man > random(7)

random(7)                         Miscellaneous Information Manual                         random(7)

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 genera‐
       tor (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_RAN‐
       DOM 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 com‐
       plexity.

   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 num‐
       bers.  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 ob‐
       tain 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 out‐  │ Never blocks   │ Returns output     │
       │               │ put          │                │ from uninitialized │
       │               │              │                │ CSPRNG (may be low │
       │               │              │                │ entropy and un‐    │
       │               │              │                │ suitable for cryp‐ │
       │               │              │                │ tography)          │
       ├───────────────┼──────────────┼────────────────┼────────────────────┤
       │ 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
       getrandom(2), getauxval(3), getentropy(3), random(4), urandom(4), signal(7)

Linux man-pages 6.7                          2023-10-31                                    random(7)