PKIX Working Group M. Cooper
Internet Draft Orion Security
Solutions
Document: draft-ietf-pkix-certpathbuild-02.txt Y. Dzambasow
Expires: May 2004 A&N Associates
P. Hesse
Gemini Security
Solutions
S. Joseph
DigitalNet
R. Nicholas
DigitalNet
November 2003
Internet X.509 Public Key Infrastructure:
Certification Path Building
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC 2026].
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Abstract
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This document was written to provide 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 Overview of PKI Structures.................................7
1.4.1 Hierarchical Structures.............................7
1.4.2 Mesh Structures.....................................9
1.4.3 Bi-lateral Cross-Certified Structures..............10
1.4.4 Bridge Structures..................................11
1.5 Bridge Structures and Certification Path Processing.......12
2. Certification Path Building...................................13
2.1 Introduction to Certification Path Building...............13
2.2 Criteria for Path Building................................14
2.3 Path Building Algorithms..................................15
2.4 How to Build a Certification Path.........................20
2.4.1 Certificate Repetition.............................22
2.4.2 Introduction to Path Building Optimization.........23
2.5 Building Certification Paths for CRL Signers..............28
2.6 Suggested Path Building Software Components...............30
2.7 Inputs to the Path Building Module........................32
2.7.1 Required Inputs....................................33
2.7.2 Optional Inputs....................................33
3. Optimizing Path Building......................................34
3.1 Optimized Path Building...................................34
3.2 Sorting vs. Elimination...................................36
3.3 Representing The Decision Tree Programmatically...........39
3.3.1 Node Representation For CA Entities................40
3.3.2 Using Nodes to Iterate Over All Paths..............40
3.4 Implementing Path Building Optimization...................43
3.5 Selected Methods for Sorting Certificates.................44
3.5.1 basicConstraints is Present and cA Equals True.....45
3.5.2 Recognized Signature Algorithms....................46
3.5.3 keyUsage is Correct................................46
3.5.4 Time (T) Falls within the Certificate Validity.....47
3.5.5 Certificate Was Previously Validated...............47
3.5.6 Previously Verified Signatures.....................48
3.5.7 Path Length Constraints............................48
3.5.8 Name Constraints...................................48
3.5.9 Certificate is Not Revoked.........................49
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3.5.10 Issuer Found in the Path Cache.....................50
3.5.11 Matching Key Identifiers (KIDs)....................50
3.5.12 Policy Processing..................................51
3.5.13 Policies Intersect The Sought Policy Set...........52
3.5.14 Endpoint Distinguished Name Matching...............52
3.5.15 Relative Distinguished Name (RDN) Matching.........53
3.5.16 Certificates are Retrieved from cACertificate......53
3.5.17 Consistent Public Key and Signature Algorithms.....54
3.5.18 Similar Issuer and Subject Names...................54
3.5.19 Certificates in the Certification Cache............55
3.5.20 Current CRL Found in Local Cache...................55
4. Forward Policy Chaining.......................................56
4.1 Simple Intersection.......................................56
4.2 Policy Mapping............................................57
4.3 Assigning Scores for Forward Policy Chaining..............58
5. Avoiding Path Building Errors.................................59
5.1 Dead-ends.................................................59
5.2 Loop Detection............................................60
5.3 Use of Key Identifiers....................................61
5.4 Distinguished Name Encoding...............................61
6. Retrieval Methods.............................................61
6.1 Directories Using LDAP....................................62
6.2 Authority Information Access..............................64
6.3 Subject Information Access................................64
6.4 CRL Distribution Points...................................64
6.5 Proprietary Mechanisms....................................65
7. Improving Retrieval Performance...............................65
7.1 Caching...................................................65
7.2 Retrieval Order...........................................66
8. Security Considerations.......................................67
Normative References.............................................68
Informative References...........................................68
Acknowledgments..................................................69
Author's Addresses...............................................69
1. Introduction
[X.509] digital certificates have become an accepted method for
securely binding the identity of an individual or device to a public
key, for the purpose of supporting public key cryptographic
operations such as digital signature verification, and public key-
based encryption and decryption. However, prior to using the public
key contained in a digital certificate, an application has to first
determine the authenticity of that digital certificate, and
specifically, the validity of all the certificates leading to a
trusted root certificate. It is through validating this
certification path that the assertion of the binding made between the
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identity and the public key in each of the digital certificate can be
traced back to a single point of trust.
The process by which an application determines this authenticity of a
digital certificate is called certification path processing.
Certification path processing establishes a chain of trust between a
trusted public key and a digital certificate. This chain of trust is
composed of a series of digital certificates known as a certification
path. A certification path begins with a certificate whose signature
can be verified using a trusted public key and ends with the target
digital certificate. Path processing entails building and validating
the certification path to determine the degree of trust to place in
the target digital certificate(s). See section 3.2 of [RFC 3280] for
more information on certification paths and trust.
