155 lines
4.1 KiB
Groff
155 lines
4.1 KiB
Groff
.\"
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.\" Copyright (c) 2000
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" 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 DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\" "
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.Dd September 25, 2000
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.Os
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.Dt RANDOM 9
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.Sh NAME
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.Nm arc4rand ,
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.Nm arc4random ,
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.Nm random ,
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.Nm read_random ,
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.Nm srandom
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.Nd supply pseudo-random numbers
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.Sh SYNOPSIS
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.In sys/libkern.h
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.Ft void
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.Fn srandom "u_long seed"
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.Ft u_long
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.Fn random "void"
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.Ft void
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.Fn arc4rand "void *ptr" "u_int length" "int reseed"
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.Ft u_int32_t
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.Fn arc4random "void"
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.Pp
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.In sys/random.h
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.Ft int
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.Fn read_random "void *buffer" "int count"
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.Sh DESCRIPTION
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The
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.Fn random
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function will by default produce a sequence of numbers that can be duplicated
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by calling
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.Fn srandom
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with
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.Ql 1
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as the
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.Fa seed .
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The
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.Fn srandom
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function may be called with any arbitrary
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.Fa seed
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value to get slightly more unpredictable numbers.
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It is important to remember that the
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.Fn random
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function is entirely predictable, and is therefore not of use where
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knowledge of the sequence of numbers may be of benefit to an attacker.
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.Pp
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The
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.Fn arc4rand
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function will return very good quality random numbers, slightly better
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suited for security-related purposes.
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The random numbers from
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.Fn arc4rand
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are seeded from the entropy device if it is available.
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Automatic reseeds happen after a certain timeinterval and after a
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certain number of bytes have been delivered.
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A forced reseed can be forced by passing a non-zero value in the
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.Fa reseed
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argument.
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.Pp
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The
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.Fn read_random
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function is used to return entropy directly from the entropy device
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if it has been loaded. If the entropy device is not loaded, then
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the
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.Fa buffer
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is filled with output generated by
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.Fn random .
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The
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.Fa buffer
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is filled with no more than
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.Fa count
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bytes. It is advised that
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.Fn read_random
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is not used; instead use
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.Fn arc4rand
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.Pp
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All the bits generated by
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.Fn random ,
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.Fn arc4rand
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and
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.Fn read_random
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are usable.
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For example,
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.Sq Li random()&01
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will produce a random binary value.
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.Pp
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The
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.Fn arc4random
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is a convenience function which calls
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.Fn arc4rand
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to return a 32 bit pseudo-random integer.
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.Sh RETURN VALUES
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The
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.Fn random
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function
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uses a non-linear additive feedback random number generator employing a
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default table of size 31 long integers to return successive pseudo-random
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numbers in the range from 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 period of this random number generator is very large, approximately
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.if t 16\(mu(2\u\s731\s10\d\(mi1).
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.if n 16*((2**31)\(mi1).
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.Pp
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The
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.Fn arc4rand
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function uses the RC4 algorithm to generate successive pseudo-random
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bytes.
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The
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.Fn arc4random
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function
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uses
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.Fn arc4rand
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to generate pseudo-random numbers in the range from 0 to
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.if t 2\u\s732\s10\d\(mi1.
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.if n (2**32)\(mi1.
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.Pp
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The
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.Fn read_random
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function returns the number of bytes placed in
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.Fa buffer .
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.Sh AUTHORS
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.An Dan Moschuk
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wrote
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.Fn arc4random .
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.An Mark R V Murray
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wrote
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.Fn read_random .
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