1. Windows Dev Random
2. Using Dev Random

Name

• Using cat to read from /dev/urandom is a bad idea, because it will try to read /dev/urandom to the end - but it does not end. You can use head. But take care to read by byte, not by line - because lines would be randomly separated by random newline bytes. So, to read 30 random bytes into a file random.bytes, use: head -c 30 /dev/urandom.
• C programs with output showing usage of operators, loops, functions, arrays, performing operations on strings, files, pointers. Download executable files and execute them without compiling the source file. Code::Blocks IDE is used to write programs, most of these will work with GCC and Dev C compilers. The first program, prints 'Hello World.'

In C, the generation algorithm used by rand is guaranteed to only be advanced by calls to this function. In C, this constraint is relaxed, and a library implementation is allowed to advance the generator on other circumstances (such as calls to elements of random ).

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 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.

Configuration

Windows Dev Random

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 aselect(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 eachread, the latter was generated once.

ioctl(2) interface

The following ioctl(2) requests are defined on file descriptors connected to either /dev/random or /dev/urandom. All requests performedwill interact with the input entropy pool impacting both /dev/random and /dev/urandom. The CAP_SYS_ADMIN capability is required for allrequests except RNDGETENTCNT.

RNDGETENTCNT

Retrieve the entropy count of the input pool, the contents will be the same as the entropy_avail file under proc. The result will be stored in theint pointed to by the argument.

RNDADDTOENTCNT

Increment or decrement the entropy count of the input pool by the value pointed to by the argument.

RNDGETPOOL

Removed in Linux 2.6.9.

RNDADDENTROPY

Add some additional entropy to the input pool, incrementing the entropy count. This differs from writing to /dev/random or /dev/urandom, whichonly adds some data but does not increment the entropy count. The following structure is used:

struct rand_pool_info { int entropy_count; int buf_size; __u32 buf[0]; };

Here entropy_count is the value added to (or subtracted from) from the entropy count, and buf is the buffer of size buf_size which getsadded to the entropy pool.

RNDZAPENTCNT, RNDCLEARPOOL

Zero the entropy count of all pools and add some system data (such as wall clock) to the pools.

Files

/dev/random
/dev/urandom

See Also

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

Referenced By

cryptsetup(8)

C programs with output showing usage of operators, loops, functions, arrays, performing operations on strings, files, pointers. Download executable files and execute them without compiling the source file. Code::Blocks IDE is used to write programs, most of these will work with GCC and Dev C++ compilers. The first program, prints 'Hello World.'

C programming examples with output

Example 1 - C hello world program
/** My first C program */

#include <stdio.h>
int main()
{
printf('Hello Worldn');
return0;
}

Output of program:
'Hello World'

Example 2 - C program to get input from a user using scanf

#include <stdio.h>

int main()
{
int x;

printf('Input an integern');
scanf('%d',&x);// %d is used for an integer

printf('The integer is: %dn', x);

return0;
}

Output:
Input an integer
7897
The integer is: 7897

Example 3 - using if else control instructions

#include <stdio.h>

int main()
{
int n;

printf('Enter a numbern');
scanf('%d',&n);

if(n >0)
printf('Greater than zero.n');
else
printf('Less than or equal to zero.n');

return0;
}

Output:
Enter a number
-45
Less than or equal to zero.

Example 4 - while loop example

#include <stdio.h>
int main()
{
int c =1;// Initializing variable
while(c <=10)// While loop will execute till the condition is true
{
printf('%d ', c);// Note the space after %d for gap in the numbers we want in output
c++;
}
return0;
}

Output:
1 2 3 4 5 6 7 8 9 10

Example 5 - C program check if an integer is prime or not

#include <stdio.h>
int main()
{
int n, c;
printf('Enter a numbern');
scanf('%d',&n);
if(n 2)
printf('Prime number.n');
else
{
for(c =2; c <= n -1; c++)
{
if(n % c 0)
break;
}
if(c != n)
printf('Not prime.n');
else
printf('Prime number.n');
}
return0;
}

Example 6 - command line arguments

#include <stdio.h>

int main(int argc,char*argv[])
{
int c;

printf('Number of command line arguments passed: %dn', argc);

for(c =0; c < argc; c++)
printf('%d argument is %sn', c +1, argv[c]);

return0;
}

This program prints the number of arguments and their contents.

