lightweight encryption of text file

[Daniel Fetchinson]

>>>>> I have a plain text file which I would like to protect in a very
>>>>> simple minded, yet for my purposes sufficient, way. I'd like to
>>>>> encrypt/convert it into a binary file in such a way that possession of
>>>>> a password allows anyone to convert it back into the original text
>>>>> file while not possessing the password one would only see the
>>>>> following with the standard linux utility 'file':
>>>>>
>>>>> [fetchinson at fetch ~]$ file encrypted.data
>>>>> encrypted.data: data
>>>>>
>>>>> and the effort required to convert the file back to the original text
>>>>> file without the password would be equivalent to guessing the
>>>>> password.
>>>>>
>>>>> I'm fully aware of the security implications of this loose
>>>>> specification, but for my purposes this would be a good solution.
>>>>>
>>>>> What would be the simplest way to achieve this using preferably stock
>>>>> python without 3rd party modules? If a not too complex 3rd party
>>>>> module made it really simple that would be acceptable too.
>>>>
>>>> Paul Rubin's p3.py algorithm is probably the most straightforward way to
>>>> meet
>>>> these requirements. It's not a standard crypto algorithm by any means,
>>>> but
>>>> Paul
>>>> knows his stuff and has devised it with these deployment restrictions in
>>>> mind.
>>>>
>>>>     http://www.nightsong.com/phr/crypto/p3.py
>>>
>>> Thanks a lot, currently I'm having trouble using this code on python
>>> 2.6 but probably some small tweaking will fix it.
>>
>> Actually, it also doesn't work with python 2.5 and currently I don't
>> have access to anything older. array.array raises a
>>
>> ValueError: string length not a multiple of item size
>>
>> Does anyone recall a change to array.array?
>>
>> The full traceback is
>>
>> Traceback (most recent call last):
>>    File "p3.py", line 163, in<module>
>>      _test()
>>    File "p3.py", line 143, in _test
>>      c1 = e(plain,key)
>>    File "p3.py", line 69, in p3_encrypt
>>      xkey = _expand_key(k_enc, n+4)
>>    File "p3.py", line 41, in _expand_key
>>      return array ('L', j)
>> ValueError: string length not a multiple of item size
>
> Are you on a 64-bit platform? Unfortunately, array's integer typecodes are
> platform-specific, but p3.py requires a 32-bit integer and was written on a
> 32-bit platform. It's reasonably straightforward to fix. Put this bit of
> (untested) code at the top of the file and replace occurrences of 'L' with
> uint32:
>
> # Find the typecode of a 32-bit unsigned integer.
> for typecode in 'IL':
>      if len(array(typecode, [0]).tostring()) == 4:
>          uint32 = typecode
>          break
> else:
>      raise RuntimeError("Neither 'I' nor 'L' are unsigned 32-bit integers.")

Thanks!
That was exactly the problem, I'm on a 64 bit machine and your fix
seems to work indeed.

Cheers,
Daniel


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