And to show my objections are not just theoretical I wrote a little program to check:
#include <time.h>
#include <stdio.h>
static int estimate_entropy(long l) {
int bits = 1; /* for the sign bit */
if (l < 0) l = -l;
while (l > 0) {
++bits;
l >>= 1;
}
return bits;
}
int main() {
struct timespec ts;
if (clock_getres(CLOCK_REALTIME, &ts) != 0) {
perror("clock_getres");
return 1;
}
printf("Clock resolution: %ld.%09ld\n", (long) ts.tv_sec, (long) ts.tv_nsec);
#define N 50 /* number of samples */
struct timespec samples[N];
for (int i = 0; i < N; ++i) {
clock_gettime(CLOCK_REALTIME, &samples[i]);
}
printf("Deltas (ns):");
long deltas[N - 1];
for (int i = 0; i < N - 1; ++i) {
deltas[i] =
(samples[i + 1].tv_sec - samples[i].tv_sec)*1000000000L
+ (samples[i + 1].tv_nsec - samples[i].tv_nsec);
printf(" %4ld", deltas[i]);
}
printf("\n");
long entropy = 0;
printf("Deltas of deltas: ");
for (int i = 0; i < N - 2; ++i) {
long dd = deltas[i + 1] - deltas[i];
printf(" %4ld", dd);
entropy += estimate_entropy(dd);
}
printf("\n");
printf("Maximum entropy: %lld\n", entropy);
}
On my system this prints:
Clock resolution: 0.000000001
Deltas (ns): 55 51 23 23 25 24 24 24 24 24 25 25 24 24 24 24 24 25 24 24 24 25 25 24 24 23 25 24 24 25 24 23 25 25 26 23 25 24 24 25 26 24 23 25 25 26 24 25 24
Deltas of deltas: -4 -28 0 2 -1 0 0 0 0 1 0 -1 0 0 0 0 1 -1 0 0 1 0 -1 0 -1 2 -1 0 1 -1 -1 2 0 1 -3 2 -1 0 1 1 -2 -1 2 0 1 -2 1 -1
Maximum entropy: 92
So no, 50 iterations of that loop does not provide 256 bits of entropy due to random fluctuations in nanontime between calls.