1.1 Motivation
Many other documents (such as [RFC 3280]) cover certification path
validation requirements and procedures in detail but do not discuss
certification path building because how the path is found 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
becoming greater. Many PKIs are now using complex structures (see
section 1.4) 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 information that will be helpful in 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 one particular way to best
perform certification path building. This document draws upon the
authors' experience with existing complex certification paths to
offer insights and recommendations to developers integrating support
for [X.509] digital 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
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path building capabilities, the algorithm is no more than a suggested
approach. Other approaches (e.g., building complete spanning trees of
the PKI.) exist and may be shown to be more effective under certain
conditions. Certification path validation is described in detail in
both [X.509] and [RFC 3280] and is not repeated in this document.
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 lines between nodes.
Certificate Processing System: An application or device that
performs the functions of certification path building and
certification path validation.
Certificate Signer: A Certification Authority (represented by a
certificate) that has issued another certificate.
Certification Authority (CA): An entity that issues and manages
digital certificates.
Certification Path: An ordered list of certificates starting with a
certificate signed by a trusted public key 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.
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Certification Path Validation: The process that verifies the binding
between the subject and the subject-public-key defined in the target
certificate, using a trusted public key and set of known constraints.
CRL Signer: The specific certificate that may be used for verifying
the signature on a CRL issued by, or on behalf of, a specific
certificate signer.
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.
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, and whose identity is defined as the Subject of the
certificate. Human end entities are often called "subscribers".
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.
Public Key Infrastructure (PKI): The set of hardware, software,
personnel, policy, and procedures used by a Certification Authority
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.
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.)
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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 [RFC 3280].
Trust List: A list of certificates or combinations of public keys and
names that are considered trustworthy.
Trust Anchor: The combination of a trusted public key and the name of
the entity to which the corresponding private key belongs.
Trusted Root Certificate: A certificate issued to a trust anchor
which 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.
1.4 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 multi-bridged mesh architectures. These
structures define the types of certification paths that might be
built and validated by an application. This section describes four
common PKI structures.
1.4.1 Hierarchical Structures
A hierarchical PKI, depicted in Figure 1, is one in which all of the
end entities and relying parties trust a single "root" CA. 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.
+---------+
+---| root CA |---+
| +---------+ |
| |
| |
v v
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+----+ +----+
+-----| CA | +-----| CA |------+
| +----+ | +----+ |
| | |
v v v
+----+ +----+ +----+
+--| CA |-----+ | CA |-+ +---| CA |---+
| +----+ | +----+ | | +----+ |
| | | | | | | |
| | | | | | | |
v v v v v v v v
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
| EE | | EE | | EE | | EE | | EE | | EE | | EE | | EE |
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
Figure 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 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. 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.
+-------------------------------------------------------+
| Trust List |
| |
| +---------+ +---------+ +---------+ |
| +--| Root CA | | Root CA | | Root CA | |
| | +---------+ +---------+ +---------+ |
| | | | | |
+--|------|----------------|---------------- |----------+
| | | |
| | | |
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| | 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
1.4.2 Mesh Structures
In a typical mesh style PKI (depicted in Figure 3), each end entity
trusts the CA that issued their own certificate(s). The CAs in this
environment have a peer relationship and are neither superior nor
subordinate to each other. 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
unidirectional and bi-directional cross-certifications (called a
partial mesh).
+---------------------------------+
| |
+-----------+----------------------+ |
| v v |
| +-------+ +------+ |
| +--->| CA B |<------------->| CA C |<--+ |
| | +-------+ +------+ | |
| | | ^ ^ | | |
| | v | | | | |
| | +----+ | | | | |
| | | EE | +----+ +--------+ v | |
| | +----+ | | +----+ | |
| | | | | EE | | |
v v v v +----+ v v
+------+ +------+ +------+
| CA E |<----------->| CA A |<----------->| CA D |
+------+ +------+ +------+
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| ^ ^ ^ ^ |
| | | | | |
v | +------------------------------------+ | v
+----+ | | +----+
| EE | | +---------+ | | EE |
+----+ +---------------| Root CA |---------------+ +----+
+---------+
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 trusted root certificate 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 paths between the Root CA and the EE issued by D
without repeating any certificates.
1.4.3 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,
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
| +------+
| +-----| CA |-----+
| | +------+ |
| | PKI 1 Root |
| v v
| +------+ +------+
v PKI 2 Root +-| CA |-+ | CA |
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+------+ | +------+ | +------+
+------->| 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
restrictive policies.
1.4.4 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 (known as a "principal" CA [PCA]) 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
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discussed in section 2.3, non-hierarchical PKIs can be considered
hierarchical, depending upon perspective.)
