MSC30-C. Do not use the rand function

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 needs to 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 will be 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 */
/* ... */

The example then uses the BSD function random() function and uses time(0) as seed. With a trivial seed like time(0), however, the results from rand() or random() are equally predictable.

Compliant Solution (UNIX)

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 rand48 family of functions provides another alternative.

Note. These 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. For additional randomness, Linux users can use the character devices /dev/random or /dev/urandom, but it is advisable to retrieve only a small number of characters from these devices. (The device /dev/random may block for a long time if there are not enough events going on to generate sufficient randomness; /dev/urandom does not block.)

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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