: The process that verifies the binding
between the subject and the subject-public-key defined in the
target certificate, using a trust anchor and set of known
constraints.

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Certificate Revocation List (CRL): A signed, time stamped list
identifying a set of certificates that are no longer considered
valid by the certificate issuer.

CRL Signer Certificate: The specific certificate that may be used for
verifying the signature on a CRL issued by, or on behalf of, a
specific CA.

Cross-Certificate: A certificate issued by one CA to another CA for
the purpose of establishing a trust relationship between the two
CAs.

Cross-Certification: The act of issuing cross-certificates.

Decision Tree: When the path-building software has multiple
certificates to choose from, and must make a decision, the
collection of possible choices is called a decision tree.

Directory: Generally used to refer an LDAP accessible repository for
certificates and PKI information. The term may also be used
generically to refer to any certificate storing repository.

End Entity: The holder of a private key and corresponding
certificate, whose identity is defined as the Subject of the
certificate. Human end entities are often called "subscribers".

Is-revocation-signer indicator: A boolean flag furnished to the
path-building software. If set, this indicates that the target
certificate is a Revocation Signer certificate for a specific CA.
For example, if building a certification path for an indirect CRL
Signer certificate, this flag would be set.

Local PKI: The set of PKI components and data (certificates,
directories, CRLs, etc.) that are created and used by the
certificate using organization. In general, this concept refers
to the components that are in close proximity to the certificate
using application. The assumption is that the local data is more
easily accessible and/or inexpensive to retrieve than non-local
PKI data.

Local Realm: See Local PKI.

Node (in a certificate graph): The collection of certificates having
identical subject distinguished names.

Online Certificate Status Protocol (OCSP): An Internet protocol used
by a client to obtain the revocation status of a certificate from
a server.

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OCSP Response Signer Certificate: The specific certificate that may
be used for verifying the signature on an OCSP response. This
response may be provided by the CA, on behalf of the CA, or by a
different signer as determined by the Relying Party's local
policy.

Public Key Infrastructure (PKI): The set of hardware, software,
personnel, policy, and procedures used by a CA to issue and manage
certificates.

Relying Party (RP): An application or entity that processes
certificates for the purpose of 1) verifying a digital signature,
2) authenticating another entity, or 3) establishing confidential
communications.

Revocation Signer Certificate: Refers collectively to either a CRL
Signer Certificate or OCSP Response Signer Certificate.

Target Certificate: The certificate that is to be validated by a
Relying Party. It is the "Certificate targeted for validation".
Although frequently this is the End Entity or a leaf node in the
PKI structure, this could also be a CA certificate if a CA
certificate is being validated. (e.g., This could be for the
purpose of building and validating a certification path for the
signer of a CRL.)

Trust (of public keys): In the scope of this document, a public key
is considered trustworthy if the certificate containing the public
key can be validated according to the procedures in [RFC3280].

Trust List: A list of trust anchors.

Trust Anchor: The combination of a trusted public key and the name of
the entity to which the corresponding private key belongs.

Trust Anchor Certificate: A self-signed certificate for a trust
anchor that is used in certification path processing.

User: An individual that is using a certificate processing system.
This document refers to some cases in which users may or may not
be prompted with information or requests, depending upon the
implementation of the certificate processing system.

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1.4. Notation

This document makes use of a few common notations that are used in
the diagrams and examples.

The first is the arrow symbol (->) which represents the issuance of a
certificate from one entity to another. For example, if entity H
were to issue a certificate to entity K, this is denoted as H->K.

Sometimes it is necessary to specify the subject and issuer of a
given certificate. If entity H were to issue a certificate to entity
K this can be denoted as K(H).

These notations can be combined to denote complicated certification
paths such as C(D)->B(C)->A(B).

1.5. Overview of PKI Structures

When verifying [X.509] public key certificates, often the application
performing the verification has no knowledge of the underlying Public
Key Infrastructure (PKI) that issued the certificate. PKI structures
can range from very simple, hierarchical structures to complex
structures such as mesh architectures involving multiple bridges (see
Section 1.5.4). These structures define the types of certification
paths that might be built and validated by an application [MINHPKIS].
This section describes four common PKI structures.

1.5.1. Hierarchical Structures

A hierarchical PKI, depicted in Figure 1, is one in which all of the
end entities and relying parties use a single "Root CA" as their
trust anchor. If the hierarchy has multiple levels, the Root CA
certifies the public keys of intermediate CAs (also known as
subordinate CAs). These CAs then certify end entities'
(subscribers') public keys or may, in a large PKI, certify other CAs.
In this architecture, certificates are issued in only one direction,
and a CA never certifies another CA "superior" to itself. Typically,
only one superior CA certifies each CA.

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+---------+
+---| Root 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 1 - Sample Hierarchical PKI

Certification path building in a hierarchical PKI is a
straightforward process that simply requires the relying party to
successively retrieve issuer certificates until a certificate that
was issued by the trust anchor (the "Root CA" in Figure 1) is
located.

A widely used variation on the single-rooted hierarchical PKI is the
inclusion of multiple CAs as trust anchors. (See Figure 2.) Here,
end entity certificates are validated using the same approach as with
any hierarchical PKI. The difference is that a certificate will be
accepted if it can be verified back to any of the set of trust
anchors. Popular web browsers use this approach, and are shipped
with trust lists containing dozens to more than one hundred CAs.
While this approach simplifies the implementation of a limited form
of certificate verification, it also may introduce certain security
vulnerabilities. For example, the user may have little or no idea of
the policies or operating practices of the various trust anchors, and
may not be aware of which root was used to verify a given
certificate. Additionally, the compromise of any trusted CA private
key or the insertion of a rogue CA certificate to the trust list may
compromise the entire system. Conversely, if the trust list is
properly managed and kept to a reasonable size, it can be an
efficient solution to building and validating certification paths.

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+-------------------------------------------------------+
| Trust List |
| |
| +---------+ +---------+ +---------+ |
| +--| Root CA | | Root CA | | Root CA | |
| | +---------+ +---------+ +---------+ |
| | | | | |
+--|------|----------------|---------------- |----------+
| | | |
| | | |
| | v |
| | +----+ |
| | +----| CA |---+ |
| | | +----+ | |
| | | | |
| | v v v
| | +----+ +----+ +----+
| | | CA |---+ | CA |-+ | CA |---+
| | +----+ | +----+ | +----+ |
| | | | | | | |
| | | | | | | |
v v v v v v v v
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
| EE | | EE | | EE | | EE | | EE | | EE | | EE | | EE |
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+

Figure 2 - Multi-Rooted Hierarchical PKI

1.5.2. Mesh Structures

In a typical mesh style PKI (depicted in Figure 3), each end entity
trusts the CA that issued their own certificate(s). Thus, there is
no 'Root CA' for the entire PKI. The CAs in this environment have
peer relationships; they are neither superior nor subordinate to one
another. In a mesh, CAs in the PKI cross-certify. That is, each CA
issues a certificate to, and is issued a certificate by, peer CAs in
the PKI. The figure depicts a mesh PKI that is fully cross-certified
(sometimes called a full mesh). However, it is possible to architect
and deploy a mesh PKI with a mixture of uni-directional and bi-
directional cross-certifications (called a partial mesh). Partial
meshes may also include CAs that are not cross-certified with other
CAs in the mesh.

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+---------------------------------+
| |
+-----------+----------------------+ |
| v v |
| +-------+ +------+ |
| +--->| CA B |<------------->| CA C |<--+ |
| | +-------+ +------+ | |
| | | ^ ^ | | |
| | v | | | | |
| | +----+ | | | | |
| | | EE | +----+ +--------+ v | |
| | +----+ | | +----+ | |
| | | | | EE | | |
v v v v +----+ v v
+------+ +------+ +------+
| CA E |<----------->| CA A |<----------->| CA D |
+------+ +------+ +------+
| ^ ^ ^ ^ |
| | | | | |
v | +------------------------------------+ | v
+----+ | | +----+
| EE | | +------+ | | EE |
+----+ +----------------| CA F |-----------------+ +----+
+------+

Figure 3 - Mesh PKI

Certification path building in a mesh PKI is more complex than in a
hierarchical PKI due to the likely existence of multiple paths
between a relying party's trust anchor and the certificate to be
verified. These multiple paths increase the potential for creating
"loops", "dead ends", or invalid paths while building the
certification path between a trust anchor and a target certificate.
In addition, in cases where no valid path exists, the total number of
paths traversed by the path-building software in order to conclude
"no path exists" can grow exceedingly large. For example, if
ignoring everything except the structure of the graph, the Mesh PKI
figure above has 22 non-self issued CA certificates and a total of
5,092,429 certification paths between CA F and the EE issued by CA D
without repeating any certificates.

1.5.3. Bi-Lateral Cross-Certified Structures

PKIs can be connected via cross-certification to enable the relying
parties of each to verify and accept certificates issued by the other
PKI. If the PKIs are hierarchical, cross-certification will
typically be accomplished by each Root CA issuing a certificate for
the other PKI's Root CA. This results in a slightly more complex,

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but still essentially hierarchical environment. If the PKIs are mesh
style, then a CA within each PKI is selected, more or less
arbitrarily, to establish the cross-certification, effectively
creating a larger mesh PKI. Figure 4 depicts a hybrid situation
resulting from a hierarchical PKI cross-certifying with a mesh PKI.

PKI 1 and 2 cross-certificates
+-------------------------------+
| |
| v
| +---------+
| +----| Root CA |---+
| | +---------+ |
| | PKI 1 |
| v v
| +------+ +------+
v PKI 2 +-| CA |-+ | CA |
+------+ | +------+ | +------+
+------->| CA |<-----+ | | | | |
| +------+ | | | | | |
| | | | v v v v v
| | | | +----+ +----+ +----+ +----+ +----+
| v v | | EE | | EE | | EE | | EE | | EE |
| +----+ +----+ | +----+ +----+ +----+ +----+ +----+
| | EE | | EE | |
| +----+ +----+ |
v v
+------+ +------+
| CA |<-------------->| CA |------+
+------+ +------+ |
| | | | |
| | | | |
v v v v v
+----+ +----+ +----+ +----+ +----+
| EE | | EE | | EE | | EE | | EE |
+----+ +----+ +----+ +----+ +----+

Figure 4 - Hybrid PKI

In current implementations, this situation creates a concern that the
applications used under the hierarchical PKIs will not have path
building capabilities robust enough to handle this more complex
certificate graph. As the number of cross-certified PKIs grows, the
number of the relationships between them grows exponentially. Two
principal concerns about cross-certification are the creation of
unintended certification paths through transitive trust, and the
dilution of assurance when a high-assurance PKI with restrictive
operating policies is cross-certified with a PKI with less

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restrictive policies. (Proper name constraints and certificate
policies processing can help mitigate the problem of assurance
dilution.)

1.5.4. Bridge Structures

Another approach to the interconnection of PKIs is the use of a
"bridge" certification authority (BCA). A BCA is a nexus to
establish trust paths among multiple PKIs. The BCA cross-certifies
with one CA in each participating PKI. Each PKI only cross-certifies
with one other CA (i.e., the BCA), and the BCA cross-certifies only
once with each participating PKI. As a result, the number of cross
certified relationships in the bridged environment grows linearly
with the number of PKIs whereas the number of cross-certified
relationships in mesh architectures grows exponentially. However,
when connecting PKIs in this way, the number and variety of PKIs
involved results in a non-hierarchical environment, such as the one
as depicted in Figure 5. (Note: as discussed in Section 2.3, non-
hierarchical PKIs can be considered hierarchical, depending upon
perspective.)

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PKI 1 cross-certified with Bridge
+-------------------------------+
| |
v v
+-----------+ +---------+
| Bridge CA | +---| Root CA |-----+
+-----------+ | +---------+ |
^ | PKI 1 |
PKI 2 cross|cert with Bridge v v
| +------+ +------+
v PKI 2 +-| CA |-+ | CA |
+------+ | +------+ | +------+
+------->| CA |<-----+ | | | | |
| +------+ | | | | | |
| | | | v v v v v
| | | | +----+ +----+ +----+ +----+ +----+
| v v | | EE | | EE | | EE | | EE | | EE |
| +----+ +----+ | +----+ +----+ +----+ +----+ +----+
| | EE | | EE | |
| +----+ +----+ |
v v
+------+ +------+
| CA |<-------------->| CA |------+
+------+ +------+ |
| | | | |
| | | | |
v v v v v
+----+ +----+ +----+ +----+ +----+
| EE | | EE | | EE | | EE | | EE |
+----+ +----+ +----+ +----+ +----+

Figure 5 - Cross-Certification with a Bridge CA

1.6. Bridge Structures and Certification Path Processing

Developers building certificate-enabled applications intended for
widespread use throughout various sectors are encouraged to consider
supporting a Bridge PKI structure because implementation of
certification path processing functions to support a Bridge PKI
structure requires support of all the PKI structures (e.g.,
hierarchical, mesh, hybrid) which the Bridge may connect. An
application that can successfully build valid certification paths in
all Bridge PKIs will therefore have implemented all of the processing
logic required to support the less complicated PKI structures. Thus,
if an application fully supports the Bridge PKI structure, it can be
deployed in any standards-compliant PKI environment and will perform
the required certification path processing properly.

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2. Certification Path Building

Certification path building is the process by which the certificate
processing system obtains the certification path between a trust
anchor and the target certificate. Different implementations can
build the certification path in different ways; therefore, it is not
the intent of this document to recommend a single "best" way to
perform this function. Rather, guidance is provided on the technical
issues that surround the path-building process, and on the
capabilities path-building implementations need in order to build
certification paths successfully, irrespective of PKI structures.

2.1. Introduction to Certification Path Building

A certification path is an ordered list of certificates starting with
a certificate that can be validated by one of the relying party's
trust anchors, and ending with the certificate to be validated. (The
certificate to be validated is referred to as the "target
certificate" throughout this document.) Though not required, as a
matter of convenience these trust anchors are typically stored in
trust anchor certificates. The intermediate certificates that
comprise the certification path may be retrieved by any means
available to the validating application. These sources may include
LDAP, HTTP, SQL, a local cache or certificate store, or as part of
the security protocol itself as is common practice with signed S/MIME
messages and SSL/TLS sessions.

Figure 6 shows an example of a certification path. In this figure,
the horizontal arrows represent certificates, and the notation B(A)
signifies a certificate issued to B, signed by A.

