1RANDOM(4)                  Linux Programmer's Manual                 RANDOM(4)
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NAME

6       random, urandom - kernel random number source devices
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DESCRIPTION

9       The character special files /dev/random and /dev/urandom (present since
10       Linux 1.3.30) provide an interface to the kernel's random number gener‐
11       ator.  File /dev/random has major device number 1 and minor device num‐
12       ber 8.  File /dev/urandom has major device number 1  and  minor  device
13       number 9.
14
15       The  random  number  generator  gathers environmental noise from device
16       drivers and other sources into an entropy  pool.   The  generator  also
17       keeps  an  estimate of the number of bits of noise in the entropy pool.
18       From this entropy pool random numbers are created.
19
20       When read, the /dev/random device will only return random bytes  within
21       the estimated number of bits of noise in the entropy pool.  /dev/random
22       should be suitable for uses that need very high quality randomness such
23       as  one-time  pad  or  key generation.  When the entropy pool is empty,
24       reads from /dev/random will block until additional environmental  noise
25       is gathered.
26
27       A  read  from  the  /dev/urandom device will not block waiting for more
28       entropy.  As a result, if  there  is  not  sufficient  entropy  in  the
29       entropy  pool,  the  returned  values are theoretically vulnerable to a
30       cryptographic attack on the algorithms used by the  driver.   Knowledge
31       of  how to do this is not available in the current unclassified litera‐
32       ture, but it is theoretically possible that such an attack  may  exist.
33       If this is a concern in your application, use /dev/random instead.
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35   Usage
36       If  you  are  unsure  about  whether  you  should  use  /dev/random  or
37       /dev/urandom, then probably you want to use the latter.  As  a  general
38       rule,  /dev/urandom  should  be  used  for everything except long-lived
39       GPG/SSL/SSH keys.
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41       If a seed file is saved across reboots as recommended above (all  major
42       Linux  distributions have done this since 2000 at least), the output is
43       cryptographically secure against attackers without local root access as
44       soon as it is reloaded in the boot sequence, and perfectly adequate for
45       network encryption session keys.   Since  reads  from  /dev/random  may
46       block,  users will usually want to open it in nonblocking mode (or per‐
47       form a read with timeout), and provide some sort of  user  notification
48       if the desired entropy is not immediately available.
49
50       The  kernel  random-number  generator  is  designed  to produce a small
51       amount of high-quality seed material to seed  a  cryptographic  pseudo-
52       random  number  generator  (CPRNG).   It  is designed for security, not
53       speed, and is poorly suited to generating large amounts of random data.
54       Users  should  be  very  economical in the amount of seed material that
55       they read from /dev/urandom (and  /dev/random);  unnecessarily  reading
56       large  quantities  of data from this device will have a negative impact
57       on other users of the device.
58
59       The amount of seed material required to generate  a  cryptographic  key
60       equals  the effective key size of the key.  For example, a 3072-bit RSA
61       or Diffie-Hellman private key has an effective key size of 128 bits (it
62       requires about 2^128 operations to break) so a key generator only needs
63       128 bits (16 bytes) of seed material from /dev/random.
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65       While some safety margin above that minimum is reasonable, as  a  guard
66       against flaws in the CPRNG algorithm, no cryptographic primitive avail‐
67       able today can hope to promise more than 256 bits of  security,  so  if
68       any  program reads more than 256 bits (32 bytes) from the kernel random
69       pool per invocation, or per reasonable reseed interval (not  less  than
70       one  minute),  that  should be taken as a sign that its cryptography is
71       not skilfully implemented.
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73   Configuration
74       If your system does  not  have  /dev/random  and  /dev/urandom  created
75       already, they can be created with the following commands:
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77           mknod -m 644 /dev/random c 1 8
78           mknod -m 644 /dev/urandom c 1 9
79           chown root:root /dev/random /dev/urandom
80
81       When  a  Linux  system starts up without much operator interaction, the
82       entropy pool may be in a fairly predictable state.   This  reduces  the
83       actual  amount  of  noise  in  the entropy pool below the estimate.  In
84       order to counteract this effect, it helps to carry entropy pool  infor‐
85       mation  across shut-downs and start-ups.  To do this, add the following
86       lines to an appropriate script which is run  during  the  Linux  system
87       start-up sequence:
88
89           echo "Initializing random number generator..."
90           random_seed=/var/run/random-seed
91           # Carry a random seed from start-up to start-up
92           # Load and then save the whole entropy pool
93           if [ -f $random_seed ]; then
94               cat $random_seed >/dev/urandom
95           else
96               touch $random_seed
97           fi
98           chmod 600 $random_seed
99           poolfile=/proc/sys/kernel/random/poolsize
100           [ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
101           dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
102
103       Also,  add  the  following  lines in an appropriate script which is run
104       during the Linux system shutdown:
105
106           # Carry a random seed from shut-down to start-up
107           # Save the whole entropy pool
108           echo "Saving random seed..."
109           random_seed=/var/run/random-seed
110           touch $random_seed
111           chmod 600 $random_seed
112           poolfile=/proc/sys/kernel/random/poolsize
113           [ -r $poolfile ] && bytes=`cat $poolfile` || bytes=512
114           dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
115
116   /proc Interface
117       The files  in  the  directory  /proc/sys/kernel/random  (present  since
118       2.3.16) provide an additional interface to the /dev/random device.
119
120       The  read-only  file  entropy_avail  gives the available entropy.  Nor‐
121       mally, this will be 4096 (bits), a full entropy pool.
122
123       The file poolsize gives the size of the entropy pool.  The semantics of
124       this file vary across kernel versions:
125
126              Linux 2.4:  This  file  gives  the  size  of the entropy pool in
127                          bytes.  Normally, this file will have the value 512,
128                          but  it is writable, and can be changed to any value
129                          for which an algorithm is  available.   The  choices
130                          are 32, 64, 128, 256, 512, 1024, or 2048.
131
132              Linux 2.6:  This  file  is  read-only, and gives the size of the
133                          entropy pool in bits.  It contains the value 4096.
134
135       The file read_wakeup_threshold contains the number of bits  of  entropy
136       required  for  waking  up processes that sleep waiting for entropy from
137       /dev/random.  The default is 64.  The file write_wakeup_threshold  con‐
138       tains  the  number  of bits of entropy below which we wake up processes
139       that do a select(2) or poll(2) for write access to /dev/random.   These
140       values can be changed by writing to the files.
141
142       The  read-only  files  uuid  and  boot_id  contain  random strings like
143       6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.  The former is  generated  afresh
144       for each read, the latter was generated once.
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FILES

147       /dev/random
148       /dev/urandom
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SEE ALSO

151       mknod (1)
152       RFC 1750, "Randomness Recommendations for Security"
153

COLOPHON

155       This  page  is  part of release 3.25 of the Linux man-pages project.  A
156       description of the project, and information about reporting  bugs,  can
157       be found at http://www.kernel.org/doc/man-pages/.
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161Linux                             2008-06-20                         RANDOM(4)
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