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random

, srandom, initstate, setstate random number generator


  1. random.3.man
  2. random.4.man
  3. random.6.man


1. random.3.man

Manpage of RANDOM

RANDOM

Section: Linux Programmer's Manual (3)
Updated: 2010-09-20
Index Return to Main Contents
 

NAME

random, srandom, initstate, setstate - random number generator  

SYNOPSIS

#include <stdlib.h>

long int random(void);

void srandom(unsigned int seed);

char *initstate(unsigned int seed, char *state, size_t n);

char *setstate(char *state);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

random(), srandom(), initstate(), setstate():

_SVID_SOURCE || _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
 

DESCRIPTION

The random() function uses a nonlinear additive feedback random number generator employing a default table of size 31 long integers to return successive pseudo-random numbers in the range from 0 to RAND_MAX. The period of this random number generator is very large, approximately 16 * ((2^31) - 1).

The srandom() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by random(). These sequences are repeatable by calling srandom() with the same seed value. If no seed value is provided, the random() function is automatically seeded with a value of 1.

The initstate() function allows a state array state to be initialized for use by random(). The size of the state array n is used by initstate() to decide how sophisticated a random number generator it should use --- the larger the state array, the better the random numbers will be. seed is the seed for the initialization, which specifies a starting point for the random number sequence, and provides for restarting at the same point.

The setstate() function changes the state array used by the random() function. The state array state is used for random number generation until the next call to initstate() or setstate(). state must first have been initialized using initstate() or be the result of a previous call of setstate().  

RETURN VALUE

The random() function returns a value between 0 and RAND_MAX. The srandom() function returns no value. The initstate() function returns a pointer to the previous state array. The setstate() function returns a pointer to the previous state array, or NULL on error.  

ERRORS

EINVAL
A state array of less than 8 bytes was specified to initstate().
 

CONFORMING TO

4.3BSD, POSIX.1-2001.  

NOTES

Current "optimal" values for the size of the state array n are 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to the nearest known amount. Using less than 8 bytes will cause an error.

This function should not be used in cases where multiple threads use random() and the behavior should be reproducible. Use random_r(3) for that purpose.

Random-number generation is a complex topic. Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University Press, 2007, 3rd ed.) provides an excellent discussion of practical random-number generation issues in Chapter 7 (Random Numbers).

For a more theoretical discussion which also covers many practical issues in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's The Art of Computer Programming, volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.  

SEE ALSO

drand48(3), rand(3), random_r(3), srand(3)  

COLOPHON

This page is part of release 3.32 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
SEE ALSO
COLOPHON

This document was created by man2html using the manual pages.
Time: 17:32:09 GMT, October 23, 2013

2. random.4.man

Manpage of RANDOM

RANDOM

Section: Linux Programmer's Manual (4)
Updated: 2010-08-29
Index Return to Main Contents
 

NAME

random, urandom - kernel random number source devices  

DESCRIPTION

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 only return random bytes 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.

A read from the /dev/urandom 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 /dev/random instead.  

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 only needs 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 skilfully 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 644 /dev/random c 1 8
    mknod -m 644 /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 following 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 ] && bytes=`cat $poolfile` || bytes=512
    dd if=/dev/urandom of=$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 ] && bytes=`cat $poolfile` || bytes=512
    dd if=/dev/urandom of=$random_seed count=1 bs=$bytes
 

/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.  

FILES

/dev/random
/dev/urandom  

SEE ALSO

mknod (1)
RFC 1750, "Randomness Recommendations for Security"  

COLOPHON

This page is part of release 3.32 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
DESCRIPTION
Usage
Configuration
/proc Interface
FILES
SEE ALSO
COLOPHON

This document was created by man2html using the manual pages.
Time: 17:32:09 GMT, October 23, 2013

3. random.6.man

Manpage of RANDOM

RANDOM

Section: Games and Demos (6)
Index Return to Main Contents

BSD mandoc
 

NAME

random - random lines from a file or random numbers  

SYNOPSIS

[-er ] [denominator ]  

DESCRIPTION

reads lines from the standard input and copies them to the standard output with a probability of 1/denominator. The default value for denominator is 2.

The options are as follows:

-e
If the -e option is specified, does not read or write anything, and simply exits with a random exit value of 0 to denominator - 1, inclusive.
-r
The -r option guarantees that the output is unbuffered.

 

SEE ALSO

fortune(6)


 

Index

NAME
SYNOPSIS
DESCRIPTION
SEE ALSO

This document was created by man2html using the manual pages.
Time: 17:32:09 GMT, October 23, 2013

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