ARC4RANDOM(9)

NAME
arc4random - kernel random subsystem

SYNOPSIS
#include <dev/rndvar.h>
void
add_true_randomness(int);
void
add_timer_randomness(int);
void
add_mouse_randomness(int);
void
add_tty_randomness(int);
void
add_net_randomness(int);
void
add_disk_randomness(int);
void
add_audio_randomness(int);
u_int32_t
arc4random(void);
void
arc4random_buf(void *buf, size_t nbytes);
u_int32_t
arc4random_uniform(u_int32_t upper_bound);

DESCRIPTION
The add_random() functions below mix input into the system entropy pool, which is then used to create a key for the ChaCha stream cipher used in the arc4random() series of functions.

The add_mouse_randomness(), add_tty_randomness(), add_net_randomness(), add_disk_randomness() and add_audio_randomness() routines are used to supply data for the random data source device for further processing. The processing involves calculating inter-event timedelta and inserting it into a pool which is cryptographically mixed multiple times.

add_true_randomness() does not involve the usual timing calculations, and causes the supplied data argument to be added to the entropy pool, increasing the entropy counter by 32 bits.

add_timer_randomness() will not cause the entropy counter to rise. It is used to change the state of the pool periodically, mostly by means of timing the random driver's operations.

arc4random() and arc4random_buf() provide random numbers and are intended to be called in any circumstance where random numbers are required.

arc4random_uniform() will return a uniformly distributed random number less than upper_bound, avoiding "modulo bias" when the upper bound is not a power of two. In the worst case, this function may consume multiple iterations to ensure uniformity; see the source code to understand the problem and solution.

arc4random(), arc4random_buf(), and arc4random_uniform() may be called in nearly any kernel context.

SEE ALSO
arc4random(3), pchb(4), random(4)