+---------+ +-----+ +-----+ +-----+ +--------+
| Trust |----->| CA |---->| CA |---->| CA |---->| Target |
| Anchor | : | A | : | B | : | C | : | EE |
+---------+ : +-----+ : +-----+ : +-----+ : +--------+
: : : :
: : : :
Cert 1 Cert 2 Cert 3 Cert 4
A(Trust Anchor) B(A) C(B) Target(C)

Figure 6 - Example Certification Path

Unlike certification path validation, certification path building is
not addressed by the standards that define the semantics and
structure of a PKI. This is because the validation of a
certification path is unaffected by the method in which the
certification path was built. However, the ability to build a valid
certification path is of paramount importance for applications that

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rely on a PKI. Without valid certification paths, certificates
cannot be validated according to [RFC3280] and therefore cannot be
trusted. Thus, the ability to build a path is every bit as important
as the ability to validate it properly.

There are many issues that can complicate the path-building process.
For example, building a path through a cross-certified environment
could require the path-building module to traverse multiple PKI
domains spanning multiple directories, using multiple algorithms, and
employing varying key lengths. A path-building client may also need
to manage a number of trust anchors, partially populated directory
entries (e.g., missing issuedToThisCA entries in the
crossCertificatePair attribute), parsing of certain certificate
extensions (e.g., authorityInformationAccess) and directory
attributes (e.g., crossCertificatePair), and error handling such as
loop detection.

In addition, a developer has to decide whether to build paths from a
trust anchor (the reverse direction) to the target certificate or
from the target certificate (the forward direction) to a trust
anchor. Some implementations may even decide to use both. The
choice a developer makes should be dependent on the environment and
the underlying PKI for that environment. More information on making
this choice can be found in Section 2.3.

2.2. Criteria for Path Building

From this point forward, this document will be discussing specific
algorithms and mechanisms to assist developers of certification
path-building implementations. To provide justification for these
mechanisms, it is important to denote what the authors considered the
criteria for a path-building implementation.

Criterion 1: The implementation is able to find all possible paths,
excepting paths containing repeated subject name/public key pairs.
This means that all potentially valid certification paths between the
trust anchor and the target certificate which may be valid paths can
be built by the algorithm. As discussed in Section 2.4.2, we
recommend that subject names and public key pairs are not repeated in
paths.

Criterion 2: The implementation is as efficient as possible. An
efficient certification path-building implementation is defined to be
one that builds paths that are more likely to validate following
[RFC3280], before building paths that are not likely to validate,
with the understanding that there is no way to account for all
possible configurations and infrastructures. This criterion is
intended to ensure implementations that can produce useful error

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information. If a particular path is entirely valid except for a
single expired certificate, this is most likely the 'right' path. If
other paths are developed that are invalid for multiple obscure
reasons, this provides little useful information.

The algorithms and mechanisms discussed henceforth are chosen because
the authors consider them to be good methods for meeting the above
criteria.

2.3. Path-Building Algorithms

It is intuitive for people familiar with the Bridge CA concept or
mesh type PKIs to view path building as traversing a complex graph.
However, from the simplest viewpoint, writing a path-building module
can be nothing more than traversal of a spanning tree, even in a very
complex cross-certified environment. Complex environments as well as
hierarchical PKIs can be represented as trees because certificates
are not permitted to repeat in a path. If certificates could be
repeated, loops can be formed such that the number of paths and
number of certificates in a path both increase without bound (e.g., A
issues to B, B issues to C, and C issues to A). Figure 7 below
illustrates this concept from the trust anchor's perspective.

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+---------+ +---------+
| Trust | | Trust |
| Anchor | | Anchor |
+---------+ +---------+
| | | |
v v v v
+---+ +---+ +---+ +---+
| A |<-->| C | +--| A | | C |--+
+---+ +---+ | +---+ +---+ |
| | | | | |
| +---+ | v v v v
+->| B |<-+ +---+ +---+ +---+ +---+
+---+ | B | | C | | A | | B |
| +---+ +---+ +---+ +---+
v | | | |
+----+ v v v v
| EE | +----+ +---+ +---+ +----+
+----+ | EE | | B | | B | | EE |
+----+ +---+ +---+ +----+
A certificate graph with | |
bi-directional cross-cert. v v
between CAs A and C. +----+ +----+
| EE | | EE |
+----+ +----+

The same certificate graph
rendered as a tree - the
way path-building software
could see it.

Figure 7 - Simple Certificate Graph - From Anchor Tree Depiction

When viewed from this perspective, all PKIs look like hierarchies
emanating from the trust anchor. An infrastructure can be depicted
in this way regardless of its complexity. In Figure 8, the same
graph is depicted from the end entity (EE) (the target certificate in
this example). It would appear this way if building in the forward
(from EE or from target) direction. In this example, without knowing
any particulars of the certificates, it appears at first that
building from EE has a smaller decision tree than building from the
trust anchor. While it is true that there are fewer nodes in the
tree, it is not necessarily more efficient in this example.

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+---------+ +---------+
| Trust | | Trust |
| Anchor | | Anchor |
+---------+ +---------+
^ ^
| |
| |
+---+ +---+
| A | | C |
+---+ +---+
+---------+ ^ ^ +---------+
| Trust | | | | Trust |
| Anchor | | | | Anchor |
+---------+ | | +---------+
^ | | ^
| +---+ +---+ |
+-------| C | | A |---------+
+---+ +---+
^ ^
| |
| +---+ |
+---------| B |------+
+---+
^
|
|
+----+
| EE |
+----+

The same certificate graph rendered
as a tree but from the end entity
rather than the trust anchor.

Figure 8 - Certificate Graph - From Target Certificate Depiction

Suppose a path-building algorithm performed no optimizations. That
is, the algorithm is only capable of detecting that the current
certificate in the tree was issued by the trust anchor, or that it
issued the target certificate (EE). From the tree above, building
from the target certificate will require going through two
intermediate certificates before encountering a certificate issued by
the trust anchor 100% of the time (e.g., EE chains to B, which then
chains to C, which is issued by the Trust Anchor). The path-building
module would not chain C to A because it can recognize that C has a
certificate issued by the Trust Anchor (TA).

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On the other hand, in the first tree (Figure 7: from anchor
depiction), there is a 50% probability of building a path longer than
needed (e.g., TA to A to C to B to EE rather than the shorter TA to A
to B to EE). However, even given our simplistic example, the path-
building software, when at A, could be designed to recognize that B's
subject distinguished name (DN) matches the issuer DN of the EE.
Given this one optimization, the builder could prefer B to C. (B's
subject DN matches that of the EE's issuer whereas C's subject DN
does not.) So, for this example, assuming the issuedByThisCA
(reverse) and issuedToThisCA (forward) elements were fully populated
in the directory and our path-building module implemented the
aforementioned DN matching optimization method, path building from
either the trust anchor or the target certificate could be made
roughly equivalent. A list of possible optimization methods is
provided later in this document.

A more complicated example is created when the path-building software
encounters a situation when there are multiple certificates from
which to choose while building a path. We refer to this as a large
decision tree, or a situation with high fan-out. This might occur if
an implementation has multiple trust anchors to choose from, and is
building in the reverse (from trust anchor) direction. Or, it may
occur in either direction if a Bridge CA is encountered. Large
decision trees are the enemy of efficient path-building software. To
combat this problem, implementations should make careful decisions
about the path-building direction, and should utilize optimizations
such as those discussed in Section 3.1 when confronted with a large
decision tree.

Irrespective of the path-building approach for any path-building
algorithm, cases can be constructed that make the algorithm perform
poorly. The following questions should help a developer decide from
which direction to build certification paths for their application:

1) What is required to accommodate the local PKI environment and the
PKI environments with which interoperability will be required?

a. If using a directory, is the directory [RFC2587] compliant
(specifically, are the issuedToThisCA [forward] cross-
certificates and/or the cACertificate attributes fully
populated in the directory)? If yes, you are able to build in
the forward direction.

b. If using a directory, does the directory contain all the
issuedByThisCA (reverse) cross-certificates in the
crossCertificatePair attribute, or, alternately, are all
certificates issued from each CA available via some other
means? If yes, it is possible to build in the reverse

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direction. Note: [RFC2587] does not require the issuedByThisCA
(reverse) cross-certificates to be populated; if they are
absent it will not be possible to build solely in the reverse
direction.

c. Are all issuer certificates available via some means other than
a directory (e.g., the authorityInformationAccess extension is
present and populated in all certificates)? If yes, you are
able to build in the forward direction.

2) How many trust anchors will the path-building and validation
software be using?

a. Are there (or will there be) multiple trust anchors in the
local PKI? If yes, forward path building may offer better
performance.

b. Will the path-building and validation software need to place
trust in trust anchors from PKIs that do not populate reverse
cross-certificates for all intermediate CAs? If no, and the
local PKI populates reverse cross-certificates, reverse path
building is an option.

2.4. How to Build a Certification Path

As was discussed in the prior section, path building is essentially a
tree traversal. It was easy to see how this is true in a simple
example, but how about a more complicated one? Before taking a look
at more a complicated scenario, it is worthwhile to address loops and
what constitutes a loop in a certification path. [X.509] specifies
that the same certificate may not repeat in a path. In a strict
sense, this works well as it is not possible to create an endless
loop without repeating one or more certificates in the path.
However, this requirement fails to adequately address Bridged PKI
environments.

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+---+ +---+
| F |--->| H |
+---+ +---+
^ ^ ^
| \ \
| \ \
| v v
| +---+ +---+
| | G |--->| I |
| +---+ +---+
| ^
| /
| /
+------+ +-----------+ +------+ +---+ +---+
| TA W |<----->| Bridge CA |<------>| TA X |-->| L |-->| M |
+------+ +-----------+ +------+ +---+ +---+
^ ^ \ \
/ \ \ \
/ \ \ \
v v v v
+------+ +------+ +---+ +---+
| TA Y | | TA Z | | J | | N |
+------+ +------+ +---+ +---+
/ \ / \ | |
/ \ / \ | |
/ \ / \ v v
v v v v +---+ +----+
+---+ +---+ +---+ +---+ | K | | EE |
| A |<--->| C | | O | | P | +---+ +----+
+---+ +---+ +---+ +---+
\ / / \ \
\ / / \ \
\ / v v v
v v +---+ +---+ +---+
+---+ | Q | | R | | S |
| B | +---+ +---+ +---+
+---+ |
/\ |
/ \ |
v v v
+---+ +---+ +---+
| E | | D | | T |
+---+ +---+ +---+

Figure 9 - Four Bridged PKIs

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Figure 9 depicts four root certification authorities cross-certified
with a Bridge CA (BCA). While multiple trust anchors are shown in
the Figure, our examples all consider TA Z as the trust anchor. The
other trust anchors serve different relying parties. By building
certification paths through the BCA, trust can be extended across the
four infrastructures. In Figure 9, the BCA has four certificates
issued to it; one issued from each of the trust anchors in the graph.
If stored in the BCA directory system, the four certificates issued
to the BCA would be stored in the issuedToThisCA (forward) entry of
four different crossCertificatePair structures. The BCA also has
issued four certificates, one to each of the trust anchors. If
stored in the BCA directory system, those certificates would be
stored in the issuedByThisCA (reverse) entry of the same four
crossCertificatePair structures. (Note that the cross-certificates
are stored as matched pairs in the crossCertificatePair attribute.
For example, a crossCertificatePair structure might contain both A(B)
and B(A), but not contain A(C) and B(A).) The four
crossCertificatePair structures would then be stored in the BCA's
directory entry in the crossCertificatePair attribute.

2.4.1. Certificate Repetition

[X.509] requires that certificates are not repeated when building
paths. For instance, from the figure above, do not build the path TA
Z->BCA->Y->A->C->A->C->B->D. Not only is the repetition unnecessary
to build the path from Z to D, but it also requires the reuse of a
certificate (the one issued from C to A), which makes the path non-
compliant with [X.509].

What about the following path from TA Z to EE?

TA Z->BCA->Y->BCA->W->BCA->X->L->N->EE

Unlike the first example, this path does not require a developer to
repeat any certificates; therefore, it is compliant with [X.509].
Each of the BCA certificates is issued from a different source and is
therefore a different certificate. Suppose now that the bottom left
PKI (in Figure 9) had double arrows between Y and C, as well as
between Y and A. The following path could then be built:

TA Z->BCA->Y->A->C->Y->BCA->W->BCA->X->L->N->EE

A path such as this could become arbitrarily complex and traverse
every cross-certified CA in every PKI in a cross-certified
environment while still remaining compliant with [X.509]. As a
practical matter, the path above is not something an application
would typically want or need to build for a variety of reasons:

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- First, certification paths like the example above are generally
not intended by the PKI designers and should not be necessary in
order to validate any given certificate. If a convoluted path
such as the example above is required (there is no corresponding
simple path) in order to validate a given certificate, this is
most likely indicative of a flaw in the PKI design.

- Second, the longer a path becomes, the greater the potential
dilution of trust in the certification path. That is, with each
successive link in the infrastructure (i.e., certification by
CAs and cross-certification between CAs) some amount of
assurance may be considered lost.

- Third, the longer and more complicated a path, the less likely
it is to validate because of basic constraints, policies or
policy constraints, name constraints, CRL availability, or even
revocation.

- Lastly, and certainly not least important from a developer's or
user's perspective, is performance. Allowing paths like the one
above dramatically increases the number of possible paths for
every certificate in a mesh or cross-certified environment.
Every path built may require one or more of the following:
validation of certificate properties, CPU intensive signature
validations, CRL retrievals, increased network load, and local
memory caching. Eliminating the superfluous paths can greatly
improve performance, especially in the case where no path
exists.

There is a special case involving certificates with the same
distinguished names but differing encodings required by [RFC3280].
This case should not be considered a repeated certificate. See
Section 5.4 for more information.

2.4.2. Introduction to Path-Building Optimization

How can these superfluous paths be eliminated? Rather than only
disallowing identical certificates from repeating, it is recommended
that a developer disallow the same public key and subject name pair
from being repeated. For maximum flexibility, the subject name
should collectively include any subject alternative names. Using
this approach, all of the intended and needed paths should be
available, and the excess and diluted paths should be eliminated.
For example, using this approach, only one path exists from the TA Z
to EE in the diagram above: TA Z->BCA->X->L->N->EE.

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Given the simplifying rule of not repeating pairs of subject names
(including subject alternative names) and public keys, and only using
certificates found in the cACertificate and forward (issuedToThisCA)
element of the crossCertificatePair attributes, Figure 10 depicts the
forward path-building decision tree from the EE to all reachable
nodes in the graph. This is the ideal graph for a path builder
attempting to build a path from TA Z to EE.

