...
This noncompliant code example generates a sequence of 10 pseudorandom numbers using the rand() function. When rand() is not seeded, it uses 1 as a default seed. No matter how many times this code is executed, it always produces the same sequence.
| Code Block |
|---|
|
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,
|
...
Use srand() before rand() to seed the random sequence generated by rand(). The code produces different random number sequences at different calls.
| Code Block |
|---|
|
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,
...
|
...
This noncompliant code example generates a sequence of 10 pseudorandom numbers using the random() function. When random() is not seeded, it behaves like rand(), thus producing the same sequence of random numbers at different calls.
| Code Block |
|---|
|
int i=0;
for (i=0; i<10; 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,
|
...
Use srandom() before random() to seed the random sequence generated by random(). The code produces different random number sequences at different calls.
| Code Block |
|---|
|
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("%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,
...
|
...
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.
\\ |
| Code Block |
|---|
|
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,
...
|
...
