# Re: Decimation

• From: "Joseph Ashwood" <ashwood@xxxxxxx>
• Date: Tue, 22 Dec 2009 00:05:22 -0800

"Mok-Kong Shen" <mok-kong.shen@xxxxxxxxxxx> wrote in message news:hgo74e\$kdq\$00\$1@xxxxxxxxxxxxxxxxxxxx

Decimation in crypto means selecting every tenth, and in general, selecting every nth element from a sequence in order to hide
exploitable patterns in it. (See http://www.ciphersbyritter.com/GLOSSARY.HTM#Decimation).

A tiny generalization in my humble view would be choosing elements from
a sequence with a probability p by a statistically good PRNG. Suppose
one chooses p=0.1, pseudo-randomly pick a starting point and "decimate"
thus the digit sequence of Pi, is there any conceivable yet practical
way that an anylist could succeed to do prediction in such cases?

Absolutely. Although it does increase the difficulty, it does not change an insecure PRNG to a cryptographically secure PRNG.
Joe

.

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