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Date: Fri, 26 Jun 2015 10:33:15 +1000
Subject: Re: Pure Python Data Mangling or Encrypting
From: Chris Angelico <rosuav@gmail.com>
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On Fri, Jun 26, 2015 at 1:26 AM, Jon Ribbens
<jon+usenet@unequivocal.co.uk> wrote:
>> There are only 256 possible values for n, one of which doesn't transform the
>> data at all (ROT-0). If you're thinking of attacking this by pencil and
>> paper, 255 transformations sounds like a lot. For a computer, that's barely
>> harder than a single transformation.
>
> Well, it means you need to send 256 times as much data, which is a
> start. If you're instead using a 256-byte translation table then
> an attack becomes utterly impractical.
>

Utterly impractical? Maybe, if you attempt a pure brute-force approach
- there are 256! possible translation tables, which is roughly e500
attempts [1], and at roughly four a microsecond [2] that'd still take
a ridiculously long time. But there are two gigantic optimizations you
could do. Firstly, there are frequency-based attacks, and byte value
duplicates will tell you a lot - classic cryptographic work. And
secondly, you can simply take the first few bytes of a file - let's
say 16, although a lot of files can be recognized in less than that.
Even if there are no duplicate bytes, that'd be a maximum of 16!
translation tables that truly matter, or just 2e13. At the same speed,
that makes about a million seconds of computing time required. Divide
that across a bunch of separate computers (the job is embarrassingly
parallel after all), and you could get that result pretty easily. Cut
the prefix to just 8 bytes and you have a mere 40K encryption keys to
try - so quick that you wouldn't even see it happen. Nope, a simple
substitution cipher is still not secure. Even the famous Enigma
machine was a lot more than just letter-for-letter substitution - a
double letter in the cleartext wouldn't be represented by a double
letter in the result - and once the machine's secrets were figured
out, the day's key could be reassembled fairly readily.

ChrisA

[1] It's actually closer to 8.6e506, if you care.
[2] timeit result from my laptop - you could do better, but that's a
reasonable average