RANDOM(4)						     Linux Programmer's Manual							 RANDOM(4)

NAME

       random, urandom - kernel random number source devices

DESCRIPTION

       The  character  special	files /dev/random and /dev/urandom (present since Linux 1.3.30) provide 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  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 unclassified literature, but it is theoretically possible that such an attack may  exist.
       If this is a concern in your application, use /dev/random instead.

   Usage
       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 below (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 ade-
       quate 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 immediately available.

       The  kernel random-number generator is designed to produce a small amount of high-quality seed material to seed a cryptographic pseudo-ran-
       dom 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
       random pool per invocation, or per reasonable reseed interval (not less than one minute), that should be taken as a sign that its  cryptog-
       raphy is not skilfully implemented.

   Configuration
       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 informa-
       tion  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..."
	   random_seed=/var/run/random-seed
	   # 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
	   else
	       touch $random_seed
	   fi
	   chmod 600 $random_seed
	   poolfile=/proc/sys/kernel/random/poolsize
	   [ -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..."
	   random_seed=/var/run/random-seed
	   touch $random_seed
	   chmod 600 $random_seed
	   poolfile=/proc/sys/kernel/random/poolsize
	   [ -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.  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.

FILES

       /dev/random
       /dev/urandom

SEE ALSO

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

COLOPHON

       This  page is part of release 3.27 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								    2010-08-29								 RANDOM(4)