Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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Manual page, using -mandoc macros
$FreeBSD: releng/10.2/lib/libc/gen/arc4random.3 231564 2012-02-12 18:29:56Z ed $
.Dd April 15, 1997 .Dt ARC4RANDOM 3 .Os .Sh NAME .Nm arc4random , .Nm arc4random_buf , .Nm arc4random_uniform , .Nm arc4random_stir , .Nm arc4random_addrandom .Nd arc4 random number generator .Sh LIBRARY .Lb libc .Sh SYNOPSIS n stdlib.h .Ft uint32_t .Fn arc4random "void" .Ft void .Fn arc4random_buf "void *buf" "size_t nbytes" .Ft uint32_t .Fn arc4random_uniform "uint32_t upper_bound" .Ft void .Fn arc4random_stir "void" .Ft void .Fn arc4random_addrandom "unsigned char *dat" "int datlen" .Sh DESCRIPTION The .Fn arc4random function uses the key stream generator employed by the arc4 cipher, which uses 8*8 8 bit S-Boxes. The S-Boxes can be in about states. The .Fn arc4random function returns pseudo-random numbers in the range of 0 to and therefore has twice the range of .Xr rand 3 and .Xr random 3 .
p .Fn arc4random_buf function fills the region .Fa buf of length .Fa nbytes with ARC4-derived random data.
p .Fn arc4random_uniform will return a uniformly distributed random number less than .Fa upper_bound . .Fn arc4random_uniform is recommended over constructions like .Dq Li arc4random() % upper_bound as it avoids "modulo bias" when the upper bound is not a power of two.
p The .Fn arc4random_stir function reads data from
a /dev/urandom and uses it to permute the S-Boxes via .Fn arc4random_addrandom .
p There is no need to call .Fn arc4random_stir before using .Fn arc4random functions family, since they automatically initialize themselves. .Sh EXAMPLES The following produces a drop-in replacement for the traditional .Fn rand and .Fn random functions using .Fn arc4random :
p .Dl "#define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))" .Sh SEE ALSO .Xr rand 3 , .Xr random 3 , .Xr srandomdev 3 .Sh HISTORY
a RC4 has been designed by RSA Data Security, Inc. It was posted anonymously to the USENET and was confirmed to be equivalent by several sources who had access to the original cipher. Since
a RC4 used to be a trade secret, the cipher is now referred to as
a ARC4 .