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random(4)
NAME
  random, urandom - Kernel random number source devices

DESCRIPTION
  The /dev/random and /dev/urandom character device special files provide an
  interface to the kernel random number generator. The random number
  generator gathers environmental data from device drivers and other sources
  into an entropy pool. The generator keeps an estimate of the number of bits
  of data in the entropy pool. Random numbers are generated from this entropy
  pool.

  Writing data to the /dev/random or /dev/urandom device will mix the new
  data into the entrophy pool to make the random data better, but it will not
  increase the entrophy estimate. This prevents a user from writing lots of
  low-entrophy data to the pool and having the /dev/random device generate
  poor random data.

FILES
  /dev/random
      A character device special file that provides an interface to the
      kernel random number generator.  When read, the /dev/random file will
      return only random bytes within the estimated number of bits of noise
      in the entropy pool. This is suitable for uses that need high quality
      randomness, such as one-time pad or key generation. If the entropy pool
      is empty or does not contain enough bytes, reads to the /dev/random
      file will be blocked until additional environment noise is gathered.

  /dev/urandom
      A character device special file that provides an interface to the
      kernel random number generator. When read, the /dev/urandom file will
      return as many random bytes as are requested. As a result, if there are
      not sufficient bytes in the entropy pool, reads to the /dev/urandom
      will not be blocked and the returned values are theoretically
      vulnerable to a cryptographic attack on the algorithms used by the
      driver. If this is a concern for your application, use the /dev/random
      device instead.

EXAMPLES
   1.  To generate a 16-byte random value and display it in hexadecimal
       characters, enter:
	    % dd bs=16 count=1 < /dev/random | od -An -tx2
	    dfd5 7b74 0049 359f 35ed b2db e70d e629

   2.  To generate eight random printable characters, enter:
	    % dd bs=10 count=10 < /dev/random | tr -cd '[:print:]' | head -c8
	    sHp3#t6w

   3.  To select 4 random words from a document, for example, Project
       Gutenberg's "Complete Works of Shakespeare" (this example is for csh),
       enter:
	    % tr -cs '[:alpha:]' '\n' < shaks12.txt > shaks12.temp
	    % set wc=`wc -l < shaks12.temp`
	    % set words=(`dd bs=16 count=1 < /dev/random | od -tu4 -An`)
	    % set w1=`expr $words[1] % $wc + 1`
	    % set w2=`expr $words[2] % $wc + 1`
	    % set w3=`expr $words[3] % $wc + 1`
	    % set w4=`expr $words[4] % $wc + 1`
	    % sed -n -e "$w1 p" -e "$w2 p" -e "$w3 p" -e "$w4 p" < shaks12.temp
	    wrested
	    affected
	    Cyprus
	    least

LEGAL NOTICES
  Derived from linux/drivers/char/random.c 2.4.0-test9

  random.c -- A strong random number generator

  Version 1.89, last modified 19-Sep-99

  Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999.	All rights
  reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are met:

   1.  Redistributions of source code must retain the above copyright notice,
       and the entire permission notice in its entirety, including the
       disclaimer of warranties.

   2.  Redistributions in binary form must reproduce the above copyright
       notice, this list of conditions and the following disclaimer in the
       documentation and/or other materials provided with the distribution.

   3.  The name of the author may not be used to endorse or promote products
       derived from this software without specific prior written permission.

  ALTERNATIVELY, this product may be distributed under the terms of the GNU
  Public License, in which case the provisions of the GPL are required
  INSTEAD OF the above restrictions.  (This clause is necessary due to a
  potential bad interaction between the GPL and the restrictions contained in
  a BSD-style copyright.)

  THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
  INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
  AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF WHICH ARE HEREBY DISCLAIMED.
  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH	 DAMAGE.

Index for Section 4

Alphabetical listing for R

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