[rc5] Re: One Time Pad

Phil Reed pcr at ic.net
Mon Oct 27 19:35:51 EST 1997


>Now, I've thought of a good approximation to a true OTP system:  
>Use an arbitrarily long key (however many bits you want), and 
>use that to generate a unique irrational number, the digits of 
>which become your random key stream. That would be a 
>virutually unbreakable system (statistical methods would 
>not work), with a suitably long key. I really don't know if it's 
>even feasible, but hey, it's just a thought.

You've just described a stream cypher. The problem is that the 
irrational number isn't really random. In fact, it's 100% predictable.

The art of developing stream cyphers is deep and complex. It's 
VERY HARD to create a stream cypher that doesn't have some
pattern to it to the "random" component.

Look at "Applied Cryptography" (2nd Ed), section 9.4 (starts on
page 197).

                                           ...phil
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