Error in encryption in C #

Hello!  I already tried to parse the code but I can not find the answer.

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

/// <summary>
/// Summary description for StringEncritacao
/// </summary>
namespace Seguranca
{
    public static  class StringEncritacao
    {
        public static string  Encritacao(string sourceData)
        {
            //define a chave e inicializa o valor do vetor
            byte[] key = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 };
            byte[] iv = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 };
            try
            {
                //converte o dado para array
                byte[] sourceDataBytes = System.Text.ASCIIEncoding.ASCII.GetBytes(sourceData);
                //obter fluxo de memoria
                MemoryStream tempStream = new MemoryStream();
                //apanha o codificador e o fluxo de codificao
                DESCryptoServiceProvider encryptor = new DESCryptoServiceProvider();
                CryptoStream encryptionStream = new CryptoStream(tempStream, encryptor.CreateDecryptor(key, iv),
                CryptoStreamMode.Write);
            //dado de encriptacao
            encryptionStream.Write(sourceDataBytes, 0,sourceDataBytes.Length);
            encryptionStream.FlushFinalBlock();

            //poe o byte no array
            byte[] encryptedDataBytes = tempStream.GetBuffer();

            //converte o dado de encriptacao para string
            return Convert.ToBase64String(encryptedDataBytes,0,(int)tempStream.Length);
        }
        catch 
        {
            throw new StringEncritacaoExcepion("Incapaz de encriptrar dados");
        }

        //
        // TODO: Add constructor logic here
        //

    }
    public static string Decriptacao(string sourceData) 
    {
        //define a chave inicializacao  valores vecto
        byte[] key = new byte[] { 1,2,3,4,5,6,7,8 };
        byte[] iv = new byte []{ 1,2,3,4,5,6,7,8 };
        try
        {
            //convert o dado para array de byte
            byte[] encryptedDataBytes = Convert.FromBase64String(sourceData);
            //apanha o codigo do fluxo memoria e enche 
            MemoryStream tempStream = new MemoryStream(encryptedDataBytes, 0, encryptedDataBytes.Length);

            //apanha o decriptador e decriptar o fluxo
            DESCryptoServiceProvider decryptor = new DESCryptoServiceProvider();
            CryptoStream decryptionStream = new CryptoStream(tempStream, decryptor.CreateDecryptor(key, iv), CryptoStreamMode.Read);

            //desicriptar
            StreamReader allDataReader = new StreamReader(decryptionStream);
            return allDataReader.ReadToEnd();
        }
        catch
        {
            throw new StringEncritacaoExcepion("Impossivel desencriptar dados.");
        }

    }
}

When I run this error comes

  

Server Error in '/' Application.

     

Length of the data to decrypt is invalid.

     

Description: An unhandled exception occurred during the execution of   the current web request. Please review the stack trace for more   information about the error and where it originated in the code.

     

Exception Details:   System.Security.Cryptography.CryptographicException: Length of the   data to decrypt is invalid.

     

Source Error:

     

Line 142: // writes encrypted data to memorystream   Line 143: _cryptoStream.Write (plainByte, 0,   plainByte.Length); Line 144:
  _cryptoStream.FlushFinalBlock (); Line 145: // search for the encrypted data size Line 146: byte [] cryptoByte =   _memoryStream.ToArray ();