bsd/man/man4/random.4

   1 .Dd September 6, 2001
   2 .Dt RANDOM 4
   3 .Os Darwin
   4 .Sh NAME
   5 .Nm random
   6 ,
   7 .Nm urandom
   8 .Nd random data source devices.
   9 .Sh SYNOPSIS
  10 .Cd "pseudo-device random"
  11 .Sh DESCRIPTION
  12 The
  13 .Nm
  14 device produces uniformly distributed random byte values
  15 of potentially high quality.
  16 .Pp
  17 To obtain random bytes, open
  18 .Nm /dev/random
  19 for reading and read from it.
  20 .Pp
  21 To add entropy to the random generation system, open
  22 .Nm /dev/random
  23 for writing and write data that you believe to be somehow random.
  24 .Pp
  25 .Nm /dev/urandom
  26 is a compatibility nod to Linux. On Linux,
  27 .Nm /dev/urandom
  28 will produce lower quality output if the entropy pool drains, while
  29 .Nm /dev/random
  30 will prefer to block and wait for additional entropy to be collected.
  31 With Yarrow, this choice and distinction is not necessary, and
  32 the two devices behave identically. You may use either.
  33 .Sh OPERATION
  34 The
  35 .Nm
  36 device implements the
  37 .Nm Yarrow
  38 pseudo random number generator algorithm and maintains its entropy pool.
  39 Additional entropy is fed to the generator regularly by the
  40 .Nm SecurityServer
  41 daemon from random jitter measurements of the kernel.
  42 .Nm SecurityServer
  43 is also responsible for periodically saving some entropy to disk
  44 and reloading it during startup to provide entropy in early system
  45 operation.
  46 .Pp
  47 You may feed additional entropy to the generator by writing it to the
  48 .Nm
  49 device, though this is not required in a normal operating environment.
  50 .Sh LIMITATIONS AND WARNINGS
  51 .Nm Yarrow
  52 is a fairly resilient algorithm, and is believed
  53 to be resistant to non-root.
  54 The quality of its output is however dependent on regular addition
  55 of appropriate entropy. If the
  56 .Nm SecurityServer
  57 system daemon fails for any reason, output quality will suffer
  58 over time without any explicit indication from the
  59 .Nm
  60 device itself.
  61 .Pp
  62 Paranoid programmers can counteract this risk somewhat by collecting
  63 entropy of their choice (e.g. from keystroke or mouse timings)
  64 and seeding it into
  65 .Nm
  66 directly before obtaining important random numbers.
  67 .Sh FILES
  68 .Bl -tag -width /dev/urandom -compact
  69 .It Pa /dev/random
  70 .It Pa /dev/urandom
  71 .El
  72 .Sh HISTORY
  73 A
  74 .Nm
  75 device appeared in the Linux operating system.