[cryptography] New Hand Cipher - The Drunken Bishop

Peter Maxwell peter at allicient.co.uk
Thu Dec 26 19:25:47 EST 2013

On 26 December 2013 19:56, Aaron Toponce <aaron.toponce at gmail.com> wrote:

> On Thu, Dec 26, 2013 at 02:53:06PM -0500, Jeffrey Walton wrote:
> > On Thu, Dec 26, 2013 at 2:44 PM, Aaron Toponce <aaron.toponce at gmail.com>
> wrote:
> > BBS is not practical in practice due to the size of the moduli
> > required. You could probably go outside, take an atmospheric reading,
> > and then run it through sha1 quicker. See, for example,
> >
> http://crypto.stackexchange.com/questions/3454/blum-blum-shub-vs-aes-ctr-or-other-csprngs
> .
> Understood. BBS was only an example of some way to modify the algorithm to
> introduce non-linearity into the system. I thought I had it, but it's
> apparent I don't. I'm just grateful I'm not getting shamed and flamed by
> cryptographers on this list much stronger in the field than I. :)
​Ok, I've only skim-read the blog page that describes the algorithm but on
a cursory reading it seems trivially weak/breakable.

If you view the moving-the-bishop as an s-box lookup, and apply it to
itself three times (composition), you end up with another s-box of the same
size, lets call it S.  Given S doesn't change, things should be rather easy
indeed.  If your cipher is then roughly akin to C[n] = P[n] + S[ C[n-1] ]
with all operations taken modulo 2^6 the problem should now be a little
more obvious.

​While I very much like the idea of using a standard chessboard to run a
cipher​ - it's innocuous and the key could be hidden almost in plain-sight
- the actual cipher isn't much use, at least not if I've got the gist of
it.  If I've misunderstood the description, please correct me (preferably
in a more terse description).

​Can I suggest doing some preliminary reading on group theory and
finite-field maths, and also paying more attention ​to how existing strong
steam ciphers are constructed.  One of the reasons Solitaire is useful is
because you can mathematically prove certain properties about the cipher
operation; also you'll note the entire internal state of Solitaire changes,
while your design stays static.
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