ns/hill_cipher_2*2.cpp

#include <iostream>
#include<cstring>
#include <map>


using namespace std;

int main() {
    
    string capital_alphabets="ABCDEFGHIJKLMNOPQRSTUVWXYZ", small_alphabets="abcdefghijklmnopqrstuvwxyz";
    
    string plain_text,cipher_text="",decrypted_plain_text="";
    int x,y,i,j,mul_factor,index,count;
    map<int,int> flag;
    
    cout<<"Enter the plain text: "<<endl;
    cin>>plain_text;
    
    cout<<"Enter the size of matrix"<<endl;
    cin>>x>>y;
    
    int key_matrix[x][y],plain_text_matrix[plain_text.length()],cipher_text_matrix[plain_text.length()],temporary_matrix[plain_text.length()][plain_text.length()],key_inverse_matrix[plain_text.length()][plain_text.length()],final_matrix[plain_text.length()];
    
    cout<<"Enter the elements of matrix"<<endl;
    
    for(i=0;i<x;i++)
        for(j=0;j<y;j++)
            cin>>key_matrix[i][j];
    
    for(i=0;i<plain_text.length();i++)
    {
        cipher_text_matrix[i]=0;
        final_matrix[i]=0;
        if(plain_text[i]>='A'&&plain_text[i]<='Z')
        {
            plain_text_matrix[i]=plain_text[i]-'A';
            flag[i]=1;
        }
        
        if(plain_text[i]>='a'&&plain_text[i]<='z')
            plain_text_matrix[i]=plain_text[i]-'a';
        
        
        //cout<<<<plain_text_matrix[i]<<endl;
        
    }
    
    for(i=0;i<plain_text.length();i++)
        for(j=0;j<plain_text.length();j+=1)
            cipher_text_matrix[i]+=plain_text_matrix[j]*key_matrix[j][i];
    
    for(i=0;i<plain_text.length();i++)
    {
        //while(( cipher_text_matrix[i]%26)<=26)
        cipher_text_matrix[i]=cipher_text_matrix[i]%26;
        
        if(flag[i]==1)
            cipher_text+=capital_alphabets[cipher_text_matrix[i]];
        
        else
            cipher_text+=small_alphabets[cipher_text_matrix[i]];
        
    }
    
    cout<<"Cipher text is "<<cipher_text<<endl;
    
    temporary_matrix[0][0]=key_matrix[1][1];
    temporary_matrix[1][1]=key_matrix[0][0];
    temporary_matrix[0][1]=-1*key_matrix[0][1];
    temporary_matrix[1][0]=-1*key_matrix[1][0];
    
    mul_factor=(key_matrix[0][0]*key_matrix[1][1])-(key_matrix[0][1]*key_matrix[1][0]);
    
    int a=mul_factor%26,x1,determinant;
    
    
    
    //while(a>26)
    a=a%26;
    
    if(a<0)
        a=a+26;
    
    for(x1=1;x1<=26;x1++)
        if(((a*x1)%26)==1)
            break;
    
    determinant = x1;
    
    
    for(i=0;i<plain_text.length();i++)
        for(j=0;j<plain_text.length();j++)
            key_inverse_matrix[i][j]=temporary_matrix[i][j]*determinant;
    
    
    
    for(i=0;i<plain_text.length();i++)
        for(j=0;j<plain_text.length();j++)
            final_matrix[i]+=cipher_text_matrix[j]*key_inverse_matrix[j][i];
    
    
    for(i=0;i<plain_text.length();i++)
    {
        //while((final_matrix[i]%26)<=26)
        final_matrix[i]=final_matrix[i]%26;
        
        if(flag[i]==1)
            decrypted_plain_text+=capital_alphabets[final_matrix[i]];
        
        else
            decrypted_plain_text+=small_alphabets[final_matrix[i]];
    }
    
    cout<<"Decrypted Text is "<<decrypted_plain_text<<endl;
    
    
}

/*
 OUTPUT:-
 
 Enter the plain text: 
 ME
 Enter the size of matrix
 2 2
 Enter the elements of matrix
 9 4 5 7
 Cipher text is YY
 Decrypted Text is ME
 
 */