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 rand()
function makes no guarantees as to the quality of the random sequence produced. The numbers generated by some implementations of rand()
have a comparatively short cycle and the numbers can be predictable. Applications that have strong pseudorandom number requirements must use a generator that is known to be sufficient for their needs.
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.
#include <stdio.h> #include <stdlib.h> enum { len = 12 }; void func(void) { /* * id will hold the ID, starting with the characters * "ID" followed by a random integer. */ char id[len]; int r; int num; /* ... */ r = rand(); /* Generate a random integer */ num = snprintf(id, len, "ID%-d", r); /* Generate the ID */ /* ... */ } |
This compliant solution replaces the rand()
function with the POSIX random()
function:
#include <stdio.h> #include <stdlib.h> #include <time.h> enum { len = 12 }; void func(void) { /* * id will hold the ID, starting with the characters * "ID" followed by a random integer. */ char id[len]; int r; int num; /* ... */ struct timespec ts; if (timespec_get(&ts, TIME_UTC) == 0) { /* Handle error */ } srandom(ts.tv_nsec ^ ts.tv_sec); /* Seed the PRNG */ /* ... */ r = random(); /* Generate a random integer */ num = snprintf(id, len, "ID%-d", r); /* Generate the ID */ /* ... */ } |
The POSIX random()
function is a better pseudorandom number generator. Although on some platforms the low dozen bits generated by rand()
go through a cyclic pattern, all the bits generated by random()
are usable. The rand48
family of functions provides another alternative for pseudorandom numbers.
Although not specified by POSIX, arc4random()
is another possibility for systems that support it. The arc4random(3)
manual page [OpenBSD] states
... provides higher quality of data than those 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 is crucial and speed is not an issue, as in the creation of strong cryptographic keys, use a true entropy source, such as /dev/random
, or a hardware device capable of generating random numbers. The /dev/random
device can block for a long time if there are not enough events going on to generate sufficient entropy.
On Windows platforms, the BCryptGenRandom()
function can be used to generate cryptographically strong random numbers. The Microsoft Developer Network BCryptGenRandom()
reference [MSDN] states:
The default random number provider implements an algorithm for generating random numbers that complies with the NIST SP800-90 standard, specifically the CTR_DRBG portion of that standard.
#include <Windows.h> #include <bcrypt.h> #include <stdio.h> #pragma comment(lib, "Bcrypt") void func(void) { BCRYPT_ALG_HANDLE Prov; int Buffer; if (!BCRYPT_SUCCESS( BCryptOpenAlgorithmProvider(&Prov, BCRYPT_RNG_ALGORITHM, NULL, 0))) { /* handle error */ } if (!BCRYPT_SUCCESS(BCryptGenRandom(Prov, (PUCHAR) (&Buffer), sizeof(Buffer), 0))) { /* handle error */ } printf("Random number: %d\n", Buffer); BCryptCloseAlgorithmProvider(Prov, 0); } |
The use of the rand()
function can result in predictable random numbers.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
MSC30-C | Medium | Unlikely | Low | P6 | L2 |
Tool | Version | Checker | Description |
---|---|---|---|
Astrée | stdlib-use-rand | Fully checked | |
Axivion Bauhaus Suite | CertC-MSC30 | ||
Clang | cert-msc30-c | Checked by clang-tidy | |
CodeSonar | BADFUNC.RANDOM.RAND | Use of rand | |
Compass/ROSE | |||
Coverity | DONTCALL | Implemented - weak support | |
CC2.MSC30 | Fully implemented | ||
Helix QAC | C5022 C++5029 | ||
Klocwork | CERT.MSC.STD_RAND_CALL | ||
LDRA tool suite | 44 S | Enhanced enforcement | |
Parasoft C/C++test | CERT_C-MSC30-a | Do not use the rand() function for generating pseudorandom numbers | |
PC-lint Plus | 586 | Fully supported | |
Polyspace Bug Finder | CERT C: Rule MSC30-C | Checks for vulnerable pseudo-random number generator (rule fully covered) | |
RuleChecker | stdlib-use-rand | Fully checked |
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Key here (explains table format and definitions)
Taxonomy | Taxonomy item | Relationship |
---|---|---|
CERT C | MSC50-CPP. Do not use std::rand() for generating pseudorandom numbers | Prior to 2018-01-12: CERT: Unspecified Relationship |
CERT Oracle Secure Coding Standard for Java | MSC02-J. Generate strong random numbers | Prior to 2018-01-12: CERT: Unspecified Relationship |
CWE 2.11 | CWE-327, Use of a Broken or Risky Cryptographic Algorithm | 2017-05-16: CERT: Rule subset of CWE |
CWE 2.11 | CWE-330, Use of Insufficiently Random Values | 2017-06-28: CERT: Rule subset of CWE |
CWE 2.11 | CWE-338, Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) | 2017-06-28: CERT: Rule subset of CWE |
CWE 2.11 | CWE-676 | 2017-05-18: CERT: Rule subset of CWE |
Key here for mapping notes
CWE-338 = Union( MSC30-C, list) where list =
Independent( MSC30-C, MSC32-C, CON33-C)
CWE-330 = Union( MSC30-C, MSC32-C, CON33-C, list) where list = other improper use or creation of random values. (EG the would qualify)
MSC30-C, MSC32-C and CON33-C are independent, they have no intersections. They each specify distinct errors regarding PRNGs.
[MSDN] | |
[OpenBSD] | arc4random() |