PKI 1 cross certified with Bridge
+-------------------------------+
| |
v v
+-----------+ +------+
| Bridge CA | +-----| CA |-----+
+-----------+ | +------+ |
^ | PKI 1 Root |
PKI 2 cross|cert with Bridge v v
| +------+ +------+
v PKI 2 Root +-| 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
1.5 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 which the Bridge
may connect. (e.g., hierarchical, mesh, hybrid) 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
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deployed in any standards compliant PKI environment and will perform
the required certification path processing properly.
2. Certification Path Building
Certification path building is the process by which the certificate
processing system obtains the certification path between a trusted
public key and the target certificate. Different implementations can
build the certification path in different ways; therefore, it is not
the intent of this paper 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 required 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 anchros are typically stored in
self signed certificates which are frequently called trusted root
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.
Figure 6 shows an example of a certification path. In this figure,
the horizontal arrows represent certificates.
+---------+ +-----+ +-----+ +-----+ +--------+
| Trust |----->| CA |---->| CA |---->| CA |---->| Target |
| Anchor | | | A | | | B | | | C | | | EE |
+---------+ | +-----+ | +-----+ | +-----+ | +--------+
| | | |
| | | |
v v v v
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
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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
rely on a PKI. Absent valid certification paths, the certificates
cannot be validated according to [RFC 3280] and therefore cannot be
trusted. Thus the ability to build a path is every bit as important
as the ability to properly validate them.
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, for
example, need to manage a number of trusted root certificates,
partially populated directory entries (e.g., missing issuedToThisCA
entries in the cross certificates.), 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. For example, if the
infrastructure is compliant with the Internet [X.509] Public Key
Infrastructure LDAPv2 Schema [RFC 2587], a developer can always build
certification paths in the forward (from target) direction. However,
not all PKIs are compliant with [RFC 2587]. An infrastructure may
not populate the issuedToThisCA (forward) cross-certificates and
instead only populate the issuedByThisCA (reverse) entries, in which
case building in reverse will be a developer's only viable option.
Note that a PKI compliant with [RFC 2587] may or may not populate the
optional issuedByThisCA (reverse) entry; so building in reverse may
or may not work in [RFC 2587] compliant systems.
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.
By this, it is meant that all possible certification paths between
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the trust anchor and the target certificate which may be valid paths
can be built by the algorithm.
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 [RFC
3280], 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.
The algorithms and mechanisms discussed henceforth are chosen because
they are considered by the authors to be good methods to meet 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. As a result, every potential
valid path has a terminus, a leaf on the tree. (If certificates were
allowed to repeat, paths could have infinite length and therefore no
terminus.) Figure 7 below illustrates this concept from the trust
anchor's perspective.
+---------+ +---------+
| Trust | | Trust |
| Anchor | | Anchor |
+---------+ +---------+
| | | |
| | | |
v v v v
+---+ +---+ +---+ +---+
| A |<-->| C | +--| A | | C |--+
+---+ +---+ | +---+ +---+ |
| | | | | |
| +---+ | v v v v
+->| B |<-+ +---+ +---+ +---+ +---+
+---+ | B | | C | | A | | B |
| +---+ +---+ +---+ +---+
| | | | |
| | | | |
v v | | v
+----+ +----+ | | +----+
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| EE | | EE | | | | EE |
+----+ +----+ | | +----+
v v
A certificate graph with +---+ +---+
bi-directional cross cert. | B | | B |
Between CAs A and C. +---+ +---+
| |
| |
v v
+----+ +----+
| 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 how complex it is - this greatly simplifies
software design. 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.
+---------+ +---------+
| Trust | | Trust |
| Anchor | | Anchor |
+---------+ +---------+
^ ^
| |
| |
+---+ +---+
| A | | C |
+---+ +---+
+---------+ ^ ^ +---------+
| Trust | | | | Trust |
| Anchor | | | | Anchor |
+---------+ | | +---------+
^ | | ^
| +---+ +---+ |
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+-------| 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, it 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).
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 matches the issuer distinguished name
of the EE. Given this one optimization, the builder could prefer B
to C. (B's subject distinguished name matches that of the EE's
issuer whereas C's subject distinguished name 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
distinguished name 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.
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A more complicated example involves an environment where more than
one trust anchor exists. The number of trust anchors should weigh
heavily upon the decision to build in the reverse direction. If, for
sake of argument, it required a fixed amount of network transfer time
to build all the possible paths either to or from a given CA, what
should be expected if there are four trust anchors? Suppose that
building paths either from the target certificate or from the trust
anchor for any given trust anchor or target certificate will require
N time. In the from anchor (reverse) direction, the path building
software potentially needs to build paths from all 4 trusted CAs, so
as much as 4*N. Assuming a path exists, if N were 10 seconds, and
assuming an even probability of the path beginning with any one of
the four trust anchors, an average delay of 25 [(10+20+30+40)/4]
seconds is expected when building from the trust anchor. In the from
target certificate (forward) direction (and using the same example),
path building software would only require 10 (1*N) seconds, only 40%
of the average time required when building from the trust anchor. As
the number of trust anchors increases, so does the average time it
takes to find a path if one does exist. Additionally, in the
degenerate case where no path exists, attempting to build from the
same four trust anchors would consume 40 seconds, whereas building
from the end entity would consume only 10 seconds.
