[cryptography] New Hand Cipher - The Drunken Bishop

andrew cooke andrew at acooke.org
Thu Dec 26 14:37:00 EST 2013


On Thu, Dec 26, 2013 at 04:30:31PM -0300, Andrew Cooke wrote:
> On Thu, Dec 26, 2013 at 04:28:06PM -0300, Andrew Cooke wrote:
> > 
> > the way that you use the plaintext to avoid short cycles (the "output number"
> > etc) is worrying - it might open you up to a chosen plaintext attack in some
> > way.
> 
> replying to myself, sorry (but at least not top-posting this time) i would at
> least add counter in there.  so you add the plaintext value plus some counter.
> 
> or use the counter to rotate (or otherwise permute) the bits.  everything is
> horribly linear at the moment.

sorry, last comment.  a counter isn't enough because the plaintext could
simply count backwards.

the problem is that the only thing that is avoid short cycles is the input
text itself.  and that makes you very vulnerable to chosen plaintext. 

it's very hard to see how to change this wihout adding more state.  perhaps
there are two pieces moving, which interact in some way?  and the rules for
the other piece use the plaintext in a different way, so that it cannot work
against you for both pieces...

andrew

> 
> andrew
> 
>  
> > and thinking about chosen plaintexts - if you encode a message that is all
> > zeroes, what does that reveal?  it seems like it might leak information about
> > the board you are using.
> > 
> > these aren't attacks, but they are obvious places where i (with, admittedly,
> > very little experience of attacking ciphers) would start.
> > 
> > andrew
> > 
> > 
> > 
> > On Thu, Dec 26, 2013 at 12:05:24PM -0700, Aaron Toponce wrote:
> > > I created a new hand cipher over the past few weeks, and announced it on my
> > > blog yesterday. I'm curious what people on this list think.
> > > 
> > >     https://pthree.org/2013/12/25/the-drunken-bishop-cipher/
> > > 
> > > The idea comes from taking an 8x8 chessboard, and assigning the values
> > > 0-63 randomly and uniquely to each square on the board. This gives a total
> > > keyspace of 64!. The bishop takes a "random walk" around the board, based
> > > on the square value he's landed on. Details are in the post.
> > > 
> > > The idea comes from wanting a hand cipher that wasn't a strenuous as the
> > > Solitaire Cipher by Bruce Schneier, and takes the idea of the random walk
> > > from the SSH key ASCII art.
> > > 
> > > The algorithm is a base-64 pseudorandom number generator which is applied
> > > to the plain text, and added modulo 64. It's an output mode stream cipher.
> > > 
> > > I haven't done any cryptanalysis on the cipher yet. Lowest hanging fruit
> > > seems to be frequency analysis with output biases and generating a "heat
> > > map" of each square the bishop has landed on for a given key and plaintext
> > > to discover any internal biases.
> > > 
> > > I'm not a professional cryptographer, just an amateur hobbyist. I think I
> > > have a lot of my ducks in a row, but I would be interested in greater
> > > feedback on how I can improve the cipher, and to discover any weaknesses.
> > > 
> > > If this list isn't appropriate for this sort of thing, my apologies. Please
> > > point me the right direction to discuss this.
> > > 
> > > Thanks,
> > > 
> > > -- 
> > > . o .   o . o   . . o   o . .   . o .
> > > . . o   . o o   o . o   . o o   . . o
> > > o o o   . o .   . o o   o o .   o o o
> > 
> > 
> > 
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> > 
> 


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