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RANDOM(4) Linux Programmer's Manual RANDOM(4)
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 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 unclassified 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 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 below (all major Linux
distributions 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 nonblocking mode (or perform a read with timeout),
and provide some sort of user notification 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 generating 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 private 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 reseed interval (not less than one minute), that should be taken as
a sign that its cryptography is not skilfully implemented.
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
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.
Normally, this file will have the value 512, but it is
writable, and can be changed to any value for which an
algorithm is available. 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.
/dev/random
/dev/urandom
mknod (1)
RFC 1750, "Randomness Recommendations for Security"
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-08-29 RANDOM(4)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface