The key size warning

Dmitri dmitri at
Sat Mar 30 06:14:02 CET 2002

On Fri, 2002-03-29 at 19:59, V Alex Brennen wrote:

> 2048bits is good, but be careful about just bumping up keysize.
> There is much more to it than just the CPU time the math takes.
> Is there enough entropy available on the average PC to support the 
> generation of strong default 2048bit keys?  Yes, most likely. 
> 4096bits? Yes, most likely. 8192bits? Probably. But, at what
> keysize does that become usually not the case?

Entropy is not in short supply :-) On key sizes that you mention,
probably the user will need to wiggle the mouse couple of times. For
megabit-sized keys, the user will need to play a game or two in Unreal
Tournament. For gigabit-sized keys he will need a local source of
randomness, something like a PCI card with a white noise generator and
the necessary A-D converter. The RF noise from the sky is quite random
as well - a stream of samples from Seti at Home will do just fine :-)

A popular idea, since "Johnny Mnemonic" the movie, is to use broadcast
TV as source of randomness. This is especially useful because the MPEG-2
compressed stream is very random (since this is the point of
compression). Anyone with HDTV receiver already has *gigabits* of
randomness, just strip the MPEG framing, since it is regular.

> I recall reading predictions of 4GB (~32Gb) keys to ensure some 
> strength in the presence of quantum computers.  I'm unsure that 
> the average PC could generate strong 4GB keys.

Probably not. How would you publish them? :-)


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