man/man4/random.4

.\"
.\" random.c -- A strong random number generator
.\"
.\" Version 0.92, last modified 21-Sep-95
.\"
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.\" $FreeBSD: src/usr.sbin/rndcontrol/random.4,v 1.9.2.2 2001/11/24 16:14:18 dd Exp $
.\"
.Dd December 31, 2021
.Dt RANDOM 4
.Os
.Sh NAME
.Nm random ,
.Nm urandom
.Nd random number devices
.Sh DESCRIPTION
This device gathers environmental noise from device drivers, etc.,
and returns good random numbers, suitable for cryptographic use.
Besides the obvious cryptographic uses, these numbers are also good
for seeding TCP sequence numbers, and other places where it is
desirable to have numbers which are not only random, but hard to
predict by an attacker.
.Ss Theory of operation
Computers are very predictable devices.  Hence it is extremely hard
to produce truly random numbers on a computer \(em as opposed to
pseudo-random numbers, which can easily be generated by using an
algorithm.  Unfortunately, it is very easy for attackers to guess
the sequence of pseudo-random number generators, and for some
applications this is not acceptable.  So instead, we must try to
gather "environmental noise" from the computer's environment, which
must be hard for outside attackers to observe, and use that to
generate random numbers.  In a Unix environment, this is best done
from inside the kernel.
.Pp
Previous and contemporary
.Nm
implementations typically used
an "entropy" pool which was processed with a cryptographic hash
function such as MD5. However, at the time of this writing security
issues have been discovered in some of these functions
(MD4, MD5, SHA0, SHA1).
.Pp
This implementation uses a CSPRNG (Cryptographically Secure Pseudo
Random Number Generator) which is continuously reseeded as described above.
.Pp
The user interface consists of two character devices
.Pa /dev/random
and
.Pa /dev/urandom .
The
.Pa /dev/random
device is suitable for use when very high quality randomness is desired
(e.g. for key generation).  Previous implementations of this device
attempted to limit the number of returned bytes based on a guess as to
the secureness of the pool.  However, this resulted in the interface being
so undependable that most programs just started using /dev/urandom
instead of /dev/random.
The current
.Dx
implementation will return all requested bytes but the system reserves the
right in the future to limit the transfer rate to maintain the high quality
of randomness requested.
.Pp
The
.Pa /dev/urandom
device uses a different and much faster algorithm, but one which is not
considered to be as secure (though for all practical purposes it probably
is good enough).
.Pp
Root may write entropy to
.Pa /dev/random
to seed the random number generator only if the securelevel is less than
or equal to zero and the
.Va kern.seedenable
sysctl is non-zero.  A certain
degree of entropy is added by RC scripts during the boot sequence.
.Sh FILES
.Bl -tag -width Pa -compact
.It Pa /dev/random
.It Pa /dev/urandom
.El
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr drand48 3 ,
.Xr getentropy 3 ,
.Xr rand 3 ,
.Xr RAND_add 3 ,
.Xr RAND_bytes 3 ,
.Xr random 3 ,
.Xr rndcontrol 8
.Sh HISTORY
The
.Pa random ,
.Pa urandom
files appeared in
.Fx 2.1.5 .
.Sh AUTHORS
.An -nosplit
.An Mark Murray
wrote the
.Xr rndcontrol 8
utility for
.Fx .
.Pp
The
.Em IBAA
CSPRNG was developed by
.An Bob Jenkins
and is used by
.Pa /dev/urandom .
The
.Dx
implementation was ported by
.An Matthew Dillon
from initial work done by
.An Robin Carey .
The
.Pa /dev/random
generator uses a choice of
.Em IBAA
and a
.Em Fortuna
-based CSPRNG implemented by
.An Alex Hornung .