+------+ +-----------+ +------+ +---+
| TA W |<------| Bridge CA |<-------| TA X |<--| L |
+------+ +-----------+ +------+ +---+
/ \ ^
/ \ \
/ \ \
v v \
+------+ +------+ +---+
| TA Y | | TA Z | | N |
+------+ +------+ +---+
^
\
\
+----+
| EE |
+----+

Figure 10 - Forward (From Entity) Decision Tree

It is not possible to build forward direction paths into the
infrastructures behind CAs W, Y, and Z, because W, Y, and Z have not
been issued certificates by their subordinate CAs. (The subordinate
CAs are F and G, A and C, and O and P, respectively.) If simplicity
and speed are desirable, the graph in Figure 10 is a very appealing
way to structure the path-building algorithm. Finding a path from
the EE to one of the four trust anchors is reasonably simple.
Alternately, a developer could choose to build in the opposite
direction, using the reverse cross-certificates from any one of the
four trust anchors around the BCA. The graph in Figure 11 depicts
all possible paths as a tree emanating from TA Z. (Note: it is not
recommended that implementations attempt to determine all possible
paths, this would require retrieval and storage of all PKI data
including certificates and CRLs! This example is provided to
demonstrate the complexity which might be encountered.)

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+---+ +---+
| I |--->| H |
+---+ +---+
^
| +---+ +---+
| | H |--->| I |
| +---+ +---+
+---+ ^
| G | / +---+ +---+ +---+
+---+ / | F |--->| H |--->| I |
^ / +---+ +---+ +---+
\ / ^
\/ /
+---+ +---+ +---+ +---+ +---+
| F | | G |--->| I |--->| H | | M |
+---+ +---+ +---+ +---+ +---+
^ ^ ^
| / |
+------+ +-----------+ +------+ +---+
| TA W |<------| Bridge CA |-------->| TA X |-->| L |
+------+ +-----------+ +------+ +---+
/ ^ \ \
v \ v v
+------+ +------+ +---+ +---+
| TA Y | | TA Z | | J | | N |
+------+ +------+ +---+ +---+
/ \ / \ \ \
v v v v v v
+---+ +---+ +---+ +---+ +---+ +----+
| A | | C | | O | | P | | K | | EE |
+---+ +---+ +---+ +---+ +---+ +----+
/ \ / \ / \ \
v v v v v v v
+---+ +---+ +---+ +---+ +---+ +---+ +---+
| B | | C | | A | | B | | Q | | R | | S |
+---+ +---+ +---+ +---+ +---+ +---+ +---+
/ \ \ \ \ \ \
v v v v v v v
+---+ +---+ +---+ +---+ +---+ +---+ +---+
| E | | D | | B | | B | | E | | D | | T |
+---+ +---+ +---+ +---+ +---+ +---+ +---+
/ | | \
v v v v
+---+ +---+ +---+ +---+
| E | | D | | E | | D |
+---+ +---+ +---+ +---+

Figure 11 - Reverse (From Anchor) Decision Tree

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Given the relative complexity of this decision tree, it becomes clear
that making the right choices while navigating the tree can make a
large difference in how quickly a valid path is returned. The path-
building software could potentially traverse the entire graph before
choosing the shortest path: TA Z->BCA->X->L->N->EE. With a decision
tree like the one above, the basic depth first traversal approach
introduces obvious inefficiencies in the path-building process. To
compensate for this, a path-building module needs to decide not only
in which direction to traverse the tree, but also which branches of
the tree are more likely to yield a valid path.

The path-building algorithm then ideally becomes a tree traversal
algorithm with weights or priorities assigned to each branch point to
guide the decision making. If properly designed, such an approach
would effectively yield the "best path first" more often than not.
(The terminology "best path first" is quoted because the definition
of the "best" path may differ from PKI to PKI. That is ultimately to
be determined by the developer, not by this document.) Finding the
"best path first" is an effort to make the implementation efficient,
which is one of our criteria as stated in Section 2.2.

So how would a developer go about finding the best path first? Given
the simplifying idea of addressing path building as a tree traversal,
path building could be structured as a depth first search. A simple
example of depth first tree traversal path building is depicted in
Figure 12, with no preference given to sort order.

Note: The arrows in the lower portion of the figure do not indicate
the direction of certificate issuance; they indicate the direction of
the tree traversal from the target certificate (EE).

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+----+ +----+ +----+
| TA | | TA | | TA |
+----+ +----+ +----+
/ \ ^ ^
/ \ | |
v v +---+ +---+
+---+ +---+ | A | | C |
| A |<->| C | +---+ +---+
+---+ +---+ ^ ^
^ ^ +----+ | | +----+
\ / | TA | | | | TA |
v v +----+ | | +----+
+---+ ^ | | ^
| B | \ | | /
+---+ \ | | /
/ \ +---+ +---+
/ \ | C | | A |
v v +---+ +---+
+---+ +---+ ^ ^
| E | | D | | /
+---+ +---+ | /
+---+
Infrastructure | B |
+---+
^
|
+----+
| EE |
+----+

The Same Infrastructure
Represented as a Tree

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+----+ +----+
| TA | | TA |
+----+ +----+
^ ^
| |
+---+ +---+
| A | | C |
+---+ +---+
+----+ ^ ^ +----+
| TA | | | | TA |
+----+ | | +----+
^ | | ^
\ | | /
+---+ +---+ +---+ +---+
| C | | C | | A | | A |
+---+ +---+ +---+ +---+
^ ^ ^ ^
| | / /
| | / /
+---+ +---+ +---+ +---+
| B | | B | | B | | B |
+---+ +---+ +---+ +---+
^ ^ ^ ^
| | | |
| | | |
+----+ +----+ +----+ +----+
| EE | | EE | | EE | | EE |
+----+ +----+ +----+ +----+

All possible paths from EE to TA
using a depth first decision tree traversal

Figure 12 - Path Building Using a Depth First Tree Traversal

Figure 12 illustrates that four possible paths exist for this
example. Suppose that the last path (TA->A->B->EE) is the only path
that will validate. This could be for any combination of reasons
such as name constraints, policy processing, validity periods, or
path length constraints. The goal of an efficient path-building
component is to select the fourth path first by testing properties of
the certificates as the tree is traversed. For example, when the
path-building software is at entity B in the graph, it should examine
both choices A and C to determine which certificate is the most
likely best choice. An efficient module would conclude that A is the
more likely correct path. Then, at A, the module compares
terminating the path at TA, or moving to C. Again, an efficient
module will make the better choice (TA) and thereby find the "best
path first".

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What if the choice between CA certificates is not binary as it was in
the previous example? What if the path-building software encounters
a branch point with some arbitrary number of CA certificates thereby
creating the same arbitrary number of tree branches? (This would be
typical in a mesh style PKI CA, or at a Bridge CA directory entry, as
each will have multiple certificates issued to itself from other
CAs.) This situation actually does not change the algorithm at all,
if it is structured properly. In our example, rather than treating
each decision as binary (i.e., choosing A or C), the path-building
software should sort all the available possibilities at any given
branch point, and then select the best choice from the list. In the
event the path could not be built through the first choice, then the
second choice should be tried next upon traversing back to that point
in the tree. Continue following this pattern until a path is found
or all CA nodes in the tree have been traversed. Note that the
certificates at any given point in the tree should only be sorted at
the time a decision is first made. Specifically, in the example, the
sorting of A and C is done when the algorithm reached B. There is no
memory resident representation of the entire tree. Just like any
other recursive depth first search algorithm, the only information
the algorithm needs to keep track of is what nodes (entities) in the
tree lie behind it on the current path, and for each of those nodes,
which arcs (certificates) have already been tried.

2.5. Building Certification Paths for Revocation Signer Certificates

Special consideration is given to building a certification path for
the Revocation Signer certificate because it may or may not be the
same as the Certification Authority certificate. For example, after
a CA performs a key rollover, the new CA certificate will be the CRL
Signer certificate, whereas the old CA certificate is the
Certification Authority certificate for previously issued
certificates. In the case of indirect CRLs, the CRL Signer
certificate will contain a different name and key than the
Certification Authority certificate. In the case of OCSP, the
Revocation Signer certificate may represent an OCSP Responder that is
not the same entity as the Certification Authority.

When the Revocation Signer certificate and the Certification
Authority certificate are identical, no additional consideration is
required from a certification path-building standpoint. That is, the
certification path built (and validated) for the Certification
Authority certificate can also be used as the certification path for
the Revocation Signer certificate. In this case, the signature on
the revocation data (e.g., CRL or OCSP response) is verified using
the same certificate, and no other certification path building is
required. An efficient certification path validation algorithm
should first try all possible CRLs issued by the Certification

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Authority to determine if any of the CRLs (a) cover the certificate
in question, (b) are current, and (c) are signed using the same key
used to sign the certificate.

When the Revocation Signer certificate is not identical to the
Certification Authority certificate, a certification path must be
built (and validated) for the Revocation Signer certificate. In
general, the certification path-building software may build the path
as it would for any other certificate. However, this document also
outlines methods in later sections for greatly improving path
building efficiency for Revocation Signer certificate case.

2.6. Suggested Path-Building Software Components

There is no single way to define an interface to a path-building
module. It is not the intent of this document to prescribe a
particular method or semantic; rather, it is up to the implementer to
decide. There are many ways this could be done. For example, a
path-building module could build every conceivable path and return
the entire list to the caller. Or, the module could build until it
finds just one that validates and then terminate the procedure. Or,
it could build paths in an iterative fashion, depending on validation
outside of the builder and successive calls to the builder to get
more paths until one valid path is found or all possible paths have
been found. All of these are possible approaches, and each of these
may offer different benefits to a particular environment or
application.

Regardless of semantics, a path-building module needs to contain the
following components:

1) The logic for building and traversing the certificate graph.

2) Logic for retrieving the necessary certificates (and CRLs and/or
other revocation status information if the path is to be
validated) from the available source(s).

Assuming a more efficient and agile path-building module is desired,
the following is a good starting point and will tie into the
remainder of this document. For a path-building module to take full
advantage of all the suggested optimizations listed in this document,
it will need all of the components listed below.

1) A local certificate and CRL cache.

a. This may be used by all certificate-using components; it does
not need to be specific to the path-building software. A local
cache could be memory resident, stored in an operating system

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or application certificate store, stored in a database, or even
stored in individual files on the hard disk. While the
implementation of this cache is beyond the scope of this
document, some design considerations are listed below.

2) The logic for building and traversing the certificate graph/tree.

a. This performs sorting functionality for prioritizing
certificates (thereby optimizing path building) while
traversing the tree.

b. There is no need to build a complete graph prior to commencing
path building. Since path building can be implemented as a
depth first tree traversal, the path builder only needs to
store the current location in the tree along with the points
traversed to the current location. All completed branches can
be discarded from memory and future branches are discovered as
the tree is traversed.

3) Logic for retrieving the necessary certificates from the available
certificate source(s):

a. Local cache.

i. Be able to retrieve all certificates for an entity by
subject name, as well as individual certificates by
issuer and serial number tuple.

ii. Tracking which directory attribute (including
issuedToThisCA and issuedByThisCA
for split crossCertificatePair attributes) each
certificate was found in may be useful. This allows for
functionality such as retrieving only forward cross-
certificates, etc.

iii. A "freshness" timestamp (cache expiry time) can be used
to determine when the directory should be searched
again.

b. LDAPv3 directory for certificates and CRLs.

i. Consider supporting multiple directories for general
queries.

ii. Consider supporting dynamic LDAP connections for
retrieving CRLs using an LDAP URI [RFC3986] in the CRL
distribution point certificate extension.

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iii. Support LDAP referrals. This is typically only a matter
of activating the appropriate flag in the LDAP API.

c. HTTP support for CRL distribution points and authority
information access (AIA) support.

i. Consider HTTPS support, but be aware that this may create
an unbounded recursion when the implementation tries to
build a certification path for the server's certificate if
this in turn requires an additional HTTPS lookup.

4) A certification path cache that stores previously validated
relationships between certificates. This cache should include:

a. A configurable expiration date for each entry. This date can
be configured based upon factors such as the expiry of the
information used to determine the validity of an entry,
bandwidth, assurance level, storage space, etc.

b. Support to store previously verified issuer certificate to
subject certificate relationships.

i. Since the issuer DN and serial number tuple uniquely
identifies a certificate, a pair of these tuples (one for
both the issuer and subject) is an effective method of
storing this relationship.

c. Support for storing "known bad" paths and certificates. Once a
certificate is determined to be invalid, implementations can
decide not to retry path development and validation.

2.7. Inputs to the Path-Building Module

[X.509] specifically addresses the list of inputs required for path
validation but makes no specific suggestions concerning useful inputs
to path building. However, given that the goal of path building is
to find certification paths that will validate, it follows that the
same inputs used for validation could be used to optimize path
building.

2.7.1. Required Inputs

Setting aside configuration information such as repository or cache
locations, the following are required inputs to the certification
path-building process:

1) The Target Certificate: The certificate that is to be validated.
This is one endpoint for the path. (It is also possible to

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provide information used to retrieve a certificate for a target,
rather than the certificate itself.)

2) Trust List: This is the other endpoint of the path, and can
consist of either:

a. Trusted CA certificates

b. Trusted keys and DNs; a certificate is not necessarily required

2.7.2. Optional Inputs

In addition to the inputs listed in Section 2.7.1, the following
optional inputs can also be useful for optimizing path building.
However, if the path-building software takes advantage of all of the
optimization methods described later in this document, all of the
following optional inputs will be required.

1) Time (T): The time for which the certificate is to be validated
(e.g., if validating a historical signature from one year ago, T
is needed to build a valid path)

a. If not included as an input, the path-building software should
always build for T equal to the current system time.

2) Initial-inhibit-policy-mapping indicator

3) Initial-require-explicit-policy indicator

4) Initial-any-policy-inhibit indicator

5) Initial user acceptable policy set

6) Error handlers (call backs or virtual classes)

7) Handlers for custom certificate extensions

8) Is-revocation-provider indicator

a. IMPORTANT: When building a certification path for an OCSP
Responder certificate specified as part of the local
configuration, this flag should not be set. It is set when
building a certification path for a CRL Signer certificate or
for an OCSP Responder Signer certificate discovered using the
information asserted in an authorityInformationAccess
certificate extension.

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9) The complete certification path for the Certification Authority
(if Is-revocation-provider is set)

10) Collection of certificates that may be useful in building the
path

11) Collection of certificate revocation lists and/or other
revocation data

The last two items are a matter of convenience. Alternately,
certificates and revocation information could be placed in a local
cache accessible to the path-building module prior to attempting to
build a path.

3. Optimizing Path Building

This section recommends methods for optimizing path-building
processes.

3.1. Optimized Path Building

Path building can be optimized by sorting the certificates at every
decision point (at every node in the tree) and then selecting the
most promising certificate not yet selected as described in Section
2.4.2. This process continues until the path terminates. This is
roughly equivalent to the concept of creating a weighted edge tree,
where the edges are represented by certificates and nodes represent
subject DNs. However, unlike the weighted edge graph concept, a
certification path builder need not have the entire graph available
in order to function efficiently. In addition, the path builder can
be stateless with respect to nodes of the graph not present in the
current path, so the working data set can be relatively small.

