An X.509 certificate is a structured grouping of information about an individual, a device, or anything one can imagine. An X.509 CRL (certificate revocation list) is a tool to help determine if a certificate is still valid. The exact definition of those can be found in the X.509 document from ITU-T, or in RFC3280 from PKIX. In OpenSSL, the type X509 is used to express such a certificate, and the type X509_CRL is used to express a CRL.
A related structure is a certificate request, defined in PKCS#10 from RSA Security, Inc, also reflected in RFC2896. In OpenSSL, the type X509_REQ is used to express such a certificate request.
To handle some complex parts of a certificate, there are the types X509_NAME (to express a certificate name), X509_ATTRIBUTE (to express a certificate attribute), X509_EXTENSION (to express a certificate extension) and a few more.
Finally, there's the supertype X509_INFO, which can contain a CRL, a certificate and a corresponding private key.
X509_XXX, d2i_X509_XXX, and i2d_X509_XXX functions handle X.509 certificates, with some exceptions, shown below.
X509_CRL_XXX, d2i_X509_CRL_XXX, and i2d_X509_CRL_XXX functions handle X.509 CRLs.
X509_REQ_XXX, d2i_X509_REQ_XXX, and i2d_X509_REQ_XXX functions handle PKCS#10 certificate requests.
X509_NAME_XXX functions handle certificate names.
X509_ATTRIBUTE_XXX functions handle certificate attributes.
X509_EXTENSION_XXX functions handle certificate extensions.
X509_NAME_ENTRY_get_object(3), X509_NAME_add_entry_by_txt(3), X509_NAME_add_entry_by_NID(3), X509_NAME_print_ex(3), X509_NAME_new(3), PEM_X509_INFO_read(3), d2i_X509(3), d2i_X509_ALGOR(3), d2i_X509_CRL(3), d2i_X509_NAME(3), d2i_X509_REQ(3), d2i_X509_SIG(3), crypto(7)
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