File Method

The X509_LOOKUP_file() method loads all the certificates or CRLs present in a file into memory at the time the file is added as a lookup source. The file format is ASCII text which contains concatenated PEM certificates and CRLs. This method should be used by applications which work with a small set of CAs.

Hashed Directory Method

X509_LOOKUP_hash_dir is a more advanced method which loads certificates and CRLs on demand, and caches them in memory once they are loaded. As of OpenSSL 1.0.0, it also checks for newer CRLs upon each lookup, so that newer CRLs are used as soon as they appear in the directory. The directory should contain one certificate or CRL per file in PEM format, with a filename of the form hash.N for a certificate, or hash.rN for a CRL. The hash is the value returned by the X509_NAME_hash(3) function applied to the subject name for certificates or issuer name for CRLs. The hash can also be obtained via the -hash option of the openssl(1) x509 or crl commands. The N suffix is a sequence number that starts at zero and is incremented consecutively for each certificate or CRL with the same hash value. Gaps in the sequence numbers are not supported. It is assumed that there are no more objects with the same hash beyond the first missing number in the sequence. Sequence numbers make it possible for the directory to contain multiple certificates with the same subject name hash value. For example, it is possible to have in the store several certificates with the same subject or several CRLs with the same issuer (and, for example, a different validity period). When checking for new CRLs, once one CRL for a given hash value is loaded, hash_dir lookup method checks only for certificates with sequence number greater than that of the already cached CRL. Note that the hash algorithm used for subject name hashing changed in OpenSSL 1.0.0, and all certificate stores have to be rehashed when moving from OpenSSL 0.9.8 to 1.0.0.

Memory Method

The X509_LOOKUP_mem() method supports loading PEM-encoded certificates and revocation lists that are already stored in memory, using the function X509_LOOKUP_add_mem(3). This is particularly useful in processes using chroot(2).