RSADigest.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Security.Cryptography;
using System.IO;


namespace ConsoleTester
{
    public class BitConverter
    {
        static public string EncodeTo64(string toEncode)
        {
            byte[] toEncodeAsBytes
                  = System.Text.ASCIIEncoding.ASCII.GetBytes(toEncode);
            string returnValue
                  = System.Convert.ToBase64String(toEncodeAsBytes);
            return returnValue;
        }
        static public string DecodeFrom64(string encodedData)
        {
            byte[] encodedDataAsBytes
                = System.Convert.FromBase64String(encodedData);
            string returnValue =
               System.Text.ASCIIEncoding.ASCII.GetString(encodedDataAsBytes);
            return returnValue;
        }

        static public byte[] GetBytes(string s)
        { 
            return Convert.FromBase64String(EncodeTo64(s));
        }

        static public string GetString(byte[] b)
        {
            return DecodeFrom64(Convert.ToBase64String(b));
        }
    }

    class Program
    {
        
        static void Main(string[] args)
        {
            string data = "ccccssssss/.'{}#$%^&*";


            byte[] binaryData = BitConverter.GetBytes(data);

            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            rsa.FromXmlString(File.ReadAllText("key"));   //key created by rsa.ToXmlString(true) and key.pub created by rsa.rsa.ToXmlString(false);
            

            var binaryDigest = rsa.SignData(binaryData, new SHA1CryptoServiceProvider());
            string digest = Convert.ToBase64String(binaryDigest);   //digest use base64 string to get fix length of digest

            string stringPassInInternet = digest + data;
            ReceiverVerifyData(stringPassInInternet);

            Console.ReadKey();
        }

        static void ReceiverVerifyData(string passStr)
        {
            //receiver knows other people can't sign the data, because other people do not have the private key(only trusted people has privatek key)

            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            rsa.FromXmlString(File.ReadAllText("key.pub"));

            string digest = passStr.Substring(0, 172);
            string data = passStr.Substring(172);

            var bResult = rsa.VerifyData(BitConverter.GetBytes(data), new SHA1CryptoServiceProvider(), Convert.FromBase64String(digest));
            if (bResult)
            {
                Console.WriteLine("send by trusted/verify person");    
            }
            else
            {
                Console.WriteLine("send by untrusted/unverify person");    
            }
        }
    }
}