MSC30-C. Do not use the rand() function for generating pseudorandom numbers

Pseudorandom number generators use mathematical algorithms to produce a sequence of numbers with good statistical properties, but the numbers produced are not genuinely random.

The C Standard function rand() makes no guarantees as to the quality of the random sequence produced. The numbers generated by some implementations of rand() have a comparatively short cycle, and the numbers can be predictable. Applications that have strong pseudorandom number requirements should use a generator that is known to be sufficient for their needs.

Noncompliant Code Example

The following code generates an ID with a numeric part produced by calling the rand() function. The IDs produced are predictable and have limited randomness.

#include <stdio.h>
#include <stdlib.h>
 
void func(void) {
  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;
  /* ... */
  r = rand();  /* Generate a random integer */
  num = snprintf(id, len, "ID%-d", r);  /* Generate the ID */
  /* ... */
}

Compliant Solution (POSIX)

The POSIX random() function is a better pseudorandom number generator. Although the low dozen bits generated by rand() go through a cyclic pattern, all the bits generated by random() are usable.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
void func(void) {
  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;
  /* ... */
  time_t now = time(NULL);
  if (now == (time_t)-1) {
    /* Handle error */
  }
  srandom(now);  /* 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.

Although not specified by POSIX, arc4random() is an option on systems that support it. From the arc4random(3) manual page [OpenBSD]:

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, 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 can block for a long time if there are not enough events going on to generate sufficient entropy.

Compliant Solution (Windows)

On Windows platforms, the CryptGenRandom() function can be used to generate cryptographically strong random numbers. Note that the exact details of the implementation are unknown, including, for example, what source of entropy CryptGenRandom() uses. From the Microsoft Developer Network CryptGenRandom() reference [MSDN]:

If an application has access to a good random source, it can fill the pbBuffer buffer with some random data before calling CryptGenRandom(). The CSP [cryptographic service provider] then uses this data to further randomize its internal seed. It is acceptable to omit the step of initializing the pbBuffer buffer before calling CryptGenRandom().

#include <Windows.h>
#include <wincrypt.h>
#include <stdio.h>
 
void func(void) {
  HCRYPTPROV prov;
  if (CryptAcquireContext(&prov, NULL, NULL, PROV_RSA_FULL, 0)) {
    long int li = 0;
    if (CryptGenRandom(prov, sizeof(li), (BYTE *)&li))
      printf("Random number: %ld\n", li);
    CryptReleaseContext(prov, 0);
  }
}

Risk Assessment

Using the rand() function leads to possibly predictable random numbers.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography