man4/random.4

.\" Copyright (c) 1997 John S. Kallal (kallal@voicenet.com)
.\"
.\" This is free documentation; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License as
.\" published by the Free Software Foundation; either version 2 of
.\" the License, or (at your option) any later version.
.\"
.\" Some changes by tytso and aeb.
.\"
.\" 2004-12-16, John V. Belmonte/mtk, Updated init and quit scripts
.\" 2004-04-08, AEB, Improved description of read from /dev/urandom
.\" 2008-06-20, George Spelvin <linux@horizon.com>,
.\"             Matt Mackall <mpm@selenic.com>
.\"     Add a Usage subsection that recommends most users to use
.\"     /dev/urandom, and emphasizes parsimonious usage of /dev/random.
.\"
.TH RANDOM 4 2010-08-29 "Linux" "Linux Programmer's Manual"
.SH NAME
random, urandom \- kernel random number source devices
.SH DESCRIPTION
The character special files \fI/dev/random\fP and
\fI/dev/urandom\fP (present since Linux 1.3.30)
provide an interface to the kernel's random number generator.
File \fI/dev/random\fP has major device number 1
and minor device number 8.
File \fI/dev/urandom\fP has major device number 1 and minor device number 9.
.LP
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.
.LP
When read, the \fI/dev/random\fP device will only return random bytes
within the estimated number of bits of noise in the entropy
pool.
\fI/dev/random\fP 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 \fI/dev/random\fP will block
until additional environmental noise is gathered.
.LP
A read from the \fI/dev/urandom\fP 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 \fI/dev/random\fP
instead.
.SS Usage
If you are unsure about whether you should use
.IR /dev/random
or
.IR /dev/urandom ,
then probably you want to use the latter.
As a general rule,
.IR /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
.I /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
.IR /dev/urandom
(and
.IR /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
.IR /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
.I not
skilfully implemented.
.SS Configuration
If your system does not have
\fI/dev/random\fP and \fI/dev/urandom\fP created already, they
can be created with the following commands:

.nf
    mknod \-m 644 /dev/random c 1 8
    mknod \-m 644 /dev/urandom c 1 9
    chown root:root /dev/random /dev/urandom
.fi

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:

.nf
    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
.fi

Also, add the following lines in an appropriate script which is
run during the Linux system shutdown:

.nf
    # 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
.fi
.SS "/proc Interface"
The files in the directory
.I /proc/sys/kernel/random
(present since 2.3.16) provide an additional interface to the
.I /dev/random
device.
.LP
The read-only file
.I entropy_avail
gives the available entropy.
Normally, this will be 4096 (bits),
a full entropy pool.
.LP
The file
.I poolsize
gives the size of the entropy pool.
The semantics of this file vary across kernel versions:
.RS
.TP 12
Linux 2.4:
This file gives the size of the entropy pool in
.IR 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.
.TP
Linux 2.6:
This file is read-only, and gives the size of the entropy pool in
.IR bits .
It contains the value 4096.
.RE
.LP
The file
.I read_wakeup_threshold
contains the number of bits of entropy required for waking up processes
that sleep waiting for entropy from
.IR /dev/random .
The default is 64.
The file
.I write_wakeup_threshold
contains the number of bits of entropy below which we wake up
processes that do a
.BR select (2)
or
.BR poll (2)
for write access to
.IR /dev/random .
These values can be changed by writing to the files.
.LP
The read-only files
.I uuid
and
.I boot_id
contain random strings like 6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.
The former is generated afresh for each read, the latter was
generated once.
.SH FILES
/dev/random
.br
/dev/urandom
.\" .SH AUTHOR
.\" The kernel's random number generator was written by
.\" Theodore Ts'o (tytso@athena.mit.edu).
.SH "SEE ALSO"
.BR mknod (1)
.br
RFC\ 1750, "Randomness Recommendations for Security"