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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 esti-
mated 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 genera-
tion. 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 non-clas-
sified 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 above (all major Linux dis-
tributions 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 non-block-
ing mode (or perform a read with timeout), and provide some sort of user notifica-
tion 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 generat-
ing 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 pri-
vate 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 re-seed
interval (not less than one minute), that should be taken as a sign that its cryp-
tography 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. Nor-
mally, this file will have the value 512, but it is writable,
and can be changed to any value for which an algorithm is avail-
able. 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 3.22 of the Linux man-pages project. A description of
the project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2008-06-20 RANDOM(4)
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