Secure Programming Cookbook for C and C++

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

Tip

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.^[1]) 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 ...