HardenedBSD/share/man/man9/random.9

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.\"	Mark R V Murray
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.Dd July 16, 2015
.Dt RANDOM 9
.Os
.Sh NAME
.Nm arc4rand ,
.Nm arc4random ,
.Nm random ,
.Nm read_random ,
.Nm read_random_uio ,
.Nm srandom
.Nd supply pseudo-random numbers
.Sh SYNOPSIS
.In sys/libkern.h
.Ft void
.Fn srandom "u_long seed"
.Ft u_long
.Fn random "void"
.Ft void
.Fn arc4rand "void *ptr" "u_int length" "int reseed"
.Ft uint32_t
.Fn arc4random "void"
.Pp
.In sys/random.h
.Ft int
.Fn read_random "void *buffer" "int count"
.Ft int
.Fn read_random_uio "struct uio *uio" "bool nonblock"
.Sh DESCRIPTION
The
.Fn random
function will by default produce
a sequence of numbers
that can be duplicated
by calling
.Fn srandom
with some constant
as the
.Fa seed .
The
.Fn srandom
function may be called with any arbitrary
.Fa seed
value to get slightly more unpredictable numbers.
It is important to remember that the
.Fn 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.
.Pp
The
.Fn arc4rand
function will return very good quality random numbers,
better suited
for security-related purposes.
The random numbers from
.Fn 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
.Fa reseed
argument.
.Pp
The
.Fn 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
.Fa buffer
is ignored
and zero is returned.
The
.Fa buffer
is filled with no more than
.Fa count
bytes.
It is strongly advised that
.Fn read_random
is not used;
instead use
.Fn arc4rand
unless it is
necessary to know
that no entropy
has been returned.
.Pp
The
.Fn read_random_uio
function behaves identically to
.Xr read 2
on
.Pa /dev/random .
The
.Fa 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
.Fa nonblock
argument is true.
.Pp
All the bits returned by
.Fn random ,
.Fn arc4rand ,
.Fn read_random ,
and
.Fn read_random_uio
are usable.
For example,
.Sq Li random()&01
will produce a random binary value.
.Pp
The
.Fn arc4random
is a convenience function which calls
.Fn arc4rand
to return a 32 bit pseudo-random integer.
.Sh RETURN VALUES
The
.Fn 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
.if t 2\u\s731\s10\d\(mi1.
.if n (2**31)\(mi1.
The period of this random number generator
is very large,
approximately
.if t 16\(mu(2\u\s731\s10\d\(mi1).
.if n 16*((2**31)\(mi1).
.Pp
The
.Fn arc4rand
function uses the RC4 algorithm
to generate successive pseudo-random bytes.
The
.Fn arc4random
function uses
.Fn arc4rand
to generate pseudo-random numbers
in the range from 0 to
.if t 2\u\s732\s10\d\(mi1.
.if n (2**32)\(mi1.
.Pp
The
.Fn read_random
function returns
the number of bytes placed in
.Fa buffer .
.Pp
.Fn read_random_uio
returns zero when successful,
otherwise an error code is returned.
.Sh ERRORS
.Fn read_random_uio
may fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
.Fa uio
points to an invalid memory region.
.It Bq Er EWOULDBLOCK
The random device is unseeded and
.Fa nonblock
is true.
.El
.Sh AUTHORS
.An Dan Moschuk
wrote
.Fn arc4random .
.An Mark R V Murray
wrote
.Fn read_random .