🚀 go-pugleaf

In article <1992Jul31.153719.28374@ncsu.edu> Webbge@che17.ncsu.EDU (Greg Earl Webb) writes:
>
>I've seen some interesting posts here recently, but I have yet to
>see the topic of padding data with garbage.
>[...]
>			Greg Webb

I've recently had an idea along these lines, though no doubt someone's
thought of it before...

The goal is to make the first several blocks of plaintext be (almost)
totally random.  With an error propogating cypher, this may help to
thwart a guessed plaintext attack.  Useful for encrypting something
with fixed headers.

First, you need a cryptographically strong random number generator.
It spits out a sequence of bytes many of which will eventually be
prepended to your message.  You also decide (at random, but within
given parameters) how many bytes you wish prepended.

Next, define a property that you can check each byte for.  A simple
property might be:  Is on this list of N byte values.  A more complex
property could change based on the set of bytes output so far.

Next, you say that your padding will be over when Q bytes satisfy the
above property, where Q is a small constant like 5.

Now, start generating bytes.  If a byte doesn't have the property,
just spit it out.  If it does, and the previous Q-1 bytes also had the
property, and you're not done with your padding yet, discard the byte.
When you're done with the padding, just reject bytes that don't have
the property until you have spit out Q that do.

Questions:  what would be good values for Q and N (persumably based on
the cypher block size and the desired (approximate) number of random
blocks)?  I would guess that you would want N chosen so that the
probablity of seeing Q bytes in a row with the property within your
padding is roughly .5.

Is it worth doing this?  Would it be just as secure to just say that
you'll always prepend 4 blocks of random pad to the plaintext before
encyphering?

-- eric messick

eric@parallax.com  uunet!parallax!eric



-- 
-- eric messick

eric@parallax.com  uunet!parallax!eric