man7/random.7

.\" Copyright (C) 2008, George Spelvin <linux@horizon.com>,
.\" and Copyright (C) 2008, Matt Mackall <mpm@selenic.com>
.\" and Copyright (C) 2016, Laurent Georget <laurent.georget@supelec.fr>
.\" and Copyright (C) 2016, Nikos Mavrogiannopoulos <nmav@redhat.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of
.\" this manual under the conditions for verbatim copying, provided that
.\" the entire resulting derived work is distributed under the terms of
.\" a permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume.
.\" no responsibility for errors or omissions, or for damages resulting.
.\" from the use of the information contained herein.  The author(s) may.
.\" not have taken the same level of care in the production of this.
.\" manual, which is licensed free of charge, as they might when working.
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.\" The following web page is quite informative:
.\" http://www.2uo.de/myths-about-urandom/
.\"
.TH RANDOM 7 2017-03-13 "Linux" "Linux Programmer's Manual"
.SH NAME
random \- overview of interfaces for obtaining randomness
.SH DESCRIPTION
The kernel random-number generator relies on entropy gathered from
device drivers and other sources of environmental noise to seed
a cryptographically secure pseudorandom number generator (CSPRNG).
It is designed for security, rather than speed.
.PP
The following interfaces provide access to output from the kernel CSPRNG:
.IP * 3
The
.I /dev/urandom
and
.I /dev/random
devices, both described in
.BR random (4).
These devices have been present on Linux since early times,
and are also available on many other systems.
.IP *
The Linux-specific
.BR getrandom (2)
system call, available since Linux 3.17.
This system call provides access either to the same source as
.I /dev/urandom
(called the
.I urandom
source in this page)
or to the same source as
.I /dev/random
(called the
.I random
source in this page).
The default is the
.I urandom
source; the
.I random
source is selected by specifying the
.BR GRND_RANDOM
flag to the system call.
(The
.BR getentropy (3)
function provides a slightly more portable interface on top of
.BR getrandom (2).)
.\"
.SS Initialization of the entropy pool
The kernel collects bits of entropy from the environment.
When a sufficient number of random bits has been collected, the
entropy pool is considered to be initialized.
.SS Choice of random source
Unless you are doing long-term key generation (and most likely not even
then), you probably shouldn't be reading from the
.IR /dev/random
device or employing
.BR getrandom (2)
with the
.BR GRND_RANDOM
flag.
Instead, either read from the
.IR /dev/urandom
device or employ
.BR getrandom (2)
without the
.B GRND_RANDOM
flag.
The cryptographic algorithms used for the
.IR urandom
source are quite conservative, and so should be sufficient for all purposes.
.PP
The disadvantage of
.B GRND_RANDOM
and reads from
.I /dev/random
is that the operation can block for an indefinite period of time.
Furthermore, dealing with the partially fulfilled
requests that can occur when using
.B GRND_RANDOM
or when reading from
.I /dev/random
increases code complexity.
.\"
.SS Monte Carlo and other probabilistic sampling applications
Using these interfaces to provide large quantities of data for
Monte Carlo simulations or other programs/algorithms which are
doing probabilistic sampling will be slow.
Furthermore, it is unnecessary, because such applications do not
need cryptographically secure random numbers.
Instead, use the interfaces described in this page to obtain
a small amount of data to seed a user-space pseudorandom
number generator for use by such applications.
.\"
.SS Comparison between getrandom, /dev/urandom, and /dev/random
The following table summarizes the behavior of the various
interfaces that can be used to obtain randomness.
.B GRND_NONBLOCK
is a flag that can be used to control the blocking behavior of
.BR getrandom (2).
The final column of the table considers the case that can occur
in early boot time when the entropy pool is not yet initialized.
.ad l
.TS
allbox;
lbw13 lbw12 lbw14 lbw18
l l l l.
Interface	Pool	T{
Blocking
\%behavior
T}	T{
Behavior when pool is not yet ready
T}
T{
.I /dev/random
T}	T{
Blocking pool
T}	T{
If entropy too low, blocks until there is enough entropy again
T}	T{
Blocks until enough entropy gathered
T}
T{
.I /dev/urandom
T}	T{
CSPRNG output
T}	T{
Never blocks
T}	T{
Returns output from uninitialized CSPRNG (may be low entropy and unsuitable for cryptography)
T}
T{
.BR getrandom ()
T}	T{
Same as
.I /dev/urandom
T}	T{
Does not block once is pool ready
T}	T{
Blocks until pool ready
T}
T{
.BR getrandom ()
.B GRND_RANDOM
T}	T{
Same as
.I /dev/random
T}	T{
If entropy too low, blocks until there is enough entropy again
T}	T{
Blocks until pool ready
T}
T{
.BR getrandom ()
.B GRND_NONBLOCK
T}	T{
Same as
.I /dev/urandom
T}	T{
Does not block once is pool ready
T}	T{
.B EAGAIN
T}
T{
.BR getrandom ()
.B GRND_RANDOM
+
.B GRND_NONBLOCK
T}	T{
Same as
.I /dev/random
T}	T{
.B EAGAIN
if not enough entropy available
T}	T{
.B EAGAIN
T}
.TE
.ad
.\"
.SS Generating cryptographic keys
The amount of seed material required to generate a cryptographic key
equals the effective key size of the key.
For example, a 3072-bit RSA
or Diffie-Hellman private key has an effective key size of 128 bits
(it requires about 2^128 operations to break) so a key generator
needs only 128 bits (16 bytes) of seed material from
.IR /dev/random .
.PP
While some safety margin above that minimum is reasonable, as a guard
against flaws in the CSPRNG algorithm, no cryptographic primitive
available today can hope to promise more than 256 bits of security,
so if any program reads more than 256 bits (32 bytes) from the kernel
random pool per invocation, or per reasonable reseed interval (not less
than one minute), that should be taken as a sign that its cryptography is
.I not
skillfully implemented.
.\"
.SH SEE ALSO
.BR getrandom (2),
.BR getauxval (3),
.BR getentropy (3),
.BR random (4),
.BR urandom (4),
.BR signal (7)