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ARC4RANDOM(3)             OpenBSD Programmer's Manual            ARC4RANDOM(3)

NAME
     arc4random, arc4random_buf, arc4random_uniform, arc4random_stir,
     arc4random_addrandom - arc4 random number generator

SYNOPSIS
     #include <stdlib.h>

     u_int32_t
     arc4random(void);

     void
     arc4random_buf(void *buf, size_t nbytes);

     u_int32_t
     arc4random_uniform(u_int32_t upper_bound);

     void
     arc4random_stir(void);

     void
     arc4random_addrandom(u_char *dat, int datlen);

DESCRIPTION
     The arc4random() function provides a high quality 32-bit pseudo-random
     number very quickly.  arc4random() seeds itself on a regular basis from
     the kernel strong random number subsystem described in random(4).  On
     each call, an ARC4 generator is used to generate a new result.  The
     arc4random() function uses the ARC4 cipher key stream generator, which
     uses 8*8 8-bit S-Boxes.  The S-Boxes can be in about (2**1700) states.

     arc4random() fits into a middle ground not covered by other subsystems
     such as the strong, slow, and resource expensive random devices described
     in random(4) versus the fast but poor quality interfaces described in
     rand(3), random(3), and drand48(3).

     arc4random_buf() fills the region buf of length nbytes with ARC4-derived
     random data.

     arc4random_uniform() will return a uniformly distributed random number
     less than upper_bound.  arc4random_uniform() is recommended over con-
     structions like ``arc4random() % upper_bound'' as it avoids "modulo bias"
     when the upper bound is not a power of two.

     The arc4random_stir() function reads data using sysctl(3) from
     kern.arandom and uses it to permute the S-Boxes via
     arc4random_addrandom().

     There is no need to call arc4random_stir() before using arc4random(),
     since arc4random() automatically initializes itself.

RETURN VALUES
     These functions are always successful, and no return value is reserved to
     indicate an error.

SEE ALSO
     rand(3), rand48(3), random(3)

HISTORY
     An algorithm called RC4 was designed by RSA Data Security, Inc.  It was
     considered a trade secret.  Because it was a trade secret, it obviously
     could not be patented.  A clone of this was posted anonymously to USENET
     and confirmed to be equivalent by several sources who had access to the
     original cipher.  Because of the trade secret situation, RSA Data Securi-
     ty, Inc. could do nothing about the release of the `Alleged RC4' algo-
     rithm.  Since RC4 was trademarked, the cipher is now referred to as ARC4.

     These functions first appeared in OpenBSD 2.1.

OpenBSD 4.7                    December 23, 2008                             2

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | SEE ALSO | HISTORY

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