CERT C: Rule MSC30-C

Do not use the rand() function for generating pseudorandom numbers

Description

Rule Definition

Do not use the rand() function for generating pseudorandom numbers.¹

Polyspace Implementation

The rule checker checks for Use of rand() for Generating Pseudorandom Number.

Examples

Use of rand() for Generating Pseudorandom Number

Issue

This issue occurs when you use the function rand for generating pseudorandom numbers.

Risk

The function rand is cryptographically weak. That is, the numbers generated by rand can be predictable. Do not use pseudorandom numbers generated from rand for security purposes. When a predictable random value controls the execution flow, your program is vulnerable to attacks.

Fix

Use more cryptographically sound pseudorandom number generators (PRNG), such as CryptGenRandom (Windows), OpenSSL/RAND_bytes(Linux/UNIX), or random (POSIX).

Example - Random Loop Numbers

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

volatile int rd = 1;
int main(int argc, char *argv[])
{   
	int j, r, nloops;
	struct random_data buf;
	int i = 0;
	
	nloops = rand(); //Noncompliant
	
	for (j = 0; j < nloops; j++) {
		i = rand(); //Noncompliant
		printf("random_r: %ld\n", (long)i);
	}
	return 0;
}

This example uses rand to generate random numbers nloops and i. The predictability of these variables makes these function vulnerable to attacks.

Correction — Use Stronger PRNG

One possible correction is to replace the vulnerable PRNG with a stronger random number generator. For instance, this code uses the PRNG random() from POSIX library. random is a much stronger PRNG because it can be seeded by a different number every time it is called.

  
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define TIME_UTC 1
volatile int rd = 1;
int randomWrapper(){
	struct timespec ts;
  if (timespec_get(&ts, TIME_UTC) == 0) {
    /* Handle error */
  }
  srandom(ts.tv_nsec ^ ts.tv_sec);  /* Seed the PRNG */
	return random();
}
int main(int argc, char *argv[])
{   
    int j, r, nloops;
    struct random_data buf;
    int i = 0;
    
    nloops = randomWrapper();
    
    for (j = 0; j < nloops; j++) {
		i = randomWrapper();
        printf("random_r: %ld\n", (long)i);
    }
    return 0;
}

Check Information

Group: Rule 48. Miscellaneous (MSC)

Version History

Introduced in R2019a