RANDOM
Section: Linux Programmer's Manual (7)
Updated: 2017-03-13
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NAME
random - overview of interfaces for obtaining randomness
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.
The following interfaces provide access to output from the kernel CSPRNG:
- *
-
The
/dev/urandom
and
/dev/random
devices, both described in
random(4).
These devices have been present on Linux since early times,
and are also available on many other systems.
- *
-
The Linux-specific
getrandom(2)
system call, available since Linux 3.17.
This system call provides access either to the same source as
/dev/urandom
(called the
urandom
source in this page)
or to the same source as
/dev/random
(called the
random
source in this page).
The default is the
urandom
source; the
random
source is selected by specifying the
GRND_RANDOM
flag to the system call.
(The
getentropy(3)
function provides a slightly more portable interface on top of
getrandom(2).)
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.
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
/dev/random
device or employing
getrandom(2)
with the
GRND_RANDOM
flag.
Instead, either read from the
/dev/urandom
device or employ
getrandom(2)
without the
GRND_RANDOM
flag.
The cryptographic algorithms used for the
urandom
source are quite conservative, and so should be sufficient for all purposes.
The disadvantage of
GRND_RANDOM
and reads from
/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
GRND_RANDOM
or when reading from
/dev/random
increases code complexity.
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.
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.
GRND_NONBLOCK
is a flag that can be used to control the blocking behavior of
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.
| Interface | Pool |
Blocking
behavior
|
Behavior when pool is not yet ready
|
|
/dev/random
|
Blocking pool
|
If entropy too low, blocks until there is enough entropy again
|
Blocks until enough entropy gathered
|
|
/dev/urandom
|
CSPRNG output
|
Never blocks
|
Returns output from uninitialized CSPRNG (may be low entropy and unsuitable for cryptography)
|
|
getrandom()
|
Same as
/dev/urandom
|
Does not block once is pool ready
|
Blocks until pool ready
|
|
getrandom()
GRND_RANDOM
|
Same as
/dev/random
|
If entropy too low, blocks until there is enough entropy again
|
Blocks until pool ready
|
|
getrandom()
GRND_NONBLOCK
|
Same as
/dev/urandom
|
Does not block once is pool ready
|
EAGAIN
|
|
getrandom()
GRND_RANDOM
+
GRND_NONBLOCK
|
Same as
/dev/random
|
EAGAIN
if not enough entropy available
|
EAGAIN
|
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
/dev/random.
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
not
skillfully implemented.
SEE ALSO
getrandom(2),
getauxval(3),
getentropy(3),
random(4),
urandom(4),
signal(7)
COLOPHON
This page is part of release 5.11 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- DESCRIPTION
-
- Initialization of the entropy pool
-
- Choice of random source
-
- Monte Carlo and other probabilistic sampling applications
-
- Comparison between getrandom, /dev/urandom, and /dev/random
-
- Generating cryptographic keys
-
- SEE ALSO
-
- COLOPHON
-
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