Calling a random 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, RNG) that is not seeded results in generating the same sequence of random numbers in different runs of the program. Suppose an RNG function is called 10 times consecutively Consider a PRNG function that is seeded with some initial seed value and is consecutively called to produce a sequence of 10 random numbers. Suppose also that this RNG is not seeded. Running the code for the first time produces the sequence S = <r1, r2, r3, r4, r5, r6, r7, r8, r9, r10>. Running the code a second time produces the exact same sequence S. Generally, any subsequent runs of the code , S. If the PRNG is subsequently seeded with the same initial seed value, then it will generate the same sequence S.

As a result, after the first run of the RNGan improperly seeded PRNG, an attacker can predict the sequence of random numbers that will be generated in the future runs. This can  Improperly seeding or failing to seed the PRNG can lead to many vulnerabilities, especially in security protocols.

As a solution, you should always ensure that your RNG is properly seeded. Seeding an RNG means that it will generate different sequences of random numbers at any call.

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

The current rule (MSC32-C) examines, in terms of seeding, all three RNGs mentioned in rule MSC30-C. Noncompliant code examples correspond to the use of an RNG without a seed, while compliant solutions correspond to the same RNG being properly seeded. Rule MSC32-C complies to 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.

The solution is to ensure that the PRNG is always properly seeded. A properly seeded PRNG will generate a different sequence of random numbers each time it is run.

Not all random number generators can be seeded. True random number generators that rely on hardware to produce completely unpredictable results do not need to be and cannot be seeded. Some high-quality PRNGs, such as the /dev/random device on some UNIX systems, also cannot be seeded. This rule applies only to algorithmic pseudorandom number generators that can be seeded.

Noncompliant Code Example (POSIX)

...

This noncompliant code example generates a sequence of 10 pseudorandom numbers using the randrandom() function. When randrandom() is not seeded, it uses 1 as a default seed. No matter how many times this code is executed, it always produces the same sequencebehaves like rand(), producing the same sequence of random numbers each time any program that uses it is run.

Code Block
bgColor #FFCCCC lang c
#include <stdio.h> int i=0; for (i=0; i<10; i++#include <stdlib.h>   void func(void) { 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,

Noncompliant Code Example

Use srand() before rand() to seed the random sequence generated by rand(). The code produces different random number sequences at different calls.

srand(time(NULL)); /* Create seed based on current time */
int i=0;
for (i=0; i<10; i++for (unsigned int i = 0; i < 10; ++i) {
  printf("%d, ", rand()); /* GeneratesAlways differentgenerates sequencesthe atsame different runssequence */
}

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 may be predictable (see MSC30-C. Do not use the rand() function for generating pseudorandom numbers).

Noncompliant Code Example (POSIX)

This noncompliant code example generates a sequence of 10 pseudorandom numbers using the random() function. When random() is not seeded, it behaves like rand() and thus produces the same sequence of random numbers at different calls.

int i=0;
for (i=0; i<10; i++) {
  printf("%ld, ", random()); /*
  }
}

The output is as follows:

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,

Compliant Solution (POSIX)

Use Call srandom() before invoking random() to seed the random sequence generated by random(). The code This compliant solution produces different random number sequences at different calls.each time the function is called, depending on the resolution of the system clock:

srandom(time(NULL));#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
void func(void) {
  struct timespec ts;
  if (timespec_get(&ts, TIME_UTC) == 0) {
    /* CreateHandle seederror based*/
 on current} timeelse counted{
 as seconds from 01/01/1970 */
int i=0;
for (i=0; i<10; i++ srandom(ts.tv_nsec ^ ts.tv_sec);
    for (unsigned int i = 0; i < 10; ++i) {
   printf("%ld, ", random()); /* Generates different sequences at different runs */
      printf("%ld, ", random());
    }
  }
}

The output is as follows:

:
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,
...

This may not be sufficiently random for concurrent execution, which may lead to correlated generated series in different threadsIn 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 01/01/1970. Depending on the application and the desirable desired level of security, a programmer may choose alternative ways to seed RNGsPRNGs. In general, hardware is more capable than software of generating real random numbers (for example, generate a sequence of bits by  by sampling the thermal noise of a diode and use this as a seed).

