Issue Porting XOR Encryption Code

[mattn]

Hi,

I recently started using Basic4Android and have been trying to port a simple vb.net XOR encryption program. I have thoroughly read the topic about VB to Basic4Android however it only covers basic stuff.

The VB.net code i'm trying to port is:

 Dim ch As Char
        Dim strCode As String
        Dim Code, Number As Short
        Dim Decrypt As String = ""

        OpenFileDialog1.Filter = "MN SecuPad Files (*.mnsec)|*.mnsec"
        OpenFileDialog1.ShowDialog()
        If OpenFileDialog1.FileName <> "" Then
            Try
                strCode = InputBox("Encryption Key:")
                If strCode = "" Then Exit Sub
                Code = CShort(strCode)
                FileOpen(1, OpenFileDialog1.FileName, OpenMode.Input)
                Do Until EOF(1)
                    Input(1, Number)
                    ch = Chr(Number Xor Code)
                    Decrypt = Decrypt & ch
                Loop
                RichTextBox1.Text = Decrypt
                RichTextBox1.Select(1, 0)
                '                MenuItem4.Enabled = False
            Catch ex As Exception
                MsgBox("Incorrect Encryption Key. Access Denied!!")
            Finally
                FileClose(1)
            End Try
        End If

Currently i've got this as the Basic4Android code:

  Dim encfile As InputDialog
   Dim fd As FileDialog
   Dim readable As String
   Dim inputcode1 As String
   Dim inputc1 As Short
         
   fd.FastScroll = True
   fd.FilePath = File.DirInternal
   fd.FileFilter = ".mnsec" 

   ret = fd.Show("Select A File:", "OK", "Cancel", "", Null)   
   If ret = -3 OR fd.ChosenName = "" Then 
      Return
   End If
   If File.Exists(File.DirInternal, fd.ChosenName) = False Then
      Msgbox(fd.ChosenName & " does not exist.", "")
      Return
   End If
   Dim TR As TextReader
   Dim s As String
   Dim ch As Char
   Dim Number As Short
   Dim Decrypt As String
   Dim c As Cipher
   
   TR.Initialize(File.OpenInput(File.DirInternal, fd.ChosenName))
      s = TR.ReadLine
      'add decryption and display file contents code here
      encfile.Show("Enter Encryption Code","Code","OK","Canel","Negative",Null)
      inputcode1 = encfile.Input
      
      Dim Code As Short
            inputc1 = encfile.Input
            Code = inputc1
         'Code = inputcode1
            File.OpenInput(File.DirInternal,fd.ChosenName)
            'Do Until s.Length
         'Number = s
         Decrypt = inputc1 + Code
         'Loop
         Msgbox("Incorrect Encryption Key. Access Denied!!","ACCESS DENIED!!!!")
         'EditText1.Text = s
            EditText1.Text = Decrypt
         Msgbox(Decrypt,"Test")
         'EditText1.Select(1, 0)
            'MenuItem4.Enabled = False
      'asndkajsndjkas
   TR.Close

It will load the file but it won't decrypt it.

Any help would be great and very much appreciated.

 

[Erel]

How do you ensure that this line returns a valid chr code?

Chr(Number Xor Code)

For example if the result will be 0 then the string will be truncated.

Note that you should use the & operator to concatenate strings.

 

[mattn]

That code you mentioned is VB.NET code that use XOR which is a VB.NET function. I'm trying to port the VB.NET code to Basic4Android. The problem is that I have no idea how to convert:

Code = CShort(strCode)

Chr(Number Xor Code)

Do Until EOF(1)

to Basic4Android code.

EOF is end of file in VB.NET

 

[Erel]

Yes, I understood it. However as I understand it this solution might fail in some cases. It is better to convert the string to bytes and work with bytes, not characters.

Code = CShort(strCode) 
=>
Dim Code As Short
Code = strCode

Chr(Number Xor Code)
=>
Chr(Bit.Xor(Number, Code))

Do Until EOF(1)
=>
s = File.ReadString(...)
For i = 0 To s.Length - 1
 Dim ch as Character
 ch = s.CharAt(i)
 ...
Next

 

[mattn]

Hi Erel,

Thanks for being so helpful. I get what you're saying about it not always working, its just that this porting has been frustrating me for a while. I need your help with one more thing (hopefully the last thing that needs porting):

I need to convert the data stored in the value 's' to short so that I can be compared in the XOR function.

Currently:

s = TR.ReadLine

and

Dim Number As Short

...
'Number = s

This is currently commented until I can get the data contained by 's' converted to Short.

 

[Erel]

Your (commented) code is correct.

 

[mattn]

Below is a link to a screenshot of the problem (why my commented code won't work). Basically the issue is I need to convert the string (including the spaces) to short then i'll be able to make Number = s. I'm not sure how to convert 's' from string to short.

http://i90.photobucket.com/albums/k259/matthewnirenberg/Problem.jpg

 

[Erel]

"15 30 8" is not a valid number. You should use Regex.Split to split the string.

 

[mattn]

Not too sure how to implement Regex.Split as the numbers are mathematically modified character addresses, hence they can vary in length depending on the numerical code entered by the user.

 

[Erel]

Is space always the separator character? If yes then you can do it easily with:

Dim numbers() As Int
numbers = Regex.Split(" ", YourString)
For i = 0 To numbers.Length - 1
   ...
Next