random, urandom - Strong random number generator device





The /dev/random and /dev/urandom files are special files that are a
source for random bytes generated by the kernel random number generator
device. The /dev/random and /dev/urandom files are suitable for
applications requiring high quality random numbers for cryptographic

The generator device produces random numbers from data and devices
available to the kernel and estimates the amount of randomness (or
"entropy") collected from these sources. The entropy level determines the
amount of high quality random numbers that are produced at a given time.

Applications retrieve random bytes by reading /dev/random or
/dev/urandom. The /dev/random interface returns random bytes only when
sufficient amount of entropy has been collected. If there is no entropy
to produce the requested number of bytes, /dev/random blocks until more
entropy can be obtained. Non-blocking I/O mode can be used to disable the
blocking behavior. The /dev/random interface also supports poll(2). Note
that using poll(2) will not increase the speed at which random numbers
can be read.

Bytes retrieved from /dev/random provide the highest quality random
numbers produced by the generator, and can be used to generate long term
keys and other high value keying material.

The /dev/urandom interface returns bytes regardless of the amount of
entropy available. It does not block on a read request due to lack of
entropy. While bytes produced by the /dev/urandom interface are of lower
quality than bytes produced by /dev/random, they are nonetheless suitable
for less demanding and shorter term cryptographic uses such as short term
session keys, paddings, and challenge strings.

Data can be written to /dev/random and /dev/urandom. Data written to
either special file is added to the generator's internal state. Data that
is difficult to predict by other users may contribute randomness to the
generator state and help improve the quality of future generated random

/dev/random collects entropy from providers that are registered with the
kernel-level cryptographic framework and implement random number
generation routines. The cryptoadm(8) utility allows an administrator to
configure which providers will be used with /dev/random.


O_NDELAY or O_NONBLOCK was set and no random bytes are
available for reading from /dev/random.

A signal was caught while reading and no data was

open(2) request failed on /dev/random because no entropy
provider is available.





See attributes(7) for descriptions of the following attributes:

|Interface Stability | Evolving |


open(2), poll(2), attributes(7), cryptoadm(8)


/dev/random can be configured to use only the hardware-based providers
registered with the kernel-level cryptographic framework by disabling the
software-based provider using cryptoadm(8). You can also use cryptoadm(8)
to obtain the name of the software-based provider.

Because no entropy is available, disabling all randomness providers
causes read(2) and poll(2) on /dev/random to block indefinitely and
results in a warning message being logged and displayed on the system
console. However, read(2) and poll(2) on /dev/urandom continue to work
in this case.

An implementation of the /dev/random and /dev/urandom kernel-based random
number generator first appeared in Linux 1.3.30.

A /dev/random interface for Solaris first appeared as part of the
CryptoRand implementation.

September 1, 2008 RANDOM(4D)