ns/RSA.cpp

#include<iostream>
#include<cmath>

using namespace std;

long double find_pow(long double a,long double b, long double c)
{
    long double i,final=1;
    for(i=0;i<b;i++)
        final=fmodl(final*fmodl(a,c),c);
        return fmodl(final,c);
}

long double find_d(long double e,long double phi_of_n)
{

    int i;
    e=fmodl(e,phi_of_n);
    
    for(i=0;i<phi_of_n;i++)
        if(fmodl((e*i),phi_of_n)==1)
            break;
  
    return i;
}

int main()
{
    long double M,p,q,e,n,phi_of_n,C,d;
    cout<<"\nEnter the plain text\n";
    cin>>M;
    
    cout<<"\nEnter the two prime numbers\n";
    cin>>p>>q;
    
    cout<<"\nEnter e\n";
    cin>>e;
    
    n=p*q;
    
    phi_of_n = (p-1)*(q-1);

    C=find_pow(M,e,n);
    cout<<"\nEncrypted Text is "<<C;
    
    d=find_d(e,phi_of_n);
    
    M=find_pow(C,d,n);
    cout<<"\nDecrypted Text is "<<M<<endl;
    
    return 0;
}


/*
 OUTPUT:-
 
 Enter the plain text
 88
 
 Enter the two prime numbers
 17 11
 
 Enter e
 7
 
 Encrypted Text is 11
 Decrypted Text is 88
 
 */