## JavaTM Certification Path API Programmer's Guide

Author: Sean Mullan
Last Modified: 11 February 2003

## Changes in J2SE 1.4.2

## Overview

#### Acknowledgments
Who Should Read This Document
Related Documentation
Introduction

## Core Classes and Interfaces

### Basic Classes

- The CertPath Class

- The CertificateFactory Class

- The CertPathParameters Interface

### Certification Path Validation Classes

- The CertPathValidator Class

- The CertPathValidatorResult Interface

### Certification Path Building Classes

- The CertPathBuilder Class

- The CertPathBuilderResult Interface

### Certificate/CRL Storage Classes

- The CertStore Class

- The CertStoreParameters Interface

  - The LDAPCertStoreParameters Class

  - The CollectionCertStoreParameters Class

- The CertSelector and CRLSelector Interfaces

  - The X509CertSelector Class

  - The X509CRLSelector Class

## PKIX Classes

- The TrustAnchor Class

- The PKIXParameters Class

- The PKIXCertPathValidatorResult Class

- The PolicyNode Interface and PolicyQualifierInfo Class

- The PKIXBuilderParameters Class

- The PKIXCertPathBuilderResult Class

- The PKIXCertPathChecker Class

## Implementing a Service Provider

## Appendix A: Standard Names

## Appendix B: The "SUN" Provider

## Changes in J2SE 1.4.2

Several enhancements have been made to the CertPath implementation in the SUN provider:

- Compliance with RFC 3280.

- Performance improvements. Several changes were made to improve the speed of validations. The performance improvement depends on the usage scenario; however, validations may be twice as fast as before. As part of these performance enhancements, the LDAP CertStore has been equipped with a cache. For details see Appendix B: The "SUN" Provider.

- Limited support for the `CRLDistributionPoints` extension. For backwards compatibility, it is disabled by default. See Appendix B: The "SUN" Provider for more information and instructions on how to enable it.

## Overview

The JavaTM Certification Path API consists of classes and interfaces for handling certification paths (also known as "certificate chains"). A certification path is an ordered list of certificates. If a certification path meets certain validation rules, it may be used to securely establish the mapping of a public key to a subject.

This API defines interfaces and abstract classes for creating, building, and validating certification paths. Implementations may be plugged in using a provider-based interface. The API is based on the Cryptographic Service Provider architecture, described in the Java Cryptography Architecture Document.

The API also includes algorithm-specific classes for building and validating X.509 certification paths according to the PKIX standards. The PKIX standards are developed by the IETF PKIX working group.

This API is being reviewed using the Java Community ProcessSM program as JavaTM Specification Request (JSR) 000055. The API is included in the JavaTM 2 SDK, Standard Edition (JDK), v 1.4. Please refer to the JSR 055 Home Page for more information on the JSR and the latest status.

> ## Acknowledgments

> The author would like to thank the individuals who contributed to the Certification Path API and provided useful comments and technical advice. Special thanks to the members of the Sun Microsystems Laboratories team who designed and developed the Certification Path API which provided the basis of the work for the Java Community Process. This team includes Anne Anderson, Yassir Elley, Geoff Goodell, Steve Hanna, Sean Mullan, Radia Perlman, and Seth Proctor.

> The expert group helped improve and refine the API using the Java Community Process and includes the following members:

> - Peter Hesse, CygnaCom Solutions, An Entrust Technologies Company
> - Sean Mullan, Sun Microsystems (Specification Lead)
> - Anthony Nadalin, IBM
> - Bob Naugle, Bluestone Software
> - Hemma Prafullchandra, VeriSign
> - Ming Yung, DSTC
> Maxine Erlund, Steve Hanna, Phil Rosenzweig and Bob Sproull of Sun Microsystems provided leadership and vision. The members of the Java Security, Networking and Naming Group of Sun Microsystems contributed invaluable comments and support, especially Sharon Liu, Jeff Nisewanger, Gary Ellison, and Andreas Sterbenz. Helpful comments and advice were received from many in the technical community, especially Mary Dageforde, Edward Dobner, Tom Gindin, Jan Luehe, David Kuehr-McLaren, Parag Salvi, Alexei Semidetnov, and Yanni Zhang.

> ## Who Should Read This Document

> This document is intended for two classes of experienced developers:

> 1. those who want to design secure applications that build or validate certification paths.

> 2. those who want to write a service provider implementation for building or validating certification paths.

> ## Related Documentation

> This document assumes you have already read the following documents (available online at http://java.sun.com/j2se/1.4/docs/guide/security/index.html):

> - X.509 Certificates and Certificate Revocation Lists (CRLs)

> - Java Cryptography Architecture

> - How to Implement a Provider for the Java Cryptography Architecture

> ## Introduction

> Users of public key applications and systems must be confident that a subject's public key is genuine, i.e., that the associated private key is owned by the subject. Public key certificates are used to establish this trust. A public key (or identity) certificate is a binding of a public key to an identity, which is digitally signed by the private key of another entity, often called a Certification Authority (CA). We shall use the term CA to refer to an entity that signs a certificate for the remainder of this section.

> If the user does not have a trusted copy of the public key of the CA that signed the subject's public key certificate, then another public key certificate vouching for the signing CA is required. This logic can be applied recursively, until a chain of certificates (or a certification path) is discovered from a trust anchor or a most-trusted CA to the target subject (commonly referred to as the end-entity). The most-trusted CA is usually specified by a certificate issued to a CA that the user directly trusts. In general, a certification path is an ordered list of certificates, usually comprised of the end-entity's public key certificate and zero or more additional certificates. A certification path typically has one or more encodings, allowing it to be safely transmitted across networks and to different operating system architectures.

> Figure 1 illustrates a certification path from a most-trusted CA's public key (CA 1) to the target subject (Alice). The certification path establishes trust in Alice's public key through an intermediate CA named CA2.

> > Figure 1: Certification Path

> A certification path must be validated before it can be relied on to establish trust in a subject's public key. Validation can consist of various checks on the certificates contained in the certification path, such as verifying the signatures and checking that each certificate has not been revoked. The PKIX standards define an algorithm for validating certification paths consisting of X.509 certificates.

> Often a user may not have a certification path from a most-trusted CA to the subject. Providing services to build or discover certification paths is an important feature of public key enabled systems. RFC 2587 defines an LDAP (Lightweight Directory Access Protocol) schema definition which facilitates the discovery of X.509 certification paths using the LDAP directory service protocol.

> Building and validating certification paths is an important part of many standard security protocols such as SSL/TLS, S/MIME, and IPSEC. The JavaTM Certification Path API provides a set of classes and interfaces for developers who need to integrate this functionality into their applications. This API benefits two types of developers: those who need to write service provider implementations for a specific certification path building or validation algorithm; and those who need to access standard algorithms for creating, building, and validating certification paths in an implementation-independent manner.

## Core Classes and Interfaces

The core classes of the Java Certification Path API consist of interfaces and classes that support certification path functionality in an algorithm- and implementation -independent manner. The API also includes a set of algorithm-specific classes for the PKIX standards that are discussed in the section titled PKIX Classes. The API builds on and extends the existing JavaTM 2 SDK, Standard Edition (JDK) `java.security.cert` package for handling certificates. The core classes can be broken up into 4 class categories: Basic, Validation, Building, and Storage:

- Basic Certification Path Classes

  - `CertPath`, `CertificateFactory`, `CertPathParameters`

- Certification Path Validation Classes

  - `CertPathValidator`, `CertPathValidatorResult`

- Certification Path Building Classes

  - `CertPathBuilder`, `CertPathBuilderResult`

- Certificate/CRL Storage Classes

  - `CertStore`, `CertStoreParameters`, `CertSelector`, `CRLSelector`

The following sections describe the most commonly used methods of each class and interface. Usage examples for some of the classes are interspersed throughout the guide. The complete reference documentation for the relevant Certification Path API classes can be found in:

- `java.security.cert` package summary

Most of the classes and interfaces in the CertPath API are not thread-safe. However, there are some exceptions which will be noted in this guide and the API specification. Multiple threads that need to access a single non-thread-safe object concurrently should synchronize amongst themselves and provide the necessary locking. Multiple threads each manipulating separate objects need not synchronize.

## Basic Certification Path Classes

The basic certification path classes provide fundamental functionality for encoding and representing certification paths. The key basic class in the Java Certification Path API is `CertPath`, which encapsulates the universal aspects shared by all types of certification paths. An application uses an instance of the `CertificateFactory` class to create a `CertPath` object.

## The CertPath Class

The `CertPath` class is an abstract class for certification paths. It defines the functionality shared by all certification path objects. Various certification path types can be implemented by subclassing the `CertPath` class, even though they may have different contents and ordering schemes. All `CertPath` objects are serializable, immutable and thread-safe and share the following characteristics:

- A type
  This corresponds to the type of the certificates in the certification path, for example: X.509. The type of a `CertPath` is obtained using the method:

          public String getType()

  See Appendix A in the Java Cryptography Architecture API Specification & Reference for information about standard certificate types.

- A list of certificates
  The `getCertificates` method returns the list of certificates in the certification path:

          public abstract List getCertificates()

  This method returns a `List` of zero or more `java.security.cert.Certificate` objects. The returned `List` and the `Certificates` contained within it are immutable, in order to protect the contents of the `CertPath` object. The ordering of the certificates returned depends on the type. By convention, the certificates in a `CertPath` object of type X.509 are ordered starting with the target certificate and ending with a certificate issued by the trust anchor. That is, the issuer of one certificate is the subject of the following one. The certificate representing the `TrustAnchor` should not be included in the certification path. Unvalidated X.509 `CertPath`s may not follow this convention. PKIX `CertPathValidator`s will detect any departure from these conventions that cause the certification path to be invalid and throw a `CertPathValidatorException`.

- One or more encodings
  Each `CertPath` object supports one or more encodings. These are external encoded forms for the certification path, used when a standard representation of the path is needed outside the Java Virtual Machine (as when transmitting the path over a network to some other party). Each path can be encoded in a default format, the bytes of which are returned using the method:

          public abstract byte[] getEncoded()

  Alternatively, the `getEncoded(String)` method returns a specific supported encoding by specifying the encoding format as a `String` (ex: "PKCS7"). A list of standard encoding formats are defined in Appendix A.

          public abstract byte[] getEncoded(String encoding)

  Also, the `getEncodings` method returns an iterator over the supported encoding format `String`s (the default encoding format is returned first):

          public abstract Iterator getEncodings()

All `CertPath` objects are also `Serializable`. `CertPath` objects are resolved into an alternate `CertPathRep` object during serialization. This allows a `CertPath` object to be serialized into an equivalent representation regardless of its underlying implementation.

`CertPath` objects are generated from an encoded byte array or list of `Certificate`s using a `CertificateFactory`. Alternatively, a `CertPathBuilder` may be used to try to find a `CertPath` from a most-trusted CA to a particular subject. Once a `CertPath` object has been created, it may be validated by passing it to the `validate` method of `CertPathValidator`. Each of these concepts are explained in more detail in subsequent sections.

