The Server Certificate Validation Protocol (SCVP) provides a mechanism to request a certificate chain from a server, which can eliminate these requirements. This requires that the validator keep a database of intermediate certificates or that the protocol using the certificate supply the needed intermediates. If not, the authority certificate is treated as a target certificate, and Step 1 is called recursively until it returns a chain to a trusted certificate or fails.Ĭonstructing the complete certificate path requires that the validator is in possession of all the certificates in that path. If the signature matches and the authority certificate is a trusted certificate, the constructed chain is then subjected to Steps 2–4. If the signature check fails, the validation process can be stopped, and the target certificate deemed invalid. (Note that, because of potentially revoked intermediate certificates, multiple chains may need to be constructed and examined through Steps 2 and 3 to find the actual valid chain.) Once the proper authority certificate is found, the validator checks the signature on the target certificate using the public key in the authority certificate. If multiple certificates still match, the most recently issued candidate certificate can be used. If multiple certificates match, the validator can search the matching certificates for a Subject Key Identifier extension that matches the Issuer Key Identifier extension in the candidate certificates. This is done by searching the intermediate certificates and certificate store for a certificate with a subject field that matches the issuer field of the target certificate.
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To do so, the certificate for the authority that signed the target certificate must be located. The contents of the target certificate cannot be trusted until the signature on the certificate is validated, so the first step is to check the signature. Validation Step 1: Construct the Chain and Validate Signatures IETF RFC 3280 6 presents a complete specification for certificate validation, and RFC 4158 7 presents a specification for constructing a certification path in environments where nonhierarchical certification structures are used. In practice, the introduction of bridge CAs and other nonhierarchical certification models have led to more complex validation procedures. The following steps are a simplified outline of how certificates are typically validated. The first is the certificate to be validated, the second is any intermediate certificates acquired by the applications, and the third is a store containing the root and intermediate certificates trusted by the application. The certificate validation process typically proceeds in three steps and typically takes three inputs. It is not a complete description and is purposely simplified. However, all these interfaces offer a variety of options, and understanding the validation process is essential to properly using these interfaces.Ī complete specification of the certificate validation process would require hundreds of pages, so here we supply just a sketch of what happens during certificate validation.
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The Server-based Certificate Validity Protocol (SCVP) can also be used to validate a certificate.
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Microsoft CryptoAPI, OpenSSL, and the Java JCE all provide certificate validation interfaces. It is rare for an application to implement certificate validation, since there are several APIs and libraries available to perform this task. Various applications will often require different validation techniques, depending on the application’s security policy. This section outlines a typical set of steps involved in validating a certificate, but it is not an exhaustive catalog of the possible methods that can be used. X.509 certificate validation is a complex process and can be done to several levels of confidence. Terence Spies, in Computer and Information Security Handbook, 2009 5.