Re: Decimation

Greg Rose wrote:
In article <4b30b227@xxxxxxxxxxxxxxxxx>,
Cristiano <cristiano.pi@xxxxxxxxxx> wrote:
Joseph Ashwood wrote:
[...] Although it does increase the difficulty, it does not
change an insecure PRNG to a cryptographically secure PRNG.

If you decimate the output of a LFSR (which is "an insecure PRNG")
you get a cryptographically secure PRNG (self-shrinking LFSR).

No you don't. There are attacks against the SSG.

There are attacks against many ciphers, but it doesn't mean that they are
not cryptographically secure.
Here:
http://en.wikipedia.org/wiki/Self-shrinking_generator#Cryptanalysis
I read that there is an attack against the SSG which requires 2^(0.7*L)
steps. If you take, say, L=256 or longer, the time needed to break that SSG
will be very big. I would call that SSG cryptographically secure PRNG.

Cristiano


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