+-----+ +-----+ +-----+ +-----+
| TR1 | | TR2 | | TR3 | | TR4 |
+-----+ +-----+ +-----+ +-----+
/ \ / \ / \ / \
/ \ / \ / \ / \
v v v v v v v v
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
| R |<->| S | | F | | G | | F |<->| G | |<A>| | C |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
\ / / \ \ | / | /\ |
\ / / \ \ | / | / \ |
v v v v v v v v v v v
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
| T | | B | | D | | E | | H |<->| I | | B | |<D>| | E |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
/ \ \ \ / \ |
/ \ \ \ / \ |
v v v v v v v
+---+ +---+ +---+ +---+ +---+ +----+
| U | | V | | F | | J |<->| K | |<EE>|
+---+ +---+ +---+ +---+ +---+ +----+
Building from the anchor (reverse) may require traversal of
multiple PKIs and unneeded paths to find the target EE.
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+-----+ +-----+ +-----+ +-----+
| TR1 | | TR2 | | TR3 | | TR4 |
+-----+ +-----+ +-----+ +-----+
/
/
v
+---+
|<A>|
+---+
\
\
v
+---+
|<D>|
+---+
|
|
v
+----+
|<EE>|
+----+
Building from the target certificate eliminates
inapplicable PKIs (those that are not cross certified
with the required one) from the path building process.
Figure 9 - Reverse and Forward Path Building
with Multiple Trust Anchors
As Figure 9 depicts, when multiple trust anchors are present, it can
be many times more efficient to build certification paths starting
from the target certificate. As the number of trust anchors
increases, so does the inefficiency of building paths from the trust
anchors. As a result, any certificate using system supporting
multiple trusted CAs is encouraged to consider developing paths in
the forward (from target certificate) direction.
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 [RFC 2587] compliant
(Specifically, are the issuedToThisCA [forward] cross-
certificates and/or the cACertificate attributes fully
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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
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 trust
root certificates 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.
+---+ +---+
| F |--->| H |
+---+ +---+
^ ^ ^
| \ \
| \ \
| v v
| +---+ +---+
| | G |--->| I |
| +---+ +---+
| ^
| /
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| /
+------+ +-----------+ +------+ +---+ +---+
| 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 10 - Four Bridged PKIs
Figure 10 depicts four root certification authorities cross-certified
with a Bridge CA (BCA). By building certification paths through the
BCA, trust can be extended across the four infrastructures. When
looking at Figure 10, note that the boxes do not represent
certificates, but rather entities (i.e. identical subject
distinguished and alternate names). The certificates are represented
by the arrows between the entities in the boxes. Thus, 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
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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 be repeated when building
paths. For instance, from the figure above, do not build the path E-
>B->C->A->C->A->Y. Not only is the repetition unnecessary to build
the path from E to Y, 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 EE to TA Z?
EE->N->L->X->BCA->W->BCA->Y->BCA->Z
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 10) had double arrows between Y and C, as well as
between Y and A. The following path could then be built:
EE->N->L->X->BCA->W->BCA->Y->C->A->Y->BCA->Z
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:
- 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
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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 [RFC 3280].
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 EE to
Z in the diagram above: EE->N->L->X->BCA->Z.
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 11 depicts all
the possible paths 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 EE to Z.
+------+ +-----------+ +------+ +---+
| TA W |<------| Bridge CA |<-------| TA X |<--| L |
+------+ +-----------+ +------+ +---+
/ \ ^
/ \ \
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/ \ \
v v \
+------+ +------+ +---+
| TA Y | | TA Z | | N |
+------+ +------+ +---+
^
\
\
+----+
| EE |
+----+
Figure 11 - 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 is desirable, the graph in Figure 11 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 12 depicts
all possible paths as a tree emanating from Z.
+---+ +---+
| I |--->| H |
+---+ +---+
^
| +---+ +---+
| | H |--->| I |
| +---+ +---+
+---+ ^
| G | / +---+ +---+ +---+
+---+ / | F |--->| H |--->| I |
^ / +---+ +---+ +---+
\ / ^
\/ /
+---+ +---+ +---+ +---+ +---+
| F | | G |--->| I |--->| H | | M |
+---+ +---+ +---+ +---+ +---+
^ ^ ^
| / |
+------+ +-----------+ +------+ +---+
| TA W |<------| Bridge CA |-------->| TA X |-->| L |
+------+ +-----------+ +------+ +---+
/ ^ \ \
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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 12 - Reverse (From Anchor) Decision Tree
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: 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 not only needs to decide
in which direction to traverse the tree, but it should also decide
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 stated as one of our criteria in section 2.2.
