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 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 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  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 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..."
           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 3.35 of the Linux man-pages project.  A description of the project, and information about reporting  bugs,
       can be found at http://man7.org/linux/man-pages/.
 

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