EVP_Seal... and EVP_Open... provide public key encryption and decryption to implement digital ``envelopes''.
The EVP_Sign... and EVP_Verify... functions implement digital signatures.
Symmetric encryption is available with the EVP_Encrypt... functions. The EVP_Digest... functions provide message digests.
Algorithms are loaded with OpenSSL_add_all_algorithms(3).
All the symmetric algorithms (ciphers) and digests can be replaced by ENGINE modules providing alternative implementations. If ENGINE implementations of ciphers or digests are registered as defaults, then the various EVP functions will automatically use those implementations automatically in preference to built in software implementations. For more information, consult the engine(3) man page.