Quor's cipher

[Antonomasia]

Antonomasia <ant at notatla.demon.co.uk> wrote:

> My attack takes a long chunk of known text and looks for repetition.
> 
> ppppppppppppppp.11.pppppppppppppppppppppp
> ccccccccccccccc.22.cccccccccccccccccccccc
> 
> When a two neighbouring p-c pairs are the same you can test
> whether they have the same value of a and b.
> (That is a_n == a_n+1 and b_n == b+n+1,   a != b usually.)
> 
> This involves 16 inputs to each byte - very cheap.
> What I really want next is to know "a".


nobody at REPLAY.COM wrote:

> Wouldn't this only happen (on average) in one out of every 65536 p-c
> pairs?

Yes (counting only those we test).

>         Since the state array is changed entirely with every 128 bytes
> encrypted, 1 out of 2^16 doesn't seem to help much.

This finding doesn't uncover a great deal, I agree, and what it does
uncover is transient.

1 out of 2^16 is the number of pairs we test to find a point
with the property we're looking for.  The measures to compare
this to are:
          message length                 several times 65k is fairly low
          cost of testing                low
          benefit of test when passed    also low  :-(

The changes are regular though, progressing through the array in two
places.  That's why I proposed testing neighbouring p-c pairs because
you can be almost sure the relevant state (and it's which part of the
state array is relevant that's uncertain) is the same for the two.  In
fact it would be nice to spot this:

ppppppppppppppp.11.ppppppppppppppppppppp
ccccccccccccccc.22.ccccccccccccccccccccc   

and see that a and b were the same, but the underlying
state array changed in a relevant way.  This would be indicated by
a slightly less than total match, but still rare enough not to be
a fluke.  This could allow you to test possible values of a and b,
and perhaps get 4 bits of a.  But a and b don't persist over nearby
bytes so I'm not clear what I'd do if I got them.

What it would be nice to have next is a more damaging result emerge
from this or some other observation so that we could determine more
of the state without going to long range comparisons.  This cypher
is constructed differently from RC4 or fish - not as simple as WAKE
but broadly comparable to Sapphire II.   I think I'll enlist the help
of a search engine.  It's more than likely there other things waiting
to be done to this.  Quor's friend 'nobody' has actually put some crypto
on the list -let's see what we can do with/to it.


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# Antonomasia   ant at notatla.demon.co.uk                      #
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