HMAC
HMAC(EVP_sha1(), cookie_secret, COOKIE_SECRET_LENGTH,
if (HMAC(EVP_sha1(), cookie_secret, COOKIE_SECRET_LENGTH,
if (HMAC(digest, salt, salt_len, secret, secret_len, out_key, &len) ==
unsigned char *HMAC(const EVP_MD *evp_md, const void *key, int key_len,