In the virtual world, it is difficult to create randomness, i$.$e$.,$ software alone is hard to create random numbers.

Most systems resort to the physical world to gain the randomness. Linux gains the randomness from the

following physical resources:

void add$\_$keyboard$\_$randomness$($unsigned char scancode$)$;

void add$\_$mouse$\_$randomness$(\_\_$u$32$ mouse$\_$data$)$;

void add$\_$interrupt$\_$randomness$($int irq$)$;

void add$\_$blkdev$\_$randomness$($int major$)$;

The first two are quite straightforward to understand: the first one uses the timing between key presses;

the second one uses mouse movement and interrupt timing; the third one gathers random numbers using

the interrupt timing. Of course, not all interrupts are good sources of randomness. For example, the timer

interrupt is not a good choice, because it is predictable. However, disk interrupts are a better measure. The

last one measures the finishing time of block device requests.

The randomness is measured using entropy, which is different from the meaning of entropy in the information theory. Here, it simply means how many bits of random numbers the system currently has. Yo