var time = DateTimeOffset.UtcNow.ToUnixTimeSeconds() * 1000;

static string HashString(string input)
        {
            using (SHA1Managed sha1 = new SHA1Managed())
            {
                var hash = sha1.ComputeHash(Encoding.UTF8.GetBytes(input));
                var sb = new StringBuilder(hash.Length * 2);

                foreach (byte b in hash)
                {
                    sb.Append(b.ToString("x2"));
                }

                return sb.ToString();
            }
        }

var Signatur = HashString("GET https://api.spreadshirt.net/api/v1/users/115417255/designs " + time + " " + SecurityRessources.getSecretKey()))