urandom

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 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 bit of the 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 to /dev/random will block until additional environmental noise is
       gathered.

       When  read,  /dev/urandom  device  will  return  as  many  bytes as are
       requested.  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.

CONFIGURING
       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 kernel random number generator..."
            # Initialize kernel random number generator with random seed
            # from last shut-down (or start-up) to this start-up.  Load and
            # then save 512 bytes, which is the size of the entropy pool.
            if [ -f /var/random-seed ]; then
                 cat /var/random-seed >/dev/urandom
            fi
            dd if=/dev/urandom of=/var/random-seed count=1

       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 for the random
            # number generator.  Save 512 bytes, which is the size of the
            # random number generator's entropy pool.
            echo "Saving random seed..."
            dd if=/dev/urandom of=/var/random-seed count=1

FILES
       /dev/random
       /dev/urandom

AUTHOR
       The  kernel's  random  number  generator  was  written by Theodore Ts'o
       (tytso@athena.mit.edu).

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
















Linux                             1997-08-01                         RANDOM(4)