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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 13, 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).
+----+ +----+ +----+
| 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
+----+ +----+
| TA | | TA |
+----+ +----+
^ ^
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| |
+---+ +---+
| 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 tree traversal
Figure 13 - Path Building Using a Depth First Tree Traversal
Figure 13 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".
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
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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 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 edges (certificates) have already been tried.
2.5 Building Certification Paths for CRL Signers
The CRL Signer certificate may or may not be the same as the
Certificate Signer certificate. For example, after a CA performs a
key rollover, the new CA certificate will be the CRL Signer, whereas
the old CA certificate is the Certificate Signer for previously
issued certificates. In the case of indirect CRLs, the CRL Signer
will have a different name and key from the Certificate Signer.
In the case where the CRL Signer certificate (and certification path)
is not identical to the Certificate Signer certificate (and
certification path), special care should be exercised when building
the CRL Signer certification path.
If special consideration is not given to building CRL Signer paths, a
CRL signer path could be constructed that terminates with a foreign
root or through a foreign certification path to the same root. If
this behavior is not prevented, the relying party may end up checking
the wrong CRL, or even a maliciously substituted CRL, resulting in
denial of service or security breach.
For example, suppose the following certification path is built for E
and is valid for the "high assurance" policy.
A->B->C->E
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When the building/validation routine attempts to verify that E is not
revoked, C is referred to as the Certificate Signer. 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" but with a different
key than the key that was used previously 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:
A->X->Y->RogueCA->C2
If a path such as the one above is permitted, nothing can be
concluded about the revocation status of E. The RogueCA may have
revoked or "unrevoked" E and this breach would go undetected.
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 Certificate Signer certificate, the Certificate Signer path
should be used to verify the CRL; no additional path building is
required. If the CRL Signer certificate is not identical to the
Certificate Signer certificate, a second path should be built for the
CRL Signer 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 emanate
from the same trust anchor (subject name and trusted public key) as
the Certificate Signer's certification path.
2. CA Name Matching: The subject distinguished names of all the CA
certificates in the two certification paths should match on a one-to-
one basis (without regard to self-issued certificates) for the entire
length of the shorter of the two paths. The CRL Signer certificate
should not be included in the comparison.
3. CRL Signer Certification Path Length: The length of the CRL
Signer certification path should be equal to or less than the length
of the Certificate Signer certification path plus (+) one. This
allows a given Certificate Signer to issue a certificate to a
delegated/subordinate CRL Signer. The latter configuration represents
the maximum certification path length for a CRL Signer.
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. In combination with the second guideline, the first
guideline prevents a malicious individual from creating a mock
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infrastructure that maps on a one-to-one subject distinguished name
basis to the infrastructure they seek to compromise.
The second guideline prevents roaming certification paths such as the
previously described example CRL Signer path for A->B->C->E. It is
especially important that the "without regard to self-issued
certificates" is noted and 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
distinguished name 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 Certificate Signer 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 distinguished name 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 distinguished
names, specifically, A, B, or C.
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 paper 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
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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 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 (and 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.
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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 <forward> and issuedByThisCA <reverse>
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 in the CRL
distribution point certificate extension.
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 AIA support.
i. Consider HTTPS support
4) A certification path cache that stores previously validated
relationships between certificates. This cache should include:
a. A configurable expiration date for each entry.
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.
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 as to what could be
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.
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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 end point for the path.
2) Trust List - This is the other endpoint of the path, and can
consist of either:
a. Trusted CA certificates
b. Trusted keys and distinguished names - a certificate is not
necessarily required
2.7.2 Optional Inputs
In addition to the inputs defined 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 1 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) Collection of certificates that may be useful in building the
path
9) Collection of certificate revocation lists and/or other
revocation data
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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 in the manner 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 distinguished names. 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
which sorting provides. Consider the following diagram:
+---+
| R |
+---+
^
/
v
+---+ +---+ +---+ +---+ +----+
| A |<----->| E |<---->| D |--->| Z |--->| EE |
+---+ +---+ +---+ +---+ +----+
^ ^ ^ ^
\ / \ /
\ / \ /
v v v v
+---+ +---+
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| B |<----->| C |
+---+ +---+
Figure 14 - 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 & 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).
Now 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.
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
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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 make it last or eliminate it from this node since
certificates can not 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 anchor keys 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)
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
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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 on another point in
the tree; at that point it may or may not be eliminated. The prior
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
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,
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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.
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 which 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, it is up to the developer to determine how to handle the
tradeoff 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 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
c. Do not check anything that can not be checked as part of the
iterative process of traversing the tree
d. Keep a detailed log, if this feature is enabled
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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 [RFC 3280] 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 - return this to the user in the event
no valid path is found. Since the path building
software was designed to return the "best path first",
this is the path that 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
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. Second, it is fairly uncommon in typical
application environments to encounter a revoked key; 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 Programmatically
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.
