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.Dd $Mdocdate: May 29 2020 $
.Dt RANDOM 4
.Os
.Sh NAME
.Nm random ,
.Nm urandom
.Nd random data source device
.Sh DESCRIPTION
The
.Nm urandom
device produces high quality pseudo-random output data without ever
blocking.
.Pp
Entropy data stored previously is provided to the kernel during the
boot sequence and used as inner-state of a stream cipher.
High quality data is available immediately upon kernel startup.
System activity (such as disk, network, and clock device interrupts),
and hardware random generator output is collected, whitened with a crc
and hash, then periodically folded together with stream cipher
inner-state and outer-state to create a new inner state.
Reads from all consumers (including the kernel itself, which makes many
requests per second) are sliced from the same output stream, which carves
the stream cipher output unpredictably and helps improve forward and
backtracking protection beyond the strength of the stream cipher.
.Pp
The
.Nm urandom
device is intended to be used in scripts.
In C programs, use the
.Xr arc4random 3
family of functions instead, which can be called in almost all
coding environments, including
.Xr pthreads 3 ,
.Xr chroot 2 ,
.Xr pledge 2 ,
and
.Xr unveil 2 ,
and which avoids accessing a filesystem device every time.
.Pp
For portability reasons, never use
.Pa /dev/random .
On
.Ox ,
it is an alias for
.Pa /dev/urandom ,
but on many other systems misbehaves by blocking because their
random number generators lack a robust boot-time initialization sequence.
.Sh FILES
.Bl -tag -width /dev/urandom -compact
.It Pa /dev/urandom
.El
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr amdpm 4 ,
.Xr glxsb 4 ,
.Xr pchb 4 ,
.Xr arc4random 9
.Sh HISTORY
A
.Nm
device first appeared in Linux in 1994.
The
.Nm random
and
.Nm urandom
devices have been available since
.Ox 2.0 .