The EVP library provides a high-level interface to cryptographic
EVP_Seal... and EVP_Open... provide public key encryption
and decryption to implement digital envelopes.
The EVP_Sign... and EVP_Verify... functions implement
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
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.