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 rand() function 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 must use a generator that is known to be sufficient for their needs.

Noncompliant Code Example

The following noncompliant 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>
 
enum { len = 12 };
 
void func(void) {
  /*
   * id will hold the ID, starting with the characters
   *  "ID" followed by a random integer.
   */
  char id[len];  
  int r;
  int num;
  /* ... */
  r = rand();  /* Generate a random integer */
  num = snprintf(id, len, "ID%-d", r);  /* Generate the ID */
  /* ... */
}

Compliant Solution (POSIX)

This compliant solution replaces the rand() function with the POSIX random() function:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

enum { len = 12 }; 

void func(void) {
  /*
   * id will hold the ID, starting with the characters
   *  "ID" followed by a random integer.
   */
  char id[len];  
  int r;
  int num;
  /* ... */
  struct timespec ts;
  if (timespec_get(&ts, TIME_UTC) == 0) {
    /* Handle error */
  }
  srandom(ts.tv_nsec ^ ts.tv_sec);  /* Seed the PRNG */
  /* ... */
  r = random();  /* Generate a random integer */
  num = snprintf(id, len, "ID%-d", r);  /* Generate the ID */
  /* ... */
}

The POSIX random() function is a better pseudorandom number generator. Although on some platforms the low dozen bits generated by rand() go through a cyclic pattern, all the bits generated by random() are usable. The rand48 family of functions provides another alternative for pseudorandom numbers.

Although not specified by POSIX, arc4random() is another possibility for systems that support it. The arc4random(3) manual page [OpenBSD] states

... provides higher quality of data than those 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 is crucial 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. 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. The exact details of the implementation are unknown, including, for example, what source of entropy CryptGenRandom() uses. The Microsoft Developer Network CryptGenRandom() reference [MSDN] states

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);
    } else {
      /* Handle error */
    }
    if (!CryptReleaseContext(prov, 0)) {
      /* Handle error */
    }
  } else {
    /* Handle error */
  }
}

Risk Assessment

The use of the rand() function can result in predictable random numbers.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography