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ARC4RANDOM(3) Library Functions Manual ARC4RANDOM(3)

arc4random, arc4random_buf, arc4random_uniform, arc4random_stir, arc4random_addrandomarc4 random number generator

#include <stdlib.h>


arc4random_buf(void *buf, size_t nbytes);

arc4random_uniform(u_int32_t upper_bound);


arc4random_addrandom(u_char *dat, int datlen);

The () 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.

() 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).

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

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

There is no need to call () 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.

December 23, 2008 OpenBSD-5.1