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.Dd August 27, 2024
.Dt ARC4RANDOM 3
.Os
.Sh NAME
.Nm arc4random ,
.Nm arc4random_uniform ,
.Nm arc4random_buf ,
.Nm arc4random_stir ,
.Nm arc4random_addrandom
.Nd random number generator
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft uint32_t
.Fn arc4random "void"
.Ft uint32_t
.Fn arc4random_uniform "uint32_t bound"
.Ft void
.Fn arc4random_buf "void *buf" "size_t len"
.Ft void
.Fn arc4random_stir "void"
.Ft void
.Fn arc4random_addrandom "unsigned char *buf" "int len"
.Sh DESCRIPTION
The
.Nm
family of functions provides a cryptographic pseudorandom number
generator automatically seeded from the system entropy pool and safe to
use from multiple threads.
.Nm
is designed to prevent an adversary from guessing outputs,
unlike
.Xr rand 3
and
.Xr random 3 ,
and is faster and more convenient than reading from
.Pa /dev/urandom
directly.
.Pp
.Fn arc4random
returns an integer in [0, 2^32) chosen independently with uniform
distribution.
.Pp
.Fn arc4random_uniform
returns an integer in [0,
.Fa bound )
chosen independently with uniform distribution.
.Pp
.Fn arc4random_buf
stores
.Fa len
bytes into the memory pointed to by
.Fa buf ,
each byte chosen independently from [0, 256) with uniform
distribution.
.Pp
.Fn arc4random_stir
draws entropy from the operating system and incorporates it into the
library's PRNG state to influence future outputs.
.Pp
.Fn arc4random_addrandom
incorporates
.Fa len
bytes, which must be nonnegative, from the buffer
.Fa buf ,
into the library's PRNG state to influence future outputs.
.Pp
It is not necessary for an application to call
.Fn arc4random_stir
or
.Fn arc4random_addrandom
before calling other
.Nm
functions.
The first call to any
.Nm
function will initialize the PRNG state unpredictably from the system
entropy pool.
.Sh SECURITY MODEL
The
.Nm
functions provide the following security properties against three
different classes of attackers, assuming enough entropy is provided by
the operating system:
.Bl -enum -offset abcd
.It
An attacker who has seen some outputs of any of the
.Nm
functions cannot predict past or future unseen outputs.
.It
An attacker who has seen the library's PRNG state in memory cannot
predict past outputs.
.It
An attacker who has seen one process's PRNG state cannot predict past
or future outputs in other processes, particularly its parent or
siblings.
.El
.Pp
One
.Sq output
means the result of any single request to an
.Nm
function, no matter how short it is.
.Pp
The second property is sometimes called
.Sq forward secrecy ,
.Sq backtracking resistance ,
or
.Sq key erasure after each output .
.Sh IMPLEMENTATION NOTES
The
.Nm
functions are currently implemented using the ChaCha20 pseudorandom
function family.
For any 32-byte string
.Fa s ,
.Pf ChaCha20_ Fa s
is a function from 16-byte strings to 64-byte strings.
It is conjectured that if
.Fa s
is chosen with uniform distribution, then the distribution on
.Pf ChaCha20_ Fa s
is indistinguishable to a computationally bounded adversary from a
uniform distribution on all functions from 16-byte strings to 64-byte
strings.
.Pp
The PRNG state is a 32-byte ChaCha20 key
.Fa s .
Each request to
an
.Nm
function
.Bl -bullet -offset abcd -compact
.It
computes the 64-byte quantity
.Fa x
=
.Pf ChaCha20_ Fa s Ns Pq 0 ,
.It
splits
.Fa x
into two 32-byte quantities
.Fa s'
and
.Fa k ,
.It
replaces
.Fa s
by
.Fa s' ,
and
.It
uses
.Fa k
as output.
.El
.Pp
.Fn arc4random
yields the first four bytes of
.Fa k
as output directly.
.Fn arc4random_buf
either yields up to 32 bytes of
.Fa k
as output directly, or, for longer
requests, uses
.Fa k
as a ChaCha20 key and yields the concatenation
.Pf ChaCha20_ Fa k Ns Pq 0
||
.Pf ChaCha20_ Fa k Ns Pq 1
|| ... as output.
