DESCRIPTIONThe 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_DigestSign... and EVP_DigestVerify... functions implement digital signatures and Message Authentication Codes (MACs). Also see the older EVP_Sign... and EVP_Verify... functions.
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. To create a new EVP_PKEY see EVP_PKEY_new(3). EVP_PKEYs can be associated with a private key of a particular algorithm by using the functions described on the EVP_PKEY_set1_RSA(3) page, or new keys can be generated using EVP_PKEY_keygen(3). EVP_PKEYs can be compared using EVP_PKEY_cmp(3), or printed using EVP_PKEY_print_private(3).
The EVP_PKEY functions support the full range of asymmetric algorithm operations:
- For key agreement see EVP_PKEY_derive(3)
- For signing and verifying see EVP_PKEY_sign(3), EVP_PKEY_verify(3) and EVP_PKEY_verify_recover(3). However, note that these functions do not perform a digest of the data to be signed. Therefore normally you would use the EVP_DigestSign... functions for this purpose.
- For encryption and decryption see EVP_PKEY_encrypt(3) and EVP_PKEY_decrypt(3) respectively. However, note that these functions perform encryption and decryption only. As public key encryption is an expensive operation, normally you would wrap an encrypted message in a "digital envelope" using the EVP_Seal... and EVP_Open... functions.
The EVP_BytesToKey(3) function provides some limited support for password based encryption. Careful selection of the parameters will provide a PKCS#5 PBKDF1 compatible implementation. However, new applications should not typically use this (preferring, for example, PBKDF2 from PCKS#5).
The EVP_Encode... and EVP_Decode... functions implement base 64 encoding and decoding.
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.