Re: (newbie) question on modification of polyalphabetic substitution cipher

vedaal_at_hush.com
Date: 03/31/04 

Date: 30 Mar 2004 14:17:40 -0800

"Gary Shannon" <gary@fiziwig.com> wrote in message news:<56923ad3505051182cc3cec4ece4f915@news.teranews.com>...
> <vedaal@hush.com> wrote in message
> news:52a1833e.0403290551.1d6cfae6@posting.google.com...
> > Joe Peschel <jpeschel@no.spam.org> wrote in message
> news:<Xns94BB930DDE73fa0khgj7ji8i8jo9@216.168.3.44>...
> > > vedaal@hush.com (vedaal@hush.com) wrote in
> > > news:52a1833e.0403280826.6e0f2efe@posting.google.com:
>
> > > Then you would have a plaintext autokey cipher.
> >
> > yes,
> > but somewhat different than the ones i've read about,
> > in that there is no repetition of the key anywhere, no matter how long
> > the plaintext
>
> Not so. Try any function you wish. As long as you take it mod 256 the
> function will have a repeating period of at most 256, possibly after some
> aperiodic part of less than 256 steps in length. Most of the time the
> period will much shorter than 256.

ok,

Thanks for answering and pointing this out,

would it be possible to prevent the periodicity, by modifying it as
follows:

instead of f(n)mod 256,

consider |[Z-f(n)]|mod 256) ,
where Z is the integer obtained by truncating pi
between 'l + n' and 'm + n' places,

then mod 256 of the absolute value of Z-f(n)

the correspondents agree to the initial values of the constants, 'l'
and 'm',
and include directions within the plaintext of how to change them for
the next message (as well as change the other constants of f(n) )

would this modification work?

TIA,

vedaal