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RANDOM(4)			    Linux Programmer's 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) pro-
       vide an interface to the kernel's random number generator.   File  /dev/random  has  major
       device  number  1  and minor device number 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  num-
       ber  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 theoreti-
       cally vulnerable to a cryptographic attack on the algorithms used by the  driver.   Knowl-
       edge  of how to do this is not available in the current unclassified literature, but it is
       theoretically possible that such an attack may exist.  If this is a concern in your appli-
       cation, 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 nonblocking mode (or perform a read with
       timeout), and provide some sort of user notification if the desired entropy is not immedi-
       ately 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 ran-
       dom 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 gen-
       erator 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 ker-
       nel random pool per invocation, or per reasonable  reseed  interval  (not  less	than  one
       minute),  that  should  be  taken  as a sign that its cryptography is not skilfully imple-

       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 information 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 sys-
       tem 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 addi-
       tional interface to the /dev/random device.

       The read-only file entropy_avail gives the available entropy.  Normally, 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 contains 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 3.25 of the Linux man-pages project.  A description of the
       project,  and  information  about  reporting  bugs,  can  be  found   at   http://www.ker-

Linux					    2008-06-20					RANDOM(4)
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