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C Dev Urandom

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In the following piece of code, which simply generates a sequence of
(random) octal codes, I'm surprised by the apparent non-randomness of
/dev/random. It's not noticeable unless RAND_LENGTH is largish. I was
under the assumption that /dev/random was 'more random' than
/dev/urandom, and that it would block if it ran out of entropy until it
got more. Why am I seeing so many zeroes in my output?
#include <stdio.h>
#include <fcntl.h>
#define RAND_LEN 1024
void
read_random( const char* dev ) {
int i, fd;
char dat[RAND_LEN];
fd = open( dev, O_RDONLY );
dat[RAND_LEN] = '0';
if( fd != -1 ) {
read( fd, dat, RAND_LEN );
for( i = 0; i < RAND_LEN; i++ ) {
dat[i] = (dat[i] >> 4 & 0x07) + 48;
}
printf( '%s: %snn', dev, dat );
} else {
exit( 1 );
}
close( fd );
}
int
main( void ) {
read_random( '/dev/random' );
read_random( '/dev/urandom' );
return( 0 );
}
506$ ./test
/dev/random: 20005061706352643760251034665507247403176742630660 50462223206040017306150002434274350703035033674561 20754173510400040000000050000000000007737204577300 00177356040504450022043404000034041773610464046404 00000000000000000000000000000000000000000000000000 00000000000000000000000000640423461000000000000000 00000000000000000000000000002404000000004100700000 00634610000000000000000000000000000000000000000000 00000000000000000000000000005604000000000000340437 73040037730504300000004773000000000000000000000000 00003000100000000000000000000000000000000000000000 00000000000000000000000000000450045004000000002000 70041304330407040000170434041704050427730704050400 30160457732404000067040000000000000000000070042000 64040000340400006404677300045004000000005773200070 04130433040704000017043404640440004773070440000770 00007773240400005704000000000000000000000000000034 04710007047773220417046704000000000000777300000000 00000000077007735404050416046304070457733773020470 00200040044773577311046414641437736504641457733404 000054041773220464045704
/dev/urandom: 10313700276075133047145322754731744715330540305544 55772411741125544374737071744665152471472301655103 21673032612147044376361336471103425426601777564215 27537506506760532046755677545002343626131545501130 60140067534031123241466461750705230751645572440071 57744425536470274352332415164131435115125573101442 74556544717777054520152351764241666207571007700122 35623563012311717741547124617262165454244161061517 74201651063200446176135471662402411424412532245563 11545142476223664702172000553652612247511550424514 72612104741103553240132614710710775524572432011176 15642610621436346764640170271720054525337367306022 20367455521635765367573615753123615162510360064030 26113642124742630634422243216772330177360713406462 47534536645354770361405340770217451441530701353725 67322557767603402406511316575637667506305617702430 35222250564070154307150656704155003661646277201332 46523045641760343566266442731667313365016666076430 23673210310073545356647564170524251663066260134503 70701275067537603265673560312322526077022157223404 211323047413633213326054
--
Ron Peterson
Network & Systems Manager
Mount Holyoke College

Name

/dev/random and /dev/urandom are convenient, but not as strong as using a HRNG (or downloading a large dump from a HRNG). Also worth noting that /dev/random refills via entropy, so it can block for quite a while depending on circumstances. @kasperd The question asks (in the title) how to read N random characters from /dev/urandom but in the body it asks for reading bytes. I agree there is an inconsistency. I came to this question from Google for reading characters and I think that it may help the future visitors as an option. – Umur Kontacı Jan 4.

random, urandom - kernel random number source devices

Synopsis

#include <linux/random.h>

int ioctl(fd, RNDrequest,param);

Description

The character special files /dev/random and /dev/urandom (present since Linux 1.3.30) provide an interface to the kernel's random numbergenerator. File /dev/random has major device number 1 and minor device number 8. File /dev/urandom has major device number 1 and minor devicenumber 9.

The random number generator gathers environmental noise from device drivers and other sources into an entropy pool. The generator also keeps an estimate ofthe 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/randomshould 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 Little snitch camera video. device will not block waiting for more entropy. As a result, if there is not sufficient entropy in the entropy pool, thereturned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver. Knowledge of how to do this is not available inthe 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.

Writing to /dev/random or /dev/urandom will update the entropy pool with the data written, but this will not result in a higher entropy count.This means that it will impact the contents read from both files, but it will not make reads from /dev/random faster.

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 iscryptographically secure against attackers without local root access as soon as it is reloaded in the boot sequence, and perfectly adequate for networkencryption session keys. Since reads from /dev/random may block, users will usually want to open it in nonblocking mode (or perform a read withtimeout), 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 numbergenerator (CPRNG). It is designed for security, not speed, and is poorly suited to generating large amounts of random data. Users should be very economical inthe amount of seed material that they read from /dev/urandom (and /dev/random); unnecessarily reading large quantities of data from this devicewill 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-Hellmanprivate 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 seedmaterial 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 canhope 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 perreasonable reseed interval (not less than one minute), that should be taken as a sign that its cryptography is not skillfully implemented.

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Configuration

C Dev Urandom Song

See Also

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

C Dev Urandom Youtube

Referenced By

cryptsetup

C Dev Urandom 2017

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