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Annotated edit history of random(4) version 2, including all changes. View license author blame.
Rev Author # Line
1 perry 1 RANDOM
2 !!!RANDOM
3 NAME
4 DESCRIPTION
5 CONFIGURING
6 FILES
7 AUTHOR
8 SEE ALSO
9 ----
10 !!NAME
11
12
13 random, urandom - kernel random number source devices
14 !!DESCRIPTION
15
16
17 The character special files __/dev/random__ and
2 StuartYeates 18 __/dev/urandom__ (present since [Linux] 1.3.30) provide an
1 perry 19 interface to the kernel's random number generator. File
20 __/dev/random__ has major device number 1 and minor
21 device number 8. File __/dev/urandom__ has major device
22 number 1 and minor device number 9.
23
24
25 The random number generator gathers environmental noise from
26 device drivers and other sources into an entropy pool. The
27 generator also keeps an estimate of the number of bit of the
28 noise in the entropy pool. From this entropy pool random
29 numbers are created.
30
31
32 When read, the __/dev/random__ device will only return
33 random bytes within the estimated number of bits of noise in
34 the entropy pool. __/dev/random__ should be suitable for
35 uses that need very high quality randomness such as one-time
36 pad or key generation. When the entropy pool is empty, reads
37 to __/dev/random__ will block until additional
38 environmental noise is gathered.
39
40
41 When read, __/dev/urandom__ device will return as many
42 bytes as are requested. As a result, if there is not
43 sufficient entropy in the entropy pool, the returned values
44 are theoretically vulnerable to a cryptographic attack on
45 the algorithms used by the driver. Knowledge of how to do
46 this is not available in the current non-classified
47 literature, but it is theoretically possible that such an
48 attack may exist. If this is a concern in your application,
49 use __/dev/random__ instead.
50 !!CONFIGURING
51
52
53 If your system does not have __/dev/random__ and
54 __/dev/urandom__ created already, they can be created
55 with the following commands:
56
57
58 mknod -m 644 /dev/random c 1 8
2 StuartYeates 59 mknod -m 644 /dev/urandom c 1 9
60 chown root:root /dev/random /dev/urandom
61
1 perry 62 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:
63
2 StuartYeates 64 echo "Initializing kernel random number generator..."
65 # Initialize kernel random number generator with random seed
66 # from last shut-down (or start-up) to this start-up. Load and
67 # then save 512 bytes, which is the size of the entropy pool.
68 if [[ -f /var/random-seed ]; then
69 cat /var/random-seed >/dev/urandom
70 fi
71 dd if=/dev/urandom of=/var/random-seed count=1
72
73 Also, add the following lines in an appropriate script which is run during the Linux system shutdown:
1 perry 74
2 StuartYeates 75 # Carry a random seed from shut-down to start-up for the random
76 # number generator. Save 512 bytes, which is the size of the
77 # random number generator's entropy pool.
78 echo "Saving random seed..."
79 dd if=/dev/urandom of=/var/random-seed count=1
1 perry 80
81
82 !!FILES
83
84
85 /dev/random
86 /dev/urandom
87 !!AUTHOR
88
89
2 StuartYeates 90 The kernel's RandomNumberGenerator was written by Theodore
1 perry 91 Ts'o (tytso@athena.mit.edu).
92 !!SEE ALSO
93
94
2 StuartYeates 95 mknod(1)
1 perry 96 RFC 1750,
97 ----
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