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The current rule (MSC32-C) examines these three RNGs in terms of seeding. 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 addresses all three RNGs mentioned in rule MSC30-C for completeness. 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(), at least, it should be properly seeded.
Noncompliant Code Example
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,
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Compliant Solution (C Standard)
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 counted as seconds from 01/01/1970 */
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,
...
|
Noncompliant Code Example
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 produces the same sequence of random numbers at different calls.
| Code Block | ||
|---|---|---|
| ||
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,
|
Compliant Solution (POSIX)
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("%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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Compliant Solution (Windows)
Risk Assessment
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
MSC18-C |
| likely |
|
|
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Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Other Languages
This recommendation appears in the C++ Secure Coding Standard as MSC32-CPP. Ensure your random number generator is properly seeded.
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