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RANDOM(4)		   Linux Programmers Manual		    RANDOM(4)

       random, urandom - kernel random number source devices

       The character special files /dev/random and /dev/urandom (present since
       Linux 1.3.30) provide an interface to the kernels random number gener
       ator.  File /dev/random has major device number 1 and minor device num
       ber 8.  File /dev/urandom has major device number 1  and  minor	device
       number 9.

       The  random  number  generator  gathers environmental noise from device
       drivers and other sources into an entropy  pool.   The  generator  also
       keeps  an  estimate of the number of bits of noise in the entropy pool.
       From this entropy pool random numbers are created.

       When read, the /dev/random device will only return random bytes	within
       the estimated number of bits of noise in the entropy pool.  /dev/random
       should be suitable for uses that need very high quality randomness such
       as  one-time  pad  or  key generation.  When the entropy pool is empty,
       reads from /dev/random will block until additional environmental  noise
       is gathered.

       A  read	from  the  /dev/urandom device will not block waiting for more
       entropy.  As a result, if  there  is  not  sufficient  entropy  in  the
       entropy	pool,  the  returned  values are theoretically vulnerable to a
       cryptographic attack on the algorithms used by the  driver.   Knowledge
       of how to do this is not available in the current non-classified liter
       ature, but it is theoretically possible that such an attack may	exist.
       If this is a concern in your application, use /dev/random instead.

       If  you	are  unsure  about  whether  you  should  use  /dev/random  or
       /dev/urandom, then probably you want to use the latter.	As  a  general
       rule,  /dev/urandom  should  be	used  for everything except long-lived
       GPG/SSL/SSH keys.

       If a seed file is saved across reboots as recommended above (all  major
       Linux  distributions have done this since 2000 at least), the output is
       cryptographically secure against attackers without local root access as
       soon as it is reloaded in the boot sequence, and perfectly adequate for
       network encryption session keys.   Since  reads	from  /dev/random  may
       block, users will usually want to open it in non-blocking mode (or per
       form a read with timeout), and provide some sort of  user  notification
       if the desired entropy is not immediately available.

       The  kernel  random-number  generator  is  designed  to produce a small
       amount of high-quality seed material to seed  a	cryptographic  pseudo-
       random  number  generator  (CPRNG).   It  is designed for security, not
       speed, and is poorly suited to generating large amounts of random data.
       Users  should  be  very	economical in the amount of seed material that
       they read from /dev/urandom (and  /dev/random);	unnecessarily  reading
       large  quantities  of data from this device will have a negative impact
       on other users of the device.

       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 only needs
       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	CPRNG  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 ran
       dom pool per invocation, or per reasonable re-seed interval  (not  less
       than  one minute), that should be taken as a sign that its cryptography
       is not skilfuly implemented.

       If your system does  not  have  /dev/random  and  /dev/urandom  created
       already, they can be created with the following commands:

	   mknod -m 644 /dev/random c 1 8
	   mknod -m 644 /dev/urandom c 1 9
	   chown root:root /dev/random /dev/urandom

       When  a	Linux  system starts up without much operator interaction, the
       entropy pool may be in a fairly predictable state.   This  reduces  the
       actual  amount  of  noise  in  the entropy pool below the estimate.  In
       order to counteract this effect, it helps to carry entropy pool	infor
       mation  across shut-downs and start-ups.  To do this, add the following
       lines to an appropriate script which is run  during  the  Linux	system
       start-up sequence:

	   echo "Initializing random number generator..."
	   # Carry a random seed from start-up to start-up
	   # Load and then save the whole entropy pool
	   if [ -f $random_seed ]; then
	       cat $random_seed >/dev/urandom
	       touch $random_seed
	   chmod 600 $random_seed
	   [ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
	   dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

       Also,  add  the	following  lines in an appropriate script which is run
       during the Linux system shutdown:

	   # Carry a random seed from shut-down to start-up
	   # Save the whole entropy pool
	   echo "Saving random seed..."
	   touch $random_seed
	   chmod 600 $random_seed
	   [ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
	   dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

   /proc Interface
       The files  in  the  directory  /proc/sys/kernel/random  (present  since
       2.3.16) provide an additional interface to the /dev/random device.

       The  read-only  file  entropy_avail  gives the available entropy.  Nor
       mally, this will be 4096 (bits), a full entropy pool.

       The file poolsize gives the size of the entropy pool.  The semantics of
       this file vary across kernel versions:

	      Linux 2.4:  This	file  gives  the  size	of the entropy pool in
			  bytes.  Normally, this file will have the value 512,
			  but  it is writable, and can be changed to any value
			  for which an algorithm is  available.   The  choices
			  are 32, 64, 128, 256, 512, 1024, or 2048.

	      Linux 2.6:  This	file  is  read-only, and gives the size of the
			  entropy pool in bits.  It contains the value 4096.

       The file read_wakeup_threshold contains the number of bits  of  entropy
       required  for  waking  up processes that sleep waiting for entropy from
       /dev/random.  The default is 64.  The file write_wakeup_threshold  con
       tains  the  number  of bits of entropy below which we wake up processes
       that do a select(2) or poll(2) for write access to /dev/random.	 These
       values can be changed by writing to the files.

       The  read-only  files  uuid  and  boot_id  contain  random strings like
       6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.  The former is  generated	afresh
       for each read, the latter was generated once.


       mknod (1)
       RFC 1750, "Randomness Recommendations for Security"

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-06-20			     RANDOM(4)

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