The concept of statelessness with respect to nodes not in the current
path is instrumental to using the sorting optimizations listed in
this document. Initially, it may seem that sorting a given group of
certificates for a CA once and then preserving that sorted order for
later use would be an efficient way to write the path builder.
However, maintaining this state can quickly eliminate the efficiency
that sorting provides. Consider the following diagram:

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+---+
| R |
+---+
^
/
v
+---+ +---+ +---+ +---+ +----+
| A |<----->| E |<---->| D |--->| Z |--->| EE |
+---+ +---+ +---+ +---+ +----+
^ ^ ^ ^
\ / \ /
\ / \ /
v v v v
+---+ +---+
| B |<----->| C |
+---+ +---+

Figure 13 - Example of Path-Building Optimization

In this example, the path builder is building in the forward (from
target) direction for a path between R and EE. The path builder has
also opted to allow subject name and key to repeat. (This will allow
multiple traversals through any of the cross-certified CAs, creating
enough complexity in this small example to illustrate proper state
maintenance. Note that a similarly complex example could be designed
by using multiple keys for each entity and prohibiting repetition.)

The first step is simple; the builder builds the path Z(D)->EE(Z).
Next the builder adds D and faces a decision between two
certificates. (Choose between D(C) or D(E)). The builder now sorts
the two choices in order of priority. The sorting is partially based
upon what is currently in the path.

Suppose the order the builder selects is [D(E), D(C)]. The current
path is now D(E)->Z(D)->EE(Z). Currently the builder has three nodes
in the graph (EE, Z, and D) and should maintain the state, including
sort order of the certificates at D, when adding the next node, E.
When E is added, the builder now has four certificates to sort: E(A),
E(B), E(C), and E(D). In this case, the example builder opts for the
order [E(C), E(B), E(A), E(D)]. The current path is now E(C)->D(E)->
Z(D)->EE(Z) and the path has four nodes; EE, Z, D, and E.

Upon adding the fifth node, C, the builder sorts the certificates
(C(B), C(D), and C(E)) at C, and selects C(E). The path is now
C(E)->E(C)->D(E)->Z(D)->EE(Z) and the path has five nodes: EE, Z, D,
E, and C.

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Now the builder finds itself back at node E with four certificates.
If the builder were to use the prior sort order from the first
encounter with E, it would have [E(C), E(B), E(A), E(D)]. In the
current path's context, this ordering may be inappropriate. To begin
with, the certificate E(C) is already in the path so it certainly
does not deserve first place.

The best way to handle this situation is for the path builder to
handle this instance of E as a new (sixth) node in the tree. In
other words, there is no state information for this new instance of E
- it is treated just as any other new node. The certificates at the
new node are sorted based upon the current path content and the first
certificate is then selected. For example, the builder may examine
E(B) and note that it contains a name constraint prohibiting "C". At
this point in the decision tree, E(B) could not be added to the path
and produce a valid result since "C" is already in the path. As a
result, the certificate E(B) should placed at the bottom of the
prioritized list.

Alternatively, E(B) could be eliminated from this new node in the
tree. It is very important to see that this certificate is
eliminated only at this node and only for the current path. If path
building fails through C and traverses back up the tree to the first
instance of E, E(B) could still produce a valid path that does not
include C; specifically R->A->B->E->D->Z->EE. Thus the state at any
node should not alter the state of previous or subsequent nodes.
(Except for prioritizing certificates in the subsequent nodes.)

In this example, the builder should also note that E(C) is already in
the path and should make it last or eliminate it from this node since
certificates cannot be repeated in a path.

If the builder eliminates both certificates E(B) and E(C) at this
node, it is now only left to select between E(A) and E(D). Now the
path has six nodes: EE, Z, D, E(1), C, and E(2). E(1) has four
certificates, and E(2) has two, which the builder sorts to yield
[E(A), E(D)]. The current path is now E(A)->C(E)->E(C)->D(E)->
Z(D)->EE(Z). A(R) will be found when the seventh node is added to
the path and the path terminated because one of the trust anchors has
been found.

In the event the first path fails to validate, the path builder will
still have the seven nodes and associated state information to work
with. On the next iteration, the path builder is able to traverse
back up the tree to a working decision point, such as A, and select
the next certificate in the sorted list at A. In this example, that
would be A(B). (A(R) has already been tested.) This would dead end,
and the builder traverse back up to the next decision point, E(2)

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where it would try D(E). This process repeats until the traversal
backs all the way up to EE or a valid path is found. If the tree
traversal returns to EE, all possible paths have been exhausted and
the builder can conclude no valid path exists.

This approach of sorting certificates in order to optimize path
building will yield better results than not optimizing the tree
traversal. However, the path-building process can be further
streamlined by eliminating certificates, and entire branches of the
tree as a result, as paths are built.

3.2. Sorting vs. Elimination

Consider a situation when building a path in which three CA
certificates are found for a given target certificate and must be
prioritized. When the certificates are examined, as in the previous
example, one of the three has a name constraint present that will
invalidate the path built thus far. When sorting the three
certificates, that one would certainly go to the back of the line.
However, the path-building software could decide that this condition
eliminates the certificate from consideration at this point in the
graph, thereby reducing the number of certificate choices by 33% at
this point.

NOTE: It is important to understand that the elimination of a
certificate only applies to a single decision point during the tree
traversal. The same certificate may appear again at another point in
the tree; at that point it may or may not be eliminated. The
previous section details an example of this behavior.

Elimination of certificates could potentially eliminate the traversal
of a large, time-consuming infrastructure that will never lead to a
valid path. The question of whether to sort or eliminate is one that
pits the flexibility of the software interface against efficiency.

To be clear, if one eliminates invalid paths as they are built,
returning only likely valid paths, the end result will be an
efficient path-building module. The drawback to this is that unless
the software makes allowances for it, the calling application will
not be able to see what went wrong. The user may only see the
unrevealing error message: "No certification path found."

On the other hand, the path-building module could opt to not rule out
any certification paths. The path-building software could then
return any and all paths it can build from the certificate graph. It
is then up to the validation engine to determine which are valid and
which are invalid. The user or calling application can then have
complete details on why each and every path fails to validate. The

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drawback is obviously one of performance, as an application or end
user may wait for an extended period of time while cross-certified
PKIs are navigated in order to build paths that will never validate.

Neither option is a very desirable approach. One option provides
good performance for users, which is beneficial. The other option
though allows administrators to diagnose problems with the PKI,
directory, or software. Below are some recommendations to reach a
middle ground on this issue.

First, developers are strongly encouraged to output detailed log
information from the path-building software. The log should
explicitly indicate every choice the builder makes and why. It
should clearly identify which certificates are found and used at each
step in building the path. If care is taken to produce a useful log,
PKI administrators and help desk personnel will have ample
information to diagnose a problem with the PKI. Ideally, there would
be a mechanism for turning this logging on and off, so that it is not
running all the time. Additionally, it is recommended that the log
contain information so that a developer or tester can recreate the
paths tried by the path-building software, to assist with diagnostics
and testing.

Secondly, it is desirable to return something useful to the user.
The easiest approach is probably to implement a "dual mode" path-
building module. In the first mode [mode 1], the software eliminates
any and all paths that will not validate, making it very efficient.
In the second mode [mode 2], all the sorting methods are still
applied, but no paths are eliminated based upon the sorting methods.
Having this dual mode allows the module to first fail to find a valid
path, but still return one invalid path (assuming one exists) by
switching over to the second mode long enough to generate a single
path. This provides a middle ground -- the software is very fast,
but still returns something that gives the user a more specific error
than "no path found".

Third, it may be useful to not rule out any paths, but instead limit
the number of paths that may be built given a particular input.
Assuming the path-building module is designed to return the "best
path first", the paths most likely to validate would be returned
before this limit is reached. Once the limit is reached the module
can stop building paths, providing a more rapid response to the
caller than one which builds all possible paths.

Ultimately, the developer determines how to handle the trade-off
between efficiency and provision of information. A developer could
choose the middle ground by opting to implement some optimizations as
elimination rules and others as not. A developer could validate

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certificate signatures, or even check revocation status while
building the path, and then make decisions based upon the outcome of
those checks as to whether to eliminate the certificate in question.

This document suggests the following approach:

1) While building paths, eliminate any and all certificates that do
not satisfy all path validation requirements with the following
exceptions:

a. Do not check revocation status if it requires a directory
lookup or network access

b. Do not check digital signatures (see Section 8.1, General
Considerations for Building A Certification Path, for
additional considerations).

c. Do not check anything that cannot be checked as part of the
iterative process of traversing the tree.

d. Create a detailed log, if this feature is enabled.

e. If a path cannot be found, the path builder shifts to "mode 2"
and allows the building of a single bad path.

i. Return the path with a failure indicator, as well as
error information detailing why the path is bad.

2) If path building succeeds, validate the path in accordance with
[X.509] and [RFC3280] with the following recommendations:

a. For a performance boost, do not re-check items already checked
by the path builder. (Note: if pre-populated paths are supplied
to the path-building system, the entire path has to be fully
re-validated.)

b. If the path validation failed, call the path builder again to
build another path.

i. Always store the error information and path from the
first iteration and return this to the user in the event
that no valid path is found. Since the path-building
software was designed to return the "best path first",
this path should be shown to the user.

As stated above, this document recommends that developers do not
validate digital signatures or check revocation status as part of the
path-building process. This recommendation is based on two

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assumptions about PKI and its usage. First, signatures in a working
PKI are usually good. Since signature validation is costly in terms
of processor time, it is better to delay signature checking until a
complete path is found and then check the signatures on each
certificate in the certification path starting with the trust anchor
(see Section 8.1). Second, it is fairly uncommon in typical
application environments to encounter a revoked certificate;
therefore, most certificates validated will not be revoked. As a
result, it is better to delay retrieving CRLs or other revocation
status information until a complete path has been found. This
reduces the probability of retrieving unneeded revocation status
information while building paths.

3.3. Representing the Decision Tree

There are a multitude of ways to implement certification path
building and as many ways to represent the decision tree in memory.

The method described below is an approach that will work well with
the optimization methods listed later in this document. Although
this approach is the best the authors of this document have
implemented, it is by no means the only way to implement it.
Developers should tailor this approach to their own requirements or
may find that another approach suits their environment, programming
language, or programming style.

3.3.1. Node Representation for CA Entities

A "node" in the certification graph is a collection of CA
certificates with identical subject DNs. Minimally, for each node,
in order to fully implement the optimizations to follow, the path-
building module will need to be able to keep track of the following
information:

1. Certificates contained in the node

2. Sorted order of the certificates

3. "Current" certificate indicator

4. The current policy set (It may be split into authority and user
constrained sets, if desired.)

- It is suggested that encapsulating the policy set in an object
with logic for manipulating the set such as performing
intersections, mappings, etc., will simplify implementation.

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5. Indicators (requireExplicitPolicy, inhibitPolicyMapping,
anyPolicyInhibit) and corresponding skipCert values

6. A method for indicating which certificates are eliminated or
removing them from the node.

- If nodes are recreated from the cache on demand, it may be
simpler to remove eliminated certificates from the node.

7. A "next" indicator that points to the next node in the current
path

8. A "previous" indicator that points to the previous node in the
current path

3.3.2. Using Nodes to Iterate Over All Paths

In simplest form, a node is created, the certificates are sorted, the
next subject DN required is determined from the first certificate,
and a new node is attached to the certification path via the next
indicator (Number 7 above). This process continues until the path
terminates. (Note: end entity certificates may not contain subject
DNs as allowed by [RFC3280]. Since end entity certificates by
definition do not issue certificates, this has no impact on the
process.)

Keeping in mind that the following algorithm is designed to be
implemented using recursion, consider the example in Figure 12 and
assume that the only path in the diagram is valid for E is TA->A->
B->E:

If our path-building module is building a path in the forward
direction for E, a node is first created for E. There are no
certificates to sort because only one certificate exists, so all
initial values are loaded into the node from E. For example, the
policy set is extracted from the certificate and stored in the node.

Next, the issuer DN (B) is read from E, and new node is created for B
containing both certificates issued to B -- B(A) and B(C). The
sorting rules are applied to these two certificates and the sorting
algorithm returns B(C);B(A). This sorted order is stored and the
current indicator is set to B(C). Indicators are set and the policy
sets are calculated to the extent possible with respect to B(C). The
following diagram illustrates the current state with the current
certificate indicated with a "*".

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Next, a node is created for C and all three certificates are added to
it. The sorting algorithm happens to return the certificates sorted
in the following order: C(TA);C(A);C(B)

Recognizing that the trust anchor has been found, the path
(TA->C->B->E) is validated but fails. (Remember that the only valid
path happens to be TA->A->B->E.) The path-building module now moves
the current certificate indicator in node 3 to C(A), and adds the
node for A.

The path TA->A->C->B->E is validated and it fails. The path-building
module now moves the current indicator in node 4 to A(C) and adds a
node for C.

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+-------------+ +---------------+ +------------------+
| Node 1 | | Node 2 | | Node 3 |
| Subject: E |--->| Subject: B |--->| Subject: C |
| Issuers: B | | Issuers: C*,A | | Issuers: TA,A*,B |
+-------------+ +---------------+ +------------------+
|
v
+------------------+ +------------------+
| Node 5 | | Node 4 |
| Subject: C |<---| Subject: A |
| Issuers: TA*,A,B | | Issuers: TA,C*,B |
+------------------+ +------------------+

At this juncture, the decision of whether to allow repetition of name
and key comes to the forefront. If the certification path-building
module will NOT allow repetition of name and key, there are no
certificates in node 5 that can be used. (C and the corresponding
public key is already in the path at node 3.) At this point, node 5
is removed from the current path and the current certificate
indicator on node 4 is moved to A(B).

If instead, the module is only disallowing repetition of
certificates, C(A) is eliminated from node 5 since it is in use in
node 3, and path building continues by first validating TA->C->A->
C->B->E, and then continuing to try to build paths through C(B).
After this also fails to provide a valid path, node 5 is removed from
the current path and the current certificate indicator on node 4 is
moved to A(B).

Now a new node 5 is created for B. Just as with the prior node 5, if
not repeating name and key, B also offers no certificates that can be
used (B and B's public key is in use in node 2) so the new node 5 is
also removed from the path. At this point all certificates in node 4
have now been tried, so node 4 is removed from the path, and the
current indicator on node 3 is moved to C(B).