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3.3.1 Node Representation For CA Entities
A "node" in the certification graph is a collection of CA
certificates with identical subject distinguished names. 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
- Could be stored as an array format
2. Sorted order of the certificates
- Sorting could be performed on aforementioned array
3. "Current" certificate indicator
- May be an index number on the array
4. The current policy set. (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
5. Indicators (requireExplicitPolicy, inhibitPolicyMapping,
anyPolicyInhibit)
- skipCert values should also be stored
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.
- If using the array approach, eliminated certificates could
be placed at the front of the array with the current
indicator set to the first certificate not eliminated
7. A "next" indicator that allows the software to locate the next
node in the current path
- May simply be a pointer assigned the address of the next
node
8. A "previous" indicator that allows the software to locate the
previous node in the current path
- This is to simplify path validation. It may simply be a
pointer assigned the address of the previous node
3.3.2 Using Nodes to Iterate Over All Paths
In simplest form, a node is created, the certificates are sorted, the
next subject distinguished name required is determined from the first
certificate, and a new node is attached to the certification path via
the next indicator. (Number seven above.) This process continues
until the path terminates.
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Keeping in mind while that the following algorithm is designed to be
implemented using recursion, consider the example in Figure 13 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 distinguished name (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 "*".
+-------------+ +---------------+
| Node 1 | | Node 2 |
| Subject: E |--->| Subject: B |
| Issuers: B* | | Issuers: C*,A |
+-------------+ +---------------+
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)
+-------------+ +---------------+ +------------------+
| Node 1 | | Node 2 | | Node 3 |
| Subject: E |--->| Subject: B |--->| Subject: C |
| Issuers: B | | Issuers: C*,A | | Issuers: TA*,A,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.
+-------------+ +---------------+ +------------------+
| Node 1 | | Node 2 | | Node 3 |
| Subject: E |--->| Subject: B |--->| Subject: C |
| Issuers: B | | Issuers: C*,A | | Issuers: TA,A*,B |
+-------------+ +---------------+ +------------------+
|
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v
+------------------+
| Node 4 |
| Subject: A |
| Issuers: TA*,C,B |
+------------------+
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.
+-------------+ +---------------+ +------------------+
| 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).
+-------------+ +---------------+ +------------------+
| Node 1 | | Node 2 | | Node 3 |
| Subject: E |--->| Subject: B |--->| Subject: C |
| Issuers: B | | Issuers: C*,A | | Issuers: TA,A*,B |
+-------------+ +---------------+ +------------------+
|
v
+------------------+
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| Node 4 |
| Subject: A |
| Issuers: TA,C,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).
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
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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.
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 that 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, it is up to the implementer to
decide 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
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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. It is up to the
implementer to decide which methods to use and what weights to assign
them. As noted previously, this list is also not exhaustive.
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.
While the path builder may certainly be designed to operate in this
fashion, it is also 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
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Number of possible values: Binary
Components required: None
Forward Method: Certificates with basicConstraints present and
cA=TRUE have priority. Certificates without basicConstraints, or
with basicConstraints and cA=FALSE 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 [RFC 3280], basicConstraints is required
to be present with cA=TRUE in all CA certificates. 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 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 can not be built through a
CA certificate with inappropriate keyUsage. Note that
digitalSignature is NOT required to be set in a CA certificate.
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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.
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).
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. There is some probability that the path from that
certificate to a trust anchor is still 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.
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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
said public key have higher priority. Other certificates may be
eliminated or set to zero priority.
Reverse Method: Does not apply. (Although the path cache does
contain one to one relationships in reverse, nothing can be concluded
about the likelihood of finding a given target certificate down one
branch versus another using this method.)
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, but the
benefit is likely less than that of the forward direction. 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
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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 lower priority (or perhaps eliminated).
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.
Reverse Method: Same as Forward.
Alternately, the certificate may be eliminated if the CRL is
verified. That is, fully verify the CRL signature and relationship
to the certificate in question in accordance with [RFC 3280]. While
this is viable, the signature verifications required make it less
attractive as an elimination method. It is suggested that this method
only be used for sorting and that CRLs 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
validation is performed post path validation - CRLs 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, 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 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)
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.
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Justification: 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 [RFC 3280], it is extremely
important that KIDs be used ONLY as sorting criteria or hint 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.
3.5.12 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
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.
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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.
3.5.13 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 trusted root certificate 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 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 the path is drawing near to the target's
domain.
3.5.14 Endpoint Distinguished Name 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.
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Justification: In the forward direction, if a certificate's issuer
DN matches a trust anchor's DN, then it may complete the path. In
the reverse direction, if the certificate's subject DN matches the
issuer DN of the target certificate, this may be the last certificate
required to complete the path.
