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 ker-
       nel'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  num-
       bers 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 theo-
       retically 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 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 immediately available.

       The kernel random-number generator is designed to produce a small amount of high-quality seed material to seed a  crypto-
       graphic pseudo-random number generator (CPRNG).  It is designed for security, not speed, and is poorly suited to generat-
       ing 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 exam-
       ple,  a  3072-bit RSA or Diffie-Hellman private key has an effective key size of 128 bits (it requires about 2^128 opera-
       tions 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  crypto-
       graphic  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 cryptography 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 com-
       mands:

           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..."
           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 3.25 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)