C#

public static string Decrypt(string encrypted)
       {
           byte[] encryptedbytes = Convert.FromBase64String(encrypted);
           AesCryptoServiceProvider aes = new AesCryptoServiceProvider();
           aes.BlockSize = 128;
           aes.KeySize = 256;
           aes.Key = System.Text.ASCIIEncoding.ASCII.GetBytes(Key);
           aes.IV = System.Text.ASCIIEncoding.ASCII.GetBytes(IV);
           aes.Padding = PaddingMode.PKCS7;
           aes.Mode = CipherMode.CBC;
           ICryptoTransform crypto = aes.CreateDecryptor(aes.Key, aes.IV);
           byte[] secret = crypto.TransformFinalBlock(encryptedbytes, 0, encryptedbytes.Length);

           crypto.Dispose();
           return System.Text.ASCIIEncoding.ASCII.GetString(secret);
       }

The below code i found in some websites but it always return error 

multicast_id":6719893729607364541,"success":0,"failure":1,"canonical_ids":0,"results":[{"error":"InvalidRegistration"}]}

Code is:

		WebRequest tRequest = WebRequest.Create("https://fcm.googleapis.com/fcm/send");
        tRequest.Method = "post";
        //serverKey - Key from Firebase cloud messaging server  
        tRequest.Headers.Add(string.Format("Authorization: key={0}", "Server key"));
        //Sender Id - From firebase project setting  
        tRequest.Headers.Add(string.Format("Sender: id={0}", "Sender Id"));
        tRequest.ContentType = "application/json";
        var payload = new
        {
            to = "//topics//news", // even "/topics/news:
			
            priority = "high",
            content_available = true,
            notification = new
            {
                body = "Test",
                title = "Test",
                badge = 1
            },
            data = new 
            {
                key1 = "value1",
                key2 = "value2"
            }

        };

        string postbody = JsonConvert.SerializeObject(payload).ToString();
        Byte[] byteArray = Encoding.UTF8.GetBytes(postbody);
        tRequest.ContentLength = byteArray.Length;
        using (Stream dataStream = tRequest.GetRequestStream())
        {
            dataStream.Write(byteArray, 0, byteArray.Length);
            using (WebResponse tResponse = tRequest.GetResponse())
            {
                using (Stream dataStreamResponse = tResponse.GetResponseStream())
                {
                    if (dataStreamResponse != null) using (StreamReader tReader = new StreamReader(dataStreamResponse))
                        {
                            String sResponseFromServer = tReader.ReadToEnd();
                            //result.Response = sResponseFromServer;
							this.Label.Text = sResponseFromServer;
                        }
                }
            }