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       random, urandom - kernel random number source devices


       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

       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 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.

       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 system 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 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.




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


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