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RANDOM(4)                BSD Kernel Interfaces Manual                RANDOM(4)

NAME
     random , urandom -- random data source devices.

SYNOPSIS
     pseudo-device random

DESCRIPTION
     The random device produces uniformly distributed random byte values of
     potentially high quality.

     To obtain random bytes, open /dev/random for reading and read from it.

     To add entropy to the random generation system, open /dev/random for
     writing and write data that you believe to be somehow random.

     /dev/urandom is a compatibility nod to Linux. On Linux, /dev/urandom will
     produce lower quality output if the entropy pool drains, while
     /dev/random will prefer to block and wait for additional entropy to be
     collected.  With Yarrow, this choice and distinction is not necessary,
     and the two devices behave identically. You may use either.

OPERATION
     The random device implements the Yarrow pseudo random number generator
     algorithm and maintains its entropy pool.  Addditional entropy is fed to
     the generator regularly by the SecurityServer daemon from random jitter
     measurements of the kernel.  SecurityServer is also responsible for peri-
     odically saving some entropy to disk and reloading it during startup to
     provide entropy in early system operation.

     You may feed additional entropy to the generator by writing it to the
     random device, though this is not required in a normal operating environ-
     ment.

LIMITATIONS AND WARNINGS
     Yarrow is a fairly resilient algorithm, and is believed to be resistant
     to non-root.  The quality of its output is however dependent on regular
     addition of appropriate entropy. If the SecurityServer system daemon
     fails for any reason, output quality will suffer over time without any
     explicit indication from the random device itself.

     Paranoid programmers can counter-act this risk somewhat by collecting
     entropy of their choice (e.g. from keystroke or mouse timings) and seed-
     ing it into random directly before obtaining important random numbers.

FILES
     /dev/random
     /dev/urandom

HISTORY
     A random device appeared in Linux operating system.

Darwin                         September 6, 2001                        Darwin