The EVP library provides a high-level interface to cryptographic functions.
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.
The EVP_PKEY... functions provide a high level interface to asymmetric algorithms.
Algorithms are loaded with OpenSSL_add_all_algorithms(3).
All the symmetric algorithms (ciphers), digests and asymmetric algorithms (public key algorithms) 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.
Although low level algorithm specific functions exist for many algorithms their use is discouraged. They cannot be used with an ENGINE and ENGINE versions of new algorithms cannot be accessed using the low level functions. Also makes code harder to adapt to new algorithms and some options are not cleanly supported at the low level and some operations are more efficient using the high level interface.