RANDOM(4) Linux Programmers 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 kernels random number gener
ator. File /dev/random has major device number 1 and minor device num
ber 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 non-classified liter
ature, 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 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 non-blocking mode (or per
form 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 ran
dom pool per invocation, or per reasonable re-seed interval (not less
than one minute), that should be taken as a sign that its cryptography
is not skilfuly 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 infor
mation 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. Nor
mally, 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 con
tains 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 3.05 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 2008-06-20 RANDOM(4)
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