ARC4RANDOM(3) BSD Programmer's Manual ARC4RANDOM(3)
arc4random, arc4random_stir, arc4random_addrandom, arc4random_push,
arc4random_pushb, arc4random_pushk - arc4 random number generator
#include <stdlib.h>
u_int32_t
arc4random(void);
void
arc4random_stir(void);
void
arc4random_addrandom(u_char *dat, int datlen);
void
arc4random_push(int value);
uint32_t
arc4random_pushb(void *buf, size_t len);
uint32_t
arc4random_pushk(void *buf, size_t len);
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).
The arc4random_stir() function reads data from /dev/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 initialises itself.
The arc4random_push() and arc4random_pushb() functions can be used by
processes to add entropy into the kernel pool by means of a circular ad-
dition buffer. This interface can be disabled by a system administrator.
The passed value is also used to stir the internal userland arc4random()
pool value, more or less directly as well as with the value returned by
the kernel after adding the arguments to the kernel pool. At the moment,
the kernel accepts up to 256 bytes. The arc4random_pushb() returns a 32-
bit random value like arc4random() which can be used by the caller if
desired.
The arc4random_pushk() function works exactly like the arc4random_pushb()
function, except in that it does not pull in all the other arc4random
functions, yielding a smaller executable size, dependencies, and faster
speed. Use this in applications that do not use the arc4random function
suite themselves, but want to push back data to the kernel. It also re-
turns a 32-bit random value. If more than 256 octets are passed, they are
XOR-folded. The stdlib.h header file defines a macro with the same name
as this function, which you can use to see if it's available.
rand(3), rand48(3), random(3), random(9)
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. can do nothing about the release of the ARC4 algorithm. Since
RC4 used to be a trade secret, the cipher is now referred to as ARC4.
These functions first appeared in OpenBSD 2.1. arc4random_push() first
appeared in MirOS #8. arc4random_pushb() first appeared in MirOS #10.
arc4random_pushk() first appeared in MirOS #11.
MirOS BSD #10-current April 6, 2008 1
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