| RANDOM(9) | Kernel Developer's Manual | RANDOM(9) |
arc4rand,
arc4random, random,
read_random,
read_random_uio, srandom
— supply pseudo-random numbers
#include
<sys/libkern.h>
void
srandom(u_long
seed);
u_long
random(void);
void
arc4rand(void
*ptr, u_int length,
int reseed);
uint32_t
arc4random(void);
#include <sys/random.h>
int
read_random(void
*buffer, int
count);
int
read_random_uio(struct
uio *uio, bool
nonblock);
The
random()
function will by default produce a sequence of numbers that can be
duplicated by calling
srandom()
with some constant as the seed. The
srandom() function may be called with any arbitrary
seed value to get slightly more unpredictable numbers.
It is important to remember that the random()
function is entirely predictable, and is therefore not of use where
knowledge of the sequence of numbers may be of benefit to an attacker.
The
arc4rand()
function will return very good quality random numbers, better suited for
security-related purposes. The random numbers from
arc4rand() are seeded from the entropy device if it
is available. Automatic reseeds happen after a certain timeinterval and
after a certain number of bytes have been delivered. A forced reseed can be
forced by passing a non-zero value in the reseed
argument.
The
read_random()
function is used to return entropy directly from the entropy device if it
has been loaded. If the entropy device is not loaded, then the
buffer is ignored and zero is returned. The
buffer is filled with no more than
count bytes. It is strongly advised that
read_random() is not used; instead use
arc4rand() unless it is necessary to know that no
entropy has been returned.
The
read_random_uio()
function behaves identically to read(2) on
/dev/random. The uio argument
points to a buffer where random data should be stored. This function only
returns data if the random device is seeded. It blocks if unseeded, except
when the nonblock argument is true.
All the bits returned by
random(),
arc4rand(), read_random(),
and read_random_uio() are usable. For example,
‘random()&01’ will produce a
random binary value.
The
arc4random()
is a convenience function which calls arc4rand() to
return a 32 bit pseudo-random integer.
The random() function uses a non-linear
additive feedback random number generator employing a default table of size
31 containing long integers to return successive pseudo-random numbers in
the range from 0 to (2**31)−1. The period of this random number
generator is very large, approximately 16*((2**31)−1).
The arc4rand() function uses the RC4
algorithm to generate successive pseudo-random bytes. The
arc4random() function uses
arc4rand() to generate pseudo-random numbers in the
range from 0 to (2**32)−1.
The read_random() function returns the
number of bytes placed in buffer.
read_random_uio() returns zero when
successful, otherwise an error code is returned.
read_random_uio() may fail if:
EFAULT]EWOULDBLOCK]Dan Moschuk wrote
arc4random().
Mark R V Murray wrote
read_random().
| July 16, 2015 | Debian |