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Also as above, if allowing repetition of name and key, B(C) is
removed from the new node 5 (B(C) is already in use in node 3) and
paths attempted through the remaining certificate B(A). After this
fails, it will lead back to removing node 5 from the path. At this
point all certificates in node 4 have now been tried, so node 4 is
removed from the path, and the current indicator on node 3 is moved
to C(B).

This process continues until all certificates in node 1 (if there
happened to be more than one) have been tried, or until a valid path
has been found. Once the process ends and in the event no valid path
was found, it may be concluded that no path can be found from E to
TA.

3.4. Implementing Path-Building Optimization

The following section describes methods that may be used for
optimizing the certification path-building process by sorting
certificates. Optimization as described earlier seeks to prioritize
a list of certificates, effectively prioritizing (weighting) branches
of the graph/tree. The optimization methods can be used to assign a
cumulative score to each certificate. The process of scoring the
certificates amounts to testing each certificate against the
optimization methods a developer chooses to implement, and then
adding the score for each test to a cumulative score for each
certificate. After this is completed for each certificate at a given
branch point in the builder's decision tree, the certificates can be
sorted so that the highest scoring certificate is selected first, the
second highest is selected second, etc.

For example, suppose the path builder has only these two simple
sorting methods:

1) If the certificate has a subject key ID, +5 to score.
2) If the certificate has an authority key ID, +10 to score.

And it then examined three certificates:

1) Issued by CA 1; has authority key ID; score is 10.
2) Issued by CA 2; has subject key ID; score is 5.
3) Issued by CA 1; has subject key ID and authority key ID; score is
15.

The three certificates are sorted in descending order starting with
the highest score: 3, 1, and 2. The path-building software should
first try building the path through certificate 3. Failing that, it
should try certificate 1. Lastly, it should try building a path
through certificate 2.

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The following optimization methods specify tests developers may
choose to perform, but does not suggest scores for any of the
methods. Rather, developers should evaluate each method with respect
to the environment in which the application will operate, and assign
weights to each accordingly in the path-building software.
Additionally, many of the optimization methods are not binary in
nature. Some are tri-valued, and some may be well suited to sliding
or exponential scales. Ultimately, the implementer decides the
relative merits of each optimization with respect to his or her own
software or infrastructure.

Over and above the scores for each method, many methods can be used
to eliminate branches during the tree traversal rather than simply
scoring and weighting them. All cases where certificates could be
eliminated based upon an optimization method are noted with the
method descriptions.

Many of the sorting methods described below are based upon what has
been perceived by the authors as common in PKIs. Many of the methods
are aimed at making path building for the common PKI fast, but there
are cases where most any sorting method could lead to inefficient
path building. The desired behavior is that although one method may
lead the algorithm in the wrong direction for a given situation or
configuration, the remaining methods will overcome the errant
method(s) and send the path traversal down the correct branch of the
tree more often than not. This certainly will not be true for every
environment and configuration, and these methods may need to be
tweaked for further optimization in the application's target
operating environment.

As a final note, the list contained in this document is not intended
to be exhaustive. A developer may desire to define additional
sorting methods if the operating environment dictates the need.

3.5. Selected Methods for Sorting Certificates

The reader should draw no specific conclusions as to the relative
merits or scores for each of the following methods based upon the
order in which they appear. The relative merit of any sorting
criteria is completely dependent on the specifics of the operating
environment. For most any method, an example can be created to
demonstrate the method is effective and a counter-example could be
designed to demonstrate that it is ineffective.

Each sorting method is independent and may (or may not) be used to
assign additional scores to each certificate tested. The implementer
decides which methods to use and what weights to assign them. As
noted previously, this list is also not exhaustive.

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In addition, name chaining (meaning the subject name of the issuer
certificate matches the issuer name of the issued certificate) is not
addressed as a sorting method since adherence to this is required in
order to build the decision tree to which these methods will be
applied. Also, unaddressed in the sorting methods is the prevention
of repeating certificates. Path builders should handle name chaining
and certificate repetition irrespective of the optimization approach.

Each sorting method description specifies whether the method may be
used to eliminate certificates, the number of possible numeric values
(sorting weights) for the method, components from Section 2.6 that
are required for implementing the method, forward and reverse methods
descriptions, and finally a justification for inclusion of the
method.

With regard to elimination of certificates, it is important to
understand that certificates are eliminated only at a given decision
point for many methods. For example, the path built up to
certificate X may be invalidated due to name constraints by the
addition of certificate Y. At this decision point only, Y could be
eliminated from further consideration. At some future decision
point, while building this same path, the addition of Y may not
invalidate the path.

For some other sorting methods, certificates could be eliminated from
the process entirely. For example, certificates with unsupported
signature algorithms could not be included in any path and validated.
Although the path builder may certainly be designed to operate in
this fashion, it is sufficient to always discard certificates only
for a given decision point regardless of cause.

3.5.1. basicConstraints Is Present and cA Equals True

May be used to eliminate certificates: Yes
Number of possible values: Binary
Components required: None

Forward Method: Certificates with basicConstraints present and
cA=TRUE, or those designated as CA certificates out-of-band have
priority. Certificates without basicConstraints, with
basicConstraints and cA=FALSE, or those that are not designated as CA
certificates out-of-band may be eliminated or have zero priority.

Reverse Method: Same as forward except with regard to end entity
certificates at the terminus of the path.

Justification: According to [RFC3280], basicConstraints is required
to be present with cA=TRUE in all CA certificates, or must be

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verified via an out-of-band mechanism. A valid path cannot be built
if this condition is not met.

3.5.2. Recognized Signature Algorithms

May be used to eliminate certificates: Yes
Number of possible values: Binary
Components required: None

Forward Method: Certificates containing recognized signature and
public key algorithms [PKIXALGS] have priority.

Reverse Method: Same as forward.

Justification: If the path-building software is not capable of
processing the signatures associated with the certificate, the
certification path cannot be validated.

3.5.3. keyUsage Is Correct

May be used to eliminate certificates: Yes
Number of possible values: Binary
Components required: None

Forward Method: If keyUsage is present, certificates with
keyCertSign set have 100% priority. If keyUsage is present and
keyCertSign is not set, the certificate may be eliminated or have
zero priority. All others have zero priority.

Reverse Method: Same as forward except with regard to end entity
certificates at the terminus of the path.

Justification: A valid certification path cannot be built through a
CA certificate with inappropriate keyUsage. Note that
digitalSignature is not required to be set in a CA certificate.

3.5.4. Time (T) Falls within the Certificate Validity

May be used to eliminate certificates: Yes
Number of possible values: Binary
Components required: None

Forward Method: Certificates that contain the required time (T)
within their validity period have 100% priority. Otherwise, the
certificate is eliminated or has priority zero.

Reverse Method: Same as forward.

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Justification: A valid certification path cannot be built if T falls
outside of the certificate validity period.

NOTE: Special care should be taken to return a meaningful error to
the caller, especially in the event the target certificate does not
meet this criterion, if this sorting method is used for elimination.
(e.g., the certificate is expired or is not yet valid).

3.5.5. Certificate Was Previously Validated

May be used to eliminate certificates: No
Number of possible values: Binary
Components required: Certification Path Cache

Forward Method: A certificate that is present in the certification
path cache has priority.

Reverse Method: Does not apply. (The validity of a certificate vs.
unknown validity does not infer anything about the correct direction
in the decision tree. In other words, knowing the validity of a CA
certificate does not indicate that the target is more likely found
through that path than another.)

Justification: Certificates in the path cache have been validated
previously. Assuming the initial constraints have not changed, it is
highly likely that the path from that certificate to a trust anchor
is still valid. (Changes to the initial constraints may cause a
certificate previously considered valid to no longer be considered
valid.)

Note: It is important that items in the path cache have appropriate
life times. For example, it could be inappropriate to cache a
relationship beyond the period the related CRL will be trusted by the
application. It is also critical to consider certificates and CRLs
farther up the path when setting cache lifetimes. For example, if
the issuer certificate expires in ten days, but the issued
certificate is valid for 20 days, caching the relationship beyond 10
days would be inappropriate.

3.5.6. Previously Verified Signatures

May be used to eliminate certificates: Yes
Number of possible values: Binary
Components required: Path Cache

Forward Method: If a previously verified relationship exists in the
path cache between the subject certificate and a public key present
in one or more issuer certificates, all the certificates containing

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said public key have higher priority. Other certificates may be
eliminated or set to zero priority.

Reverse Method: If known bad signature relationships exist between
certificates, these relationships can be used to eliminate potential
certificates from the decision tree. Nothing can be concluded about
the likelihood of finding a given target certificate down one branch
versus another using known good signature relationships.

Justification: If the public key in a certificate (A) was previously
used to verify a signature on a second certificate (B), any and all
certificates containing the same key as (A) may be used to verify the
signature on (B). Likewise, any certificates that do not contain the
same key as (A) cannot be used to verify the signature on (B). This
forward direction method is especially strong for multiply cross-
certified CAs after a key rollover has occurred.

3.5.7. Path Length Constraints

May be used to eliminate certificates: Yes
Number of possible values: Binary
Components required: None

Forward Method: Certificates with basic constraints present and
containing a path length constraint that would invalidate the current
path (the current length is known since the software is building from
the target certificate) may be eliminated or set to zero priority.
Otherwise, the priority is 100%.

Reverse Method: This method may be applied in reverse. To apply it,
the builder keeps a current path length constraint variable and then
sets zero priority for (or eliminates) certificates that would
violate the constraint.

Justification: A valid path cannot be built if the path length
constraint has been violated.

3.5.8. Name Constraints

May be used to eliminate certificates: Yes
Number of possible values: Binary
Components required: None

Forward Method: Certificates that contain nameConstraints that would
be violated by certificates already in the path to this point are
given zero priority or eliminated.

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Reverse Method: Certificates that will allow successful processing
of any name constraints present in the path to this point are given
higher priority.

Justification: A valid path cannot be built if name constraints are
violated.

3.5.9. Certificate Is Not Revoked

May be used to eliminate certificates: No
Number of possible values: Three
Components required: CRL Cache

Forward Method: If a current CRL for a certificate is present in the
CRL cache, and the certificate serial number is not on the CRL, the
certificate has priority. If the certificate serial number is
present on the CRL, it has zero priority. If an (acceptably fresh)
OCSP response is available for a certificate, and identifies the
certificate as valid, the certificate has priority. If an OCSP
response is available for a certificate, and identifies the
certificate as invalid, the certificate has zero priority.

Reverse Method: Same as Forward.

Alternately, the certificate may be eliminated if the CRL or OCSP
response is verified. That is, fully verify the CRL or OCSP response
signature and relationship to the certificate in question in
accordance with [RFC3280]. While this is viable, the signature
verification required makes it less attractive as an elimination
method. It is suggested that this method only be used for sorting
and that CRLs and OCSP responses are validated post path building.

Justification: Certificates known to be not revoked can be
considered more likely to be valid than certificates for which the
revocation status is unknown. This is further justified if CRL or
OCSP response validation is performed post path validation - CRLs or
OCSP responses are only retrieved when complete paths are found.

NOTE: Special care should be taken to allow meaningful errors to
propagate to the caller, especially in cases where the target
certificate is revoked. If a path builder eliminates certificates
using CRLs or OCSP responses, some status information should be
preserved so that a meaningful error may be returned in the event no
path is found.

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3.5.10. Issuer Found in the Path Cache

May be used to eliminate certificates: No
Number of possible values: Binary
Components required: Certification Path Cache

Forward Method: A certificate whose issuer has an entry (or entries)
in the path cache has priority.

Reverse Method: Does not apply.

Justification: Since the path cache only contains entries for
certificates that were previously validated back to a trust anchor,
it is more likely than not that the same or a new path may be built
from that point to the (or one of the) trust anchor(s). For
certificates whose issuers are not found in the path cache, nothing
can be concluded.

NOTE: This method is not the same as the method named "Certificate
Was Previously Validated". It is possible for this sorting method to
evaluate to true while the other method could evaluate to zero.

3.5.11. Issuer Found in the Application Protocol

May be used to eliminate certificates: No
Number of possible values: Binary
Components required: Certification Path Cache

Forward Method: If the issuer of a certificate sent by the target
through the application protocol (SSL/TLS, S/MIME, etc.), matches the
signer of the certificate you are looking at, then that certificate
has priority.

Reverse Method: If the subject of a certificate matches the issuer
of a certificate sent by the target through the application protocol
(SSL/TLS, S/MIME, etc.), then that certificate has priority.

Justification: The application protocol may contain certificates
that the sender considers valuable to certification path building,
and are more likely to lead to a path to the target certificate.

3.5.12. Matching Key Identifiers (KIDs)

May be used to eliminate certificates: No
Number of possible values: Three
Components required: None

Forward Method: Certificates whose subject key identifier (SKID)

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matches the current certificate's authority key identifier (AKID)
have highest priority. Certificates without a SKID have medium
priority. Certificates whose SKID does not match the current
certificate's AKID (if both are present) have zero priority. If the
current certificate expresses the issuer name and serial number in
the AKID, certificates that match both these identifiers have highest
priority. Certificates that match only the issuer name in the AKID
have medium priority.

Reverse Method: Certificates whose AKID matches the current
certificate's SKID have highest priority. Certificates without an
AKID have medium priority. Certificates whose AKID does not match
the current certificate's SKID (if both are present) have zero
priority. If the certificate expresses the issuer name and serial
number in the AKID, certificates that match both these identifiers in
the current certificate have highest priority. Certificates that
match only the issuer name in the AKID have medium priority.

Justification: Key Identifier (KID) matching is a very useful
mechanism for guiding path building (that is their purpose in the
certificate) and should therefore be assigned a heavy weight.

NOTE: Although required to be present by [RFC3280], it is extremely
important that KIDs be used only as sorting criteria or as hints
during certification path building. KIDs are not required to match
during certification path validation and cannot be used to eliminate
certificates. This is of critical importance for interoperating
across domains and multi-vendor implementations where the KIDs may
not be calculated in the same fashion.

3.5.13. Policy Processing

May be used to eliminate certificates: Yes
Number of possible values: Three
Components required: None

Forward Method: Certificates that satisfy Forward Policy Chaining
have priority. (See Section 4 entitled "Forward Policy Chaining" for
details.) If the caller provided an initial-policy-set and did not
set the initial-require-explicit flag, the weight of this sorting
method should be increased. If the initial-require-explicit-policy
flag was set by the caller or by a certificate, certificates may be
eliminated.

Reverse Method: Certificates that contain policies/policy mappings
that will allow successful policy processing of the path to this
point have priority. If the caller provided an initial-policy-set
and did not set the initial-require-explicit flag, the weight of this

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sorting method should be increased. Certificates may be eliminated
only if initial-require-explicit was set by the caller or if
require-explicit-policy was set by a certificate in the path to this
point.

