338 lines
10 KiB
Groff
338 lines
10 KiB
Groff
.\" Copyright (c) 2001 Mark R V Murray. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 3, 2004
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.Dt RANDOM 4
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.Os
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.Sh NAME
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.Nm random
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.Nd the entropy device
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.Sh SYNOPSIS
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.Cd "device random"
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.Sh DESCRIPTION
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The
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.Nm
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device
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returns an endless supply of random bytes when read.
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It also accepts and reads data
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as any ordinary (and willing) file,
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but discards data written to it.
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The device will probe for
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certain hardware entropy sources,
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and use these in preference to the fallback,
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which is a generator implemented in software.
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.Pp
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If the device is using
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the software generator,
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writing data to
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.Nm
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would perturb the internal state.
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This perturbation of the internal state
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is the only userland method of introducing
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extra entropy into the device.
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If the writer has superuser privilege,
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then closing the device after writing
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will make the software generator reseed itself.
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This can be used for extra security,
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as it immediately introduces any/all new entropy
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into the PRNG.
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The hardware generators will generate
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sufficient quantities of entropy,
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and will therefore ignore user-supplied input.
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The software
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.Nm
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device may be controlled with
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.Xr sysctl 8 .
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.Pp
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To see the current settings of the software
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.Nm
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device, use the command line:
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.Pp
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.Dl sysctl kern.random
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.Pp
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which results in something like:
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.Pp
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.Bd -literal -offset indent
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kern.random.sys.seeded: 1
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kern.random.sys.burst: 20
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kern.random.sys.harvest.ethernet: 0
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kern.random.sys.harvest.point_to_point: 0
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kern.random.sys.harvest.interrupt: 0
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kern.random.yarrow.gengateinterval: 10
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kern.random.yarrow.bins: 10
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kern.random.yarrow.fastthresh: 100
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kern.random.yarrow.slowthresh: 160
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kern.random.yarrow.slowoverthresh: 2
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.Ed
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.Pp
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(These would not be seen if a
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hardware generator is present.)
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.Pp
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All settings are read/write.
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.Pp
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The
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.Va kern.random.sys.seeded
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variable indicates whether or not the
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.Nm
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device is in an acceptably secure state
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as a result of reseeding.
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If set to 0, the device will block (on read) until the next reseed
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(which can be from an explicit write,
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or as a result of entropy harvesting).
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A reseed will set the value to 1 (non-blocking).
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.Pp
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The
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.Va kern.random.sys.burst
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variable instructs the kernel thread
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that processes the harvest queue
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to
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.Xr tsleep 9
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briefly after that many events
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have been processed.
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This helps prevent the random device
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from being so compute-bound
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that it takes over all processing ability.
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A value of 0 (zero) is treated as
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.Em infinity ,
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and will only allow the kernel to pause
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if the queue is empty.
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Only values in the range
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.Bq 0..20
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are accepted.
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.Pp
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The
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.Va kern.random.sys.harvest.ethernet
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variable is used to select LAN traffic as an entropy source.
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A 0 (zero) value means that LAN traffic
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is not considered as an entropy source.
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Set the variable to 1 (one)
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if you wish to use LAN traffic for entropy harvesting.
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.Pp
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The
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.Va kern.random.sys.harvest.point_to_point
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variable is used to select serial line traffic as an entropy source.
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(Serial line traffic includes PPP, SLIP and all tun0 traffic.)
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A 0 (zero) value means such traffic
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is not considered as an entropy source.
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Set the variable to 1 (one)
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if you wish to use it for entropy harvesting.
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.Pp
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The
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.Va kern.random.sys.harvest.interrupt
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variable is used to select hardware interrupts
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as an entropy source.
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A 0 (zero) value means interrupts
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are not considered as an entropy source.
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Set the variable to 1 (one)
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if you wish to use them for entropy harvesting.
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All interrupt harvesting is setup by the
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individual device drivers.
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.Pp
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The other variables are explained in the paper describing the
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.Em Yarrow
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algorithm at
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.Pa http://www.counterpane.com/yarrow.html .
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.Pp
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These variables are all limited
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in terms of the values they may contain:
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.Bl -tag -width "kern.random.yarrow.gengateinterval" -compact -offset indent
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.It Va kern.random.yarrow.gengateinterval
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.Bq 4..64
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.It Va kern.random.yarrow.bins
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.Bq 2..16
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.It Va kern.random.yarrow.fastthresh
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.Bq 64..256
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.It Va kern.random.yarrow.slowthresh
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.Bq 64..256
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.It Va kern.random.yarrow.slowoverthresh
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.Bq 1..5
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.El
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.Pp
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Internal
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.Xr sysctl 3
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handlers force the above variables
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into the stated ranges.
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.Sh RANDOMNESS
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The use of randomness in the field of computing
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is a rather subtle issue because randomness means
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different things to different people.
