MSC30-CPP. 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 function rand() (available in stdlib.h) does not have good random number properties. The numbers generated by rand() have a comparatively short cycle and can be predictable.

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.

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 */
/* ... */

Compliant Solution (POSIX)

In this compliant solution, a better pseudorandom number generator is the random() function. While the low-dozen bits generated by rand() go through a cyclical pattern, all the bits generated by random() are usable.

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;
/* ... */
time_t now = time(NULL);
if (now == (time_t) -1) {
  /* handle error */
}
srandom(now);  /* seed the PRNG with the current time */
/* ... */
r = random();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

The rand48 family of functions provides another alternative for pseudorandom numbers.

Although not specified by POSIX, arc4random() is an option on systems that support it. The arc4random(3) manual page says that

arc4random() fits into a middle ground not covered by other subsystems such as the strong, slow, and resource expensive random devices described in random(4) versus the fast but poor quality interfaces 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 matters and speed is not an issue, such as in the creation of strong cryptographic keys, a true entropy source such as /dev/random or a hardware device capable of generating random numbers should be used. Note that 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)

In the compliant solution, on Windows platforms, the CryptGenRandom() function can be used to generate cryptographically strong random numbers. It is important to note that the exact details of the implementation are unknown, and it is unknown what source of entropy the CryptGenRandom() uses. The Microsoft Developer Network CryptGenRandom() reference [MSDN 2010] says,

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<Wincrypt.h>

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 could lead to predictable random numbers.

Automated Detection

Related Vulnerabilities

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

Other Languages

This rule appears in the C Secure Coding Standard as MSC30-C. Do not use the rand() function for generating pseudorandom numbers.

This rule appears in the Java Secure Coding Standard as MSC02-J. Generate strong random numbers.

Bibliography

[ISO/IEC 9899:1999] Section 7.20.2.1, "The rand function"
[MITRE 2007] CWE ID 327 , "Use of a Broken or Risky Cryptographic Algorithm," CWE ID 330, "Use of Insufficiently Random Values"
[MSDN 2010] "CryptGenRandom Function."

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