#include <iostream> 
#include <stdlib.h> 
#include <math.h> 
using namespace std; 

int gcd(int a, int b)
{
    if (b == 0)
        return a;
    return gcd(b, a % b); 
}

int main() {
  double m, p, q, e, i, d;
  int flag;
  
  cout<<"Enter the number to be encrypted and decryted\n";
  cin>>m;
  
  // Choose 2 distinct random prime numbers
  cout<<"Enter first prime number\n";
  cin>>p;
  
  cout<<"Enter second prime number\n\n";
  cin>>q;
  
  // Compute n = p x q
  int n = p*q;
  
  // Compute phi
  int phi = (p-1)*(q-1);
  
  // Choose e such that it lies between 1<e<phi & gcd(e,phi)=1
  for(i=2; i<phi; i++)
  {
    if(gcd(i,phi)==1)
    {
      e = i;
      break;
    }
  }
  
  // Choose d such that (d*e) mod n = 1
  // j = (phi * i) + 1
  // d = j / e  -> iterate i till d does not come in point
  i = 1;
  while(true)
  {
    int j = (phi * i) + 1;
    if(fmod(j,e) == 0)
    {
      d=j/e;
      break;
    }
    i++;
  }
  
  cout<<"The value of phi: "<<phi<<endl;
  cout<<"The value of d: "<<d<<endl;
  cout<<"The value of e: "<<e<<endl;
  cout<<"Public encryption key: (" <<e<<" , " <<n<< ") "<<endl;
  cout<<"Private dencryption key: (" <<d<<" , " <<n<< ") "<<endl;
  
  // Ciphertext c = (m^e) mod n
  int cipher = fmod(pow(m,e), n);
  cout<<"Encrypted Ciphertext : "<<cipher<<endl;
  
  // Plaintext m = (c^d) mod n
  int plain = fmod(pow(cipher, d), n);
  cout<<"Encrypted Plaintext : "<<plain<<endl;
  
  return 0;
}
by

C++ Online Compiler

Write, Run & Share C++ code online using OneCompiler's C++ online compiler for free. It's one of the robust, feature-rich online compilers for C++ language, running on the latest version 17. Getting started with the OneCompiler's C++ compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as C++ and start coding!

Read inputs from stdin

OneCompiler's C++ online compiler supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample program which takes name as input and print your name with hello.

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    string name;
    cout << "Enter name:";
    getline (cin, name);
    cout << "Hello " << name;
    return 0;
}

About C++

C++ is a widely used middle-level programming language.

  • Supports different platforms like Windows, various Linux flavours, MacOS etc
  • C++ supports OOPS concepts like Inheritance, Polymorphism, Encapsulation and Abstraction.
  • Case-sensitive
  • C++ is a compiler based language
  • C++ supports structured programming language
  • C++ provides alot of inbuilt functions and also supports dynamic memory allocation.
  • Like C, C++ also allows you to play with memory using Pointers.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
   //code
}
else {
   //code
}

You can also use if-else for nested Ifs and If-Else-If ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to If-Else-If ladder.

switch(conditional-expression){    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
......    
    
default:     
 code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  //code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
// code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
 // code 
} while (condition);

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Function gets run only when it is called.

How to declare a Function:

return_type function_name(parameters);

How to call a Function:

function_name (parameters)

How to define a Function:

return_type function_name(parameters) {  
 // code
}