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A pseudorandom number generator (PRNG) is a deterministic algorithm capable of generating sequences of numbers that approximate the properties of random numbers. Each sequence is completely determined by the initial state of the PRNG and the algorithm for changing the state. Most PRNGs make it possible to set the initial state, also called the seed state. Setting the initial state is called seeding the PRNG.

Calling a PRNG in the same initial state, either without seeding it explicitly or by seeding it with the same value, results in generating the same sequence of random numbers in different runs of the program.

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MSC30-C. Do not use the rand() function for generating pseudorandom numbers addresses PRNGs from a different perspective, which is the cycle of the pseudorandom number sequence—that is, during a single run of a PRNG, the time interval after which the PRNG generates the same random numbers. MSC30-C deprecates the rand() function because it generates numbers that have a comparatively short cycle. The same rule proposes the use of the random() function for POSIX and the CryptGenRandom() function for Windows.

The current rule (MSC32-C) examinesThis rule examines, in terms of seeding, all three PRNGs mentioned in rule MSC30-C. Noncompliant code examples correspond to the use of a PRNG without a seed, while compliant solutions correspond to the same PRNG being properly seeded. MSC32-C complies  This rule complies with MSC30-C and does not recommend the use of the rand() function. Nevertheless, if it is unavoidable to use rand(), it should at least be properly seeded.

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int i=0;
#include <stdio.h>
#include <stdlib.h>
 
void func(void) {
  for (int i = 0; i<10i < 10; i++i) {
    printf("%d, ", rand()); /* Always generates the same sequence */
  }
}

output:
1st run: 41, 18467, 6334, 26500, 19169, 15724, 11478, 29358, 26962, 24464,
2nd run: 41, 18467, 6334, 26500, 19169, 15724, 11478, 29358, 26962, 24464,
...
nth run: 41, 18467, 6334, 26500, 19169, 15724, 11478, 29358, 26962, 24464,

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Noncompliant Code Example
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 Code Block
bgColor
#FFCCCC
lang
c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
void func(void) {
  srand(time(NULL)); /* Create seed based on current time */
  for (int i = 0;
for (i=0; i<10 < 10; i++i) {
    printf("%d, ", rand()); /* Generates different sequences at different runs */
  }

}
output:
1st run: 25121, 15571, 29839, 2454, 6844, 10186, 27534, 6693, 12456, 5756,
2nd run: 25134, 25796, 2992, 403, 15334, 25893, 7216, 27752, 12966, 13931,
3rd run: 25503, 27950, 22795, 32582, 1233, 10862, 31243, 24650, 11000, 7328,
...
 
Although the rand() function is now properly seeded, this solution is still noncompliant because the numbers generated by rand() have a comparatively short cycle, and the numbers can be predictable. (See MSC30-C. Do not use the rand() function for generating pseudorandom numbers.)
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 Code Block
bgColor
#FFCCCC
lang
c
int i=0;
#include <stdio.h>
#include <stdlib.h>
 
void func(void) {
  for (int i = 0; i < i<1010; i++i) {
    printf("%ld, ", random()); /* Always generates the same sequence */
  }

}
output:
1st run: 1804289383, 846930886, 1681692777, 1714636915, 1957747793, 424238335, 719885386, 1649760492, 596516649, 1189641421,
2nd run: 1804289383, 846930886, 1681692777, 1714636915, 1957747793, 424238335, 719885386, 1649760492, 596516649, 1189641421,
...
nth run: 1804289383, 846930886, 1681692777, 1714636915, 1957747793, 424238335, 719885386, 1649760492, 596516649, 1189641421,
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Compliant Solution (POSIX)
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 Code Block
bgColor
#ccccff
lang
c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
void func(void) {
  srandom(time(NULL)); /* Create seed based on current time counted as seconds from 01/01/1970 */
  for (int i = 0;
for (i=0; i<10 < 10; i++i) {
    printf("%ld, ", random()); /* Generates different sequences at different runs */
  }
}
output:
1st run: 198682410, 2076262355, 910374899, 428635843, 2084827500, 1558698420, 4459146, 733695321, 2044378618, 1649046624,
2nd run: 1127071427, 252907983, 1358798372, 2101446505, 1514711759, 229790273, 954268511, 1116446419, 368192457, 1297948050,
3rd run: 2052868434, 1645663878, 731874735, 1624006793, 938447420, 1046134947, 1901136083, 418123888, 836428296, 2017467418,
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In the previous examples, seeding in rand() and random() is done using the time() function, which returns the current time calculated as the number of seconds that have passed since January 1, 1970. Depending on the application and the desirable level of security, a programmer may choose alternative ways to seed PRNGs. In general, hardware is more capable than humans of generating real random numbers (for example, by generating a sequence of bits by sampling the thermal noise of a diode and using the result as a seed).
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CryptGenRandom() does not run the risk of not being properly seeded because its arguments serve as seeders. From the Microsoft Developer Network CryptGenRandom() reference [MSDN]:
The CryptGenRandom() function fills a buffer with cryptographically random bytes.
Syntax
 Code Block
BOOL WINAPI CryptGenRandom(
  __in     HCRYPTPROV hProv,
  __in     DWORD dwLen,
  __inout  BYTE *pbBuffer
);
Parameters
hProv [in]
     Handle of a cryptographic service provider (CSP) created by a call to CryptAcquireContext.
 dwLen [in]
     Number of bytes of random data to be generated.
 pbBuffer [in, out]
     Buffer to receive the returned data. This buffer must be at least dwLen bytes in length.
     Optionally, the application can fill this buffer with data to use as an auxiliary random seed.
 Code Block
bgColor
#ccccff
lang
c
#include <Windows.h>
#include <stdio.h>
 
void func(void) {
  
 Code Block
bgColor
#ccccff
lang
c
HCRYPTPROV   hCryptProv;

  /* union stores the random number generated by CryptGenRandom() */
  union  {
    BYTE bs[sizeof(long int)];
    long int li;
  } rand_buf;

  /* An example of instantiating the CSP */
  if (CryptAcquireContext(&hCryptProv, NULL, NULL, PROV_RSA_FULL, 0)) {
    printf("CryptAcquireContext succeeded.\n");
  }
 else {
    printf("Error during CryptAcquireContext!\n");
  }

  for (int i = 0; i<10i < 10; i++i) {
    if (!CryptGenRandom(hCryptProv, sizeof(rand_buf), (BYTE*) &rand_buf)) {
      printf("Error\n");
  }
  } else {
      printf("%ld, ", rand_buf.li);
    }
  }
}
output:
1st run: -1597837311, 906130682, -1308031886, 1048837407, -931041900, -658114613, -1709220953, -1019697289, 1802206541, 406505841,
2nd run: 885904119, -687379556, -1782296854, 1443701916, -624291047, 2049692692, -990451563, -142307804, 1257079211, 897185104,
3rd run: 190598304, -1537409464, 1594174739, -424401916, -1975153474, 826912927, 1705549595, -1515331215, 474951399, 1982500583,
...
 
Risk Assessment
Rule
Severity
Likelihood
Remediation Cost
Priority
Level
MSC32-C
medium
likely
low
P18
L1
Automated Detection
Tool
Version
Checker
Description
Compass/ROSE
 
 
 
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
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[MSDN]
"CryptGenRandom Function"
 
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