3.5.15 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, 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
matches to be weighted differently from exact matches. Additionally,
it should be noted that this method can require a lot of processing
if many trust anchors are present.
3.5.16 Certificates are Retrieved from cACertificate
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
Forward Method: Certificates retrieved from the cACertificate
attribute have priority over certificates retrieved from the
crossCertificatePair attribute. (See [RFC 2587])
Reverse Method: Does not apply.
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Justification: The cACertificate attribute contains certificates
issued from local sources and self issued certificates. By using the
cACertificate 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. Not only do most
of today's PKI applications spend most of their time processing
information from the local (user's own) PKI, but the local PKI is
usually very efficient to traverse due to proximity and network
speed.
3.5.17 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 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 aren't
consistent, the signature on the subject certificate will not
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.
3.5.18 Similar Issuer and Subject Names
May be used to eliminate certificates: No
Number of possible values: Sliding Scale
Components required: None
Forward Method: Certificates that match more RDNs between the
subject DN and the issuer DN 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
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generally match fewer RDNs (if any), whereas certificates in the
local domain will frequently match multiple RDNs.
3.5.19 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 repository)
Forward Method: A certificate whose issuer certificate is present in
the certificate cache (and populated with one or more certificates)
has priority. A certificate whose issuer certificate is present in
the certificate cache and fully populated with recent data (all
certificate attributes have been searched within an appropriate
timeout period - something shorter than cache expiry.) has higher
priority. (This helps reduce LDAP lookups until necessary.)
Reverse Method: If an entity named by a reverse 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
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.
3.5.20 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
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through this entity at some past point, so a path still likely
exists. This also helps reduce LDAP lookups until necessary.
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 to some 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
quite 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 been long considered a weakness inherent to
building in the forward (from the target certificate) direction.
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 / end
entity 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.
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When building in the forward direction, policy processing is handled
in much the same fashion as 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.
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 foreign 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}
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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 rest {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
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 rest {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.
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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.
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 15.
+----+ +---+
| TA | | Z |
+----+ +---+
| |
| |
V V
+---+ +---+
| C |<-----| Y |
+---+ +---+
|
|
V
+--------+
| Target |
+--------+
Figure 15 - 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,
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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.
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 16 - 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 which 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
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only need to appear once in the path (see section 2.4.1 for more
information on the reasoning behind this recommendation).
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 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 which uses the authority
distinguished name 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 distinguished
names being encoded as PrintableString. Although frequently encoded
as PrintableString, distinguished names may also appear as other
types, including BMPString or UTF8String. As a result, software
systems that are unable to process BMPString and UTF8String encoded
distinguished names may be unable to build and validate some
certification paths.
Furthermore, looking forward, [RFC 3280] compliant certificates will
be required to encode distinguished names as UTF8String as of January
1, 2004. Certification path building software should be prepared to
handle "name rollover" certificates as described in [RFC 3280]. Note
that the inclusion of a "name rollover" certificate in a
certification path does NOT constitute repetition of a distinguished
name and key. Implementations that include the "name rollover"
certificate in the path should ensure that the distinguished names
with differing encoding are regarded as dissimilar. (Implementations
may instead handle matching distinguished names of different
encodings and will therefore not need to include "name rollover"
certificates in the path.)
6. Retrieval Methods
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Building a certification path requires the availability of the
certificates that make up the path. There are many different methods
for performing this retrieval. 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. The LDAP v3 specification is found in [RFC 2251].
The LDAP v3 specification defines one attribute retrieval option, the
"binary" option. This option, when specified in an LDAP retrieval
request, was intended to force the directory to ignore any string-
based representations of directory information, and send the
requested attribute(s) in binary format. Since all PKI objects of
concern are binary objects, the "binary" option should be used.
However, not all directories support the "binary" option.
(Additionally, recent developments in the LDAP working group seem to
be leading toward the removal of the "binary" option.) Therefore,
all attribute retrievals should specify the attribute name with and
without the "binary" option. For example, if an application wishes
to retrieve the userCertificate attribute, the retrieval request
should contain the following list of attributes to retrieve:
"userCertificate, and userCertificate;binary".
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. 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.
- cACertificate: contains self-issued certificates (if any) and
any certificates issued to this certification authority by
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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: the crossCertificatePair is used to
contain certificates issued to this certification authority by
other certification authorities in other realms, as well as
certificates issued by this certification authority to other
certification authorities in other realms. 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 certification authorities. Both
elements of the crossCertificatePair are labeled optional in
the ASN.1 definition; however the LDAP v2 schema states that
the issuedToThisCA (once called the 'forward' element) is
mandatory and the issuedByThisCA (once called the 'reverse'
element) is optional. 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.
- certificateRevocationList: the certificateRevocationList
attribute contains a certificate revocation list (CRL). A CRL
is defined in [RFC 3280] 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 [RFC 3280].
