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

       random - overview of interfaces for obtaining randomness

       The  kernel random-number generator relies on entropy gathered from de-
       vice drivers and other sources of environmental noise to seed a crypto-
       graphically  secure  pseudorandom number generator (CSPRNG).  It is de-
       signed for security, rather than speed.

       The following interfaces provide  access  to  output  from  the  kernel

       *  The  /dev/urandom  and  /dev/random  devices, both described in ran-
          dom(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  geten-
          tropy(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  suf-
       ficient  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 pur-

       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  proba-
       bilistic  sampling  will  be slow.  Furthermore, it is unnecessary, be-
       cause 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 genera-
       tor 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 fi-
       nal 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 ran-
       dom 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.

       getrandom(2), getauxval(3), getentropy(3), random(4), urandom(4),  sig-

       This  page  is  part of release 5.05 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

Linux                             2017-03-13                         RANDOM(7)
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