Certificate Chain and Certification Path Validation

The certificate path validation is an algorithm that seeks to verify the binding between the public key and the subject of a certificate, using trust anchor information. The complete processing is described in RFC 5280 section 6.1, and as stated there, it verifies among other things that a prospective certification path (a sequence of n certificates) satisfies the following conditions:

1. for all x in {1, …​, n-1}, the subject of certificate x is the issuer of certificate x+1;

2. certificate 1 is issued by the trust anchor;

3. certificate n is the certificate to be validated (i.e., the target certificate); and

4. for all x in {1, …​, n}, the certificate was valid at the time in question.

Although RFC 5280 states that procedures performed to obtain the sequence of certificate that is provided to the certification path validation is outside its scope, Common PKI v2.0 part 5 section 2.1 ([R24]) provides one such possible procedure.

An intuitive approach to build a prospective certification path is to start by looking at the “Authority Information Access” (AIA) extension of the target certificate (see RFC 5280 section 4.2.2.1) which, if present, frequently includes information on how to retrieve the certificate of the issuer of that certificate. Repeating this action on the certificate retrieved can then allow to build a prospective certification path.

The wording "certificate chain" is often used interchangeably with "certification path".

In ETSI EN 319 102-1 ([R09]) however, a prospective certificate chain is defined as a sequence of certificate that satisfies the conditions a. to c. above and for which the trust anchor is trusted according the validation policy in use.

An illustration of different certificate chains/certification paths is provided in the figure below.