Certificate chain verification is the process of making sure a given certificate chain is well-formed, valid, properly signed, and trustworthy. Directory Server software uses the following steps to form and verify a certificate chain, starting with the certificate being presented for authentication:
The certificate validity period is checked against the current time provided by the verifier’s system clock.
The issuer’s certificate is located. The source can be either the verifier’s local certificate database (on that client or server) or the certificate chain provided by the subject (for example, over an SSL connection).
The certificate signature is verified using the public key in the issuer certificate.
If the issuer’s certificate is trusted by the verifier in the verifier’s certificate database, verification stops successfully here. Otherwise, the issuer’s certificate is checked to make sure it contains the appropriate subordinate CA indication in the Directory Server certificate type extension, and chain verification returns to step 1 to start again, but with this new certificate.
Figure 2–5 shows what happens when only Root CA is included in the verifier’s local database. If a certificate for one of the intermediate CAs shown in Figure 2–6, such as Engineering CA, is found in the verifier’s local database, verification stops with that certificate, as shown in the following figure.
Expired validity dates, an invalid signature, or the absence of a certificate for the issuing CA at any point in the certificate chain causes authentication to fail. For example, the following figure shows how verification fails if neither the Root CA certificate nor any of the intermediate CA certificates are included in the verifier’s local database.
For general information about the way digital signatures work, see Digital Signatures.