EVP_DecryptInit_ex(&session, EVP_bf_cbc(), NIL, _key->observe(),
init_vector().observe());
EVP_CIPHER_CTX_set_key_length(&session, _key_size);
EVP_DecryptInit_ex(&session, EVP_bf_cbc(), NIL, _key->observe(),
init_vector().observe());
EVP_CIPHER_CTX_set_key_length(&session, _key_size);