Problem - Kevin Tafuro

See Also
Recipes 11.19, 11.20
11.22 Gathering Entropy from Thread Timings
Problem
You want to collect some entropy without user intervention, hoping that there is
some inherent, measurable entropy in the environment.
Solution
In practice, timing how long it takes to start up and stop a particular number of
threads can provide a bit of entropy. For example, many Java virtual machines exclusively
use such a technique to gather entropy.
Because the thread timing data is only indirectly related to actual user input, it is
good to be extremely conservative about the quality of this entropy source. We recommend
the following methodology:
1. Launch and join on some fixed number of threads (at least 100).
2. Mix in a timestamp when all threads have returned successfully.
3. Estimate entropy based on the considerations discussed in Recipe 11.19.
4. Wait at least a second before trying again, in hopes that there is additional
entropy affecting the system later.
The following code spawns a particular number of threads that you can time, in the
hope of getting some entropy. This code works on Unix implementations that have
the pthreads library (the POSIX standard for threads). Linking is different depending
on platform; check your local pthreads documentation.
#include
static void *thread_stub(void *arg) {
return 0;
}
void spc_time_threads(unsigned int numiters) {
pthread_t tid;
}
while (numiters--)
if (!pthread_create(&tid, 0, thread_stub, 0))
pthread_join(tid, 0);
Gathering Entropy from Thread Timings | 643
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