Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        System.IO.File.Encrypt(TextBox1.Text)
    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        System.IO.File.Decrypt(TextBox1.Text)
    End Sub
End Class

System.IO.IOException was unhandled
  Message="The request is not supported. "

Private DES As New TripleDESCryptoServiceProvider
Private MD5 As New MD5CryptoServiceProvider

Public Function MD5Hash(ByVal value As String) As Byte()
    Return MD5.ComputeHash(ASCIIEncoding.ASCII.GetBytes(value))
End Function

Public Function Encrypt(ByVal stringToEncrypt As String, ByVal key As String) As String
    DES.Key = MD5Hash(key)
    DES.Mode = CipherMode.ECB
    Dim Buffer As Byte() = ASCIIEncoding.ASCII.GetBytes(stringToEncrypt)
    Return Convert.ToBase64String(DES.CreateEncryptor().TransformFinalBlock(Buffer, 0, Buffer.Length))
End Function

Public Function Decrypt(ByVal encryptedString As String, ByVal key As String) As String
    Try  
    DES.Key = MD5Hash(key)
    DES.Mode = CipherMode.ECB
    Dim Buffer As Byte() = Convert.FromBase64String(encryptedString)
    Return ASCIIEncoding.ASCII.GetString(DES.CreateDecryptor().TransformFinalBlock(Buffer, 0, Buffer.Length))
   Catch ex As Exception
         MessageBox.Show("Wrong Key Number, decryption not available!")
   End Try 
End Function