## The CertificateFactory Class

The `CertificateFactory` class is an engine class that defines the functionality of a certificate factory. In releases prior to JDK, v 1.4 it was used to generate `Certificate` and `CRL` objects. It has been enhanced in JDK, v 1.4 to also be used to generate certification path (`CertPath`) objects. A `CertificateFactory` should not be confused with a `CertPathBuilder`. A `CertPathBuilder` (discussed later) is used to discover or find a certification path when one does not exist. In contrast, a `CertificateFactory` is used when a certification path has already been discovered and the caller needs to instantiate a `CertPath` object from its contents which exist in a different form such as an encoded byte array or an array of `Certificate`s.

> ### Creating a CertificateFactory Object

> See the `CertificateFactory` section in the Java Cryptography Architecture for details on creating a `CertificateFactory` object.

> ### Generating CertPath Objects

> A `CertificateFactory` instance generates `CertPath` objects from a `List` of `Certificate` objects or from an `InputStream` that contains the encoded form of a `CertPath`. Just like a `CertPath`, each `CertificateFactory` supports a default encoding format for certification paths (ex: PKCS#7). To generate a `CertPath` object and initialize it with the data read from an input stream (in the default encoding format), use the `generateCertPath` method:

>         public final CertPath generateCertPath(InputStream inStream)

> or from a particular encoding format:

>         public final CertPath generateCertPath(InputStream inStream,
>         					   String encoding)

> To find out what encoding formats are supported, use the `getCertPathEncodings` method (the default encoding is returned first):

>         public final Iterator getCertPathEncodings()

> To generate a certification path object from a `List` of `Certificate` objects, use the following method:

>         public final CertPath generateCertPath(List certificates)

> A `CertificateFactory` always returns `CertPath` objects that consist of `Certificate`s that are of the same type as the factory. For example, a `CertificateFactory` of type X.509 returns `CertPath` objects consisting of certificates that are an instance of `java.security.cert.X509Certificate`.

> The following code sample illustrates generating a certification path from a PKCS#7 encoded certificate reply stored in a file:

>         // open an input stream to the file
>         FileInputStream fis = new FileInputStream(filename);
>         // instantiate a CertificateFactory for X.509
>         CertificateFactory cf = CertificateFactory.getInstance("X.509");
>         // extract the certification path from
>         // the PKCS7 SignedData structure
>         CertPath cp = cf.generateCertPath(fis, "PKCS7");
>         // print each certificate in the path
>         List certs = cp.getCertificates();
>         Iterator i = certs.iterator();
>         while (i.hasNext()) {
>             X509Certificate cert = (X509Certificate) i.next();
>             System.out.println(cert);
>         }

> Here's another code sample that fetches a certificate chain from a `KeyStore` and converts it to a `CertPath` using a `CertificateFactory`:

>         // instantiate a KeyStore with type JKS
>         KeyStore ks = KeyStore.getInstance("JKS");
>         // load the contents of the KeyStore
>         ks.load(new FileInputStream("./keystore"),
>         	"password".toCharArray());
>         // fetch certificate chain stored with alias "sean"
>         Certificate[] certArray = ks.getCertificateChain("sean");
>         // convert chain to a List
>         List certList = Arrays.asList(certArray);
>         // instantiate a CertificateFactory for X.509
>         CertificateFactory cf = CertificateFactory.getInstance("X.509");
>         // extract the certification path from
>         // the List of Certificates
>         CertPath cp = cf.generateCertPath(certList);

Note that there is an existing method in `CertificateFactory` named `generateCertificates` that parses a sequence of `Certificate`s. For encodings consisting of multiple certificates, use `generateCertificates` when you want to parse a collection of possibly unrelated certificates. Otherwise, use `generateCertPath` when you want to generate a `CertPath` and subsequently validate it with a `CertPathValidator` (discussed later).

## The CertPathParameters Interface

The `CertPathParameters` interface is a transparent representation of the set of parameters used with a particular certification path builder or validation algorithm. Its main purpose is to group (and provide type safety for) all certification path parameter specifications. The `CertPathParameters` interface extends the `Cloneable` interface and defines a `clone()` method that does not throw an exception. All concrete implementations of this interface should implement and override the `Object.clone()` method, if necessary. This allows applications to clone any `CertPathParameters` object.

Objects implementing the `CertPathParameters` interface are passed as arguments to methods of the `CertPathValidator` and `CertPathBuilder` classes. Typically, a concrete implementation of the `CertPathParameters` interface will hold a set of input parameters specific to a particular certification path build or validation algorithm. For example, the `PKIXParameters` class is an implementation of the `CertPathParameters` interface that holds a set of input parameters for the PKIX certification path validation algorithm. One such parameter is the set of most-trusted CAs that the caller trusts for anchoring the validation process. This parameter among others is discussed in more detail in the section discussing the `PKIXParameters` class.

## Certification Path Validation Classes

The Java Certification Path API includes classes and interfaces for validating certification paths. An application uses an instance of the `CertPathValidator` class to validate a `CertPath` object. If successful, the result of the validation algorithm is returned in an object implementing the `CertPathValidatorResult` interface.

## The CertPathValidator Class

The `CertPathValidator` class is an engine class used to validate a certification path.

> ### Creating a CertPathValidator Object

> As with all engine classes, the way to get a `CertPathValidator` object for a particular validation algorithm is to call one of the `getInstance` static factory methods on the `CertPathValidator` class:

>     	public static CertPathValidator getInstance(String algorithm)
>         	public static CertPathValidator getInstance(String algorithm,
>     						    String provider)
>         	public static CertPathValidator getInstance(String algorithm,
>     						    Provider provider)

> The `algorithm` parameter is the name of a certification path validation algorithm (for example, "PKIX"). Standard `CertPathValidator` algorithm names are listed in Appendix A.

> ### Validating a Certification Path

> Once a `CertPathValidator` object is created, paths can be validated by calling the `validate` method, passing it the certification path to be validated and a set of algorithm-specific parameters:

>     	public final CertPathValidatorResult
>     	    	validate(CertPath certPath, CertPathParameters params)
>     		throws CertPathValidatorException,
>     		       InvalidAlgorithmParameterException

> If the validation algorithm is successful, the result is returned in an object implementing the `CertPathValidatorResult` interface. Otherwise, a `CertPathValidatorException` is thrown. The `CertPathValidatorException` contains methods that return the `CertPath`, and if relevant, the index of the certificate that caused the algorithm to fail and the root exception or cause of the failure.

> Note that the `CertPath` and `CertPathParameters` passed to the `validate` method must be of a type that is supported by the validation algorithm. Otherwise, an `InvalidAlgorithmParameterException` is thrown. For example, a `CertPathValidator` instance that implements the PKIX algorithm validates `CertPath` objects of type X.509 and `CertPathParameters` that are an instance of `PKIXParameters`.

## The CertPathValidatorResult Interface

The `CertPathValidatorResult` interface is a transparent representation of the successful result or output of a certification path validation algorithm. Its main purpose is to group (and provide type safety for) all validation results. Like the `CertPathParameters` interface, `CertPathValidatorResult` extends `Cloneable` and defines a `clone()` method that does not throw an exception. This allows applications to clone any `CertPathValidatorResult` object.

Objects implementing the `CertPathValidatorResult` interface are returned by the `validate` method of `CertPathValidator` (only when successful; otherwise a `CertPathValidatorException` is thrown with a description of the failure). Typically, a concrete implementation of the `CertPathValidatorResult` interface will hold a set of output parameters specific to a particular certification path validation algorithm. For example, the `PKIXCertPathValidatorResult` class is an implementation of the `CertPathValidatorResult` interface which contains methods to get the output parameters of the PKIX certification path validation algorithm. One such parameter is the valid policy tree. This parameter among others is discussed in more detail in the section discussing the `PKIXCertPathValidatorResult` class.

Here's a simplified code sample that illustrates how to create a `CertPathValidator` and use it to validate a certification path. The sample assumes that the `CertPath` and `CertPathParameters` objects which are passed to the `validate` method have been previously created; a more complete example will be illustrated in the section describing the PKIX classes.

        // create CertPathValidator that implements the "PKIX" algorithm
        CertPathValidator cpv = null;
        try {
            cpv = CertPathValidator.getInstance("PKIX");
        } catch (NoSuchAlgorithmException nsae) {
    	System.err.println(nsae);
    	System.exit(1);
        }
        // validate certification path ("cp") with specified parameters ("params")
        try {
            CertPathValidatorResult cpvResult = cpv.validate(cp, params);
        } catch (InvalidAlgorithmParameterException iape) {
    	System.err.println("validation failed: " + iape);
    	System.exit(1);
        } catch (CertPathValidatorException cpve) {
    	System.err.println("validation failed: " + cpve);
    	System.err.println("index of certificate that caused exception: "
    		+ cpve.getIndex());
    	System.exit(1);
        }

## Certification Path Building Classes

The Java Certification Path API includes classes for building (or discovering) certification paths. An application uses an instance of the `CertPathBuilder` class to build a `CertPath` object. If successful, the result of the build is returned in an object implementing the `CertPathBuilderResult` interface.

## The CertPathBuilder Class

The `CertPathBuilder` class is an engine class used to build a certification path.

> ### Creating a CertPathBuilder Object

> As with all engine classes, the way to get a `CertPathBuilder` object for a particular build algorithm is to call one of the `getInstance` static factory method on the `CertPathBuilder` class:

>     	public static CertPathBuilder getInstance(String algorithm)
>     	public static CertPathBuilder getInstance(String algorithm,
>     						  String provider)
>         	public static CertPathBuilder getInstance(String algorithm,
>     					          Provider provider)

> The `algorithm` parameter is the name of a certification path builder algorithm (for example, "PKIX"). Standard `CertPathBuilder` algorithm names are listed in Appendix A.

> ### Building a Certification Path

> Once a `CertPathBuilder` object is created, paths can be constructed by calling the `build` method, passing it an algorithm-specific parameter specification:

>     	public final CertPathBuilderResult build(CertPathParameters params)
>     		throws CertPathBuilderException,
>     		       InvalidAlgorithmParameterException

> If the build algorithm is successful, the result is returned in an object implementing the `CertPathBuilderResult` interface. Otherwise, a `CertPathBuilderException` is thrown containing information about the failure; for example, the underlying exception (if any) and an error message.

> Note that the `CertPathParameters` passed to the `build` method must be of a type that is supported by the build algorithm. Otherwise, an `InvalidAlgorithmParameterException` is thrown.

## The CertPathBuilderResult Interface

The `CertPathBuilderResult` interface is a transparent representation of the result or output of a certification path builder algorithm. This interface contains a method to return the certification path that has been successfully built:

    	public CertPath getCertPath()

The purpose of the `CertPathBuilderResult` interface is to group (and provide type safety for) all build results. Like the `CertPathValidatorResult` interface, `CertPathBuilderResult` extends `Cloneable` and defines a `clone()` method that does not throw an exception. This allows applications to clone any `CertPathBuilderResult` object.

