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.
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 5.09 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/.