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 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 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-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.
mknod -m 644 /dev/random c 1 8 mknod -m 644 /dev/urandom c 1 9 chown root:root /dev/random /dev/urandom
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
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. Normally, this will be 512 (bytes). It can be changed to any value for which an algorithm is available. Currently the choices are 32, 64, 128, 256, 512, 1024, 2048.
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() or poll() 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.