Objects implementing the `CertPathBuilderResult` interface are returned by the `build` method of `CertPathBuilder`.

Here's a simplified code sample that illustrates how to create a `CertPathBuilder` and use it to build a certification path. The sample assumes that the `CertPathParameters` object which is passed to the `build` method has been previously created; a more complete example will be illustrated in the section describing the PKIX classes.

        // create CertPathBuilder that implements the "PKIX" algorithm
        CertPathBuilder cpb = null;
        try {
            cpb = CertPathBuilder.getInstance("PKIX");
        } catch (NoSuchAlgorithmException nsae) {
    	System.err.println(nsae);
    	System.exit(1);
        }
        // build certification path using specified parameters ("params")
        try {
            CertPathBuilderResult cpbResult = cpb.build(params);
            CertPath cp = cpbResult.getCertPath();
            System.out.println("build passed, path contents: " + cp);
        } catch (InvalidAlgorithmParameterException iape) {
    	System.err.println("build failed: " + iape);
    	System.exit(1);
        } catch (CertPathBuilderException cpbe) {
    	System.err.println("build failed: " + cpbe);
    	System.exit(1);
        }

## Certificate/CRL Storage Classes

The Java Certification Path API also includes the `CertStore` class for retrieving certificates and CRLs from a repository. This is useful because it allows a caller to specify the repository a `CertPathValidator` or `CertPathBuilder` implementation should use to find certificates and CRLs (see the `addCertStores` method of `PKIXParameters` for an example).

A `CertPathValidator` implementation may use the `CertStore` object that the caller specifies as a callback mechanism to fetch CRLs for performing revocation checks. Similarly, a `CertPathBuilder` may use the `CertStore` as a callback mechanism to fetch certitificates and, if performing revocation checks, CRLs.

## The CertStore Class

The `CertStore` class is an engine class used to provide the functionality of a certificate and certificate revocation list (CRL) repository. It can be used by `CertPathBuilder` and `CertPathValidator` implementations to find certificates and CRLs or as a general purpose certificate and CRL retrieval mechanism.

Unlike the `java.security.KeyStore` class, which provides access to a cache of private keys and trusted certificates, a `CertStore` is designed to provide access to a potentially vast repository of untrusted certificates and CRLs. For example, an LDAP implementation of `CertStore` provides access to certificates and CRLs stored in one or more directories using the LDAP protocol.

All public methods of CertStore objects are thread-safe. That is, multiple threads may concurrently invoke these methods on a single `CertStore` object (or more than one) with no ill effects. This allows a `CertPathBuilder` to search for a CRL while simultaneously searching for further certificates, for instance.

> ### Creating a CertStore Object

> As with all engine classes, the way to get a `CertStore` object for a particular repository type is to call one of the `getInstance` static factory methods on the `CertStore` class:

>     	public static CertStore getInstance(String type,
>     		CertStoreParameters params)
>     	public static CertStore getInstance(String type,
>     		CertStoreParameters params, String provider)
>     	public static CertStore getInstance(String type,
>     		CertStoreParameters params, Provider provider)

> The `type` parameter is the name of a certificate repository type (for example, "LDAP"). Standard `CertStore` types are listed in Appendix A.

> The initialization parameters (`params`) are specific to the repository type. For example, the initialization parameters for a server-based repository may include the hostname and the port of the server. An `InvalidAlgorithmParameterException` is thrown if the parameters are invalid for this `CertStore` type. The `getCertStoreParameters` method returns the `CertStoreParameters` that were used to initialize a `CertStore`:

>     	public final CertStoreParameters getCertStoreParameters()

> ### Retrieving Certificates

> Once you have created a `CertStore` object, you can retrieve certificates from the repository using the `getCertificates` method. This method takes a `CertSelector` (discussed in more detail later) object as an argument, which specifies a set of selection criteria for determining which certificates should be returned:

>     	public final Collection getCertificates(CertSelector selector)
>     		throws CertStoreException

> This method returns a `Collection` of `java.security.cert.Certificate` objects that satisfy the selection criteria. An empty `Collection` is returned if there are no matches. A `CertStoreException` is usually thrown if an unexpected error condition is encountered, such as a communications failure with a remote repository.

> For some `CertStore` implementations, it may not be feasible to search the entire repository for certificates or CRLs that match the specified selection criteria. In these instances, the `CertStore` implementation may use information that is specified in the selectors to locate certificates and CRLs. For instance, an LDAP `CertStore` may not search all entries in the directory. Instead, it may just search entries that are likely to contain the certificates it is looking for. If the `CertSelector` provided does not provide enough information for the LDAP `CertStore` to determine which entries it should look in, the LDAP `CertStore` may throw a `CertStoreException`.

> ### Retrieving CRLs

> You can also retrieve CRLs from the repository using the `getCRLs` method. This method takes a `CRLSelector` (discussed in more detail later) object as an argument, which specifies a set of selection criteria for determining which CRLs should be returned:

>     	public final Collection getCRLs(CRLSelector selector)
>     		throws CertStoreException

> This method returns a `Collection` of `java.security.cert.CRL` objects that satisfy the selection criteria. An empty `Collection` is returned if there are no matches.

## The CertStoreParameters Interface

The `CertStoreParameters` interface is a transparent representation of the set of parameters used with a particular `CertStore`. Its main purpose is to group (and provide type safety for) all certificate storage parameter specifications. The `CertStoreParameters` interface extends the `Cloneable` interface and defines a `clone` method that does not throw an exception. Implementations of this interface should implement and override the `Object.clone()` method, if necessary. This allows applications to clone any `CertStoreParameters` object.

Objects implementing the `CertStoreParameters` interface are passed as arguments to the `getInstance` method of the `CertStore` class. Two classes implementing the `CertStoreParameters` interface are defined in this API: the `LDAPCertStoreParameters` and the `CollectionCertStoreParameters` classes.

> ### The LDAPCertStoreParameters Class

> The `LDAPCertStoreParameters` class is an implementation of the `CertStoreParameters` interface and holds a set of minimal initialization parameters (host and port number of the directory server) for retrieving certificates and CRLs from a `CertStore` of type LDAP.

> Please refer to the `LDAPCertStoreParameters` API documentation for more detailed information on this class.

> ### The CollectionCertStoreParameters Class

> The `CollectionCertStoreParameters` class is an implementation of the `CertStoreParameters` interface and holds a set of initialization parameters for retrieving certificates and CRLs from a `CertStore` of type Collection.

> Please refer to the `CollectionCertStoreParameters` API documentation for more detailed information on this class.

## The CertSelector and CRLSelector Interfaces

The `CertSelector` and `CRLSelector` interfaces are a specification of the set of criteria for selecting certificates and CRLs from a collection or large group of certificates and CRLs. The interfaces group and provide type safety for all selector specifications. Each selector interface extends `Cloneable` and defines a `clone()` method that does not throw an exception. This allows applications to clone any `CertSelector` or `CRLSelector` object.

The `CertSelector` and `CRLSelector` interfaces each define a method named `match`. The `match` method takes a `Certificate` or `CRL` object as an argument and returns `true` if the object satisfies the selection criteria. Otherwise, it returns `false`. The `match` method for the `CertSelector` interface is defined as follows:

    	public boolean match(Certificate cert)

and for the `CRLSelector` interface:

    	public boolean match(CRL crl)

Typically, objects implementing these interfaces are passed as parameters to the `getCertificates` and `getCRLs` methods of the `CertStore` class. These methods return a `Collection` of `Certificate`s or `CRL`s from the `CertStore` repository that match the specified selection criteria. `CertSelector`s may also be used to specify the validation constraints on a target or end-entity certificate in a certification path (see for example, the `PKIXParameters.setTargetCertConstraints` method.

### The X509CertSelector Class

The `X509CertSelector` class is an implementation of the `CertSelector` interface that defines a set of criteria for selecting X.509 certificates. An `X509Certificate` object must match all of the specified criteria to be selected by the `match` method. The selection criteria are designed to be used by a `CertPathBuilder` implementation to discover potential certificates as it builds an X.509 certification path.

For example, the `setSubject` method of `X509CertSelector` allows a PKIX `CertPathBuilder` to filter out `X509Certificate`s that do not match the issuer name of the preceding `X509Certificate` in a partially completed chain. By setting this and other criteria in an `X509CertSelector` object, a `CertPathBuilder` is able to discard irrelevant certificates and more easily find an X.509 certification path that meets the requirements specified in the `CertPathParameters` object.

Please refer to http://www.ietf.org/rfc/rfc3280.txt for definitions of the X.509 certificate extensions mentioned in this section.

> ### Creating an X509CertSelector Object

> An `X509CertSelector` object is created by calling the default constructor:

>     	public X509CertSelector()

> No criteria are initially set (any `X509Certificate` will match).

> ### Setting Selection Criteria

> The selection criteria allow a caller to match on different components of an X.509 certificate. A few of the methods for setting selection criteria are described here. Please refer to the `X509CertSelector` API documentation for details on the other methods.

> The `setIssuer` methods set the issuer criterion:

>     	public void setIssuer(String issuerDN)
>     	public void setIssuer(byte[] issuerDN)

> The specified distinguished name (in RFC 2253 String or ASN.1 DER encoded form) must match the issuer distinguished name in the certificate. If null, any issuer distinguished name will do.

> Similarly, the `setSubject` methods set the subject criterion:

>     	public void setSubject(String subjectDN)
>     	public void setSubject(byte[] subjectDN)

> The specified distinguished name (in RFC 2253 String or ASN.1 DER encoded form) must match the subject distinguished name in the certificate. If null, any subject distinguished name will do.

> The `setSerialNumber` method sets the serialNumber criterion:

>     	public void setSerialNumber(BigInteger serial)

> The specified serial number must match the certificate serial number in the certificate. If null, any certificate serial number will do.

> The `setAuthorityKeyIdentifier` method sets the authorityKeyIdentifier criterion:

>     	public void setAuthorityKeyIdentifier(byte[] authorityKeyID)

> The certificate must contain an Authority Key Identifier extension matching the specified value. If null, no check will be done on the authorityKeyIdentifier criterion.

> The `setCertificateValid` method sets the certificateValid criterion:

>     	public void setCertificateValid(Date certValid)

> The specified date must fall within the certificate validity period for the certificate. If null, any date is valid.

> The `setKeyUsage` method sets the keyUsage criterion:

>     	public void setKeyUsage(boolean[] keyUsage)

> The certificate's Key Usage Extension must allow the specified key usage values (those which are set to true). If null, no keyUsage check will be done.

> ### Getting Selection Criteria

> The current values for each of the selection criteria can be retrieved using an appropriate `get` method. Please refer to the `X509CertSelector` API documentation for further details on these methods.

> ### Example

> Here is an example of retrieving X.509 certificates from an LDAP `CertStore` with the `X509CertSelector` class.