Compliant Solution (Windows)

[{{CryptGenRandom()}}|http://msdn.microsoft.com/en-us/library/aa379942.aspx] does not run the risk of not being properly seeded. The reason for that is that its arguments serve as seeders. From the Microsoft Developer Network {{CryptGenRandom()}} reference \[[MSDN|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-MSDN]\]:

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

Wiki Markup

hProv \[in\]     Handle of acryptographic service provider (CSP) created by a call toCryptAcquireContext. 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 leastdwLenbytes in length.     Optionally, the application can fill this buffer with data to use as an auxiliary random seed. \\

The BCryptGenRandom() function does not run the risk of not being properly seeded because its arguments serve as seeders:

#include <stdio.h>
#include <Windows.h>
#include <Bcrypt.h>
#include <Ntstatus.h>
#include <Wincrypt.h>

void func(void) {
  BCRYPT_ALG_HANDLE hAlgorithm = NULL;
  long rand_buf;
  PUCHAR pbBuffer = (PUCHAR) &rand_buf;
  ULONG cbBuffer = sizeof(rand_buf);
  ULONG dwFlags = BCRYPT_USE_SYSTEM_PREFERRED_RNG;
  NTSTATUS status;
  for (unsigned int i = 0; i < 10; ++i) {
    status = BCryptGenRandom(hAlgorithm, pbBuffer, cbBuffer, dwFlags);
    if (status == STATUS_SUCCESS) {
      printf("%ld, ", rand_buf);
    } else {
      /* Handle Error */
    }
  }
}

The output is as follows:

1st run: -683378946, 1957231690, 1933176011, -1745403355, -883473417, 882992405, 169629816, 1824800038, 899851668, 1702784647, 
2nd run: -58750553, -1921870721, -1973269161, 1512649964, -673518452, 234003619, -1622633366, 1312389688, -2125631172, 2067680022, 
3rd run: -189899579, 1220698973, 752205360, -1826365616, 79310867, 1430950090, -283206168, -941773185, 129633665, 543448789, 

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<10; 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

Automated Detection

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Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V		Supported, but no explicit checker
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-MSC32
CodeSonar	Include Page CodeSonar_V CodeSonar_V	HARDCODED.SEED MISC.CRYPTO.TIMESEED	Hardcoded Seed in PRNG Predictable Seed in PRNG
Cppcheck Premium	Include Page Cppcheck Premium_V Cppcheck Premium_V	premium-cert-msc32-c	Fully implemented
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C5031 C++5036
Klocwork	Include Page Klocwork_V Klocwork_V	CERT.MSC.SEED_RANDOM
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	2460, 2461, 2760	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rule MSC32-C	Checks for: Deterministic random output from constant seed Predictable random output from predictable seed Rule fully covered.
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-MSC32-d	Properly seed pseudorandom number generators

Related Vulnerabilities

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

Other Languages

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Related Guidelines

Key here (explains table format and definitions)

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CERT-CWE Mapping Notes

Key here for mapping notes

CWE-327 and MSC32-C

• Intersection( MSC30-C, MSC32-C) = Ø

• MSC32-C says to properly seed pseudorandom number generators. For example, if you call rand(), make sure to seed it properly by calling srand() first. So far, we haven’t found any calls to rand().

• Failure to seed a PRNG causes it to produce reproducible (hence insecure) series of random numbers.

• CWE-327 = Union( MSC32-C, list) where list =

• Invocation of broken/risky crypto algorithms that are not properly seeded

CWE-330 and MSC32-C

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.

Bibliography

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Image Added Image Added Image Added

References

\[[C+\+ Reference|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-CPPReference]\] Standard C Library
\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 327 |http://cwe.mitre.org/data/definitions/327 .html], "Use of a Broken or Risky Cryptographic Algorithm," [CWE ID 330|http://cwe.mitre.org/data/definitions/330.html], "Use of Insufficiently Random Values"
\[[MSDN|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-MSDN]\] "[CryptGenRandom Function|http://msdn.microsoft.com/en-us/library/aa379942.aspx]"

MSC31-C. Ensure that return values are compared against the proper type      49. Miscellaneous (MSC)      50. POSIX (POS)