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Calling a Random Number Generator random number generator (RNG) that is not seeded, will result results in generating the same sequence of random numbers in different runs of the program.

Suppose there is a code that calls 10 times an RNG function an RNG function is called 10 times consecutively to produce a sequence of 10 random numbers. Suppose, also, that this RNG is not seeded. Running the code for the first time will produce produces the sequence S = <r1, r2, r3, r4, r5, r6, r7, r8, r9, r10>. Running the code again for a second time will produce produces the exact same sequence S. Generally, any subsequent runs of the code will generate the same sequence S.

As a result, after the first run of the RNG, an attacker will know can predict the sequence of random numbers that will be generated in the future runs. Knowing the sequence of random numbers that will be generated beforehand This can lead to many vulnerabilities, especially when in security protocols are concerned.

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, i.e. that is the time till first collision occurs. 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 which have a comparatively short cycle. The same rule proposes the use of random() function for POSIX and CryptGenRandom() function for Windows.

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Code Block

bgColor	#FFCCCC

int i=0;
for (i=0; i<10; 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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Code Block

bgColor	#ccccff

srand(time(NULL)); /* Create seed based on current time */
int i=0;
for (i=0; i<10; 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,
...

...

Code Block

bgColor	#FFCCCC

int i=0;
for (i=0; i<10; i++) {
    printf("%d, ", 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,

...

Code Block

bgColor	#ccccff

srandom(time(NULL)); /* Create seed based on current time counted as seconds from 01/01/1970 */
int i=0;
for (i=0; i<10; i++) {
    printf("%d, ", 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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Wiki Markup

[{{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 \[in\]
&nbsp;&nbsp;&nbsp; Handle of acryptographic service provider (CSP) created by a call toCryptAcquireContext.
dwLen \[in\]
&nbsp;&nbsp;&nbsp; Number of bytes of random data to be generated.
pbBuffer \[in, out\]
&nbsp;&nbsp;&nbsp; Buffer to receive the returned data. This buffer must be at leastdwLenbytes in length.
&nbsp;&nbsp;&nbsp; Optionally, the application can fill this buffer with data to use as an auxiliary random seed.
\\

Code Block

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HCRYPTPROV   hCryptProv;

union /* 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)) /* An example of instantiating the CSP */
{
    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
	{
		   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,
...

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Rule	Severity	Likelihood	Remediation Cost	Priority	Level
MSC18-C	medium	likely	low	P18	L1

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