128 lines
3.8 KiB
Groff
128 lines
3.8 KiB
Groff
.\" $OpenBSD: arc4random.3,v 1.2 1997/04/27 22:40:25 angelos Exp $
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.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by Niels Provos.
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.\" 4. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 AUTHOR 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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.\" Manual page, using -mandoc macros
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.\" $FreeBSD$
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.\"
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.Dd April 15, 1997
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.Dt ARC4RANDOM 3
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.Os
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.Sh NAME
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.Nm arc4random ,
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.Nm arc4random_buf ,
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.Nm arc4random_uniform ,
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.Nm arc4random_stir ,
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.Nm arc4random_addrandom
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.Nd arc4 random number generator
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdlib.h
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.Ft u_int32_t
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.Fn arc4random "void"
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.Ft void
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.Fn arc4random_buf "void *buf" "size_t nbytes"
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.Ft u_int32_t
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.Fn arc4random_uniform "u_int32_t upper_bound"
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.Ft void
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.Fn arc4random_stir "void"
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.Ft void
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.Fn arc4random_addrandom "unsigned char *dat" "int datlen"
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.Sh DESCRIPTION
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The
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.Fn arc4random
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function uses the key stream generator employed by the
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arc4 cipher, which uses 8*8 8 bit S-Boxes.
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The S-Boxes
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can be in about
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.if t 2\u\s71700\s10\d
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.if n (2**1700)
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states.
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The
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.Fn arc4random
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function returns pseudo-random numbers in the range of 0 to
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.if t 2\u\s731\s10\d\(mi1,
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.if n (2**32)\(mi1,
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and therefore has twice the range of
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.Xr rand 3
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and
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.Xr random 3 .
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.Pp
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.Fn arc4random_buf
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function fills the region
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.Fa buf
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of length
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.Fa nbytes
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with ARC4-derived random data.
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.Pp
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.Fn arc4random_uniform
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will return a uniformly distributed random number less than
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.Fa upper_bound .
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.Fn arc4random_uniform
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is recommended over constructions like
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.Dq Li arc4random() % upper_bound
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as it avoids "modulo bias" when the upper bound is not a power of two.
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.Pp
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The
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.Fn arc4random_stir
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function reads data from
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.Pa /dev/urandom
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and uses it to permute the S-Boxes via
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.Fn arc4random_addrandom .
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.Pp
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There is no need to call
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.Fn arc4random_stir
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before using
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.Fn arc4random
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functions family, since
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they automatically initialize themselves.
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.Sh EXAMPLES
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The following produces a drop-in replacement for the traditional
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.Fn rand
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and
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.Fn random
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functions using
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.Fn arc4random :
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.Pp
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.Dl "#define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))"
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.Sh SEE ALSO
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.Xr rand 3 ,
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.Xr random 3 ,
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.Xr srandomdev 3
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.Sh HISTORY
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.Pa RC4
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has been designed by RSA Data Security, Inc.
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It was posted anonymously
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to the USENET and was confirmed to be equivalent by several sources who
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had access to the original cipher.
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Since
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.Pa RC4
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used to be a trade secret, the cipher is now referred to as
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.Pa ARC4 .
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