Python byte per byte XOR decryption
- by neurino
I have an XOR encypted file by a VB.net program using this function to scramble:
Public Class Crypter
...
'This Will convert String to bytes, then call the other function.
Public Function Crypt(ByVal Data As String) As String
Return Encoding.Default.GetString(Crypt(Encoding.Default.GetBytes(Data)))
End Function
'This calls XorCrypt giving Key converted to bytes
Public Function Crypt(ByVal Data() As Byte) As Byte()
Return XorCrypt(Data, Encoding.Default.GetBytes(Me.Key))
End Function
'Xor Encryption.
Private Function XorCrypt(ByVal Data() As Byte, ByVal Key() As Byte) As Byte()
Dim i As Integer
If Key.Length <> 0 Then
For i = 0 To Data.Length - 1
Data(i) = Data(i) Xor Key(i Mod Key.Length)
Next
End If
Return Data
End Function
End Class
and saved this way:
Dim Crypter As New Cryptic(Key)
'open destination file
Dim objWriter As New StreamWriter(fileName)
'write crypted content
objWriter.Write(Crypter.Crypt(data))
Now I have to reopen the file with Python but I have troubles getting single bytes, this is the XOR function in python:
def crypto(self, data):
'crypto(self, data) -> str'
return ''.join(chr((ord(x) ^ ord(y)) % 256) \
for (x, y) in izip(data.decode('utf-8'), cycle(self.key))
I had to add the % 256 since sometimes x is 256 i.e. not a single byte.
This thing of two bytes being passed does not break the decryption because the key keeps "paired" with the following data.
The problem is some decrypted character in the conversion is wrong.
These chars are all accented letters like à, è, ì but just a few of the overall accented letters. The others are all correctly restored.
I guess it could be due to the 256 mod but without it I of course get a chr exception...
Thanks for your support