Justification: In a policy-using environment, certificates that
successfully propagate policies are more likely part of an intended
certification path than those that do not.

When building in the forward direction, it is always possible that a
certificate closer to the trust anchor will set the require-
explicit-policy indicator; so giving preference to certification
paths that propagate policies may increase the probability of finding
a valid path first. If the caller (or a certificate in the current
path) has specified or set the initial-require-explicit-policy
indicator as true, this sorting method can also be used to eliminate
certificates when building in the forward direction.

If building in reverse, it is always possible that a certificate
farther along the path will set the require-explicit-policy
indicator; so giving preference to those certificates that propagate
policies will serve well in that case. In the case where require-
explicit-policy is set by certificates or the caller, certificates
can be eliminated with this method.

3.5.14. Policies Intersect the Sought Policy Set

May be used to eliminate certificates: No
Number of possible values: Additive
Components required: None

Forward Method: Certificates that assert policies found in the
initial-acceptable-policy-set have priority. Each additional
matching policy could have an additive affect on the total score.

Alternately, this could be binary; it matches 1 or more, or matches
none.

Reverse Method: Certificates that assert policies found in the
target certificate or map policies to those found in the target
certificate have priority. Each additional matching policy could
have an additive affect on the total score. Alternately, this could
be binary; it matches 1 or more, or matches none.

Justification: In the forward direction, as the path draws near to
the trust anchor in a cross-certified environment, the policies
asserted in the CA certificates will match those in the caller's
domain. Since the initial acceptable policy set is specified in the

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caller's domain, matches may indicate that the path building is
drawing nearer to a desired trust anchor. In the reverse direction,
finding policies that match those of the target certificate may
indicate that the path is drawing near to the target's domain.

3.5.15. Endpoint Distinguished Name (DN) Matching

May be used to eliminate certificates: No
Number of possible values: Binary
Components required: None

Forward Method: Certificates whose issuer exactly matches a trust
anchor subject DN have priority.

Reverse Method: Certificates whose subject exactly matches the
target entity issuer DN have priority.

Justification: In the forward direction, if a certificate's issuer
DN matches a trust anchor's DN [X.501], then it may complete the
path. In the reverse direction, if the certificate's subject DN
matches the issuer DN of the target certificate, it may be the last
certificate required to complete the path.

3.5.16. Relative Distinguished Name (RDN) Matching

May be used to eliminate certificates: No
Number of possible values: Sliding Scale
Components required: None

Forward Method: Certificates that match more ordered RDNs between
the issuer DN and a trust anchor DN have priority. When all the RDNs
match, this yields the highest priority.

Reverse Method: Certificates with subject DNs that match more RDNs
with the target's issuer DN have higher priority. When all the RDNs
match, this yields the highest priority.

Justification: In PKIs the DNs are frequently constructed in a tree
like fashion. Higher numbers of matches may indicate that the trust
anchor is to be found in that direction within the tree. Note that
in the case where all the RDNs match [X.501], this sorting method
appears to mirror the preceding one. However, this sorting method
should be capable of producing a 100% weight even if the issuer DN
has more RDNs than the trust anchor. The Issuer DN need only contain
all the RDNs (in order) of the trust anchor.

NOTE: In the case where all RDNs match, this sorting method mirrors
the functionality of the preceding one. This allows for partial

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matches to be weighted differently from exact matches. Additionally,
this method can require a lot of processing if many trust anchors are
present.

3.5.17. Certificates are Retrieved from cACertificate Directory
Attribute

May be used to eliminate certificates: No
Number of possible values: Binary
Components required: Certificate Cache with flags for the attribute
from where the certificate was retrieved and Remote Certificate
Storage/Retrieval using a directory

Forward Method: Certificates retrieved from the cACertificate
directory attribute have priority over certificates retrieved from
the crossCertificatePair attribute. (See [RFC2587].)

Reverse Method: Does not apply.

Justification: The cACertificate directory attribute contains
certificates issued from local sources and self issued certificates.
By using the cACertificate directory attribute before the
crossCertificatePair attribute, the path-building algorithm will
(depending on the local PKI configuration) tend to demonstrate a
preference for the local PKI before venturing to external cross-
certified PKIs. Most of today's PKI applications spend most of their
time processing information from the local (user's own) PKI, and the
local PKI is usually very efficient to traverse due to proximity and
network speed.

3.5.18. Consistent Public Key and Signature Algorithms

May be used to eliminate certificates: Yes
Number of possible values: Binary
Components required: None

Forward Method: If the public key in the issuer certificate matches
the algorithm used to sign the subject certificate, then it has
priority. (Certificates with unmatched public key and signature
algorithms may be eliminated.)

Reverse Method: If the public key in the current certificate matches
the algorithm used to sign the subject certificate, then it has
priority. (Certificates with unmatched public key and signature
algorithms may be eliminated.)

Justification: Since the public key and signature algorithms are not
consistent, the signature on the subject certificate will not verify

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successfully. For example, if the issuer certificate contains an RSA
public key, then it could not have issued a subject certificate
signed with the DSA-with-SHA-1 algorithm.

3.5.19. Similar Issuer and Subject Names

May be used to eliminate certificates: No
Number of possible values: Sliding Scale
Components required: None

Forward Method: Certificates encountered with a subject DN that
matches more RDNs with the issuer DN of the target certificate have
priority.

Reverse Method: Same as forward.

Justification: As it is generally more efficient to search the local
domain prior to branching to cross-certified domains, using
certificates with similar names first tends to make a more efficient
path builder. Cross-certificates issued from external domains will
generally match fewer RDNs (if any), whereas certificates in the
local domain will frequently match multiple RDNs.

3.5.20. Certificates in the Certification Cache

May be used to eliminate certificates: No
Number of possible values: Three
Components required: Local Certificate Cache and Remote Certificate
Storage/Retrieval (e.g., LDAP directory as the repository)

Forward Method: A certificate whose issuer certificate is present in
the certificate cache and populated with certificates has higher
priority. A certificate whose issuer's entry is fully populated with
current data (all certificate attributes have been searched within
the timeout period) has higher priority.

Reverse Method: If the subject of a certificate is present in the
certificate cache and populated with certificates, then it has higher
priority. If the entry is fully populated with current data (all
certificate attributes have been searched within the timeout period)
then it has higher priority.

Justification: The presence of required directory values populated
in the cache increases the likelihood that all the required
certificates and CRLs needed to complete the path from this
certificate to the trust anchor (or target if building in reverse)
are present in the cache from a prior path being developed, thereby

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eliminating the need for directory access to complete the path. In
the event no path can be found, the performance cost is low since the
certificates were likely not retrieved from the network.

3.5.21. Current CRL Found in Local Cache

May be used to eliminate certificates: No
Number of possible values: Binary
Components Required: CRL Cache

Forward Method: Certificates have priority if the issuer's CRL entry
exists and is populated with current data in the CRL cache.

Reverse Method: Certificates have priority if the subject's CRL
entry exists and is populated with current data in the CRL cache.

Justification: If revocation is checked only after a complete path
has been found, this indicates that a complete path has been found
through this entity at some past point, so a path still likely
exists. This also helps reduce remote retrievals until necessary.

3.6. Certificate Sorting Methods for Revocation Signer Certification
Paths

Unless using a locally-configured OCSP responder or some other
locally-configured trusted revocation status service, certificate
revocation information is expected to be provided by the PKI that
issued the certificate. It follows that when building a
certification path for a Revocation Signer certificate, it is
desirable to confine the building algorithm to the PKI that issued
the certificate. The following sorting methods seek to order
possible paths so that the intended Revocation Signer certification
path is found first.

These sorting methods are not intended to be used in lieu of the ones
described in the previous section; they are most effective when used
in conjunction with those in Section 3.5. Some sorting criteria below
have identical names as those in the preceding section. This
indicates that the sorting criteria described in the preceding
section are modified slightly when building the Revocation Signer
certification path.

3.6.1. Identical Trust Anchors

May be used to eliminate certificates: No
Number of possible values: Binary
Components required: Is-revocation-signer indicator and the
Certification Authority's trust anchor

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Forward Method: Not applicable.

Reverse Method: Path building should begin from the same trust
anchor used to validate the Certification Authority before trying any
other trust anchors. If any trust anchors exist with a different
public key but an identical subject DN to that of the Certification
Authority's trust anchor, they should be tried prior to those with
mismatched names.

Justification: The revocation information for a given certificate
should be produced by the PKI that issues the certificate.
Therefore, building a path from a different trust anchor than the
Certification Authority's is not desirable.

3.6.2. Endpoint Distinguished Name (DN) Matching

May be used to eliminate certificates: No
Number of possible values: Binary
Components required: Is-revocation-signer indicator and the
Certification Authority's trust anchor

Forward Method: Operates identically to the sorting method described
in 3.5.15, except that instead of performing the matching against all
trust anchors, the DN matching is performed only against the trust
anchor DN used to validate the CA certificate.

Reverse Method: No change for Revocation Signer's certification
path.

Justification: The revocation information for a given certificate
should be produced by the PKI that issues the certificate.
Therefore, building a path to a different trust anchor than the CA's
is not desirable. This sorting method helps to guide forward
direction path building toward the trust anchor used to validate the
CA certificate.

3.6.3. Relative Distinguished Name (RDN) Matching

May be used to eliminate certificates: No
Number of possible values: Sliding Scale
Components required: Is-revocation-signer indicator and the
Certification Authority's trust anchor

Forward Method: Operates identically to the sorting method described
in 3.5.16 except that instead of performing the RDN matching against
all trust anchors, the matching is performed only against the trust
anchor DN used to validate the CA certificate.

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Reverse Method: No change for Revocation Signer's certification
path.

3.6.4. Identical Intermediate Names

May be used to eliminate certificates: No
Number of possible values: Binary
Components required: Is-revocation-signer indicator and the
Certification Authority's complete certification path

Forward Method: If the issuer DN in the certificate matches the
issuer DN of a certificate in the Certification Authority's path, it
has higher priority.

Reverse Method: If the subject DN in the certificate matches the
subject DN of a certificate in the Certification Authority's path, it
has higher priority.

Justification: Following the same path as the Certificate should
deter the path-building algorithm from wandering in an inappropriate
direction. Note that this sorting method is indifferent to whether
the certificate is self-issued. This is beneficial in this situation
because it would be undesirable to lower the priority of a re-key
certificate.

4. Forward Policy Chaining

It is tempting to jump to the conclusion that certificate policies
offer little assistance to path building when building from the
target certificate. It's easy to understand the "validate as you go"
approach from the trust anchor, and much less obvious that any value
can be derived in the other direction. However, since policy
validation consists of the intersection of the issuer policy set with
the subject policy set and the mapping of policies from the issuer
set to the subject set, policy validation can be done while building
a path in the forward direction as well as the reverse. It is simply
a matter of reversing the procedure. That is not to say this is as
ideal as policy validation when building from the trust anchor, but
it does offer a method that can be used to mostly eliminate what has
long been considered a weakness inherent to building in the forward
(from the target certificate) direction.

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4.1. Simple Intersection

The most basic form of policy processing is the intersection of the
policy sets from the first CA certificate through the target
certificate. Fortunately, the intersection of policy sets will
always yield the same final set regardless of the order of
intersection. This allows processing of policy set intersections in
either direction. For example, if the trust anchor issues a CA
certificate (A) with policies {X,Y,Z}, and that CA issues another CA
certificate (B) with policies {X,Y}, and CA B then issues a third CA
certificate (C) with policy set {Y,G}, one normally calculates the
policy set from the trust anchor as follows:

1) Intersect A{X,Y,Z} with B{X,Y} to yield the set {X,Y}

2) Intersect that result, {X,Y} with C{Y,G} to yield the final set
{Y}

Now it has been shown that certificate C is good for policy Y.

The other direction is exactly the same procedure, only in reverse:

1) Intersect C{Y,G} with B{X,Y} to yield the set {Y}

2) Intersect that result, {Y} with A{X,Y,Z} to yield the final set
{Y}

Just like in the reverse direction, it has been shown that
certificate C is good for policy Y, but this time in the forward
direction.

When building in the forward direction, policy processing is handled
much like it is in reverse -- the software lends preference to
certificates that propagate policies. Neither approach guarantees
that a path with valid policies will be found, but rather both
approaches help guide the path in the direction it should go in order
for the policies to propagate.

If the caller has supplied an initial-acceptable-policy set, there is
less value in using it when building in the forward direction unless
the caller also set inhibit-policy-mapping. In that case, the path
builder can further constrain the path building to propagating
policies that exist in the initial-acceptable-policy-set. However,
even if the inhibit-policy-mapping is not set, the initial-policy-set
can still be used to guide the path building toward the desired trust
anchor.

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4.2. Policy Mapping

When a CA issues a certificate into another domain, an environment
with disparate policy identifiers to its own, the CA may make use of
policy mappings to map equivalence from the local domain's policy to
the non-local domain's policy. If in the prior example, A had
included a policy mapping that mapped X to G in the certificate it
issued to B, C would be good for X and Y:

1) Intersect A{X,Y,Z} with B{X,Y} to yield the set {X,Y}

2) Process Policy Mappings in B's certificate (X maps to G) to yield
{G,Y} (same as {Y,G})

3) Intersect that result, {G,Y} with C{Y,G} to yield the final set
{G,Y}

Since policies are always expressed in the relying party's domain,
the certificate C is said to be good for {X, Y}, not {Y, G}. This is
because "G" doesn't mean anything in the context of the trust anchor
that issued A without the policy mapping.

When building in the forward direction, policies can be "unmapped" by
reversing the mapping procedure. This procedure is limited by one
important aspect: if policy mapping has occurred in the forward
direction, there is no mechanism by which it can be known in advance
whether or not a future addition to the current path will invalidate
the policy chain (assuming one exists) by setting inhibit-policy-
mapping. Fortunately, it is uncommon practice to set this flag. The
following is the procedure for processing policy mapping in the
forward direction:

1) Begin with C's policy set {Y,G}

2) Apply the policy mapping in B's certificate (X maps to G) in
reverse to yield {Y,X} (same as {X,Y})

3) Intersect the result {X,Y} with B{X,Y} to yield the set {X,Y}

4) Intersect that result, {X,Y}, with A{X,Y,Z} to yield the final set
{X,Y}

Just like in the reverse direction, it is determined in the forward
direction that certificate C is good for policies {X,Y}. If during
this procedure, an inhibit-policy-mapping flag was encountered, what
should be done? This is reasonably easy to keep track of as well.
The software simply maintains a flag on any policies that were
propagated as a result of a mapping; just a simple Boolean kept with

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the policies in the set. Imagine now that the certificate issued to
A has the inhibit-policy-mapping constraint expressed with a skip
certificates value of zero.

