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DESCRIPTION

EVP_SealInit(3) and EVP_OpenInit(3) provide public key encryption and decryption to implement digital "envelopes".

The EVP_DigestSignInit(3) and EVP_DigestVerifyInit(3) functions implement digital signatures and Message Authentication Codes (MACs). Also see the older EVP_SignInit(3) and EVP_VerifyInit(3) functions.

Symmetric encryption is available with the EVP_EncryptInit(3) functions. The EVP_DigestInit(3) functions provide message digests.

Authenticated encryption with additional data (AEAD) is available with the EVP_AEAD_CTX_init(3) functions.

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 in 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_DigestSignInit(3) 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_SealInit(3) and EVP_OpenInit(3) 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 typically not use this (preferring, for example, PBKDF2 from PCKS#5).

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; see ENGINE_register_RSA(3) and the related manual pages for more information. If ENGINE implementations of ciphers or digests are registered as defaults, then the various EVP functions will automatically use those implementations in preference to built in software implementations.

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. Using them also makes code harder to adapt to new algorithms, some options are not cleanly supported at the low level, and some operations are more efficient using the high level interfaces.