random(7) — Linux manual page

NAME | DESCRIPTION | SEE ALSO | COLOPHON

RANDOM(7)               Linux Programmer's Manual              RANDOM(7)

NAME         top

       random - overview of interfaces for obtaining randomness

DESCRIPTION         top

       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.

SEE ALSO         top

       getrandom(2), getauxval(3), getentropy(3), random(4), urandom(4),
       signal(7)

COLOPHON         top

       This page is part of release 5.13 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/.

Linux                          2017-03-13                      RANDOM(7)

Pages that refer to this page: getrandom(2)getentropy(3)random(4)