using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Threading.Tasks;

​

namespace ConsoleApp1

{

    class Program

    {

        public static void Main(string[] args)

        {

            Console.WriteLine(TamperProofStringDecode("NjE5OQ==-rgrcZzTcPNo="));          

        }

​

        public static string TamperProofStringDecode(string value)

        {

            //errorMessage = "";

​

            if (value != null)

            {

                string key = "a1";

                string expectedEncryptedHashValue = string.Empty;

                string dataValue = "";

​

                value = value.Replace(" ", "+");

                System.Security.Cryptography.MACTripleDES mac3des = new System.Security.Cryptography.MACTripleDES();

                System.Security.Cryptography.MD5CryptoServiceProvider md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();

                mac3des.Key = md5.ComputeHash(System.Text.Encoding.UTF8.GetBytes(key));

                try

                {

                    string[] encryptedValues = value.Split('-');

​

                    if (encryptedValues.Length < 2)

                    {

                        //errorMessage = "The provided value doesn't contain Hash value.";

                       // WriteLog("\tTamperProofStringDecode\tThe provided value doesn't contain Hash value.\t" + value, LogType.Unknown);

                        dataValue = string.Empty;

                    }

                    else

                    {

​

                        dataValue = System.Text.Encoding.UTF8.GetString(Convert.FromBase64String(encryptedValues[0]));

​

                        Regex regex = new Regex(@"[^a-zA-Z0-9'{}[\]\\;':"",\./?!@#$%&*()_+=-`~\^\-|<> " + Convert.ToChar(124) + "]");

                        if (regex.Match(dataValue).Length > 0)

                        {

                           // errorMessage = "The decoded value contains some junk characters";

                            //WriteLog("\tTamperProofStringDecode\tThe decoded value contains some junk characters.\t" + value, LogType.Unknown);

                            dataValue = string.Empty;

                        }

                        else

                        {

                            expectedEncryptedHashValue = Convert.ToBase64String(mac3des.ComputeHash(System.Text.Encoding.UTF8.GetBytes(dataValue)));

                            if (encryptedValues[1].IndexOf(expectedEncryptedHashValue) != 0)

                            {

                             //   errorMessage = "Hash values do not match";

                               //WriteLog("\tTamperProofStringDecode\tHash values do not match\t" + value, LogType.Unknown);

                                dataValue = string.Empty;

                            }

                        }

                    }

​

                }

                catch (Exception ex)

                {

                  //  errorMessage = "Unexpected error : " + ex.Message;

                    //WriteLog("\tTamperProofStringDecode\t" + ex.Message + "\t" + value, LogType.Unknown);

                    dataValue = string.Empty;

                }

                //-----End of Addition by Mehar on 13-11-2013 for Elmah Issue - Invalid Base64 String

​

                return dataValue;

            }

            return string.Empty;

        }

​

    }

}

​