> First, we create the `LDAPCertStoreParameters` object that we will use to initialize the `CertStore` object with the hostname and port of the LDAP server:

>     	LDAPCertStoreParameters lcsp = new
>     		LDAPCertStoreParameters("ldap.sun.com", 389);

> Next, create the `CertStore` object, and pass it the `LDAPCertStoreParameters` object, as in the following statement:

>     	CertStore cs = CertStore.getInstance("LDAP", lcsp);

> This call creates a `CertStore` object that retrieves certificates and CRLs from an LDAP repository using the schema defined in RFC 2587.

> The following block of code establishes an `X509CertSelector` to retrieve all unexpired (as of the current date and time) end-entity certificates issued to a particular subject with 1) a key usage that allows digital signatures, and 2) a subject alternative name with a specific email address:

>     	X509CertSelector xcs = new X509CertSelector();
>     	// select only unexpired certificates
>     	xcs.setCertificateValid(new Date());
>     	// select only certificates issued to
>     	// 'CN=alice, O=xyz, C=us'
>     	xcs.setSubject("CN=alice, O=xyz, C=us");
>     	// select only end-entity certificates
>     	xcs.setBasicConstraints(-2);
>     	// select only certificates with a digitalSignature
>     	// keyUsage bit set (set the first entry in the
>     	// boolean array to true)
>     	boolean[] keyUsage = {true};
>     	xcs.setKeyUsage(keyUsage);
>     	// select only certificates with a subjectAltName of
>     	// 'alice@xyz.com' (1 is the integer value of
>     	// an RFC822Name)
>     	xcs.addSubjectAlternativeName(1, "alice@xyz.com");

> Then we pass the selector to the `getCertificates` method of our `CertStore` object that we previously created:

>     	Collection certs = cs.getCertificates(xcs);

> A PKIX `CertPathBuilder` may use similar code to help discover and sort through potential certificates by discarding those that do not meet validation constraints or other criteria.

### The X509CRLSelector Class

The `X509CRLSelector` class is an implementation of the `CRLSelector` interface that defines a set of criteria for selecting X.509 CRLs. An `X509CRL` object must match all of the specified criteria to be selected by the `match` method. The selection criteria are designed to be useful to a `CertPathValidator` or `CertPathBuilder` implementation that must retrieve CRLs from a repository to check the revocation status of certificates in an X.509 certification path.

For example, the `setDateAndTime` method of `X509CRLSelector` allows a PKIX `CertPathValidator` to filter out `X509CRL`s that have been issued after or expire before the time indicated. By setting this and other criteria in an `X509CRLSelector` object, it allows the `CertPathValidator` to discard irrelevant CRLs and more easily check if a certificate has been revoked.

Please refer to http://www.ietf.org/rfc/rfc3280.txt for definitions of the X.509 CRL fields and extensions mentioned in this section.

> ### Creating an X509CRLSelector Object

> An `X509CRLSelector` object is created by calling the default constructor:

>     	public X509CRLSelector()

> No criteria are initially set (any `X509CRL` will match).

> ### Setting Selection Criteria

> The selection criteria allow a caller to match on different components of an X.509 CRL. Most of the methods for setting selection criteria are described here. Please refer to the `X509CRLSelector` API documentation for details on the remaining methods.

> The `setIssuerNames` method sets the issuerNames criterion:

>     	public void setIssuerNames(Collection names)

> The issuer distinguished name in the CRL must match at least one of the specified distinguished names. Each entry of the `names` argument is either a `String` or a byte array (representing the name, in RFC 2253 or ASN.1 DER encoded form, respectively). If null, any issuer distinguished name will do.

> The `setMinCRLNumber` and `setMaxCRLNumber` methods set the minCRLNumber and maxCRLNumber criterion:

>     	public void setMinCRLNumber(BigInteger minCRL)
>     	public void setMaxCRLNumber(BigInteger maxCRL)

> The CRL must have a CRL Number extension whose value is greater than or equal to the specified value if the `setMinCRLNumber` method is called, and less than or equal to the specified value if the `setMaxCRLNumber` method is called. If the value passed to one of these methods is null, the corresponding check is not done.

> The `setDateAndTime` method sets the dateAndTime criterion:

>     	public void setDateAndTime(Date dateAndTime)

> The specified date must be equal to or later than the value of the thisUpdate component of the CRL and earlier than the value of the nextUpdate component. If null, no dateAndTime check will be done.

> The `setCertificateChecking` method sets the certificate whose revocation status is being checked:

>     	public void setCertificateChecking(X509Certificate cert)

> This is not a criterion. Rather, it is optional information that may help a `CertStore` find CRLs that would be relevant when checking revocation for the specified certificate. If null is specified, then no such optional information is provided. An application should always call this method when checking revocation for a particular certificate, as it may provide the `CertStore` with more information for finding the correct CRLs and filtering out irrelevant ones.

> ### Getting Selection Criteria

> The current values for each of the selection criteria can be retrieved using an appropriate `get` method. Please refer to the `X509CRLSelector` API documentation for further details on these methods.

> ### Example

> Creating an `X509CRLSelector` to retrieve CRLs from an LDAP repository is similar to the `X509CertSelector` example. Suppose we want to retrieve all current (as of the current date and time) CRLs issued by a specific CA and with a minimum CRL number. First, we create an `X509CRLSelector` object and call the appropriate methods to set the selection criteria:

>     	X509CRLSelector xcrls = new X509CRLSelector();
>     	// select CRLs satisfying current date and time
>     	xcrls.setDateAndTime(new Date());
>     	// select CRLs issued by 'O=xyz, C=us'
>     	xcrls.addIssuerName("O=xyz, C=us");
>     	// select only CRLs with a CRL number at least '2'
>     	xcrls.setMinCRLNumber(new BigInteger("2"));

> Then we pass the selector to the `getCRLs` method of our `CertStore` object (created in the `X509CertSelector` example):

>     	Collection crls = cs.getCRLs(xcrls);

## PKIX Classes

The Java Certification Path API also includes a set of algorithm-specific classes modelled for use with the PKIX certification path validation algorithm defined in RFC 3280: Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile.

> ### The TrustAnchor Class

> This class represents a "most-trusted CA", which is used as a trust anchor for validating X.509 certification paths. A most-trusted CA includes the public key of the CA, the CA's name, and any constraints upon the set of paths which may be validated using this key. These parameters can be specified in the form of a trusted `X509Certificate` or as individual parameters.

> All `TrustAnchor` objects are immutable and thread-safe. That is, multiple threads may concurrently invoke the methods defined in this class on a single `TrustAnchor` object (or more than one) with no ill effects. Requiring `TrustAnchor` objects to be immutable and thread-safe allows them to be passed around to various pieces of code without worrying about coordinating access.

> Note that although this class is described as a PKIX class it may be used with other X.509 certification path validation algorithms.

> > ### Creating a TrustAnchor Object

> > To instantiate a `TrustAnchor` object, a caller must specify "the most-trusted CA" as a trusted `X509Certificate` or public key and distinguished name pair. The caller may also optionally specify name constraints that are applied to the trust anchor by the validation algorithm during initialization. Note that support for name constraints on trust anchors is not required by the PKIX algorithm, therefore a PKIX `CertPathValidator` or `CertPathBuilder` may choose not to support this parameter and instead throw an exception. Use one of the following constructors to create a `TrustAnchor` object:

> >     	public TrustAnchor(X509Certificate trustedCert,
> >     		byte[] nameConstraints)
> >     	public TrustAnchor(String caName, PublicKey pubKey,
> >     		byte[] nameConstraints)

> > The `nameConstraints` parameter is specifed as a byte array containing the ASN.1 DER encoding of a NameConstraints extension. An `IllegalArgumentException` is thrown if the name constraints cannot be decoded (are not formatted correctly).

> > ### Getting Parameter Values

> > Each of the parameters can be retrieved using a corresponding get method:

> >     	public final X509Certificate getTrustedCert()
> >     	public final String getCAName()
> >     	public final PublicKey getCAPublicKey()
> >     	public final byte[] getNameConstraints()

> > Note that the `getTrustedCert` method returns `null` if the trust anchor was specified as a public key and name pair. Likewise, the `getCAName` and `getCAPublicKey` methods return `null` if the trust anchor was specified as an `X509Certificate`.

> ### The PKIXParameters Class

> This class (which implements the `CertPathParameters` interface) specifies the set of input parameters defined by the PKIX certification path validation algorithm. It also includes a few additional useful parameters.

> An X.509 `CertPath` object and a `PKIXParameters` object are passed as arguments to the `validate` method of a `CertPathValidator` instance implementing the PKIX algorithm. The `CertPathValidator` uses the parameters to initialize the PKIX certification path validation algorithm.

> > ### Creating a PKIXParameters Object

> > To instantiate a `PKIXParameters` object, a caller must specify "the most-trusted CA(s)" as defined by the PKIX validation algorithm. The most-trusted CAs can be specified using one of two constructors:

> >     	public PKIXParameters(Set trustAnchors)
> >     	    throws InvalidAlgorithmParameterException
> >     	public PKIXParameters(KeyStore keystore)
> >     	    throws KeyStoreException, InvalidAlgorithmParameterException

> > The first constructor allows the caller to specify the most-trusted CAs as a `Set` of `TrustAnchor` objects. Alternatively, a caller can use the second constructor and specify a `KeyStore` instance containing trusted certificate entries, each of which will be considered as a most-trusted CA.

> > ### Setting Parameter Values

> > Once a `PKIXParameters` object has been created, a caller can set (or replace the current value of) various parameters. A few of the methods for setting parameters are described here. Please refer to the `PKIXParameters` API documentation for details on the other methods.

> > The `setInitialPolicies` method sets the initial policy identifiers, as specified by the PKIX validation algorithm. The elements of the `Set` are object identifiers (OIDs) represented as a `String`. If the `initialPolicies` parameter is null or not set, any policy is acceptable:

> >     	public void setInitialPolicies(Set initialPolicies)

> > The `setDate` method sets the time for which the validity of the path should be determined. If the `date` parameter is not set or is null, the current date is used:

> >     	public void setDate(Date date)

> > The `setPolicyMappingInhibited` method sets the value of the policy mapping inhibited flag. The default value for the flag, if not specified, is false:

> >     	public void setPolicyMappingInhibited(boolean val)

> > The `setExplicitPolicyRequired` method sets the value of the explicit policy required flag. The default value for the flag, if not specified, is false:

> >     	public void setExplicitPolicyRequired(boolean val)

> > The `setAnyPolicyInhibited` method sets the value of the any policy inhibited flag. The default value for the flag, if not specified, is false:

> >     	public void setAnyPolicyInhibited(boolean val)

> > The `setTargetCertConstraints` method allows the caller to set constraints on the target or end-entity certificate. For example, the caller can specify that the target certificate must contain a specific subject name. The constraints are specified as a `CertSelector` object. If the `selector` parameter is null or not set, no constraints are defined on the target certificate:

> >     	public void setTargetCertConstraints(CertSelector selector)

> > The `setCertStores` method allows a caller to specify a `List` of `CertStore` objects that will be used by a PKIX implementation of `CertPathValidator` to find CRLs for path validation. This provides an extensible mechanism for specifying where to locate CRLs. The `setCertStores` method takes a `List` of `CertStore` objects as a parameter. The first `CertStore`s in the list may be preferred to those that appear later.