1) Begin with C's policy set {Y,G}

2) Apply the policy mapping in B's certificate and mark X as
resulting from a mapping. (X maps to G) in reverse to yield {Y,Xm}
(same as {Xm,Y})

3) Intersect the result {Xm,Y} with B{X,Y} to yield the set {Xm,Y}

4) A's certificate expresses the inhibit policy mapping constraint,
so eliminate any policies in the current set that were propagated
due to mapping (which is Xm) to yield {Y}

5) Intersect that result, {Y} with A{X,Y,Z} to yield the final set
{Y}

If in our example, the policy set had gone to empty at any point (and
require-explicit-policy was set), the path building would back up and
try to traverse another branch of the tree. This is analogous to the
path-building functionality utilized in the reverse direction when
the policy set goes to empty.

4.3. Assigning Scores for Forward Policy Chaining

Assuming the path-building module is maintaining the current forward
policy set, weights may be assigned using the following procedure:

1) For each CA certificate being scored:

a. Copy the current forward policy set.

b. Process policy mappings in the CA certificate in order to
"un-map" policies, if any.

c. Intersect the resulting set with CA certificate's policies.

The larger the policy set yielded, the larger the score for that CA
certificate.

2) If an initial acceptable set was supplied, intersect this set with
the resulting set for each CA certificate from (1).

The larger the resultant set, the higher the score is for this
certificate.

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Other scoring schemes may work better if the operating environment
dictates.

5. Avoiding Path-Building Errors

This section defines some errors that may occur during the path-
building process, as well as ways to avoid these errors when
developing path-building functions.

5.1. Dead Ends

When building certification paths in a non-hierarchical PKI
structure, a simple path-building algorithm could fail prematurely
without finding an existing path due to a "dead end". Consider the
example in Figure 14.

+----+ +---+
| TA | | Z |
+----+ +---+
| |
| |
V V
+---+ +---+
| C |<-----| Y |
+---+ +---+
|
|
V
+--------+
| Target |
+--------+

Figure 14 - Dead End Example

Note that in the example, C has two certificates: one issued by Y,
and the other issued by the Trust Anchor. Suppose that a simple
"find issuer" algorithm is used, and the order in which the path
builder found the certificates was Target(C), C(Y), Y(Z), Z(Z). In
this case, Z has no certificates issued by any other entities, and so
the simplistic path-building process stops. Since Z is not the
relying party's trust anchor, the certification path is not complete,
and will not validate. This example shows that in anything but the
simplest PKI structure, additional path-building logic will need to
handle the cases in which entities are issued multiple certificates
from different issuers. The path-building algorithm will also need
to have the ability to traverse back up the decision tree and try
another path in order to be robust.

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5.2. Loop Detection

In a non-hierarchical PKI structure, a path-building algorithm may
become caught in a loop without finding an existing path. Consider
the example below:

+----+
| TA |
+----+
|
|
+---+ +---+
| A | ->| Z |
+---+ / +---+
| / |
| / |
V / V
+---+ +---+
| B |<-----| Y |
+---+ +---+
|
|
V
+--------+
| Target |
+--------+

Figure 15 - Loop Example

Let us suppose that in this example the simplest "find issuer"
algorithm is used, and the order in which certificates are retrieved
is Target(B), B(Y), Y(Z), Z(B), B(Y), Y(Z), Z(B), B(Y), ... A loop
has formed that will cause the correct path (Target, B, A) to never
be found. The certificate processing system will need to recognize
loops created by duplicate certificates (which are prohibited in a
path by [X.509]) before they form to allow the certification path-
building process to continue and find valid paths. The authors of
this document recommend that the loop detection not only detect the
repetition of a certificate in the path, but also detect the presence
of the same subject name / subject alternative name/ subject public
key combination occurring twice in the path. A name/key pair should
only need to appear once in the path. (See Section 2.4.2 for more
information on the reasoning behind this recommendation.)

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5.3. Use of Key Identifiers

Inconsistent and/or incompatible approaches to computing the subject
key identifier and authority key identifier in public key
certificates can cause failures in certification path-building
algorithms that use those fields to identify certificates, even
though otherwise valid certification paths may exist. Path-building
implementations should use existing key identifiers and not attempt
to re-compute subject key identifiers. It is extremely important
that Key Identifiers be used only as sorting criteria or hints. KIDs
are not required to match during certification path validation and
cannot be used to eliminate certificates. This is of critical
importance for interoperating across domains and multi-vendor
implementations where the KIDs may not be calculated in the same
fashion.

Path-building and processing implementations should not rely on the
form of authority key identifier that uses the authority DN and
serial number as a restrictive matching rule, because cross-
certification can lead to this value not being matched by the cross-
certificates.

5.4. Distinguished Name Encoding

Certification path-building software should not rely on DNs being
encoded as PrintableString. Although frequently encoded as
PrintableString, DNs may also appear as other types, including
BMPString or UTF8String. As a result, software systems that are
unable to process BMPString and UTF8String encoded DNs may be unable
to build and validate some certification paths.

Furthermore, [RFC3280] compliant certificates are required to encode
DNs as UTF8String as of January 1, 2004. Certification path-building
software should be prepared to handle "name rollover" certificates as
described in [RFC3280]. Note that the inclusion of a "name rollover"
certificate in a certification path does not constitute repetition of
a DN and key. Implementations that include the "name rollover"
certificate in the path should ensure that the DNs with differing
encoding are regarded as dissimilar. (Implementations may instead
handle matching DNs of different encodings and will therefore not
need to include "name rollover" certificates in the path.)

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6. Retrieval Methods

Building a certification path requires the availability of the
certificates and CRLs that make up the path. There are many
different methods for obtaining these certificates and CRLs. This
section lists a few of the common ways to perform this retrieval, as
well as some suggested approaches for improving performance. This
section is not intended to provide a complete reference for
certificate and CRL retrieval methods or optimizations that would be
useful in certification path building.

6.1. Directories Using LDAP

Most applications utilize the Lightweight Directory Access Protocol
(LDAP) when retrieving data from directories following the X.500
model. Applications may encounter directories which support either
LDAP v2 [RFC1777] or LDAP v3 [RFC3377].

The LDAP v3 specification defines one attribute retrieval option, the
"binary" option. When specified in an LDAP retrieval request, this
option was intended to force the directory to ignore any string-based
representations of BER-encoded directory information, and send the
requested attribute(s) in BER format. Since all PKI objects of
concern are BER-encoded objects, the "binary" option should be used.
However, not all directories support the "binary" option. Therefore,
applications should be capable of requesting attributes with and
without the "binary" option. For example, if an application wishes
to retrieve the userCertificate attribute, the application should
request "userCertificate;binary". If the desired information is not
returned, robust implementations may opt to request "userCertificate"
as well.

The following attributes should be considered by PKI application
developers when performing certificate retrieval from LDAP sources:

userCertificate: contains certificates issued by one or more
certification authorities with a subject DN that matches that of
the directory entry. This is a multi-valued attribute and all
values should be received and considered during path building.
Although typically it is expected that only end entity
certificates will be stored in this attribute, (e.g., this is the
attribute an application would request to find a person's
encryption certificate) implementers may opt to search this
attribute when looking in CA entries to make their path builder
more robust. If it is empty, the overhead added by including this
attribute when already requesting one or both of the two below is
marginal.

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cACertificate: contains self-issued certificates (if any) and any
certificates issued to this certification authority by other
certification authorities in the same realm. (Realm is dependent
upon local policy.) This is a multi-valued attribute and all
values should be received and considered during path building.

crossCertificatePair: in conformant implementations, the
crossCertificatePair is used to contain all, except self-issued
certificates issued to this certification authority, as well as
certificates issued by this certification authority to other
certification authorities. Each attribute value is a structure
containing two elements. The issuedToThisCA element contains
certificates issued to this certification authority by other
certification authorities. The issuedByThisCA element contains
certificates issued by this certification authority to other
certification authorities. Both elements of the
crossCertificatePair are labeled optional in the ASN.1 definition.
If both elements are present in a single value, the issuer name in
one certificate is required to match the subject name in the other
and vice versa, and the subject public key in one certificate
shall be capable of verifying the digital signature on the other
certificate and vice versa. As this technology has evolved,
different standards have had differing requirements on where
information could be found. For example, the LDAP v2 schema
[RFC2587] states that the issuedToThisCA (once called 'forward')
element of the crossCertificatePair attribute is mandatory and the
issuedByThisCA (once called 'reverse') element is optional. In
contrast, Section 11.2.3 of [X.509] requires the issuedByThisCA
element to be present if the CA issues a certificate to another CA
if the subject is not a subordinate CA in a hierarchy. Conformant
directories behave as required by [X.509], but robust path-
building implementations may want to retrieve all certificates
from the cACertificate and crossCertificatePair attributes to
ensure all possible certification authority certificates are
obtained.

certificateRevocationList: the certificateRevocationList attribute
contains a certificate revocation list (CRL). A CRL is defined in
[RFC3280] as a time stamped list identifying revoked certificates,
which is signed by a CA or CRL issuer and made freely available in
a public repository. Each revoked certificate is identified in a
CRL by its certificate serial number. There may be one or more
CRLs in this attribute, and the values should be processed in
accordance with [RFC3280].

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authorityRevocationList: the authorityRevocationList attribute also
contains CRLs. These CRLs contain revocation information
regarding certificates issued to other CAs. There may be one or
more CRLs in this attribute, and the values should be processed in
accordance with [RFC3280].

Certification path processing systems that plan to interoperate with
varying PKI structures and directory designs should at a minimum be
able to retrieve and process the userCertificate, cACertificate,
crossCertificatePair, certificateRevocationList, and
authorityRevocationList attributes from directory entries.

6.2. Certificate Store Access via HTTP

Another possible method of certificate retrieval is using HTTP as an
interface mechanism for retrieving certificates and CRLs from PKI
repositories. A current PKIX document [CERTSTORE] provides a
protocol for a general-purpose interface capability for retrieving
certificates and CRLs from PKI repositories. Since the [CERTSTORE]
document is a work in progress as of the writing of this document, no
details are given here on how to utilize this mechanism for
certificate and CRL retrieval. Instead, refer to the [CERTSTORE]
document or its current version. Certification path processing
systems may wish to implement support for this interface capability,
especially if they will be used in environments that will provide
HTTP-based access to certificates and CRLs.

6.3. Authority Information Access

The authority information access (AIA) extension, defined within
[RFC3280], indicates how to access CA information and services for
the issuer of the certificate in which the extension appears. If a
certificate with an AIA extension contains an accessMethod defined
with the id-ad-caIssuers OID, the AIA may be used to retrieve one or
more certificates for the CA that issued the certificate containing
the AIA extension. The AIA will provide a uniform resource
identifier (URI) [RFC3986] when certificates can be retrieved via
LDAP, HTTP, or FTP. The AIA will provide a directoryName when
certificates can be retrieved via directory access protocol (DAP).
The AIA will provide an rfc822Name when certificates can be retrieved
via electronic mail. Additionally, the AIA may specify the location
of an OCSP [RFC2560] responder that is able to provide revocation
information for the certificate.

If present, AIA may provide forward path-building implementations
with a direct link to a certificate for the issuer of a given
certificate. Therefore, implementations may wish to provide support
for decoding the AIA extension and processing the LDAP, HTTP, FTP,

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DAP, or e-mail locators. Support for AIA is optional; [RFC3280]
compliant implementations are not required to populate the AIA
extension. However, implementers of path-building and validation
modules are strongly encouraged to support AIA, especially the HTTP
transport; this will provide for usability and interoperability with
many existing PKIs.

6.4. Subject Information Access

The subject information access (SIA) extension, defined within
[RFC3280], indicates how to access information and services for the
subject of the certificate in which the extension appears. If a
certificate with an SIA extension contains an accessMethod defined
with the id-ad-caRepository OID, the SIA may be used to locate one or
more certificates (and possibly CRLs) for entities issued
certificates by the subject. The SIA will provide a uniform resource
identifier (URI) [RFC3986] when data can be retrieved via LDAP, HTTP,
or FTP. The SIA will provide a directoryName when data can be
retrieved via directory access protocol (DAP). The SIA will provide
an rfc822Name when data can be retrieved via electronic mail.

If present, the SIA extension may provide reverse path-building
implementations with the certificates required to continue building
the path. Therefore, implementations may wish to provide support for
decoding the SIA extension and processing the LDAP, HTTP, FTP, DAP,
or e-mail locators. Support for SIA is optional; [RFC3280] compliant
implementations are not required to populate the SIA extension.
However, implementers of path-building and validation modules are
strongly encouraged to support SIA, especially the HTTP transport;
this will provide for usability and interoperability with many
existing PKIs.

6.5. CRL Distribution Points

The CRL distribution points (CRLDP) extension, defined within
[RFC3280], indicates how to access CRL information. If a CRLDP
extension appears within a certificate, the CRL(s) to which the CRLDP
refer are generally the CRLs that would contain revocation
information for the certificate. The CRLDP extension may point to
multiple distribution points from which the CRL information may be
obtained; the certificate processing system should process the CRLDP
extension in accordance with [RFC3280]. The most common distribution
points contain URIs from which the appropriate CRL may be downloaded,
and directory names, which can be queried in a directory to retrieve
the CRL attributes from the corresponding entry.

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If present, CRLDP can provide certificate processing implementations
with a link to CRL information for a given certificate. Therefore,
implementations may wish to provide support for decoding the CRLDP
extension and using the information to retrieve CRLs. Support for
CRLDP is optional and [RFC3280] compliant implementations need not
populate the CRLDP extension. However, implementers of path-building
and validation modules are strongly encouraged to support CRLDPs. At
a minimum, developers are encouraged to consider supporting the LDAP
and HTTP transports; this will provide for interoperability across a
wide range of existing PKIs.

6.6. Data Obtained via Application Protocol

Many application protocols, such as SSL/TLS and S/MIME, allow one
party to provide certificates and CRLs to another. Data provided in
this method is generally very valuable to path-building software
(will provide direction toward valid paths), and should be stored and
used accordingly. Note: self-signed certificates obtained via
application protocol are not trustworthy; implementations should only
consider the relying party's trust anchors when building paths.

6.7. Proprietary Mechanisms

Some certificate issuing systems and certificate processing systems
may utilize proprietary retrieval mechanisms, such as network mapped
drives, databases, or other methods that are not directly referenced
via the IETF standards. Certificate processing systems may wish to
support other proprietary mechanisms, but should only do so in
addition to supporting standard retrieval mechanisms such as LDAP,
AIA, and CRLDP (unless functioning in a closed environment).

