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ARC4RANDOM(3) Library Functions Manual ARC4RANDOM(3)

arc4random, arc4random_buf, arc4random_uniform, arc4random_stir, arc4random_addrandom
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);

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() provides a cryptographically secure pseudo-random number generator, as opposed to 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 constructions 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.

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

rand(3), rand48(3), random(3)

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 Security, Inc. could do nothing about the release of the ‘Alleged RC4’ algorithm. Since RC4 was trademarked, the cipher is now referred to as ARC4.
These functions first appeared in OpenBSD 2.1.
June 5, 2013 OpenBSD-5.4