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 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.
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
/dev/random
/dev/urandom
The kernel's random number generator was written by Theodore Ts'o
([email protected]).
mknod (1)
RFC 1750, "Randomness Recommendations for Security"
Linux 1997-08-01 RANDOM(4)
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