Re: /dev/random broken?

Valdis.Kletnieks
2003-06-29T02:22:00Z

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On Sat, 28 Jun 2003 19:10:18 PDT, Justin Pryzby said:
> /dev/urandom is what you want; it makes up its own entropy. /dev/random
> uses entropy from user input (low order bits I imagine).

Strictly speaking, urandom doesn't "make up" any entropy - it generates
a pseudorandom stream of bits of arbitrary length using a small chunk of
entropy from the entropy pool. That's why it's able to generate multi-megabyte
streams of bits even when the entropy pool is empty - it is generating a
fixed but unpredictable stream based on the initial entropy.

The distinction is important mostly to cryptographers - for almost all
practical uses, the pseudorandom stream of bits produced by urandom
is quite sufficient, much faster, and leaves the entropy pool untouched
for those applications that *do* care about the difference....

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