random, urandom - Kernel random number source devices
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 lowentrophy
data to the pool and having the /dev/random
device generate poor random data.
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. 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.
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
To generate eight random printable characters, enter: % dd
bs=10 count=10 < /dev/random | tr -cd '[:print:]' | head
-c8 sHp3#t6w
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
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: Redistributions of source code
must retain the above copyright notice, and the entire
permission notice in its entirety, including the disclaimer
of warranties. 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.
The name of the author may not be used to endorse
or promote products derived from this software without
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ALTERNATIVELY, this product may be distributed under the
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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
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random(4)
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