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RANDOM(4) Linux Programmer's Manual RANDOM(4)
random, urandom - kernel random number source devices
#include <linux/random.h>
int ioctl(fd, RNDrequest, param);
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 return random bytes only
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. If open(2) is called for
/dev/random with the flag O_NONBLOCK, a subsequent read(2) will not
block if the requested number of bytes is not available. Instead,
the available bytes are returned. If no byte is available, read(2)
will return -1 and errno will be set to EAGAIN.
A read from the /dev/urandom device will not block waiting for more
entropy. If there is not sufficient entropy, a pseudorandom number
generator is used to create the requested bytes. As a result, in
this case 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. O_NONBLOCK has no effect when opening /dev/urandom. When
calling read(2) for the device /dev/urandom, signals will not be
handled until after the requested random bytes have been generated.
Since Linux 3.16, a read(2) from /dev/urandom will return at most 32
MB. A read(2) from /dev/random will return at most 512 bytes (340
bytes on Linux kernels before version 2.6.12).
Writing to /dev/random or /dev/urandom will update the entropy pool
with the data written, but this will not result in a higher entropy
count. This means that it will impact the contents read from both
files, but it will not make reads from /dev/random faster.
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 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
needs only 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 skillfully 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 666 /dev/random c 1 8
mknod -m 666 /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
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 $poolfile ] && bits=$(cat $poolfile) || bits=4096
bytes=$(expr $bits / 8)
dd if=/dev/urandom of=$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 $poolfile ] && bits=$(cat $poolfile) || bits=4096
bytes=$(expr $bits / 8)
dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
In the above examples, we assume Linux 2.6.0 or later, where
/proc/sys/kernel/random/poolsize returns the size of the entropy pool
in bits (see below).
/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.
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.
ioctl(2) interface
The following ioctl(2) requests are defined on file descriptors
connected to either /dev/random or /dev/urandom. All requests
performed will interact with the input entropy pool impacting both
/dev/random and /dev/urandom. The CAP_SYS_ADMIN capability is
required for all requests except RNDGETENTCNT.
RNDGETENTCNT
Retrieve the entropy count of the input pool, the contents
will be the same as the entropy_avail file under proc. The
result will be stored in the int pointed to by the argument.
RNDADDTOENTCNT
Increment or decrement the entropy count of the input pool by
the value pointed to by the argument.
RNDGETPOOL
Removed in Linux 2.6.9.
RNDADDENTROPY
Add some additional entropy to the input pool, incrementing
the entropy count. This differs from writing to /dev/random
or /dev/urandom, which only adds some data but does not
increment the entropy count. The following structure is used:
struct rand_pool_info {
int entropy_count;
int buf_size;
__u32 buf[0];
};
Here entropy_count is the value added to (or subtracted from)
the entropy count, and buf is the buffer of size buf_size
which gets added to the entropy pool.
RNDZAPENTCNT, RNDCLEARPOOL
Zero the entropy count of all pools and add some system data
(such as wall clock) to the pools.
/dev/random
/dev/urandom
getrandom(2), mknod(1)
RFC 1750, "Randomness Recommendations for Security"
This page is part of release 4.04 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
www.kernel.org/doc/man-pages/.
Linux 2015-12-28 RANDOM(4)
Copyright and license for this manual page
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HTML rendering created 2015-12-31 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH.
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