Certification Path Introduction

Users of public key applications and systems must be confident that a subject's public key is genuine, for example, 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). The term CA is used for the remainder of this section to refer to an entity that signs a certificate.

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 that is 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
The certification path starts with CA1's Public Key. The next object in the chain is CA2, which has its own Public Key, a Subject of CA2 and an Issuer of CA1. The last object in the chain is Alice, who has her own Public Key, a Subject of Alice and an Issuer of CA2.

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contained in the certification path, such as verifying the signatures and checking that each certificate has not been revoked.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 Java™ Certification Path API provides a set of classes and interfaces to Java 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.