ARC4RANDOM(3) BSD Programmer's Manual ARC4RANDOM(3)
arc4random, arc4random_buf, arc4random_uniform, arc4random_stir,
arc4random_addrandom, arc4random_push, arc4random_pushb, arc4random_pushk
- arc4 random number generator
#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);
void (deprecated)
arc4random_push(int value);
uint32_t
arc4random_pushb(const void *buf, size_t len);
uint32_t (deprecated, except as macro)
arc4random_pushk(const 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).
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 construc-
tions 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 initialises itself.
The arc4random_push() and arc4random_pushb() functions can be used by
regular non-superuser processes to add entropy into the kernel pool by
means of the wrandom(4) circular addition buffer. This interface can be
disabled by a system administrator. The passed value, as well as the
current time, is passed both to the kernel and added to the internal
userland pool. At the moment, the kernel accepts up to 256 bytes; longer
arguments are XOR folded. The arc4random_pushb() returns a 32-bit random
value like arc4random() which can be used by the caller if desired.
The stdlib.h header file defines a macro arc4random_pushk() which is
aliased to arc4random_pushb() these days and can be used on an ifdef
basis as availability check.
These functions are always successful, and no return value is reserved to
indicate an error.
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. 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. arc4random_push() first
appeared in MirOS #8. arc4random_pushb() first appeared in MirOS #10.
arc4random_pushk() first appeared in MirOS #11. It originally was a func-
tion, but has been deprecated and folded into arc4random_pushb() soon
afterwards. arc4random_push() was deprecated in MirOS #11.
The arc4random_addrandom() prototype should use const u_char * and size_t
similar to arc4random_pushb(). The OpenBSD developers however do not pro-
duce const-clean code or APIs.
MirOS BSD #10-current November 29, 2009 1
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