.Dd September 6, 2001
.Dt RANDOM 4
.Os Darwin
.Sh NAME
.Nm random
,
.Nm urandom
.Nd random data source devices.
.Sh SYNOPSIS
.Cd "pseudo-device random"
.Sh DESCRIPTION
The
.Nm
device produces uniformly distributed random byte values
of potentially high quality.
.Pp
To obtain random bytes, open
.Nm /dev/random
for reading and read from it.
.Pp
To add entropy to the random generation system, open
.Nm /dev/random
for writing and write data that you believe to be somehow random.
.Pp
.Nm /dev/urandom
is a compatibility nod to Linux. On Linux,
.Nm /dev/urandom
will produce lower quality output if the entropy pool drains, while
.Nm /dev/random
will prefer to block and wait for additional entropy to be collected.
With Yarrow, this choice and distinction is not necessary, and
the two devices behave identically. You may use either.
.Sh OPERATION
The
.Nm
device implements the
.Nm Yarrow
pseudo random number generator algorithm and maintains its entropy pool.
Additional entropy is fed to the generator regularly by the
.Nm SecurityServer
daemon from random jitter measurements of the kernel.
.Nm SecurityServer
is also responsible for periodically saving some entropy to disk
and reloading it during startup to provide entropy in early system
operation.
.Pp
You may feed additional entropy to the generator by writing it to the
.Nm
device, though this is not required in a normal operating environment.
.Sh LIMITATIONS AND WARNINGS
.Nm Yarrow
is a fairly resilient algorithm, and is believed
to be resistant to non-root.
The quality of its output is however dependent on regular addition
of appropriate entropy. If the
.Nm SecurityServer
system daemon fails for any reason, output quality will suffer
over time without any explicit indication from the
.Nm
device itself.
.Pp
Paranoid programmers can counteract this risk somewhat by collecting
entropy of their choice (e.g. from keystroke or mouse timings)
and seeding it into
.Nm
directly before obtaining important random numbers.
.Sh FILES
.Bl -tag -width /dev/urandom -compact
.It Pa /dev/random
.It Pa /dev/urandom
.El
.Sh HISTORY
A
.Nm
device appeared in the Linux operating system.