EVP_CIPHER_CTX *ctc = EVP_CIPHER_CTX_new();
EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, (const unsigned char)"not a real iv");
...
EVP_DecryptUpdate(ctx, (unsigned char*)payload, &len, (unsigned char*)payload, len);
EVP_DecryptFinal_ex(ctx, (unsigned char*)payload + len, &len);