A program to show encryption and decryption using hill cipher

Based on linear algebra Hill cipher is a polygraphic substitution cipher in cryptography.

To encrypt a message: The key string and message string are represented as a matrix form. They are multiplied then, against modulo 26. The key matrix should have an inverse to decrypt the message.

To decrypt a message: The encrypted message is multiplied by inverse key matrix used for encryption against modulo 26 to get decrypt the message.

#include "stdc++.h"
#include<math.h>
using namespace std;
float en[3][1], de[3][1], a[3][3], b[3][3], msg[3][1], m[3][3];
void getKeyMatrix() { //get key and message from user
  int i, j;
  char mes[3];
  cout<<"Enter 3x3 matrix for key (should have inverse):\n";
  for(i = 0; i < 3; i++)
  for(j = 0; j < 3; j++) {
     cin>>a[i][j];
     m[i][j] = a[i][j];
  }
  cout<<"\nEnter a string of 3 letter(use A through Z): ";
  cin>>mes;
  for(i = 0; i < 3; i++)
  msg[i][0] = mes[i] - 65;
}
void encrypt() { //encrypts the message
  int i, j, k;
  for(i = 0; i < 3; i++)
  for(j = 0; j < 1; j++)
  for(k = 0; k < 3; k++)
  en[i][j] = en[i][j] + a[i][k] * msg[k][j];
  cout<<"\nEncrypted string is: ";
  for(i = 0; i < 3; i++)
  cout<<(char)(fmod(en[i][0], 26) + 65); //modulo 26 is taken for each element of the matrix obtained by multiplication
}
void inversematrix() { //find inverse of key matrix
  int i, j, k;
  float p, q;
  for(i = 0; i < 3; i++)
  for(j = 0; j < 3; j++) {
     if(i == j)
        b[i][j]=1;
     else
        b[i][j]=0;
  }
  for(k = 0; k < 3; k++) {
     for(i = 0; i < 3; i++) {
        p = m[i][k];
        q = m[k][k];
        for(j = 0; j < 3; j++) {
           if(i != k) {
              m[i][j] = m[i][j]*q - p*m[k][j];
              b[i][j] = b[i][j]*q - p*b[k][j];
           }
        }
     }
  }
  for(i = 0; i < 3; i++)
  for(j = 0; j < 3; j++)
  b[i][j] = b[i][j] / m[i][i];
  cout<<"\n\nInverse Matrix is:\n";
  for(i = 0; i < 3; i++) {
     for(j = 0; j < 3; j++)
     cout<<b[i][j]<<" ";
     cout<<"\n";
  }
}
void decrypt() { //decrypt the message
  int i, j, k;
  inversematrix();
  for(i = 0; i < 3; i++)
  for(j = 0; j < 1; j++)
  for(k = 0; k < 3; k++)
  de[i][j] = de[i][j] + b[i][k] * en[k][j];
  cout<<"\nDecrypted string is: ";
  for(i = 0; i < 3; i++)
  cout<<(char)(fmod(de[i][0], 26) + 65); //modulo 26 is taken to get the original message
  cout<<"\n";
}
int main() {
  getKeyMatrix();
  encrypt();
  decrypt();
}