7. Improving Retrieval Performance

Retrieval performance can be improved through a few different
mechanisms, including the use of caches and setting a specific
retrieval order. This section discusses a few methods by which the
performance of a certificate processing system may be improved during
the retrieval of PKI objects. Certificate processing systems that
are consistently very slow during processing will be disliked by
users and will be slow to be adopted into organizations. Certificate
processing systems are encouraged to do whatever possible to reduce
the delays associated with requesting and retrieving data from
external sources.

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7.1. Caching

Certificate processing systems operating in a non-hierarchical PKI
will often need to retrieve certificates and certificate revocation
lists (CRLs) from a source outside the application protocol.
Typically, these objects are retrieved from an X.500 or LDAP
repository, an Internet URI [RFC3986], or some other non-local
source. Due to the delays associated with establishing connections
as well as network transfers, certificate processing systems ought to
be as efficient as possible when retrieving data from external
sources. Perhaps the best way to improve retrieval efficiency is by
using a caching mechanism. Certificate processing systems can cache
data retrieved from external sources for some period of time, but not
to exceed the useful period of the data (i.e., an expired certificate
need not be cached). Although this comes at a cost of increased
memory/disk consumption by the system, the cost and performance
benefit of reducing network transmissions is great. Also, CRLs are
often issued and available in advance of the nextUpdate date in the
CRL. Implementations may wish to obtain these "fresher" CRLs before
the nextUpdate date has passed.

There are a number of different ways in which caching can be
implemented; the specifics of these methods can be used as
distinguishing characteristics between certificate processing
systems. However, some things that implementers may wish to consider
when developing caching systems are as follows:

- If PKI objects are cached, the certification path-building
mechanism should be able to examine and retrieve from the cache
during path building. This will allow the certificate
processing system to find or eliminate one or more paths quickly
without requiring external contact with a directory or other
retrieval mechanism.

- Sharing caches between multiple users (via a local area network
or LAN) may be useful if many users in one organization
consistently perform PKI operations with another organization.

- Caching not only PKI objects (such as certificates and CRLs) but
also relationships between PKI objects (storing a link between a
certificate and the issuer's certificate) may be useful. This
linking may not always lead to the most correct or best
relationship, but could represent a linking that worked in
another scenario.

- Previously built paths and partial paths are quite useful to
cache, because they will provide information on previous
successes or failures. Additionally, if the cache is safe from

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unauthorized modifications, caching validation and signature
checking status for certificates, CRLs, and paths can also be
stored.

7.2. Retrieval Order

To optimize efficiency, certificate processing systems are encouraged
to also consider the order in which different PKI objects are
retrieved, as well as the mechanism from which they are retrieved.
If caching is utilized, the caches can be consulted for PKI objects
before attempting other retrieval mechanisms. If multiple caches are
present (such as local disk and network), the caches can be consulted
in the order in which they can be expected to return their result
from fastest to slowest. For example, if a certificate processing
system wishes to retrieve a certificate with a particular subject DN,
the system might first consult the local cache, then the network
cache, and then attempt directory retrieval. The specifics of the
types of retrieval mechanisms and their relative costs are left to
the implementer.

In addition to ordering retrieval mechanisms, the certificate
processing system ought to order the relative merits of the different
external sources from which a PKI object can be retrieved. If the
AIA is present within a certificate, with a URI [RFC3986] for the
issuer's certificate, the certificate processing system (if able) may
wish to attempt to retrieve the certificate first from local cache
and then by using that URI (because it is expected to point directly
to the desired certificate) before attempting to retrieve the
certificates that may exist within a directory.

If a directory is being consulted, it may be desirable to retrieve
attributes in a particular order. A highly cross-certified PKI
structure will lead to multiple possibilities for certification
paths, which may mean multiple validation attempts before a
successful path is retrieved. Therefore, cACertificate and
userCertificate (which typically contain certificates from within the
same 'realm') could be consulted before attempting to retrieve the
crossCertificatePair values for an entry. Alternately, all three
attributes could be retrieved in one query, but cross-certificates
then tagged as such and used only after exhausting the possibilities
from the cACertificate attribute. The best approach will depend on
the nature of the application and PKI environment.

7.3. Parallel Fetching and Prefetching

Much of this document has focused on a path-building algorithm that
minimizes the performance impact of network retrievals, by preventing
those retrievals and utilization of caches. Another way to improve

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performance would be to allow network retrievals to be performed in
advance (prefetching) or at the same time that other operations are
performed (parallel fetching). For example, if an email application
receives a signed email message, it could download the required
certificates and CRLs prior to the recipient viewing (or attempting
to verify) the message. Implementations that provide the capability
of parallel fetching and/or prefetching, along with a robust cache,
can lead to greatly improved performance or user experience.

8. Security Considerations

8.1. General Considerations for Building a Certification Path

Although certification path building deals directly with security
relevant PKI data, the PKI data itself needs no special handling
because its integrity is secured with the digital signature applied
to it. The only exception to this is the appropriate protection of
the trust anchor public keys. These are to be kept safe and obtained
out of band (e.g., not from an electronic mail message or a
directory) with respect to the path-building module.

The greatest security risks associated with this document revolve
around performing certification path validation while certification
paths are built. It is therefore noted here that fully implemented
certification path validation in accordance with [RFC3280] and
[X.509] is required in order for certification path building,
certification path validation, and the certificate using application
to be properly secured. All of the Security Considerations listed in
Section 9 of [RFC3280] apply equally here.

In addition, as with any application that consumes data from
potentially untrusted network locations, certification path-building
components should be carefully implemented so as to reduce or
eliminate the possibility of network based exploits. For example, a
poorly implemented path-building module may not check the length of
the CRLDP URI [RFC3986] before using the C language strcpy() function
to place the address in a 1024 byte buffer. A hacker could use such
a flaw to create a buffer overflow exploit by encoding malicious
assembly code into the CRLDP of a certificate and then use the
certificate to attempt an authentication. Such an attack could yield
system level control to the attacker and expose the sensitive data
the PKI was meant to protect.

Path building may be used to mount a denial of service (DOS) attack.
This might occur if multiple simple requests could be performed that
cause a server to perform a number of path developments, each taking
time and resources from the server. Servers can help avoid this by
limiting the resources they are willing to devote to path building,

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and being able to further limit those resources when the load is
heavy. Standard DOS protections such as systems that identify and
block attackers can also be useful.

A DOS attack can be also created by presenting spurious CA
certificates containing very large public keys. When the system
attempts to use the large public key to verify the digital signature
on additional certificates, a long processing delay may occur. This
can be mitigated by either of two strategies. The first strategy is
to perform signature verifications only after a complete path is
built, starting from the trust anchor. This will eliminate the
spurious CA certificate from consideration before the large public
key is used. The second strategy is to recognize and simply reject
keys longer than a certain size.

A similar DOS attack can occur with very large public keys in end
entity certificates. If a system uses the public key in a
certificate before building and validating that certificate's
certification path, long processing delays may occur. To mitigate
this threat, the public key in an end entity certificate should not
be used for any purpose until a complete certification path for that
certificate is built and validated.

8.2. Specific Considerations for Building Revocation Signer
Certification Paths

If the CRL Signer certificate (and certification path) is not
identical to the Certification Authority certificate (and
certification path), special care should be exercised when building
the CRL Signer certification path.

If special consideration is not given to building a CRL Signer
certification path, that path could be constructed such that it
terminates with a different root or through a different certification
path to the same root. If this behavior is not prevented, the
relying party may end up checking the wrong revocation data, or even
maliciously substituted data, resulting in denial of service or
security breach.

For example, suppose the following certification path is built for E
and is valid for an example "high assurance" policy.

A->B->C->E

When the building/validation routine attempts to verify that E is not
revoked, C is referred to as the Certification Authority certificate.
The path builder finds that the CRL for checking the revocation
status of E is issued by C2; a certificate with the subject name "C",

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but with a different key than the key that was used to sign E. C2 is
referred to as the CRL Signer. An unrestrictive certification path
builder might then build a path such as the following for the CRL
Signer C2 certificate:

X->Y->Z->C2

If a path such as the one above is permitted, nothing can be
concluded about the revocation status of E since C2 is a different CA
from C.

Fortunately, preventing this security problem is not difficult and
the solution also makes building CRL Signer certification paths very
efficient. In the event the CRL Signer certificate is identical to
the Certification Authority certificate, the Certification Authority
certification path should be used to verify the CRL; no additional
path building is required. If the CRL Signer certificate is not
identical to the Certification Authority certificate, a second path
should be built for the CRL Signer certificate in exactly the same
fashion as for any certificate, but with the following additional
guidelines:

1. Trust Anchor: The CRL Signer's certification path should start
with the same trust anchor as the Certification Authority's
certification path. Any trust anchor certificate with a subject
DN matching that of the Certification Authority's trust anchor
should be considered acceptable though lower in priority than the
one with a matching public key and subject DN. While different
trust anchor public keys are acceptable at the beginning of the
CRL signer's certification path and the Certification Authority's
certification path, both keys must be trusted by the relying
party per the recommendations in Section 8.1.

2. CA Name Matching: The subject DNs for all CA certificates in the
two certification paths should match on a one-to-one basis
(ignoring self-issued certificates) for the entire length of the
shorter of the two paths.

3. CRL Signer Certification Path Length: The length of the CRL
Signer certification path (ignoring self-issued certificates)
should be equal to or less than the length of the Certification
Authority certification path plus (+) one. This allows a given
Certification Authority to issue a certificate to a
delegated/subordinate CRL Signer. The latter configuration
represents the maximum certification path length for a CRL Signer
certificate.

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The reasoning behind the first guideline is readily apparent.
Lacking this and the second guideline, any trusted CA could issue
CRLs for any other CA, even if the PKIs are not related in any
fashion. For example, one company could revoke certificates issued
by another company if the relying party trusted the trust anchors
from both companies. The two guidelines also prevent erroneous CRL
checks since Global uniqueness of names is not guaranteed.

The second guideline prevents roaming certification paths such as the
previously described example CRL Signer certification path for
A->B->C->E. It is especially important that the "ignoring self-
issued certificates" is implemented properly. Self-issued
certificates are cast out of the one-to-one name comparison in order
to allow for key rollover. The path-building algorithm may be
optimized to only consider certificates with the acceptable subject
DN for the given point in the CRL Signer certification path while
building the path.

The third and final guideline ensures that the CRL used is the
intended one. Without a restriction on the length of the CRL Signer
certification path, the path could roam uncontrolled into another
domain and still meet the first two guidelines. For example, again
using the path A->B->C->E, the Certification Authority C, and a CRL
Signer C2, a CRL Signer certification path such as the following
could pass the first two guidelines:

A->B->C->D->X->Y->RogueCA->C2

In the preceding example, the trust anchor is identical for both
paths and the one-to-one name matching test passes for A->B->C.
However, accepting such a path has obvious security consequences, so
the third guideline is used to prevent this situation. Applying the
second and third guideline to the certification path above, the path
builder could have immediately detected this path was not acceptable
(prior to building it) by examining the issuer DN in C2. Given the
length and name guidelines, the path builder could detect that
"RogueCA" is not in the set of possible names by comparing it to the
set of possible CRL Signer issuer DNs, specifically, A, B, or C.

Similar consideration should be given when building the path for the
OCSP Responder certificate when the CA is the OCSP Response Signer or
the CA has delegated the OCSP Response signing to another entity.

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9. Acknowledgements

The authors extend their appreciation to David Lemire for his efforts
coauthoring "Managing Interoperability in Non-Hierarchical Public Key
Infrastructures" from which material was borrowed heavily for use in
the introductory sections.

This document has also greatly benefited from the review and
additional technical insight provided by Dr. Santosh Chokhani, Carl
Wallace, Denis Pinkas, Steve Hanna, Alice Sturgeon, Russ Housley, and
Tim Polk.

10. Normative References

[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.

11. Informative References

[MINHPKIS] Hesse, P., and D. Lemire, "Managing Interoperability in
Non-Hierarchical Public Key Infrastructures", 2002
Conference Proceedings of the Internet Society Network
and Distributed System Security Symposium, February 2002.

[RFC1777] Yeong, W., Howes, T., and S. Kille, "Lightweight
Directory Access Protocol", RFC 1777, March 1995.

[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.

[RFC2587] Boeyen, S., Howes, T., and P. Richard, "Internet X.509
Public Key Infrastructure LDAPv2 Schema", RFC 2587, June
1999.

[RFC3377] Hodges, J. and R. Morgan, "Lightweight Directory Access
Protocol (v3): Technical Specification", RFC 3377,
September 2002.

[RFC3820] Tuecke, S., Welch, V., Engert, D., Pearlman, L., and M.
Thompson, "Internet X.509 Public Key Infrastructure (PKI)
Proxy Certificate Profile", RFC 3820, June 2004.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.

Cooper, et al. Informational [Page 78]

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[X.501] ITU-T Recommendation X.501: Information Technology - Open
Systems Interconnection - The Directory: Models, 1993.

[X.509] ITU-T Recommendation X.509 (2000 E): Information
Technology - Open Systems Interconnection - The
Directory: Authentication Framework, March 2000.

[PKIXALGS] Bassham, L., Polk, W. and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
Lists (CRL) Profile", RFC 3279, April 2002.

[CERTSTORE] P. Gutmann, "Internet X.509 Public Key Infrastructure
Operational Protocols: Certificate Store Access via
HTTP", Work in Progress, August 2004.

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RFC 4158 Certification Path Building September 2005

Authors' Addresses

Matt Cooper
Orion Security Solutions, Inc.
1489 Chain Bridge Rd, Ste. 300
McLean, VA 22101, USA

Phone: +1-703-917-0060
EMail: mcooper@orionsec.com

Yuriy Dzambasow
A&N Associates, Inc.
999 Corporate Blvd Ste. 100
Linthicum, MD 21090, USA

Phone: +1-410-859-5449 x107
EMail: yuriy@anassoc.com

Peter Hesse
Gemini Security Solutions, Inc.
4451 Brookfield Corporate Dr. Ste. 200
Chantilly, VA 20151, USA

Phone: +1-703-378-5808 x105
EMail: pmhesse@geminisecurity.com

Susan Joseph
Van Dyke Technologies
6716 Alexander Bell Drive
Columbia, MD 21046

EMail: susan.joseph@vdtg.com

Richard Nicholas
BAE Systems Information Technology
141 National Business Parkway, Ste. 210
Annapolis Junction, MD 20701, USA

Phone: +1-301-939-2722
EMail: richard.nicholas@it.baesystems.com

Cooper, et al. Informational [Page 80]

RFC 4158 Certification Path Building September 2005

Full Copyright Statement

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Acknowledgement

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Internet Society.

Cooper, et al. Informational [Page 81]

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