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Consider generating a password randomly,
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simulating a coin tossing experiment or
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choosing a random back-off period when a server does not respond.
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Each of these tasks requires random numbers,
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but the random numbers in each case have different requirements.
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.Pp
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Generation of passwords, session keys and the like
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requires cryptographic randomness.
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A cryptographic random number generator should be designed
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so that its output is difficult to guess,
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even if a lot of auxiliary information is known
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(such as when it was seeded, subsequent or previous output, and so on).
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On
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.Fx ,
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seeding for cryptographic random number generators is provided by the
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.Nm
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device,
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which provides real randomness.
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The
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.Xr arc4random 3
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library call provides a pseudo-random sequence
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which is generally reckoned to be suitable for
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simple cryptographic use.
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The OpenSSL library also provides functions for managing randomness
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via functions such as
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.Xr RAND_bytes 3
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and
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.Xr RAND_add 3 .
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Note that OpenSSL uses the
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.Nm
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device for seeding automatically.
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.Pp
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Randomness for simulation is required in engineering or
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scientific software and games.
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The first requirement of these applications is
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that the random numbers produced conform to some well-known,
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usually uniform, distribution.
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The sequence of numbers should also appear numerically uncorrelated,
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as simulation often assumes independence of its random inputs.
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Often it is desirable to reproduce
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the results of a simulation exactly,
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so that if the generator is seeded in the same way,
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it should produce the same results.
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A peripheral concern for simulation is
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the speed of a random number generator.
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.Pp
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Another issue in simulation is
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the size of the state associated with the random number generator, and
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how frequently it repeats itself.
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For example,
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a program which shuffles a pack of cards should have 52!\& possible outputs,
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which requires the random number generator to have 52!\& starting states.
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This means the seed should have at least log_2(52!) ~ 226 bits of state
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if the program is to stand a chance of outputting all possible sequences,
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and the program needs some unbiased way of generating these bits.
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Again,
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the
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.Nm
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device could be used for seeding here,
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but in practice, smaller seeds are usually considered acceptable.
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.Pp
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.Fx
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provides two families of functions which are considered
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suitable for simulation.
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The
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.Xr random 3
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family of functions provides a random integer
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between 0 to
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.if t 2\u\s731\s10\d\(mi1.
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.if n (2**31)\(mi1.
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The functions
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.Xr srandom 3 ,
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.Xr initstate 3
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and
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.Xr setstate 3
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are provided for deterministically setting
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the state of the generator and
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the function
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.Xr srandomdev 3
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is provided for setting the state via the
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.Nm
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device.
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The
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.Xr drand48 3
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family of functions are also provided,
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which provide random floating point numbers in various ranges.
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.Pp
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Randomness that is used for collision avoidance
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(for example, in certain network protocols)
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has slightly different semantics again.
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It is usually expected that the numbers will be uniform,
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as this produces the lowest chances of collision.
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Here again,
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the seeding of the generator is very important,
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as it is required that different instances of
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the generator produce independent sequences.
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However, the guessability or reproducibility of the sequence is unimportant,
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unlike the previous cases.
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.Pp
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One final consideration for the seeding of random number generators
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is a bootstrapping problem.
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In some cases, it may be difficult to find enough randomness to
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seed a random number generator until a system is fully operational,
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but the system requires random numbers to become fully operational.
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There is no substitute for careful thought here,
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but the
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.Fx
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.Nm
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device,
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which is based on the Yarrow system,
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should be of some help in this area.
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.Pp
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.Fx
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does also provide the traditional
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.Xr rand 3
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library call,
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for compatibility purposes.
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However,
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it is known to be poor for simulation and
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absolutely unsuitable for cryptographic purposes,
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so its use is discouraged.
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.Sh FILES
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.Bl -tag -width ".Pa /dev/random"
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.It Pa /dev/random
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.El
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.Sh SEE ALSO
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.Xr arc4random 3 ,
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.Xr drand48 3 ,
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.Xr rand 3 ,
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.Xr RAND_add 3 ,
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.Xr RAND_bytes 3 ,
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.Xr random 3 ,
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.Xr sysctl 8
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.Sh HISTORY
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A
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.Nm
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device appeared in
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.Fx 2.2 .
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The early version was taken from Theodore Ts'o's entropy driver for Linux.
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The current software implementation,
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introduced in
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.Fx 5.0 ,
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is a complete rewrite by
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.An Mark R V Murray ,
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and is an implementation of the
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.Em Yarrow
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algorithm by Bruce Schneier,
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.Em et al .
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The only hardware implementation
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currently is for the
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.Tn VIA C3 Nehemiah
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(stepping 3 or greater)
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CPU.
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More will be added in the future.
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.Pp
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The author gratefully acknowledges
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significant assistance from VIA Technologies, Inc.
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