RANDOM(4) OpenBSD Programmer's Manual RANDOM(4)
NAME
random, srandom, urandom, prandom, arandom - random data source devices
SYNOPSIS
#include <sys/types.h>
#include <dev/rndvar.h>
#include <dev/rndioctl.h>
DESCRIPTION
The various random devices produce random output data with different ran-
dom qualities. Entropy data is collected from system activity (like disk
and network device interrupts and such), and then run through various
hash or message digest functions to generate the output.
/dev/random This device is reserved for future support of hardware
random generators.
/dev/srandom Strong random data. This device returns reliable random
data. If sufficient entropy is not currently available
(i.e., the entropy pool quality starts to run low), the
driver pauses while more of such data is collected. The
entropy pool data is converted into output data using MD5.
/dev/urandom Same as above, but does not guarantee the data to be
strong. The entropy pool data is converted into output
data using MD5. When the entropy pool quality runs low,
the driver will continue to output data.
/dev/prandom Simple pseudo-random generator.
/dev/arandom As required, entropy pool data re-seeds an ARC4 generator,
which then generates high-quality pseudo-random output da-
ta. The arc4random(3) function in userland libraries
seeds itself from this device, providing a second level of
ARC4 hashed data.
SEE ALSO
arc4random(3), md5(3), random(3), pchb(4), md5(9), random(9)
FILES
/dev/random, /dev/srandom, /dev/urandom, /dev/prandom, /dev/arandom
BUGS
No randomness testing suite provided.
HISTORY
A random device first appeared in Linux operating system. This is a
cloned interface.
ARC4 routines added by David Mazieres.
OpenBSD 2.8 March 25, 1996 1