355 lines
9.3 KiB
Groff
355 lines
9.3 KiB
Groff
.\" Copyright (c) 2001-2015 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 August 17, 2015
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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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.Cd "options RANDOM_LOADABLE"
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.Cd "options RANDOM_ENABLE_UMA"
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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 file.
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.Pp
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The generator will start in an
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.Em unseeded
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state, and will block reads until
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it is seeded for the first time.
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This may cause trouble at system boot
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when keys and the like
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are generated from
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.Xr random 4
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so steps should be taken to ensure a
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seeding as soon as possible.
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.Pp
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It is also possible
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to read random bytes
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by using the KERN_ARND sysctl.
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On the command line
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this could be done by
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.Pp
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.Dl "sysctl -x -B 16 kern.arandom"
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.Pp
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This sysctl will not return
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random bytes unless
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the
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.Xr random 4
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device is seeded.
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.Pp
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This initial seeding
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of random number generators
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is a bootstrapping problem
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that needs very careful attention.
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In some cases,
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it may be difficult
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to find enough randomness
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to seed a random number generator
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until a system is fully operational,
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but the system requires random numbers
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to become fully operational.
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It is (or more accurately should be)
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critically important that the
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.Nm
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device is seeded
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before the first time it is used.
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In the case where a dummy or "blocking-only"
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device is used,
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it is the responsibility
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of the system architect
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to ensure that no blocking reads
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hold up critical processes.
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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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.Bd -literal -offset indent
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kern.random.fortuna.minpoolsize: 64
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kern.random.harvest.mask_symbolic: [HIGH_PERFORMANCE], ... ,CACHED
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kern.random.harvest.mask_bin: 00111111111
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kern.random.harvest.mask: 511
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kern.random.random_sources: 'Intel Secure Key RNG'
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.Ed
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.Pp
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Other than
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.Dl kern.random.fortuna.minpoolsize
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and
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.Dl kern.random.harvest.mask
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all settings are read-only.
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.Pp
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The
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.Pa kern.random.fortuna.minpoolsize
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sysctl is used
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to set the seed threshold.
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A smaller number gives a faster seed,
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but a less secure one.
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In practice,
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values between 64 and 256
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are acceptable.
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.Pp
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The
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.Va kern.random.harvest.mask
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bitmask is used to select
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the possible entropy sources.
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A 0 (zero) value means
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the corresponding source
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is not considered
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as an entropy source.
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Set the bit to 1 (one)
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if you wish to use
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that source.
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The
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.Va kern.random.harvest.mask_bin
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and
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.Va kern.random.harvest.mask_symbolic
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sysctls
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can be used to confirm
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that the choices are correct.
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Note that disabled items
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in the latter item
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are listed in square brackets.
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See
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.Xr random_harvest 9
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for more on the harvesting of entropy.
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.Pp
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When
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.Cd "options RANDOM_LOADABLE"
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is used,
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the
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.Pa /dev/random
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device is not created
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until an "algorithm module"
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is loaded.
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Two of these modules
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are built by default,
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.Em random_fortuna
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and
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.Em random_yarrow .
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The
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.Em random_yarrow
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module is deprecated,
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and will be removed in
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.Fx 12.
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Use of the Yarrow algorithm
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is not encouraged,
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but while still present
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in the kernel source,
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it can be selected with the
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.Cd "options RANDOM_YARROW"
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kernel option.
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Note that these loadable modules
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are slightly less efficient
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than their compiled-in equivalents.
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This is because some functions
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must be locked against
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load and unload events,
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and also must be indirect calls
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to allow for removal.
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.Pp
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When
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.Cd "options RANDOM_ENABLE_UMA"
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is used,
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the
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.Pa /dev/random
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device will obtain entropy
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from the zone allocator.
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This is potentially very high rate,
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and if so will be of questionable use.
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If this is the case,
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use of this option
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is not recommended.
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Determining this is not trivial,
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so experimenting and measurement
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using tools such as
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.Xr dtrace 1
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will be required.
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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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.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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.Xr random 9
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.Rs
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.%A Ferguson
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.%A Schneier
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.%A Kohno
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.%B Cryptography Engineering
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.%I Wiley
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.%O ISBN 978-0-470-47424-2
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.Re
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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 current software implementation,
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introduced in
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.Fx 10.0 ,
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is 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 Fortuna
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algorithm by Ferguson
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.Em et al .
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It replaces the previous
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.Em Yarrow
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implementation,
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introduced in
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.Fx 5.0 .
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The Yarrow algorithm
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is no longer supported
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by its authors,
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and is therefore deprecated.
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