using System;

using System.Linq;

using System.Text;

using System.Security.Cryptography;

public class Program

{

    public static void Main()

    {

        // Use transaction reference as secret key

        String client_secret = Convert.ToBase64String(Encoding.UTF8.GetBytes("149499755346"));

        /* 

        {

           "amount":1000,

           "reference":"149499755346",

           "phoneNumber":"254722000000",

           "telco":"MPesa",

           "sourceAccount":"0020100002618",

           "paymentReason":"pesa",

           "AmountCurrency":"KES",

           "signature":"2UjbkOIBTlL9XJI9RMW40uBIP2CevTOXvQptI0hu60o="

        }

        */ 

        // sourceAccount&amount2decimalplace&telco&phonenumber

        String input = "0020100002618&1000.00&MPesa&254722000000";

        var rs = GenerateHMACSignature(input, client_secret);

        Console.WriteLine(rs);

    }

    public static string GenerateHMACSignature(string input, string secret)

    {

        var secretKeyByteArray = Encoding.UTF8.GetBytes(secret);

        byte[] signature = Encoding.UTF8.GetBytes(input);

​

        using (HMACSHA256 hmac = new HMACSHA256(secretKeyByteArray))

        {

            byte[] signatureBytes = hmac.ComputeHash(signature);

​

            return Convert.ToBase64String(signatureBytes);

        }

    }

}