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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.

       ┌──────────────┬──────────────┬────────────────┬────────────────────┐
       │InterfacePoolBlockingBehavior when pool │
       │              │              │ behavioris 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

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

COLOPHON

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       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.