Example 7 - Array program

#include <stdio.h>

int main()
{
int array[100], n, c;
printf('Enter number of elements in arrayn');
scanf('%d',&n);
printf('Enter %d elementsn', n);
for(c =0; c < n; c++)
scanf('%d',&array[c]);
printf('The array elements are:n');
for(c =0; c < n; c++)
printf('%dn', array[c]);
return0;
}

Example 8 - function program

#include <stdio.h>

Precision tune auto eagan mn. void my_function();// Declaring a function

int main()
{
printf('Main function.n');

my_function();// Calling the function

printf('Back in function main.n');

return0;
}

// Defining the function
void my_function()
{
printf('Welcome to my function. Feel at home.n');
}

Example 9 - Using comments in a program

#include <stdio.h>
int main()
{
// Single line comment in a C program
printf('Writing comments is very useful.n');
/*
* Multi-line comment syntax
* Comments help us to understand a program later easily.
* Will you write comments while writing programs?
*/
printf('Good luck C programmer.n');
return0;
}

Example 10 - using structures in C programming

#include <stdio.h>
#include <string.h>

struct game
{
char game_name[50];
int number_of_players;
};// Note the semicolon

int main()
{
struct game g;

strcpy(g.game_name,'Cricket');
g.number_of_players=11;

printf('Name of game: %sn', g.game_name);
printf('Number of players: %dn', g.number_of_players);

return0;
}

Example 11 - C program for Fibonacci series

#include <stdio.h>

int main()
{
int n, first =0, second =1, next, c;

printf('Enter the number of termsn');
scanf('%d',&n);

printf('First %d terms of Fibonacci series are:n', n);

for(c =0; c < n; c++)
{
if(c <=1)
next = c;
else
{
next = first + second;
first = second;
second = next;
}
printf('%dn', next);
}

return0;
}

Example 12 - C graphics programming

#include <graphics.h>
#include <conio.h>

int main()
{
int gd = DETECT, gm;
initgraph(&gd,&gm,'C:TCBGI');
outtextxy(10,20,'Graphics programming is fun!');
circle(200,200,50);
setcolor(BLUE);
line(350,250,450,50);
getch();
closegraph();
return0;
}

How to compile C programs with GCC compiler?

If you are using GCC on Linux operating system, then you may need to modify the programs. For example, consider the following program that prints the first ten natural numbers.

#include <stdio.h>
#include <conio.h>

int main()
{
int c;

for(c =1; c <=10; c++)
printf('%dn', c);
getch();
return0;
}

The program includes a header file <conio.h> and uses function getch, but this file is Borland specific, so it works in Turbo C compiler but not in GCC. The program for GCC must be like:

#include <stdio.h>

int main()
{
int c;

/* for loop */

for(c =1; c <=10; c++)
printf('%dn', c);
return0;
}

If you are using GCC, save the program in a file say 'numbers.c' to compile the program, open the terminal and enter the command 'gcc numbers.c', this compile the program and to execute it enter the command './a.out' do not use quotes while executing commands. You can specify the output file name as 'gcc numbers.c -o numbers.out', to run execute './numbers.out' in the terminal.

C programming tutorial

A program consists of functions that contain instructions given to a machine to perform a task. The process of writing it includes designing an algorithm, drawing a flowchart, and then writing code. After writing it, you need to test it and debug it if it does not produce the required output.

To write a program, you need a text editor (use your favorite one) and a compiler. A compiler converts source code into machine code, which consists of zero's and one's only, ready to be executed on a machine.

An IDE (Integrated Development Environment) provides a text editor, compiler, debugger, etc. for developing programs and managing projects. Code::Blocks IDE provides an ideal environment for development. It can import Microsoft Visual C++ projects, is extendable as it uses plug-ins, open-source, and cross-platform.

How to write a C program?

A program must have at least one function which must be main. A function consists of declarations and statements. A statement is an expression followed by a semicolon. For example, a + b, printf('C program examples') are expressions and a + b; and printf('C is an easy to learn computer programming language'); are statements.

To use a variable, we must indicate its type, whether it is an integer, float, character, or others. C language has many built-in data types, and we can create our own using structures and unions. Every data type has its size that may depend on the machine; for example, an integer may be of 2 or 4 Bytes. Data is stored in a binary form, i.e., a group of bits where each bit can be '0' or '1'.

Keywords such as 'switch,' 'case,' 'default,' 'register,' are reserved words with predefined meaning and can't be used as the name of a variable or a function. Memory can be allocated at compile-time or run-time using malloc and calloc functions. C language has many features such as recursion, preprocessor, conditional compilation, portability, pointers, multi-threading by using external libraries, dynamic memory allocation. Thanks to these, it is used for making portable software programs and applications. Using networking API's users can communicate and interact with each other and share files.

C standard library contains functions for mathematical operations, characters, input/output, files, and many more. The process of making a program which is known as coding requires knowledge of programming language and logic to achieve the desired output. So you should learn C programming basics and start making programs.

Learning data structures (stacks, queues, linked lists) using C provides you a greater understanding as you learn everything in detail. A general belief is to go for high-level languages. However, it's a good idea to learn C before learning C++ or Java. C++ is object-oriented and contains all features of C, so learning C help you learn C++ quickly, then you can study Java.

C programming PDF

C programming books

1. Let Us C By Yashavant Kanetkar
2. PROGRAMMING WITH C By Byron Gottfried, Jitender Chhabra
3. The C Programming By Brian Kernighan and Dennis Ritchie

Using Dev Random

If you are a beginner, buy any one of the first two books, and if you have previous programming experience or you know the basics of C language, buy the third one.