> >     	public void setCertStores(List stores)

> > The `setCertPathCheckers` method allows a caller to extend the PKIX validation algorithm by creating implementation-specific certification path checkers. For example, this mechanism can be used to process private certificate extensions. The `setCertPathCheckers` method takes a list of `PKIXCertPathChecker` (discussed later) objects as a parameter:

> >     	public void setCertPathCheckers(List checkers)

> > The `setRevocationEnabled` method allows a caller to disable revocation checking. Revocation checking is enabled by default, since it is a required check of the PKIX validation algorithm. However, PKIX does not define how revocation should be checked. An implementation may use CRLs or OCSP, for example. This method allows the caller to disable the implementation's default revocation checking mechanism if it is not appropriate. A different revocation checking mechanism can then be specified by calling the `setCertPathCheckers` method, and passing it a `PKIXCertPathChecker` that implements the alternate mechanism.

> >     	public void setRevocationEnabled(boolean val)

> > The `setPolicyQualifiersRejected` method allows a caller to enable or disable policy qualifier processing. When a `PKIXParameters` object is created, this flag is set to `true`. This setting reflects the most common (and simplest) strategy for processing policy qualifiers. Applications that want to use a more sophisticated policy must set this flag to `false`.

> >     	public void setPolicyQualifiersRejected(boolean qualifiersRejected)

> > ### Getting Parameter Values

> > The current values for each of the parameters can be retrieved using an appropriate `get` method. Please refer to the `PKIXParameters` API documentation for further details on these methods.

> ### The PKIXCertPathValidatorResult Class

> This class (which implements the `CertPathValidatorResult` interface) represents the result of the PKIX certification path validation algorithm. It holds the valid policy tree and subject public key resulting from the validation algorithm, and includes methods (`getPolicyTree()` and `getPublicKey()`) for returning them. Instances of `PKIXCertPathValidatorResult` are returned by the `validate` method of `CertPathValidator` objects implementing the PKIX algorithm.

> Please refer to the `PKIXCertPathValidatorResult` API documentation for more detailed information on this class.

> ### The PolicyNode Interface and PolicyQualifierInfo Class

> The PKIX validation algorithm defines several outputs related to certificate policy processing. Most applications will not need to use these outputs, but all providers that implement the PKIX validation or building algorithm must support them.

> The `PolicyNode` interface represents a node of a valid policy tree resulting from a successful execution of the PKIX certification path validation. An application can obtain the root of a valid policy tree using the `getPolicyTree` method of `PKIXCertPathValidatorResult`. Policy Trees are discussed in more detail in the PKIX Certificate and CRL Profile.

> The `getPolicyQualifiers` method of `PolicyNode` returns a `Set` of `PolicyQualifierInfo` objects, each of which represents a policy qualifier contained in the Certificate Policies extension of the relevant certificate that this policy applies to.

> Most applications will not need to examine the valid policy tree and policy qualifiers. They can achieve their policy processing goals by setting the policy-related parameters in `PKIXParameters`. However, the valid policy tree is available for more sophisticated applications, especially those that process policy qualifiers.

> Please refer to the `PolicyNode` and `PolicyQualifierInfo` API documentation for more detailed information on these classes.

> ### Example of Validating a Certification Path using the PKIX algorithm

> This is an example of validating a certification path with the PKIX validation algorithm. The example ignores most of the exception handling and assumes that the certification path and public key of the trust anchor have already been created.

> First, create the `CertPathValidator`, as in the following line:

>         CertPathValidator cpv = CertPathValidator.getInstance("PKIX");

> The next step is to create a `TrustAnchor` object. This will be used as an anchor for validating the certification path. In this example, the most-trusted CA is specified as a public key and name (name constraints are not applied and are specified as `null`):

>         TrustAnchor anchor = new TrustAnchor("O=xyz,C=us", pubkey, null);

> The next step is to create a `PKIXParameters` object. This will be used to populate the parameters used by the PKIX algorithm. In this example, we pass to the constructor a `Set` containing a single element - the `TrustAnchor` we created in the previous step:

>         PKIXParameters params = new PKIXParameters(Collections.singleton(anchor));

> Next, we populate the parameters object with constraints or other parameters used by the validation algorithm. In this example, we enable the explicitPolicyRequired flag and specify a set of initial policy OIDs (the contents of the set are not shown):

>         // set other PKIX parameters here
>         params.setExplicitPolicyRequired(true);
>         params.setInitialPolicies(policyIds);

> The final step is to validate the certification path using the input parameter set we have created:

>         try {
>             PKIXCertPathValidatorResult result =
>                 (PKIXCertPathValidatorResult) cpv.validate(certPath, params);
>             PolicyNode policyTree = result.getPolicyTree();
>             PublicKey subjectPublicKey = result.getPublicKey();
>         } catch (CertPathValidatorException cpve) {
>             System.out.println("Validation failure, cert["
>                 + cpve.getIndex() + "] :" + cpve.getMessage());
>         }

> If the validation algorithm is successful, the policy tree and subject public key resulting from the validation algorithm are obtained using the `getPolicyTree` and `getPublicKey` methods of `PKIXCertPathValidatorResult`.

> Otherwise, a `CertPathValidatorException` is thrown and the caller can catch the exception and print some details about the failure, such as the error message and the index of the certificate that caused the failure.

> ### The PKIXBuilderParameters Class

> This class (which extends the `PKIXParameters` class) specifies the set of parameters to be used with `CertPathBuilder`s that build certification paths validated against the PKIX certification path validation algorithm.

> A `PKIXBuilderParameters` object is passed as an argument to the `build` method of a `CertPathBuilder` instance implementing the PKIX algorithm. All PKIX `CertPathBuilder`s must return certification paths which have been validated according to the PKIX certification path validation algorithm.

> Please note that the mechanism that a PKIX `CertPathBuilder` uses to validate a constructed path is an implementation detail. For example, an implementation might attempt to first build a path with minimal validation and then fully validate it using an instance of a PKIX `CertPathValidator`, whereas a more efficient implementation may validate more of the path as it is building it, and backtrack to previous stages if it encounters validation failures or dead-ends.

> > ### Creating a PKIXBuilderParameters Object

> > Creating a `PKIXBuilderParameters` object is similar to creating a `PKIXParameters` object. However, a caller must specify constraints on the target or end-entity certificate when creating a `PKIXBuilderParameters` object. These constraints should provide the `CertPathBuilder` with enough information to find the target certificate. The constraints are specified as a `CertSelector` object. Use one of the following constructors to create a `PKIXBuilderParameters` object:

> >     	public PKIXBuilderParameters(Set trustAnchors,
> >     		CertSelector targetConstraints)
> >     		throws InvalidAlgorithmParameterException
> >     	public PKIXBuilderParameters(KeyStore keystore,
> >     		CertSelector targetConstraints)
> >     		throws KeyStoreException, InvalidAlgorithmParameterException

> > ### Getting/Setting Parameter Values

> > The `PKIXBuilderParameters` class inherits all of the parameters that can be set in the `PKIXParameters` class. In addition, the `setMaxPathLength` method can be called to place a limit on the maximum number of certificates in a certification path:

> >     	public void setMaxPathLength(int maxPathLength)

> > The `maxPathLength` parameter specifies the maximum number of non-self-issued intermediate certificates that may exist in a certification path. A `CertPathBuilder` instance implementing the PKIX algorithm must not build paths longer than the length specified. If the value is 0, the path can only contain a single certificate. If the value is -1, the path length is unconstrained (i.e., there is no maximum). The default maximum path length, if not specified, is 5. This method is useful to prevent the `CertPathBuilder` from spending resources and time constructing long paths that may or may not meet the caller's requirements.

> > If any of the CA certificates in the path contain a Basic Constraints extension, the value of the pathLenConstraint component of the extension overrides the value of the `maxPathLength` parameter whenever the result is a certification path of smaller length. There is also a corresponding `getMaxPathLength` method for retrieving this parameter:

> >     	public int getMaxPathLength()

> > Also, the `setCertStores` method (inherited from the `PKIXParameters` class) is typically used by a PKIX implementation of `CertPathBuilder` to find Certificates for path construction as well as finding CRLs for path validation. This provides an extensible mechanism for specifying where to locate Certificates and CRLs.

> ### The PKIXCertPathBuilderResult Class

> This class (which extends the `PKIXCertPathValidatorResult` class and implements the `CertPathBuilderResult` interface) represents the successful result of the PKIX certification path construction algorithm. Instances of `PKIXCertPathBuilderResult` are returned by the `build` method of `CertPathBuilder` objects implementing the PKIX algorithm.

> The `getCertPath` method of a `PKIXCertPathBuilderResult` instance always returns a `CertPath` object validated using the PKIX certification path validation algorithm. The returned `CertPath` object does not include the most-trusted CA certificate that may have been used to anchor the path. Instead, use the `getTrustAnchor` method to get the `Certificate` of the most-trusted CA.

> Please refer to the `PKIXCertPathBuilderResult` API documentation for more detailed information on this class.

> ### Example of Building a Certification Path using the PKIX algorithm

> This is an example of of building a certification path validated against the PKIX algorithm. Some details have been left out, such as exception handling, and the creation of the trust anchors and certificates for populating the `CertStore`.

> First, create the `CertPathBuilder`, as in the following example:

>         CertPathBuilder cpb = CertPathBuilder.getInstance("PKIX");

> This call creates a `CertPathBuilder` object that returns paths validated against the PKIX algorithm.

> The next step is to create a `PKIXBuilderParameters` object. This will be used to populate the PKIX parameters used by the `CertPathBuilder`:

>         // Create parameters object, passing it a Set of
>         // trust anchors for anchoring the path
>         // and a target subject DN.
>         X509CertSelector targetConstraints = new X509CertSelector();
>         targetConstraints.setSubject("CN=alice,O=xyz,C=us");
>         PKIXBuilderParameters params =
>             new PKIXBuilderParameters(trustAnchors, targetConstraints);

> The next step is to specify the `CertStore` that the `CertPathBuilder` will use to look for certificates and CRLs. For this example, we will populate a Collection `CertStore` with the certificates and CRLs:

>         CollectionCertStoreParameters ccsp =
>     	new CollectionCertStoreParameters(certsAndCrls);
>         CertStore store = CertStore.getInstance("Collection", ccsp);
>         params.addCertStore(store);

> The next step is to build the certification path using the input parameter set we have created:

>         try {
>             PKIXCertPathBuilderResult result =
>     	    (PKIXCertPathBuilderResult) cpb.build(params);
>             CertPath cp = result.getCertPath();
>         } catch (CertPathBuilderException cpbe) {
>     	System.out.println("build failed: " + cpbe.getMessage());
>         }

> If the `CertPathBuilder` cannot build a path that meets the supplied parameters it will throw a `CertPathBuilderException`. Otherwise, the validated certification path can be obtained from the `PKIXCertPathBuilderResult` using the `getCertPath` method.

