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-'\" t
-.\" 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>
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.\" The following web page is quite informative:
-.\" http://www.2uo.de/myths-about-urandom/
-.\"
-.TH random 7 (date) "Linux man-pages (unreleased)"
-.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.
-.P
-The following interfaces provide access to output from the kernel CSPRNG:
-.IP \[bu] 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 \[bu]
-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
-.B 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
-.I /dev/random
-device or employing
-.BR getrandom (2)
-with the
-.B GRND_RANDOM
-flag.
-Instead, either read from the
-.I /dev/urandom
-device or employ
-.BR getrandom (2)
-without the
-.B GRND_RANDOM
-flag.
-The cryptographic algorithms used for the
-.I urandom
-source are quite conservative, and so should be sufficient for all purposes.
-.P
-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\[ha]128 operations to break) so a key generator
-needs only 128 bits (16 bytes) of seed material from
-.IR /dev/random .
-.P
-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)