What is the best replacement for Windows' rand_s in Linux/POSIX?

cryptography, linux, random, security

Solution

Use `/dev/random` (requires user input, eg mouse movements) or `/dev/urandom`. The latter has an entropy pool and doesn't require any user input unless the pool is empty.

You can read from the pool like this:

char buf[100];
FILE *fp;
if (fp = fopen("/dev/urandom", "r")) {
    fread(&buf, sizeof(char), 100, fp);
    fclose(fp);
}

Or something like that.

Problem

The problem is not about randomness itself (we have rand), but in cryptographically secure PRNG. What can be used on Linux, or ideally POSIX? Does NSS have something useful? Clarification: I know about /dev/random, but it may run out of entropy pool. And I'm not sure whether /dev/urandom is guaranteed to be cryptographically secure.

Original source