Re: Which paper and pencil cipher to use ?

From: Mike South (miso@someplace.net)
Date: 01/20/03 

From: "Mike South" <miso@someplace.net>
Date: Mon, 20 Jan 2003 10:32:07 -0600

"Roger Fleming" <roger_for_nntp@hotmail.com> wrote in message
news:fdbae11.0301192314.13b29f61@posting.google.com...
> Paul Rubin <phr-n2002b@NOSPAMnightsong.com> wrote in message
news:<7x4r8edscj.fsf@ruckus.brouhaha.com>...
> > Nomen Nescio <nobody@dizum.com> writes:
> > > Can someone please recommend a reasonably secure paper and pencil
cipher ?.
> > > The cipher is to be used in an area where electronic methods are
not
> > > feasible.
> > > Messages using this cipher should be secure for a period of about
3 to 6
> > > months maximum.
> > > If possible no tables are to be used - perhaps something similar
to
> > > Playfair for example, but a bit more secure - perhaps 4 square ?.
> > > Thanks for all suggestions.
> >
> > This should go in the sci.crypt faq. [...]
>
> I agree. A bit of Googling shows nearly a thousand messages on the
> topic in the last ten years. Many of them rehash the same ideas over
> and over again. Which is exactly the sort of thing to put in the FAQ.
> But I guess we first need a consensus on what exactly to put in it.
> Can we expand this thread to cover that question?
> I would start by suggesting:
> 1. historical comments on double transposition being at the limit of
> practicality, and notes on its practical use from "Between Silk and
> Cyanide".
> 2. the relative speed, security, and error rates of RC4 (over a
> reduced word size), Solitaire, double transposition, and VIC.
> 3. an analysis of the decimal Fibonacci generator used within VIC.
> 4. brief summary of a few of the suggestions made over the years (to
> prevent hearing them over again), perhaps categorised according to how
> much equipment they require.
> 5. By Shannon's estimate that normal English text has about 1.6 bits
> of entropy per letter, and that lg 26 = 4.700, enciphering a text with
> three successive, different running keys would be theroretically
> unbreakable if the running key texts were taken from the space of all
> possible texts. In practice the opponent might divide-and-conquer by
> trying all published texts for the first key, and cryptanalysing the
> remaining two. It would, however, still be extremely strong; but is
> probably well beyond practical work factors for encryption.

I agree too, let's put this in the FAQ.

1. I'll stick with double transposition, which can be broken with
enough messages by multiple anagramming. For few messages, it
is fairly secure provided the lengths of the messages are of
different lengths -- I suspect the lengths should be relatively
prime, but I've never tried to prove that.

There are people with amazing anagramming skills, and the double
transposition even with a few messages may fall to these people
if the key is not long enough. A guy named Joseph Courville wrote
a paper on solving double transpositions, which may be available
yet.

2. the book cipher used as page, line, and word indicators,
not as a running key. I think this is rather difficult to use
because another encryption must be used for words not in the
book, and practically speaking, a concordance is needed.

Also, the user is supposed to select words at random, which is
a tall order. Most of us cannot do that without mechanical
help. Maybe the concordance can arrange the indexes to a word
randomly.

I'm still skeptical. I suspect there is NO pencil and paper
cipher that is both practical and secure for the specified
period of several months. 

--
Mike

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