CWE Rule 336

Same Seed in Pseudo-Random Number Generator (PRNG)

Since R2024a

Description

Rule Description

A Pseudo-Random Number Generator (PRNG) uses the same seed each time the product is initialized.

Polyspace Implementation

The rule checker checks for Deterministic random output from constant seed.

Examples

Deterministic random output from constant seed

Issue

This issue occurs when you use standard random number generator functions that have deterministic output given a constant seed.

The checker detects this issue with the following random number generator functions:

• C Standard Library functions such as srand, srandom and initstate

• OpenSSL functions such as RAND_seed and RAND_add

• C++ Standard Library functions such as std::linear_congruential_engine<>::seed() and std::mersenne_twister_engine<>::seed() (and also the constructors of these class templates)

Risk

With constant seeds, random number generator functions produce the same output every time your program is run. A hacker can disrupt your program if they know how your program behaves.

Fix

Use a different random standard function or use a nonconstant seed.

Some standard random routines are inherently cryptographically weak, and should not be used for security purposes.

Example — Random Number Generator Initialization

#include <stdlib.h>

void random_num(void)
{
    srand(12345U); //Noncompliant
    /* ... */
}

This example initializes a random number generator using srand with a constant seed. The random number generation is deterministic, making this function cryptographically weak.

Correction — Use Different Random Number Generator

One possible correction is to use a random number generator that does not require a seed. This example uses rand_s.

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

unsigned int random_num_time(void)
{

    unsigned int number;
    errno_t err;
    err = rand_s(&number);

    if(err != 0)
    {
        return number;
    }
    else
    {
        return err;
    }
}

Check Information

Category: Others

Version History

Introduced in R2024a