Rabin Cipher.cpp

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#include <iostream>
#include <algorithm>
#include <iterator>
#include <string>
#include <vector>
#include <cstring>
using namespace std;
int p = 167, q = 151;
int n = p * q;

int mod(int k, int b, int m)
{
    int i=0;
    int a=1;
    vector<int> t;
    while(k>0){
        t.push_back(k%2);
        k=(k-t[i])/2;
        i++;
    }
    for(int j=0; j<i; j++){
        if(t[j]==1){
            a=(a*b)%m;
            b=(b*b)%m;
        } else{
            b=(b*b)%m;
        }
    }
    return a;
}

void encode(string s)
{
    vector<int>l;
    for(int i = 0; i < strlen(s.c_str()); i++) {
        l.push_back((int)s[i]);
    }
    copy(l.begin(), l.end(), ostream_iterator<int>(cout, ""));
}

int modulo (int a, int b)
{
    return a >= 0 ? a % b : ( b - abs ( a%b ) ) % b;
}

void decoding(vector<int> s)
{
    vector<char>l;
    for(int i = 0; i < s.size(); i++) {
        l.push_back(char(s[i]));
    }
    copy(l.begin(), l.end(), ostream_iterator<char>(cout, ""));
}

int encryption(int m, int n)
{
    int c = (m * m)%n;
    return c;
}



vector<int> eea(int a, int b)
{
    if (b>a) {
        int temp=a; a=b; b=temp;
    }
    int x=0;
    int y=1;
    int lastx=1;
    int lasty=0;
    while (b!=0) {
        int q= a/b;
        int temp1= a%b;
        a=b;
        b=temp1;
        int temp2 = x;
        x=lastx-q*x;
        lastx = temp2;
        int temp3 = y;
        y = lasty-q*y;
        lasty=temp3;
    }
    vector<int>arr(3);
    arr[0] = lastx;
    arr[1] = lasty;
    arr[2] = 1;
    return arr;
}

int decryption(int c, int p, int q)
{
    int mp = mod((p+1)/4, c, p);
    int mq = mod((q+1)/4, c, q);
    vector<int> arr = eea(p, q);
    int pp = arr[0]*p*mq;
    int qq = arr[1]*q*mp;
    double r = modulo((pp+qq), n);
    if( r < 128)
        return r;
    int negative_r = n - r;
    if (negative_r < 128)
        return negative_r;
    int s = modulo((pp-qq), n);
    if( s < 128)
        return s;
    int negative_s = n - s;
    if( negative_s < 128)
        return negative_s;
}

int main()
{   cout<<"\n Implementation of Rabin Cryptosystem \n\n";
    cout<<" Public Key is: "<<n;
    string msg ="Sourajita Dewasi";
    cout << "\n Message: " << msg << endl;
    int len = strlen(msg.c_str());
    vector<int>li;
    for(int i = 0; i <= len; i++)
    {
        li.push_back(encryption(msg[i], n));
    }
    cout << " Cipher Text: ";
    copy(li.begin(), li.end(), ostream_iterator<int>(cout, ""));
    vector<int>des;
    for(int i = 0; i < len; i++)
    {
        des.push_back(decryption(li[i], p, q));
    }
    cout << "\n Plain Text: ";
    decoding(des);
    return 0;
}