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The C Standard function rand() (available in stdlib.h) does not have good random number properties. The numbers generated by rand() have a comparatively short cycle, and the numbers may be predictable.

To achieve the best random numbers possible, an implementation-specific function must be used. When unpredictability really matters and speed is not an issue, such as in the creation of strong cryptographic keys, use a true entropy source such as /dev/random or a hardware device capable of generating random numbers. The /dev/random device may block for a long time if there are not enough events going on to generate sufficient entropy. From the Linux urandom(4) manual page:

A read from the /dev/urandom device will not block waiting for more entropy. As a result, if there is not sufficient entropy in the entropy pool, the returned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver. Knowledge of how to do this is not available in the current non-classified literature, but it is theoretically possible that such an attack may exist. If this is a concern in your application, use /dev/random instead.

In many cases, however, it will be acceptable to simply use a pseudorandom number generator from a cryptographic library (such as the Mersenne Twister) and seed it with data read from /dev/random.

Non-Compliant Code Example

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Compliant Solution (POSIX)

This compliant solution improves the previous non-compliant code example by adding some additional sources of information to the seed.

long int li;
FILE* fd;

if(!(fd = fopen("/dev/random", "r")) {
   /* Handle error condition */
}

if(fread(&li, sizeof(li), 1, fd) != sizeof(li)) {
   /* Handle error condition */
}

fclose(fd);

printf("Random number: %ld\n", li);enum {len = 12};
char id[len];  /* id will hold the ID, starting with the characters "ID" */
               /* followed by a random integer */
int r;
int num;
/* ... */
srandom(time(0)*getpid());  /* seed the PRNG with the current time */
/* ... */
r = random();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

The rand48 family of functions provides another alternative for pseudorandom numbers.

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arc4random() fits into a middle ground not covered by other subsystems such as the strong, slow, and resource expensive random devices described in random(4) versus the fast but poor quality interfaces described in rand(3), random(3), and drand48(3).

To achieve the best random numbers possible, an implementation-specific function must be used. When unpredictability really matters and speed is not an issue, such as in the creation of strong cryptographic keys, use a true entropy source such as /dev/random or a hardware device capable of generating random numbers. Note that the /dev/random device may block for a long time if there are not enough events going on to generate sufficient entropy.

In many cases, however, it will be acceptable to simply use a pseudorandom number generator from a cryptographic library (such as the Mersenne Twister) and seed it with data read from /dev/random.

Compliant Solution (Windows)

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