// var key = SHA256EncryptString(KEY);
var key = System.Text.Encoding.UTF8.GetBytes(KEY);
var hmac = new HMACSHA256(key);
var expiry = DateTimeOffset.UtcNow.ToUnixTimeSeconds() + EXPIRATION;
var stringToSign = $"{url}?exp={expiry}";
// var stringToSign = url;
var mac = hmac.ComputeHash(Encoding.UTF8.GetBytes(stringToSign));
var sig = BitConverter.ToString(mac);
sig = sig.Replace("-", "").ToLower();
return stringToSign + "&sig=" + sig;
}