[Tutor] Pack as HEX question

[Tom Green]

Thanks for your reply Dave.  I am capturing the data off the network
(wireshark) and saving it in WinHex for testing.  I am actually building a
socket to decrypt the data, so prior to implementing my logic in the socket,
I figured I would write the algorithm in a quick script.  Here is what I
have so far and it works.

import struct,binascii
decrypt=[]
data_count=0
key_pos=0
#packed data consists of HEX values
packed_data = binascii.unhexlify('313B372C2E2C63362E2128')
#XorKey data consists of HEX values
packed_XorKey=binascii.unhexlify('41424344')
while data_count < len(packed_data):
    if key_pos >len(packed_XorKey)-1:
        key_pos=0

decrypt.append(chr(ord(packed_data[data_count])^ord(packed_XorKey[key_pos])))
    key_pos+=1
    data_count+=1
print "".join(decrypt)

This decrypts to Python rock

This logic seems to work, but I am certain there is a better way.

Mike


On Sat, Oct 24, 2009 at 4:43 PM, Dave Angel <davea at ieee.org> wrote:

> Tom Green wrote:
>
>> Alan,
>>
>> Thanks for your response and hopefully I can clear things up.  I apologize
>> for not being more clear.
>>
>> I obtain the HEX encoded data from Winhex i.e. copy Hex values.  The HEX
>> encode data is very large and I simply paste it into my Python script
>> along
>> with the XOR key.  The data is a string of bytes represented in HEX, as I
>> showed.
>>
>> Here is the problem I ran into.
>>
>> Take the 4 byte XOR key.  If I convert them to int with Base 16 it takes
>> the
>> 4 and converts it to 0x34 when I in turn I actually need 0x41.
>>
>> Thanks for your feedback.
>>
>> Mike
>>
>> On Sat, Oct 24, 2009 at 10:24 AM, Alan Gauld <alan.gauld at btinternet.com
>> >wrote:
>>
>> <snip>
>>
>>>
>>>  Encrypted string in hex
>>>
>>>
>>>> "313B372C2E2C63362E2128"
>>>>
>>>> Four byte key
>>>> XOR key "41424344"
>>>>
>>>>
>>>>
>>> <snip>
>>>
>> If by Winhex, you mean the hex editor, then you're undoubtedly going about
> it the long way.  There's no need to convert the file to printable hex, you
> should be able to work on it directly.  Just open the file, read it into a
> string (in python2.x), then loop through it, one byte at a time.  Store your
> key in a binary string as well.  Now just loop through the two in pairs (use
> zip, and cycle) doing a chr of xor of ords.
>
>
> And when you respond, give us your python version, and show us the code
> you've got (almost) working.
>
> DaveA
>
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