Re: /dev/random vs. /dev/urandom

linux-os
2005-01-07T22:46:37Z

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On Fri, 7 Jan 2005, Andries Brouwer wrote:

> On Fri, Jan 07, 2005 at 02:05:36PM -0500, Ron Peterson wrote:
>
>> When I compile and run the code below, the string of octal characters
>> generated by reading /dev/random contains long strings of zeroes.
>
> But it is your program that invents the zeros, they are not returned
> by /dev/random. The bug in your program is failing to check the
> return value of read().
> -

Also, the following shows that the AND operation will destroy
the randomness of the data. In this case I AND with 1, which
should produce as many '1's as '0's, ... and clearly does not.

Script started on Fri 07 Jan 2005 05:36:43 PM EST
LINUX> cat >xxx.c
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#define LEN 0x20
void doit(unsigned char *buf) {
size_t i, odds, evens;
int fd, ret;
odds = evens = 0;
printf("Trying %s\n", buf);
if((fd = open(buf, O_RDONLY)) < 0)
exit(EXIT_FAILURE);
if((ret = read(fd, buf, LEN)) > 0)
{
for(i=0; i< ret; i++)
{
if(buf[i] & 1)
odds++;
else
evens++;
printf("%02x", buf[i] & 1);
}
printf("\n odds = %u evens = %u\n", odds, evens);
}
(void)close(fd);
}
int main() {
char buf[0x100];
strcpy(buf, "/dev/random");
doit(buf);
strcpy(buf, "/dev/urandom");
doit(buf);
return 0;
}

LINUX> gcc -Wall -O2 -o xxx xxx.c
LINUX> ./xxx
Trying /dev/random
0100000101010000010001000101000000000000000101000100010000000101
odds = 14 evens = 18
Trying /dev/urandom
0001010001000100000101000100010001000000000000000000010000000000
odds = 10 evens = 22
LINUX> ./xxx
Trying /dev/random
0100000100010101000101010101010101000100010000010001010000000101
odds = 20 evens = 12
Trying /dev/urandom
0100000100000101010001000101010001010001000000010101010100010000
odds = 18 evens = 14
LINUX> exi\b^[[Kit
Script done on Fri 07 Jan 2005 05:37:37 PM EST

Cheers,
Dick Johnson
Penguin : Linux version 2.6.10 on an i686 machine (5537.79 BogoMips).
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