[Tutor] Pack as HEX question

[Tom Green]

Sweet nice tip I love this list.  Thank you.

Mike

On Sat, Oct 24, 2009 at 7:40 PM, Dave Angel <davea at ieee.org> wrote:

> Tom Green wrote:
>
>> 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
>>>
>>>
>>>
>>
>>
>>
> (You top-posted, so your message is out of order)
>
> Replace your while -- loop with the following simpler version.
>
>
> for byte0, byte1 in itertools.izip(packed_data,
> itertools.cycle(packed_XorKey)):
>   decrypt.append(chr(ord(byte0) ^ ord(byte1)))
>
> (You'll need an import itertools, at beginning, of course.)
>
> itertools.cycle() repeats the pattern of the keys.  itertools.izip()
> combines two iterables into one.  Then you just loop through that one in the
> usual way, where byte0 comes from the packed_data, and byte1 comes from the
> XorKey.
>
> DaveA
>
>
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