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 purposes.
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
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 pro-
duced 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 numbers.
By default, write access is restricted to the super-user. An
administrator may change the default read/write restriction
by changing the permissions on the appropriate special
O_NDELAY or O_NONBLOCK was set and no random
bytes are available for reading from /dev/random.
EINTR A signal was caught while reading and no data was
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Availability | SUNWcsr (32-bit) |
| | SUNWcarx (64-bit) |
| Interface Stability | Evolving |
An implementation of the /dev/random and /dev/urandom
kernel-based random number generator first appeared in Linux
A /dev/random interface for Solaris first appeared as part
of the CryptoRand implementation.
Man(1) output converted with