> ### The PKIXCertPathChecker Class

> This section describes a powerful class that allows a user to extend a PKIX `CertPathValidator` or `CertPathBuilder` implementation. This is an advanced feature that most users will not need to understand. However, anyone implementing a PKIX service provider should read this section.

> The `PKIXCertPathChecker` class is an abstract class that executes one or more checks on an X.509 certificate. Developers should create concrete implementations of the `PKIXCertPathChecker` class when it is necessary to dynamically extend a PKIX `CertPathValidator` or `CertPathBuilder` implementation at runtime. The following are a few examples of when a `PKIXCertPathChecker` implementation is useful:

> - If the revocation mechanism supplied by a PKIX `CertPathValidator` or `CertPathBuilder` implementation is not adequate. For example, a developer might implement a `PKIXCertPathChecker` that uses OCSP ( RFC 2560) to check that certificates have not been revoked.

> - If the user wants to recognize certificates containing a critical private extension. Since the extension is private, it will not be recognized by the PKIX `CertPathValidator` or `CertPathBuilder` implementation and a `CertPathValidatorException` will be thrown. In this case, a developer can implement a `PKIXCertPathChecker` that recognizes and processes the critical private extension.

> - If the developer wants to record information about each certificate processed for debugging or display purposes.

> - If the user wants to reject certificates with certain policy qualifiers.

> The `setCertPathCheckers` method of the `PKIXParameters` class allows a user to pass a `List` of `PKIXCertPathChecker` objects to a PKIX `CertPathValidator` or `CertPathBuilder` implementation. Each of the `PKIXCertPathChecker` objects will be called in turn, for each certificate processed by the PKIX `CertPathValidator` or `CertPathBuilder` implementation.

> > ### Creating and using a PKIXCertPathChecker Object

> > The `PKIXCertPathChecker` class does not have a public constructor. This is intentional, since creating an instance of `PKIXCertPathChecker` is an implementation-specific issue. For example, the constructor for a `PKIXCertPathChecker` implementation that uses OCSP to check a certificate's revocation status may require the hostname and port of the OCSP server:

> >     	PKIXCertPathChecker checker = new OCSPChecker("ocsp.sun.com", 1321);

> > Once the checker has been instantiated, it can be added as a parameter using the `addCertPathChecker` method of the `PKIXParameters` class:

> >     	params.addCertPathChecker(checker);

> > Alternatively, a `List` of checkers can be added using the `setCertPathCheckers` method of the `PKIXParameters` class.

> > ### Implementing a PKIXCertPathChecker Object

> > The `PKIXCertPathChecker` class is abstract. It has four methods (`check`, `getSupportedExtensions`, `init`, and `isForwardCheckingSupported`) that all concrete subclasses must implement.

> > Implementing a `PKIXCertPathChecker` may be trivial or complex. A `PKIXCertPathChecker` implementation can be stateless or stateful. A stateless implementation does not maintain state between successive calls of the `check` method. For example, a `PKIXCertPathChecker` that checks that each certificate contains a particular policy qualifier is stateless. In contrast, a stateful implementation does maintain state between successive calls of the `check` method. The `check` method of a stateful implementation usually depends on the contents of prior certificates in the certification path. For example, a `PKIXCertPathChecker` that processes the NameConstraints extension is stateful.

> > Also, the order in which the certificates processed by a service provider implementation are presented (passed) to a `PKIXCertPathChecker` is very important, especially if the implementation is stateful. Depending on the algorithm used by the service provider, the certificates may be presented in reverse or forward order. A reverse ordering means that the certificates are ordered from the most trusted CA (if present) to the target subject, whereas a forward ordering means that the certificates are ordered from the target subject to the most trusted CA. The order must be made known to the `PKIXCertPathChecker` implementation, so that it knows how to process consecutive certificates.

> > > #### Initializing a PKIXCertPathChecker Object

> > > The `init` method initializes the internal state of the checker:

> > >     	public abstract void init(boolean forward)

> > > All stateful implementations should clear or initialize any internal state in the checker. This prevents a service provider implementation from calling a checker that is in an uninitialized state. It also allows stateful checkers to be reused in subsequent operations without reinstantiating them. The `forward` parameter indicates the order of the certificates presented to the `PKIXCertPathChecker`. If `forward` is `true`, the certificates are presented from target to trust anchor; if `false`, from trust anchor to target.

> > > #### Forward Checking

> > > The `isForwardCheckingSupported` method returns a `boolean` that indicates if the `PKIXCertPathChecker` supports forward checking:

> > >     	public abstract boolean isForwardCheckingSupported()

> > > All `PKIXCertPathChecker` implementations must support reverse checking. A `PKIXCertPathChecker` implementation may support forward checking.

> > > Supporting forward checking improves the efficiency of `CertPathBuilder`s that build forward, since it allows paths to be checked as they are built. However, some stateful `PKIXCertPathChecker`s may find it difficult or impossible to support forward checking.

> > > #### Supported Extensions

> > > The `getSupportedExtensions` method returns an immutable `Set` of OID `String`s for the X.509 extensions that the `PKIXCertPathChecker` implementation supports (i.e., recognizes, is able to process):

> > >     	public abstract Set getSupportedExtensions()

> > > The method should return `null` if no extensions are processed. All implementations should return the `Set` of OID `String`s that the `check` method may process.

> > > A `CertPathBuilder` can use this information to identify certificates with unrecognized critical extensions, even when performing a forward build with a `PKIXCertPathChecker` that does not support forward checking.

> > > #### Executing the Check

> > > The following method executes a check on the certificate:

> > >     	public abstract void
> > >     	        check(Certificate cert, Collection unresolvedCritExts)
> > >             	throws CertPathValidatorException

> > > The `unresolvedCritExts` parameter contains a collection of OIDs as `String`s. These OIDs represent the set of critical extensions in the certificate that have not yet been resolved by the certification path validation algorithm. Concrete implementations of the `check` method should remove any critical extensions that it processes from the `unresolvedCritExts` parameter.

> > > If the certificate does not pass the check(s), a `CertPathValidatorException` should be thrown.

> > > #### Cloning a PKIXCertPathChecker

> > > The `PKIXCertPathChecker` class implements the `Cloneable` interface. All stateful `PKIXCertPathChecker` implementations must override the `clone` method if necessary. The default implementation of the `clone` method calls the `Object.clone` method, which performs a simple clone by copying all fields of the original object to the new object. A stateless implementation should not override the `clone` method. However, all stateful implementations must ensure that the default `clone` method is correct, and override it if necessary. For example, a `PKIXCertPathChecker` that stores state in an array must override the `clone` method to make a copy of the array, rather than just a reference to the array.

> > > The reason that `PKIXCertPathChecker` objects are `Cloneable` is to allow a PKIX `CertPathBuilder` implementation to efficiently backtrack and try another path when a potential certification path reaches a dead end or point of failure. In this case, the implementation is able to restore prior path validation states by restoring the cloned objects.

> > ### Example

> > This is an example of a stateless `PKIXCertPathChecker` implementation. It checks if a private extension exists in a certificate and processes it according to some rules.

> >     	import java.security.cert.Certificate;
> >     	import java.security.cert.X509Certificate;
> >     	import java.util.Collection;
> >     	import java.util.Collections;
> >     	import java.util.Set;
> >     	import java.security.cert.PKIXCertPathChecker;
> >     	import java.security.cert.CertPathValidatorException;

> >     	public class MyChecker extends PKIXCertPathChecker {
> >     	    private static Set supportedExtensions =
> >     	        Collections.singleton("2.16.840.1.113730.1.1");

> >     	    /*
> >      	     * Initialize checker
> >     	     */
> >     	    public void init(boolean forward)
> >     		throws CertPathValidatorException {
> >     		// nothing to initialize
> >     	    }

> >     	    public Set getSupportedExtensions() {
> >     		return supportedExtensions;
> >     	    }

> >     	    public boolean isForwardCheckingSupported() {
> >     		return true;
> >     	    }

> >     	    /*
> >     	     * Check certificate for presence of Netscape's
> >     	     * private extension
> >     	     * with OID "2.16.840.1.113730.1.1"
> >     	     */
> >     	    public void check(Certificate cert,
> >     			      Collection unresolvedCritExts)
> >     		throws CertPathValidatorException
> >     	    {
> >     		X509Certificate xcert = (X509Certificate) cert;
> >     	        byte[] ext =
> >     		    xcert.getExtensionValue("2.16.840.1.113730.1.1");
> >     		if (ext == null)
> >     		    return;

> >     		//
> >     	      // process private extension according to some
> >     		// rules - if check fails, throw a
> >     		// CertPathValidatorException ...
> >     		// {insert code here}

> >     		// remove extension from collection of unresolved
> >     		// extensions (if it exists)
> >     		if (unresolvedCritExts != null)
> >     		    unresolvedCritExts.remove("2.16.840.1.113730.1.1");
> >     	    }
> >     	}

> > ### How a PKIX Service Provider implementation should use a PKIXCertPathChecker

> > Each `PKIXCertPathChecker` object must be initialized by a service provider implementation before commencing the build or validation algorithm, for example:

> >     	List checkers = params.getCertPathCheckers();
> >     	ListIterator li = checkers.listIterator();
> >     	while (li.hasNext()) {
> >     	    PKIXCertPathChecker checker = (PKIXCertPathChecker) li.next();
> >     	    checker.init(false);
> >     	}

> > For each certificate that it validates, the service provider implementation must call the `check` method of each `PKIXCertPathChecker` object in turn, passing it the certificate and any remaining unresolved critical extensions:

> >     	ListIterator li = checkers.listIterator();
> >     	while (li.hasNext()) {
> >     	    PKIXCertPathChecker checker = (PKIXCertPathChecker) li.next();
> >     	    checker.check(cert, unresolvedCritExts);
> >     	}

> > If any of the `check`s throw a `CertPathValidatorException`, a `CertPathValidator` implementation should terminate the validation procedure. However, a `CertPathBuilder` implementation may simply log the failure and continue to search for other potential paths. If all of the `check`s are successful, the service provider implementation should check that all critical extensions have been resolved and if not, consider the validation to have failed. For example:

> >     	if (unresolvedCritExts != null &&
> >     	    !unresolvedCritExts.isEmpty())
> >     	{
> >                 // note that a CertPathBuilder may have an enclosing
> >                 // try block to catch the exception below and continue on error
> >     	    throw new CertPathValidatorException
> >     		("Unrecognized Critical Extension");
> >     	}

> > As discussed in the previous section, a `CertPathBuilder` implementation may need to backtrack when a potential certification path reaches a dead end or point of failure. Backtracking in this context implies returning to the previous certificate in the path and checking for other potential paths. If the `CertPathBuilder` implementation is validating the path as it is building it, it will need to restore the previous state of each `PKIXCertPathChecker`. It can do this by making clones of the `PKIXCertPathChecker` objects before each certificate is processed, for example:

> >     	/* clone checkers */
> >     	List newList = new ArrayList(checkers);
> >     	ListIterator li = newList.listIterator();
> >     	while (li.hasNext()) {
> >     	    PKIXCertPathChecker checker = (PKIXCertPathChecker) li.next();
> >     	    li.set(checker.clone());
> >     	}

> ## Implementing a Service Provider

> This section is intended for experienced programmers wishing to create their own provider packages supplying certification path service implementations. This section assumes you have read the document: How to Implement a Provider for the Java Cryptography Architecture.