.Fn arc4random_uniform
repeats
.Fn arc4random
until it obtains an integer in [2^32 %
.Fa bound ,
2^32), and reduces that modulo
.Fa bound .
.Pp
The PRNG state is per-thread, unless memory allocation fails inside the
library, in which case some threads may share global PRNG state with a
mutex.
The global PRNG state is zeroed on fork in the parent via
.Xr pthread_atfork 3 ,
and the per-thread PRNG state is zeroed on fork in the child via
.Xr minherit 2
with
.Dv MAP_INHERIT_ZERO ,
so that the child cannot reuse or see the parent's PRNG state.
The PRNG state is reseeded automatically from the system entropy pool
on the first use of an
.Nm
function after zeroing.
.Pp
The first use of an
.Nm
function may abort the process in the highly unlikely event that
library initialization necessary to implement the security model fails.
Additionally,
.Fn arc4random_stir
and
.Fn arc4random_addrandom
may abort the process in the highly unlikely event that the operating
system fails to provide entropy.
.Pp
If
.Nm
detects that the sysctl variable
.Li kern.entropy.epoch
.Pq see Xr rnd 4
has changed since its last output, it reseeds itself with additional
data from the system entropy pool again before generating its next
output.
.Sh SEE ALSO
.Xr rand 3 ,
.Xr random 3 ,
.Xr rnd 4 ,
.Xr cprng 9
.Rs
.%A Daniel J. Bernstein
.%T ChaCha, a variant of Salsa20
.%D 2008-01-28
.%O Document ID: 4027b5256e17b9796842e6d0f68b0b5e
.%U http://cr.yp.to/papers.html#chacha
.Re
.Sh BUGS
There is no way to get deterministic, reproducible results out of
.Nm
for testing purposes.
.Pp
The name
.Sq arc4random
was chosen for hysterical raisins \(em it was originally implemented
using the RC4 stream cipher, which has been known since shortly after
it was published in 1994 to have observable biases in the output, and
is now known to be broken badly enough to admit practical attacks in
the real world.
.\" Bob Jenkins, sci.crypt post dated 1994-09-16, message-id
.\" <359qjg$55v$1@mhadg.production.compuserve.com>,
.\" https://groups.google.com/d/msg/sci.crypt/JsO3xEATGFA/-wO4ttv7BCYJ
.\"
.\" Andrew Roos, `A Class of Weak Keys in the RC4 Stream Cipher',
.\" sci.crypt posts dated 1995-09-22, message-ids
.\" <43u1eh$1j3@hermes.is.co.za> and <44ebge$llf@hermes.is.co.za>.
.\"
.\" Paul Crowley, `Small bias in RC4 experimentally verified', March
.\" 1998, http://www.ciphergoth.org/crypto/rc4/
Unfortunately, the library found widespread adoption and the name stuck
before anyone recognized that it was silly.
.Pp
The signature of
.Fn arc4random_addrandom
is silly.
There is no reason to require casts or accept negative lengths:
it should take a
.Vt void *
buffer and a
.Vt size_t
length.
But it's too late to change that now.
.Pp
.Fn arc4random_uniform
does not help to choose integers in [0,
.Fa n )
uniformly at random when
.Fa n
> 2^32.
.Pp
The security model of
.Nm
is stronger than many applications need, and stronger than other
operating systems provide.
For example, applications encrypting messages with random, but not
secret, initialization vectors need only prevent an adversary from
guessing future outputs, since past outputs will have been published
already.
.Pp
On the one hand,
.Nm
could be marginally faster if it were not necessary to prevent an
adversary who sees the state from predicting past outputs.
On the other hand, there are applications in the wild that use
.Nm
to generate key material, such as OpenSSH, so for the sake of
.Nx
users it would be imprudent to weaken the security model.
On the third hand, relying on the security model of
.Nm
in
.Nx
may lead you to an unpleasant surprise on another operating system
whose implementation of
.Nm
has a weaker security model.
.Pp
One may be tempted to create new APIs to accommodate different
security models and performance constraints without unpleasant
surprises on different operating systems.
This should not be done lightly, though, because there are already too
many different choices, and too many opportunities for programmers to
reach for one and pick the wrong one.