HP-UX Reference (11i v2 07/12) - 7 Device (Special) Files, 9 General Information, Index (vol 10)

r
random(7) random(7)
NAME
random, urandom, rng - strong random number generator
SYNOPSIS
#include <sys/random.h>
DESCRIPTION
The character special files /dev/random and /dev/urandom
provide an interface to the kernel-
resident random number generator,
rng.Aread()
from /dev/random is potentially blocking. A
read() from /dev/urandom is always nonblocking. Data from
/dev/urandom can potentially have
lower entropy than data from
/dev/random .
The rng module is a dynamically loadable kernel module (DLKM). That is, it can be dynamically
unconfigured or reconfigured by an administrator with root authority without rebooting the system.
A sequence from rng has unlimited entropy. In contrast, a sequence generated computationally by a pseu-
dorandom number generator, such as random(3M), has limited entropy, derived only from its initial seed.
The rng module should be considered a quality source for randomness. It has passed extensive statistical
testing, including the NIST (National Institute of Standards and Technology) tests for randomness.
The rng module uses the uncertainty in completion times of interrupt threads triggered by external
events. The rng module extracts a sequence of bits from the interrupt time stamps. Any existing bit bias
is removed to yield a sequence with uniform distribution of 0’s and 1’s. The resulting sequence is divided
between the holding buffers for the special files /dev/random and /dev/urandom . For each
read()
on /dev/random and /dev/urandom , data is retrieved from the corresponding holding buffer. A hash
function based on AES (Advanced Encryption Standard) is applied and the result is placed in the buffer pro-
vided by the user. All requests on the holding buffers are serialized to ensure that returned random data is
not shared between different requests even for simultaneous requests on a multiprocessor system.
There is no write() function associated with either /dev/random or /dev/urandom , and both dev-
ices are read-only by all users. A single ioctl() is defined for /dev/random to facilitate independent
verification of
rng production.
The file /usr/include/sys/random.h
contains the following definitions:
/* The maximum request size, for read() or ioctl(), in bytes */
#define RNG_READMAX 256
/* ioctl() to retrieve data from the entropy collector directly*/
#define RNG_GETRAW _IOR(’Q’, 0, uint8_t[RNG_READMAX])
If a
read() request is for more than RNG_READMAX bytes, it is treated as if it was for exactly
RNG_READMAX bytes. This holds for both /dev/random and /dev/urandom .
Specific Information About /dev/random
When there are a large number of requests on /dev/random within a short time interval, the demand on
the holding buffer can exceed the rate at which data is supplied by rng.A
read() on the
/dev/random device blocks the requesting thread if the random data stored in the holding buffer is too
low to complete the request. The thread blocks until the holding buffer has been updated with enough ran-
dom data to complete the request.
For /dev/random open() flags, only O_NONBLOCK and O_NDELAY have device-specific actions. If
neither of these flags is set, a read() on /dev/random will block until the amount of data requested,
up to RNG_READMAX bytes, can be returned. When the requested number of bytes is not available and
either of the above flags are set, read() returns immediately. If the O_NONBLOCK flag is set, read()
returns -1 and errno is set to [EAGAIN]. If O_NONBLOCK is not set and O_NDELAY is set, read()
returns zero.
The RNG_GETRAW ioctl() permits an application with superuser privilege to fetch RNG_READMAX
bytes of data directly from the /dev/random holding buffer, after bias has been removed but before the
AES hash. This interface is not intended to be used for cryptographic applications, rather, for statistical
testing of the randomness of the data in the /dev/random holding buffer. This RNG_GETRAW
ioctl()
blocks for the same reason as a read on /dev/random . If the requesting thread does not have
superuser authority, [EACCES] is returned.
136 Hewlett-Packard Company − 1 − HP-UX 11i Version 2: December 2007 Update