- 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 [RFC 3280].
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 (all with
and without the ;binary option).
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6.2 Authority Information Access
The authority information access (AIA) extension, defined within [RFC
3280], 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 entities that issued the certificate containing
the AIA extension. The AIA will provide a uniform resource
identifier (URI) 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 [RFC 2560] 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,
DAP, or e-mail locators. Support for AIA is optional; [RFC 3280]
compliant implementations are not required to populate the AIA
extension.
6.3 Subject Information Access
The subject information access (SIA) extension, defined within [RFC
3280], 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) when data can be retrieved via LDAP, HTTP, or FTP.
The AIA will provide a directoryName when data can be retrieved via
directory access protocol (DAP). The AIA 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; [RFC 3280]
compliant implementations are not required to populate the SIA
extension.
6.4 CRL Distribution Points
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The CRL distribution points (CRLDP) extension, defined within [RFC
3280], indicates how to access CRL information. If a CRLDP extension
appears within a certificate, the CRL(s) to which the CRLDP(s) refer
is the CRL 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 [RFC 3280]. 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.
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 [RFC 3280] 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.
6.5 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.
7.1 Caching
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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, or some other non-local source. Due to
the delays associated with both the establishing of 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 in which retrieval efficiency can often be
improved is by the use of 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.
There are a number of different ways in which caching can be
implemented, and 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.
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
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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 wished to retrieve a certificate with a particular subject
distinguished name, 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 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
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.
8. Security Considerations
Although certification path building deals directly with security
relevant PKI data, the PKI data itself needs no special handling as
the PKI data 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 [RFC 3280] and
[X.509] is required in order for certification path building,
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certification path validation, and the certificate using application
to be properly secured. All of the Security Considerations listed in
Section 9 of [RFC 3280] 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 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 using 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.
Normative References
[RFC 3280] Housley, R., W. Ford, W. Polk and D. Solo, "Internet
X.509 Public Key Infrastructure: Certificate and CRL
Profile", RFC 2459, January 1999.
[X.509] ITU-T Recommendation X.509 (1997 E): Information
Technology - Open Systems Interconnection - The
Directory: Authentication Framework, June 1997.
Informative References
[MINHPKIS] Hesse, P., Lemire, D., "Managing Interoperability
in Non-Hierarchical Public Key Infrastructures",
February 2002.
[RFC 1738] Berners-Lee, T., L. Masinter and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[RFC 2026] Bradner, S., "The Internet Standards Process -
Revision 3", RFC 2026, October 1996
[RFC 2247] Kille, S., M. Wahl, A. Grimstad, R. Huber and S.
Sataluri, "Using Domains in LDAP/X.500 Distinguished
Names", RFC 2247, January 1998.
[RFC 2251] Wahl, M., T. Howes and S. Kille,
"Lightweight Directory Access Protocol (v3) ", RFC
2251, December 1997.
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[RFC 2252] Wahl, M., A. Coulbeck, T. Howes and S. Kille,
"Lightweight Directory Access Protocol (v3):
Attribute Syntax Definitions", RFC 2252,
December 1997.
[RFC 2396] Berners-Lee, T., Fielding, R., Irving, U.C., and L.
Masinter, "Uniform Resource Identifiers (URI): Generic
Syntax", RFC 2396, August 1998.
[RFC 2560] Myers, M., R. Ankney, A. Malpani, S. Galperin and C.
Adams, "Online Certificate Status Protocal - OCSP",
June 1999.
[RFC 2587] S. Boeyen, T. Howes, P. Richard, "Internet X.509
Public Key Infrastructure LDAPv2 Schema", RFC 2587,
June 1999
[X.501] ITU-T Recommendation X.501: Information Technology -
Open Systems Interconnection - The Directory: Models,
1993.
[X.520] ITU-T Recommendation X.520: Information Technology -
Open Systems Interconnection - The Directory: Selected
Attribute Types, 1993.
[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.
Acknowledgments
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, and Alice Sturgeon.
Author's Addresses
Matt Cooper
Orion Security Solutions, Inc.
1489 Chain Bridge Rd, Ste. 300
McLean, VA 22101, USA
Phone: +1-703-917-0060
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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.
4031 University Dr. Ste. 200
Fairfax, VA 22030, USA
Phone: +1-703-934-2031
Email: pmhesse@geminisecurity.com
Susan Joseph
DigitalNet Government Solutions, LLC.
141 National Business Parkway, Ste. 210
Annapolis Junction, MD 20701, USA
Phone: +1-301-939-2705
Email: susan.joseph@digitalnet.com
Richard Nicholas
DigitalNet Government Solutions, LLC.
141 National Business Parkway, Ste. 210
Annapolis Junction, MD 20701, USA
Phone: +1-301-939-2722
Email: richard.nicholas@digitalnet.com
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