The EVP cipher routines are a high level interface to certain symmetric ciphers.
EVP_CIPHER_CTX_init() initializes cipher contex ctx.
EVP_EncryptInit_ex() sets up cipher context ctx for encryption with cipher type from ENGINE impl. ctx must be initialized before calling this function. type is normally supplied by a function such as EVP_des_cbc(). If impl is NULL then the default implementation is used. key is the symmetric key to use and iv is the IV to use (if necessary), the actual number of bytes used for the key and IV depends on the cipher. It is possible to set all parameters to NULL except type in an initial call and supply the remaining parameters in subsequent calls, all of which have type set to NULL. This is done when the default cipher parameters are not appropriate.
EVP_EncryptUpdate() encrypts inl bytes from the buffer in and writes the encrypted version to out. This function can be called multiple times to encrypt successive blocks of data. The amount of data written depends on the block alignment of the encrypted data: as a result the amount of data written may be anything from zero bytes to (inl + cipher_block_size - 1) so outl should contain sufficient room. The actual number of bytes written is placed in outl.
If padding is enabled (the default) then EVP_EncryptFinal_ex() encrypts the "final" data, that is any data that remains in a partial block. It uses standard block padding (aka PKCS padding). The encrypted final data is written to out which should have sufficient space for one cipher block. The number of bytes written is placed in outl. After this function is called the encryption operation is finished and no further calls to EVP_EncryptUpdate() should be made.
If padding is disabled then EVP_EncryptFinal_ex() will not encrypt any more data and it will return an error if any data remains in a partial block: that is if the total data length is not a multiple of the block size.
EVP_DecryptInit_ex(), EVP_DecryptUpdate() and EVP_DecryptFinal_ex() are the corresponding decryption operations. EVP_DecryptFinal() will return an error code if padding is enabled and the final block is not correctly formatted. The parameters and restrictions are identical to the encryption operations except that if padding is enabled the decrypted data buffer out passed to EVP_DecryptUpdate() should have sufficient room for ( inl + cipher_block_size) bytes unless the cipher block size is 1 in which case inl bytes is sufficient.
EVP_CipherInit_ex(), EVP_CipherUpdate() and EVP_CipherFinal_ex() are functions that can be used for decryption or encryption. The operation performed depends on the value of the enc parameter. It should be set to 1 for encryption, 0 for decryption and -1 to leave the value unchanged (the actual value of 'enc' being supplied in a previous call).
EVP_CIPHER_CTX_cleanup() clears all information from a cipher context and free up any allocated memory associate with it. It should be called after all operations using a cipher are complete so sensitive information does not remain in memory.
EVP_EncryptInit(), EVP_DecryptInit() and EVP_CipherInit() behave in a similar way to EVP_EncryptInit_ex(), EVP_DecryptInit_ex and EVP_CipherInit_ex() except the ctx paramter does not need to be initialized and they always use the default cipher implementation.
EVP_EncryptFinal(), EVP_DecryptFinal() and EVP_CipherFinal() behave in a similar way to EVP_EncryptFinal_ex(), EVP_DecryptFinal_ex() and EVP_CipherFinal_ex() except ctx is automatically cleaned up after the call.
EVP_get_cipherbyname(), EVP_get_cipherbynid() and EVP_get_cipherbyobj() return an EVP_CIPHER structure when passed a cipher name, a NID or an ASN1_OBJECT structure.
EVP_CIPHER_nid() and EVP_CIPHER_CTX_nid() return the NID of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX structure. The actual NID value is an internal value which may not have a corresponding OBJECT IDENTIFIER.
EVP_CIPHER_CTX_set_padding() enables or disables padding. By default encryption operations are padded using standard block padding and the padding is checked and removed when decrypting. If the pad parameter is zero then no padding is performed, the total amount of data encrypted or decrypted must then be a multiple of the block size or an error will occur.
EVP_CIPHER_key_length() and EVP_CIPHER_CTX_key_length() return the key length of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX structure. The constant EVP_MAX_KEY_LENGTH is the maximum key length for all ciphers. Note: although EVP_CIPHER_key_length() is fixed for a given cipher, the value of EVP_CIPHER_CTX_key_length() may be different for variable key length ciphers.
EVP_CIPHER_CTX_set_key_length() sets the key length of the cipher ctx. If the cipher is a fixed length cipher then attempting to set the key length to any value other than the fixed value is an error.
EVP_CIPHER_iv_length() and EVP_CIPHER_CTX_iv_length() return the IV length of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX. It will return zero if the cipher does not use an IV. The constant EVP_MAX_IV_LENGTH is the maximum IV length for all ciphers.
EVP_CIPHER_block_size() and EVP_CIPHER_CTX_block_size() return the block size of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX structure. The constant EVP_MAX_IV_LENGTH is also the maximum block length for all ciphers.
EVP_CIPHER_type() and EVP_CIPHER_CTX_type() return the type of the passed cipher or context. This "type" is the actual NID of the cipher OBJECT IDENTIFIER as such it ignores the cipher parameters and 40 bit RC2 and 128 bit RC2 have the same NID. If the cipher does not have an object identifier or does not have ASN1 support this function will return NID_undef.
EVP_CIPHER_CTX_cipher() returns the EVP_CIPHER structure when passed an EVP_CIPHER_CTX structure.
EVP_CIPHER_mode() and EVP_CIPHER_CTX_mode() return the block cipher mode: EVP_CIPH_ECB_MODE, EVP_CIPH_CBC_MODE, EVP_CIPH_CFB_MODE or EVP_CIPH_OFB_MODE. If the cipher is a stream cipher then EVP_CIPH_STREAM_CIPHER is returned.
EVP_CIPHER_param_to_asn1() sets the AlgorithmIdentifier "parameter" based on the passed cipher. This will typically include any parameters and an IV. The cipher IV (if any) must be set when this call is made. This call should be made before the cipher is actually "used" (before any EVP_EncryptUpdate(), EVP_DecryptUpdate() calls for example). This function may fail if the cipher does not have any ASN1 support.
EVP_CIPHER_asn1_to_param() sets the cipher parameters based on an ASN1 AlgorithmIdentifier "parameter". The precise effect depends on the cipher In the case of RC2, for example, it will set the IV and effective key length. This function should be called after the base cipher type is set but before the key is set. For example EVP_CipherInit() will be called with the IV and key set to NULL, EVP_CIPHER_asn1_to_param() will be called and finally EVP_CipherInit() again with all parameters except the key set to NULL. It is possible for this function to fail if the cipher does not have any ASN1 support or the parameters cannot be set (for example the RC2 effective key length is not supported.
EVP_CIPHER_CTX_ctrl() allows various cipher specific parameters to be determined and set. Currently only the RC2 effective key length and the number of rounds of RC5 can be set.