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How good is security via hashing

Nobody nobody at nowhere.com
Tue Jun 7 17:23:05 EDT 2011 
On Tue, 07 Jun 2011 13:27:59 +0100, Robin Becker wrote:

>> If you want the full 16 bytes of unpredictability, why don't you just
>> read 16 bytes from
>> /dev/urandom and forget about all the other stuff?
>
> I have a vague memory that the original author felt that entropy might
> run out or something like that so reading from /dev/urandom always was
> not a good idea.

The problem with /dev/urandom is that it shares the same entropy pool as
/dev/random, so you're "stealing" entropy which may be needed for tasks
which really need it (e.g. generating SSL/TLS keys).

Personally, I'd take whatever "cheap" entropy I can get and hash it.
If you're going to read from /dev/urandom, limit it to a few bytes per
minute, not per request.
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