ARC4RANDOM(3) BSD Programmer's Manual ARC4RANDOM(3)
arc4random, arc4random_buf, arc4random_uniform, arc4random_stir, arc4random_addrandom, arc4random_push, arc4random_pushb, arc4random_pushb_fast, arc4random_pushk - arcfour based stretching 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 (deprecated) arc4random_pushb(const void *buf, size_t len); uint32_t (deprecated) arc4random_pushk(const void *buf, size_t len); void arc4random_pushb_fast(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 collects data from the user-space SRNG and queued for upload, sends it to the kernel and receives new entropic data using sysctl(3) from kern.arandom and uses it to permute the S-Boxes. There is no need to call arc4random_stir() before using arc4random(), since arc4random() automatically initialises itself. Explicit calls will, however, trigger kernel pushing after arc4random_pushb_fast() has been used to queue some data for doing so. The arc4random_pushb_fast() function will queue the data passed for upload on the next stir using a (non-cryptographic) hash algorithm, which is only used for collapsing/compressing the data into the queue buffer. The deprecated arc4random_push() function behaves the same, except in- stead of being passed a buffer and its size for pushing, it takes a direct numeric argument. The deprecated arc4random_pushk() and arc4random_pushb() functions do exactly the same, namely call arc4random_pushb_fast() and then call arc4random() and return its return value. The arc4random_addrandom() function implements an old, discouraged, API with which the S-Boxes can be permuted directly from user-specified data. If merely desiring to add entropy to the pool, use arc4random_pushb_fast instead, which is much faster, unless you really want to do a KSA.
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() and arc4random_pushb_fast() first appeared in MirOS #11. All these functions were rewritten for MirOS #11 and macros for every function are now defined for easy existence checks. MirOS BSD #10-current September 21, 2010 1
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