> The following engine classes are defined in the Java Certification Path API:

> - `CertPathValidator` - used to validate certification paths

> - `CertPathBuilder` - used to build certification paths

> - `CertStore` - used to retrieve certificates and CRLs from a repository

> In addition, the pre-existing `CertificateFactory` engine class has been enhanced in the JDK, v 1.4 to support generation of certification paths.

> The application interfaces supplied by an engine class are implemented in terms of a "Service Provider Interface" (SPI). The name of each SPI class is the same as that of the corresponding engine class, followed by "Spi". For example, the SPI class corresponding to the `CertPathValidator` engine class is the `CertPathValidatorSpi` class. Each SPI class is abstract. To supply the implementation of a particular type of service, for a specific algorithm or type, a provider must subclass the corresponding SPI class and provide implementations for all the abstract methods. For example, the `CertStore` class provides access to the functionality of retrieving certificates and CRLs from a repository. The actual implementation supplied in a `CertStoreSpi` subclass would be that for a specific type of certificate repository, such as LDAP.

> ### Steps to Implement and Integrate a Provider

> Developers should follow the required steps listed in the How To Implement A Provider for the Java Cryptography Architecture document. Here are some additional rules to follow for certain steps:

> Step 3: Write your "Master Class", a subclass of Provider

> These are the properties that must be defined for the certification path services, where the algorithm name is substituted for algName, and certstore type for storeType:

> - `CertPathValidator`.algName

> - `CertPathBuilder`.algName

> - `CertStore`.storeType

> See Appendix A for the standard names that are defined for algName and storeType. The value of each property must be the fully qualified name of the class implementing the specified algorithm, or certstore type. That is, it must be the package name followed by the class name, where the two are separated by a period. For example, a provider sets the `CertPathValidator.PKIX` property to have the value "sun.security.provider.certpath.PKIXCertPathValidator" as follows:

> > `put("CertPathValidator.PKIX", "sun.security.provider.certpath.PKIXCertPathValidator")`

> In addition, service attributes can be defined for the certification path services. These attributes can be used as filters for selecting service providers. See Appendix A for the definition of some standard service attributes. For example, a provider may set the `ValidationAlgorithm` service attribute to the name of an RFC or specification that defines the PKIX validation algorithm:

> > `put("CertPathValidator.PKIX ValidationAlgorithm", "RFC3280");`

> Step 8: Document your Provider and its Supported Services

> Certification path service providers should document the following information for each SPI:

> Certificate Factories

> A provider should document what types of certification paths (and the version numbers of the certificates in the path, if relevant) can be created by the factory. A provider should describe the ordering of the certificates in the certification path, as well as the contents.

> A provider should document the list of encoding formats supported. This is not technically necessary, since the client can request them by calling the `getCertPathEncodings` method. However, the documentation should describe each encoding format in more detail and reference any standards when applicable.

> Certification Path Validators

> A provider should document any relevant information regarding the `CertPathValidator` implementation, including the types of certification paths that it validates. In particular, a PKIX `CertPathValidator` implementation should document the following information:

> - The RFC or specification it is compliant with.
> - The mechanism it uses to check that certificates have not been revoked.
> - Any optional certificate or CRL extensions that it recognizes and how it processes them.

> Certification Path Builders

> A provider should document any relevant information regarding the `CertPathBuilder` implementation, including the types of certification paths that it creates and whether or not they are validated. In particular a PKIX `CertPathBuilder` implementation should document the following information:

> - The RFC or specification it is compliant with.
> - The mechanism it uses to check that certificates have not been revoked.
> - Any optional certificate or CRL extensions that it recognizes and how it processes them.
> - Details on the algorithm it uses for finding certification paths. Ex: depth-first, breadth-first, forward (i.e., from target to trust anchor(s)), reverse (i.e., from trust anchor(s) to target).
> - The algorithm it uses to select and sort potential certificates. For example, given two certificates that are potential candidates for the next certificate in the path, what criteria are used to select one before the other? What criteria are used to reject a certificate?
> - If applicable, the algorithm it uses for backtracking or constructing another path (i.e., when potential paths do not meet constraints).
> - The types of `CertStore` implementations that have been tested. The implementation should be designed to work with any `CertStore` type, but this information may still be useful.

> All `CertPathBuilder` implementations should provide additional debugging support, in order to analyze and correct potential path building problems. Details on how to access this debugging information should be documented.

> Certificate/CRL Stores

> A provider should document what types of certificates and CRLs (and the version numbers, if relevant) are retrieved by the `CertStore`.

> A provider should also document any relevant information regarding the `CertStore` implementation (such as protocols used or formats supported). For example, an LDAP `CertStore` implementation should describe which versions of LDAP are supported and which standard attributes are used for finding certificates and CRLs. It should also document if the implementation caches results, and for how long (i.e., under what conditions are they refreshed).

> If the implementation returns the certificates and CRLs in a particular order, it should describe the sorting algorithm. An implementation should also document any additional or default initialization parameters.
> Finally, an implementation should document if and how it uses information in the `CertSelector` or `CRLSelector` objects to find certificates and CRLs.

> ### Service Interdependencies

> Here are some common types of algorithm interdependencies in certification path service implementations:

> Certification Path Validation and Signature Algorithms

> A `CertPathValidator` implementation often requires use of a signature algorithm to verify each certificate's digital signature. The `setSigProvider` method of the `PKIXParameters` class allows a user to specify a specific `Signature` provider.

> Certification Path Builders and Certificate Factories

> A `CertPathBuilder` implementation will often utilize a `CertificateFactory` to generate a certification path from a list of certificates.

> CertStores and Certificate Factories

> A `CertStore` implementation will often utilize a `CertificateFactory` to generate certificates and CRLs from their encodings. For example, an LDAP `CertStore` implementation may use an X.509 `CertificateFactory` to generate X.509 certificates and CRLs from their ASN.1 encoded form.

> ### Certification Path Parameter Specification Interfaces

> The Certification Path API contains two interfaces representing transparent specifications of parameters, the `CertPathParameters` and `CertStoreParameters` interfaces.

> Two implementations of the `CertPathParameters` interface are included, the `PKIXParameters` and `PKIXBuilderParameters` classes. If you are working with PKIX certification path validation and algorithm parameters, you can utilize these classes. If you need parameters for a different algorithm, you will need to supply your own `CertPathParameters` implementation for that algorithm.

> Two implementations of the `CertStoreParameters` interface are included, the `LDAPCertStoreParameters` and the `CollectionCertStoreParameters` classes. These classes are to be used with LDAP and Collection `CertStore` implementations, respectively. If you need parameters for a different repository type, you will need to supply your own `CertStoreParameters` implementation for that type.

> The `CertPathParameters` and `CertStoreParameters` interfaces each define a `clone` method that implementations should override. A typical implementation will perform a "deep" copy of the object, such that subsequent changes to the copy will not affect the original (and vice versa). However, this is not an absolute requirement for implementations of `CertStoreParameters`. A shallow copy implementation of `clone` is more appropriate for applications that need to hold a reference to a parameter contained in the `CertStoreParameters`. For example, since `CertStore.getInstance` makes a clone of the specified `CertStoreParamter`s, a shallow copy `clone` allows an application to hold a reference to and later release the resources of a particular `CertStore` initialization parameter, rather than waiting for the garbage collection mechanism. This should be done with the utmost care, since the `CertStore` may still be in use by other threads.

> ### Certification Path Result Specification Interfaces

> The Certification Path API contains two interfaces representing transparent specifications of results, the `CertPathValidatorResult` and `CertPathBuilderResult` interfaces.

> One implementation for each of the interfaces is included: the `PKIXCertPathValidatorResult` and `PKIXCertPathBuilderResult` classes. If you are implementing PKIX certification path service providers, you can utilize these classes. If you need certification path results for a different algorithm, you will need to supply your own `CertPathValidatorResult` or `CertPathBuilderResult` implementation for that algorithm.

> A PKIX implementation of a `CertPathValidator` or a `CertPathBuilder` may find it useful to store additional information in the `PKIXCertPathValidatorResult` or `PKIXCertPathBuilderResult`, such as debugging traces. In these cases, the implementation should implement a subclass of the appropriate result class with methods to retrieve the relevant information. These classes must be shipped with the provider classes, for example, as part of the provider JAR file.

> ### Certification Path Exception Classes

> The Certification Path API contains a set of exception classes for handling errors. `CertPathValidatorException, CertPathBuilderException`, and `CertStoreException` are subclasses of `GeneralSecurityException`.
> You may need to extend these classes in your service provider implementation. For example, a `CertPathBuilder` implementation may provide additional information such as debugging traces when a `CertPathBuilderException` is thrown. The implementation may throw a subclass of `CertPathBuilderException` that holds this information. Likewise, a `CertStore` implementation can provide additional information when a failure occurs by throwing a subclass of `CertStoreException`. Also, you may want to implement a subclass of `CertPathValidatorException` to describe a particular failure mode of your `CertPathValidator` implementation.
> In each case, the new exception classes must be shipped with the provider classes, for example, as part of the provider JAR file. Each provider should document the exception subclasses.

> ### TrustAnchor Class

> As previously mentioned, a PKIX `CertPathValidator` or `CertPathBuilder` is not required to support the `nameConstraints` parameter of the `TrustAnchor` class. Implementations should throw an `InvalidAlgorithmParameterException` if this parameter is not supported.

## Appendix A: Standard Names

The Java Certification Path API requires and utilizes a set of standard names for certification path validation algorithms, encodings and certificate storage types. It supplements the list of standard names defined in Appendix A in the Java Cryptography Architecture API Specification and Reference. Note that algorithm names are treated as case-insensitive (for comparison).

Please note that a service provider may choose to define a new name for a proprietary or non-standard algorithm not mentioned below. However, to prevent name collisions, it is recommended that the name be prefixed with the reverse Internet domain name of the provider's organization (for example: `com.sun.MyCertPathValidator`).

### CertPathValidator Algorithms

The algorithm names in this section can be specified when generating an instance of `CertPathValidator`.

> PKIX: The PKIX certification path validation algorithm as defined in the ValidationAlgorithm service attribute.

### CertPathBuilder Algorithms

The algorithm names in this section can be specified when generating an instance of `CertPathBuilder`.

> PKIX: The PKIX certification path validation algorithm as defined in the ValidationAlgorithm service attribute. The output of `CertPathBuilder` instances implementing this algorithm is a certification path validated against the PKIX validation algorithm.

### CertStore Types

The following types can be specified when generating an instance of `CertStore`:

> LDAP: A `CertStore` implementation that fetches certificates and CRLs from an LDAP directory using the schema defined in the LDAPSchema service attribute.

