Problem - Kevin Tafuro

11.3 Using the Standard Unix Randomness
Infrastructure
Problem
You want to use random numbers on a modern-day Unix machine.
Solution
On most modern Unix systems, there are two devices from which you can read: /dev/
random, which is expected to produce entropy, and /dev/urandom, which is expected
to provide cryptographically secure pseudo-random values. In reality, these expectations
may not always be met, but in practice, it seems reasonably safe to assume that
they are.
We strongly recommend accessing these devices through the API we present in Recipe
11.2.
Discussion
If you need a cryptographically strong random number source that is
nonetheless reproducible, /dev/random will not suit your purposes.
Use one of the other PRNGs discussed in this chapter.
Most modern Unix operating systems have two devices that produce random numbers:
/dev/random and /dev/urandom. In theory, /dev/random may block and should
produce data that is statistically close to pure entropy, while /dev/urandom should
return immediately, providing only cryptographic randomness.
The real world is somewhat messy, though. First, your application may need to run
on a system that does not have these devices. (In that case, see Recipe 11.19, where
we discuss solutions to this problem. *) Any reasonable version of Linux, FreeBSD,
OpenBSD, or NetBSD will have these devices. They are also present on Mac OS X
10.1 or later, Solaris 9 or later, AIX 5.2 or later, HP-UX 11i or later, and IRIX 6.5.19
or later. As of this writing, only dead or officially “about to die” Unix variants, such
as Tru64 and Ultrix, lack these devices. Note that each operating system tends to
have its own implementation of these devices. We haven’t looked at them all, so we
cannot, in general, vouch for how strong and efficient these generators are, but we
* If you want to interoperate with such platforms (there are still plenty of systems without /dev/random and
/dev/urandom), that reinforces the utility of using our API; simply link against code that implements our API
using the solution from Recipe 11.8 instead of the solution from this recipe.
Using the Standard Unix Randomness Infrastructure | 575
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