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 when pool │
│ │ │ behavior │ is not yet ready │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│/dev/random │ Blocking │ If entropy too │ Blocks until │
│ │ pool │ low, blocks │ enough entropy │
│ │ │ until there is │ gathered │
│ │ │ enough entropy │ │
│ │ │ again │ │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│/dev/urandom │ CSPRNG │ Never blocks │ Returns output │
│ │ output │ │ from uninitialized │
│ │ │ │ CSPRNG (may be low │
│ │ │ │ entropy and │
│ │ │ │ unsuitable for │
│ │ │ │ cryptography) │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│getrandom() │ Same as │ Does not block │ Blocks until pool │
│ │ /dev/urandom │ once is pool │ ready │
│ │ │ ready │ │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│getrandom() │ Same as │ If entropy too │ Blocks until pool │
│GRND_RANDOM │ /dev/random │ low, blocks │ ready │
│ │ │ until there is │ │
│ │ │ enough entropy │ │
│ │ │ again │ │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│getrandom() │ Same as │ Does not block │ EAGAIN │
│GRND_NONBLOCK │ /dev/urandom │ once is pool │ │
│ │ │ ready │ │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│getrandom() │ Same as │ EAGAIN if not │ EAGAIN │
│GRND_RANDOM + │ /dev/random │ enough entropy │ │
│GRND_NONBLOCK │ │ available │ │
└──────────────┴──────────────┴────────────────┴────────────────────┘
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.
