namespace CSEncryptDecrypt
{
    class Class1
    {
        //  Call this function to remove the key from memory after use for security
        [System.Runtime.InteropServices.DllImport("KERNEL32.DLL", EntryPoint = "RtlZeroMemory")]
        public static extern bool ZeroMemory(IntPtr Destination, int Length);

        // Function to Generate a 64 bits Key.
        static DESCryptoServiceProvider GenerateKey()
        {
            // Create an instance of Symetric Algorithm. Key and IV is generated automatically.
            DESCryptoServiceProvider desCrypto = (DESCryptoServiceProvider)DESCryptoServiceProvider.Create();

            // Use the Automatically generated key for Encryption.
            return desCrypto;
        }

        static void EncryptFile(string sInputFilename,
           string sOutputFilename,
          DESCryptoServiceProvider sKey)
        {
            FileStream fsInput = new FileStream(sInputFilename,
               FileMode.Open,
               FileAccess.Read);

            FileStream fsEncrypted = new FileStream(sOutputFilename,
               FileMode.Create,
               FileAccess.Write);
            DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
            DES.Key = sKey.Key;
            DES.IV = sKey.IV;
            ICryptoTransform desencrypt = DES.CreateEncryptor();
            CryptoStream cryptostream = new CryptoStream(fsEncrypted,
               desencrypt,
               CryptoStreamMode.Write);

            byte[] bytearrayinput = new byte[fsInput.Length];
            fsInput.Read(bytearrayinput, 0, bytearrayinput.Length);
            cryptostream.Write(bytearrayinput, 0, bytearrayinput.Length);
            cryptostream.Close();
            fsInput.Close();
            fsEncrypted.Close();
        }

        static void DecryptFile(string sInputFilename,
           string sOutputFilename,
           DESCryptoServiceProvider sKey)
        {
            DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
            //A 64 bit key and IV is required for this provider.
            //Set secret key For DES algorithm.
            DES.Key = sKey.Key;
            //Set initialization vector.
            DES.IV = sKey.IV;

            //Create a file stream to read the encrypted file back.
            FileStream fsread = new FileStream(sInputFilename,
               FileMode.Open,
               FileAccess.Read);
            //Create a DES decryptor from the DES instance.
            ICryptoTransform desdecrypt = DES.CreateDecryptor();
            //Create crypto stream set to read and do a
            //DES decryption transform on incoming bytes.
            CryptoStream cryptostreamDecr = new CryptoStream(fsread,
               desdecrypt,
               CryptoStreamMode.Read);
            //Print the contents of the decrypted file.
            FileStream fsDecrypted = new FileStream(sOutputFilename, FileMode.Create);
            byte[] bytearrayinput = new byte[1024];
            int length;
            do
            {
                length = cryptostreamDecr.Read(bytearrayinput, 0, 1024);
                fsDecrypted.Write(bytearrayinput, 0, length);
            } while (length == 1024);
            fsDecrypted.Flush();
            fsDecrypted.Close();
        }

        static void Main()
        {
            // Must be 64 bits, 8 bytes.
            // Distribute this key to the user who will decrypt this file.
            DESCryptoServiceProvider sSecretKey;

            // Get the Key for the file to Encrypt.
            sSecretKey = GenerateKey();

            // For additional security Pin the key.
            GCHandle gch = GCHandle.Alloc(sSecretKey.Key, GCHandleType.Pinned);

            // Encrypt the file.
            EncryptFile(@"d:\MyData.txt",
               @"d:\Encrypted.txt",
               sSecretKey);

            // Decrypt the file.
            DecryptFile(@"d:\Encrypted.txt",
               @"d:\Decrypted.txt",
               sSecretKey);

            gch.Free();
        }
    }
}