MSC30-C. Do not use the rand function

Pseudo random 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 (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.

Non-Compliant 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.

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 */
/* ... */

Non-Compliant Code Example (BSD)

A better pseudo random number generator is the BSD function random().

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));  /* seed the PRNG with the current time */
/* ... */
r = random();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

However, the random() function uses time(0) as seed. With a trivial seed like time(0), however, the results from random() are also predictable.

Compliant Solution (Linux, Solaris, Mac OS X, NetBSD, OpenBSD)

To generate an unpredictable number, use an unpredictable seed and a cryptographically strong mixing function. On Unix systems, for example, decent results can be obtained by reading /dev/urandom, which will not block the application.

When unpredictability really matters (session IDs and crypto keys) use a cryptographical library and seed it with data that are read from /dev/random.

The /dev/random device may block for a long time if there are not enough events going on to generate sufficient randomness; /dev/urandom does not block.)

The rand48 family of functions provides another psuedo-random alternative.

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);

Compliant Solution (Windows)

(someone on a Windows machine should test this out - avolkovi)

On Windows platforms, the CryptGenRandom() function may be used to generate cryptographically strong random numbers. It is important to note, however, that the exact details of the implementation are unknown, and it is undetermined as to what source of entropy the CryptGenRandom() uses.

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 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().

HCRYPTPROV hCryptProv;
union {
    BYTE bs[sizeof(long int)];
    long int li;
} rand_buf;

if(!CryptGenRandom(hCryptProv, sizeof(rand_buf), &rand_buf) {
    /* Handle error */
} else {
    printf("Random number: %ld\n", rand_buf.li);
}

Risk Assessment

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

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 7.20.2.1, "The rand function"

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