SSL - SSLv2/v3/v23 etc.

BIO - methods and how they plug together

MEM - memory allocation callback

CRYPTO - locking for threads

EVP - Ciphers/Digests/signatures

RSA - methods

X509 - certificate retrieval

X509 - validation

X509 - X509v3 extensions

Objects - adding object identifiers

ASN.1 - parsing

PEM - parsing