RANDOM (4)
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 bit of the 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 to /dev/random will block
until additional environmental noise is gathered.
When read, /dev/urandom device will return as many bytes as are
requested. 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-classified
literature, but it is theoretically possible that such an attack may
exist. If this is a concern in your application, use /dev/random
instead.
CONFIGURING
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 kernel random number generator..."
# Initialize kernel random number generator with random seed
# from last shut-down (or start-up) to this start-up. Load and
# then save 512 bytes, which is the size of the entropy pool.
if [ -f /var/random-seed ]; then
cat /var/random-seed >/dev/urandom
fi
dd if=/dev/urandom of=/var/random-seed count=1
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 for the random
# number generator. Save 512 bytes, which is the size of the
# random number generator's entropy pool.
echo "Saving random seed..."
dd if=/dev/urandom of=/var/random-seed count=1
FILES
AUTHOR
The kernel's random number generator was written by
Theodore Ts'o (tytso@athena.mit.edu).
SEE ALSO
- mknod (1) -
RFC 1750 "Randomness Recommendations for Security"
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