> Collection: A `CertStore` implementation that retrieves certificates and CRLs from a `Collection`. This type of `CertStore` is particularly useful in applications where certificates or CRLs are received in a bag or some sort of attachment, such as with a signed email message or in an SSL negotiation.

### CertPath Encodings

The following encodings may be passed to the `getEncoded` method of `CertPath` or the `generateCertPath(InputStream inStream, String encoding)` method of `CertificateFactory`.

> PKCS7: A PKCS#7 SignedData object, with the only significant field being certificates. In particular, the signature and the contents are ignored. If no certificates are present, a zero-length `CertPath` is assumed. Warning: PKCS#7 does not maintain the order of certificates in a certification path. This means that if a `CertPath` is converted to PKCS#7 encoded bytes and then converted back, the order of the certificates may change, potentially rendering the `CertPath` invalid. Users should be aware of this behavior.
> PkiPath: an ASN.1 DER encoded sequence of certificates, defined as follows:

> > `PkiPath ::= SEQUENCE OF Certificate`

> Within the sequence, the order of certificates is such that the subject of the first certificate is the issuer of the second certificate, etc. Each certificate in `PkiPath` shall be unique. No certificate may appear more than once in a value of `Certificate` in `PkiPath`. The `PkiPath` format is defined in defect report 279 against X.509 (2000) and is incorporated into Technical Corrigendum 1 (DTC 2) for the ITU-T Recommendation X.509 (2000) at: ftp://ftp.bull.com/pub/OSIdirectory/DefectResolution/TechnicalCorrigenda/ApprovedTechnicalCorrigendaToX.509/8%7CX.509-TC1%284th%29.pdf.

### Service Attributes

> ValidationAlgorithm: The name of the specification that defines the certification path validation algorithm that an implementation of `CertPathBuilder` or `CertPathValidator` supports. RFCs should be specified as "RFC#" (ex: "RFC3280") and Internet Drafts as the name of the draft (ex: "draft-ietf-pkix-rfc2560bis-01.txt"). Values for this attribute that are specified as selection criteria to the `Security.getProviders` method will be compared using the `String.equalsIgnoreCase` method. All PKIX implementations of `CertPathBuilder` and `CertPathValidator` should provide a value for this attribute.

> LDAPSchema: The name of the specification that defines the LDAP schema that an implementation of an LDAP CertStore uses to retrieve certificates and CRLs. The format and semantics of this attribute is the same as described for the ValidationAlgorithm attribute. All LDAP implementations of `CertStore` should provide a value for this attribute.

## Appendix B: The "SUN" Provider

The "SUN" provider which is supplied with JDK has been enhanced for version 1.4 to support the following standard algorithms, types and encodings:

- `CertificateFactory`: X.509 `CertPath` type with PKCS7 and PkiPath encodings
- `CertPathValidator`: PKIX algorithm
- `CertPathBuilder`: PKIX algorithm
- `CertStore`: LDAP and Collection `CertStore` types

Each of these service provider interface implementations is discussed in more detail below.

### CertificateFactory

The "SUN" provider for the `CertificateFactory` engine class has been enhanced to support generation of X.509 `CertPath` objects. The PKCS7 and PkiPath encodings are supported. The PKCS#7 implementation supports a subset of RFC 2315 (only the SignedData ContentInfo type is supported). The certificates in the `CertPath` are ordered in the forward direction (from target to trust anchor). Each certificate in the `CertPath` is of type `java.security.cert.X509Certificate`, and versions 1, 2 and 3 are supported.

### CertPathValidator

The "SUN" provider supplies a PKIX implementation of the `CertPathValidator` engine class. The implementation validates `CertPath`s of type X.509 and implements the certification path validation algorithm defined in RFC 3280: PKIX Certificate and CRL Profile. This implementation sets the `ValidationAlgorithm` service attribute to "RFC3280".

The PKIX Certificate and CRL Profile has many optional features. The "SUN" provider implements support for the policy mapping extension and the reason code CRL entry extension. It does not implement support for the CRL distribution point, Freshest CRL, Authority Information Access, or Subject Information Access certificate extensions. It also does not include support for the Freshest CRL, Delta CRL, and Issuing Distribution Point CRL extensions and the Certificate Issuer, Invalidity Date, and Hold Instruction Code CRL entry extensions.

The implementation supports a CRL revocation checking mechanism that conforms to section 6.3 of the PKIX Certificate and CRL Profile. An application must specify one or more `CertStore`s containing CRLs in a `PKIXParameters` object. For each certificate in the `CertPath`, the `CertPathValidator` searches the specified `CertStore`s for applicable CRLs. OCSP (RFC 2560) is not currently supported as a built in revocation checking mechanism.

The implementation does not support the `nameConstraints` parameter of the `TrustAnchor` class and the `validate` method throws an `InvalidAlgorithmParameterException` if it is specified.

### CertPathBuilder

The "SUN" provider supplies a PKIX implementation of the `CertPathBuilder` engine class. The implementation builds `CertPath`s of type X.509. Each `CertPath` is validated according to the PKIX algorithm defined in RFC 3280: PKIX Certificate and CRL Profile. This implementation sets the `ValidationAlgorithm` service attribute to "RFC3280".

The implementation requires that the `targetConstraints` parameter of a `PKIXBuilderParameters` object is an instance of `X509CertSelector` and the subject criterion is set to a non-null value. Otherwise the `build` method throws an `InvalidAlgorithmParameterException`.

The implementation builds `CertPath` objects in a forward direction using a depth-first algorithm. It backtracks to previous states and tries alternate paths when a potential path is determined to be invalid or exceeds the `PKIXBuilderParameters` `maxPathLength` parameter.

Validation of the path is performed in the same manner as the `CertPathValidator` implementation. The implementation validates most of the path as it is being built, in order to eliminate invalid paths earlier in the process. Validation checks that cannot be executed on certificates ordered in a forward direction are delayed and executed on the path after it has been constructed (but before it is returned to the application).

When two or more potential certificates are discovered that may lead to finding a path that meets the specified constraints, the implementation uses the following criteria to prioritize the certificates (in the examples below, assume a `TrustAnchor` distinguished name of "ou=D,ou=C,o=B,c=A" is specified):

1. The issuer DN of the certificate matches the DN of one of the specified `TrustAnchor`s (ex: issuerDN = "ou=D,ou=C,o=B,c=A").
2. The issuer DN of the certificate is a descendant of the DN of one of the `TrustAnchor`s, ordered by proximity to the anchor (ex: issuerDN = "ou=E,ou=D,ou=C,o=B,c=A").
3. The issuer DN of the certificate is an ancestor of the DN of one of the `TrustAnchor`s, ordered by proximity to the anchor (ex: issuerDN = "ou=C,o=B,c=A".
4. The issuer DN of the certificate is in the same namespace of one of the `TrustAnchor`s, ordered by proximity to the anchor (ex: issuerDN = "ou=G,ou=C,o=B,c=A").
5. The issuer DN of the certificate is an ancestor of the subject DN of the certificate, ordered by proximity to the subject.

These are followed by certificates which don't meet any of the above criteria.

This implementation has been tested with the LDAP and Collection `CertStore` implementations included in this release of the "SUN" provider.

Debugging support can be enabled by setting the `java.security.debug` property to `certpath`. For example:

    	java -Djava.security.debug=certpath BuildCertPath

This will print additional debugging information to standard error.

### CertStore

The "SUN" provider supports two implementations of the `CertStore` engine class: Collection and LDAP.

> ### Collection CertStore

> The Collection `CertStore` implementation can hold any objects that are an instance of `java.security.cert.Certificate` or `java.security.cert.CRL`.

> The certificates and CRLs are not returned in any particular order and will not contain duplicates.

> ### LDAP CertStore

> The LDAP `CertStore` implementation retrieves certificates and CRLs from an LDAP directory using the LDAP schema defined in RFC 2587. The LDAPSchema service attribute is set to "RFC2587".

> The implementation fetches certificates from different locations, depending on the values of the subject, issuer, and basicConstraints selection criteria specified in the `X509CertSelector`. It performs as many of the following operations as possible:

> 1. Subject non-null, basicConstraints <= -1

>    Looks for certificates in the subject DN's "userCertificate" attribute.

> 2. Subject non-null, basicConstraints >= -1

>    Looks for certificates in the forward element of the subject DN's "crossCertificatePair" attribute AND in the subject's "caCertificate" attribute.

> 3. Issuer non-null, basicConstraints >= -1

>    Looks for certificates in the reverse element of the issuer DN's "crossCertificatePair" attribute AND in the issuer DN's "caCertificate" attribute.

> In each case, certificates are checked using `X509CertSelector.match()` before adding them to the resulting collection.

> If none of the conditions specified above applies, then an exception is thrown to indicate that it was impossible to fetch certificates using the criteria supplied. Note that even if one or more of the conditions apply, the Collection returned may still be empty if there are no certificates in the directory.

> The implementation fetches CRLs from the issuer DNs specified in the `setCertificateChecking`, `addIssuerName` or `setIssuerNames` methods of the `X509CRLSelector` class. If no issuer DNs have been specified using one of these methods, the implementation throws an exception indicating it was impossible to fetch CRLs using the criteria supplied. Otherwise, the CRLs are searched as follows:

> The implementation first creates a list of issuer names. If a certificate was specified in the `setCertificateChecking` method, it uses the issuer of that certificate. Otherwise, it uses the issuer names specified using the `addIssuerName` or `setIssuerNames` methods.

> Next, the implementation iterates through the list of issuer names. For each issuer name, it searches first in the issuer's "authorityRevocationList" attribute and then, if no matching CRL was found there, in the issuer's "certificateRevocationList" attribute. One exception to the above is that if the issuer name was obtained from the certificate specified in the `setCertificateChecking` method, it only checks the issuer's "authorityRevocationList" attribute if the specified certificate is a CA certificate.

> All CRLs are checked using `X509CRLSelector.match()` before adding them to the resulting collection.

> If no CRLs satisfying the selection criteria can be found, an empty Collection is returned.

> > #### Caching

> > By default each LDAP CertStore instance caches lookups for a maximum of 30 seconds. The cache lifetime can be changed by setting the system property `sun.security.certpath.ldap.cache.lifetime` to a value in seconds. A value of `0` disables the cache completely. A value of `-1` means unlimited lifetime.

### Support for the CRL Distribution Points Extension

Limited support for the CRL Distribution Points extension is available. It is disabled by default for compatibility and can be enabled by setting the system property `com.sun.security.enableCRLDP` to the value `true`.

If active CRLs will be retrieved for the locations specified in the extension, provided the following three criteria are all met:

1. the distribution point specifies a fullName of type URI.
2. The URI is not of type LDAP.
3. TRLIssuer and reasons are not set.

This allows interoperability with many simple certification infrastructures as in use on the Internet today. We plan to remove the above limitations in a future release.

> Note: Depending on your network and firewall setup, it may be necessary to also configure your networking proxy servers as described in the networking documentation.

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