Encryption and Decryption of plaintext using Hill Cipher in C++

hillcipher.cpp

Implementation of Encryption and Decryption using Hill Cipher in C++. Hill cipher is a polygraphic substitution cipher.

Hill Cipher encrypts a group of letters called a polygraph.

This method makes use of matrices from mathematics.

Encryption: The key and plaintext are converted into matrix format according to the positions like a=0 to z=25. The matrices are multiplied against modulo 26. The key matrix should have an inverse to decrypt the message.

C = KP mod 26

where K is the Key and P is the plain text.

Decryption: The encrypted message is multiplied by the inverse of the key matrix used for encryption against modulo 26 to get the decrypted message.

P = K^-1C mod 26

where K is the Key and C is the ciphertext.

For example

Key matrix

2 3
3 6

Message string ‘ATTACK’ in matrix form −

0  19 2 
19 0  10

For encryption

After multiplying the above two matrices we get,

5  12 8
10 5  14

Which will be the encrypted message ‘FKMFIO’

For decryption

The inverse of the key matrix is

2 25
25 18

Now after multiplying the inverse matrix of the key matrix with the encrypted message matrix is

0  19 2
19 0  10

Which is the original message string is ‘ATTACK’.

Example Code :

Considering if the key matrix is 3*3.

#include
#include
using namespace std;
float e[3][1], d[3][1], a[3][3], b[3][3], ms[3][1], m[3][3];
void getKey() {
   int i, j;
   char me[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>>me;
   for(i = 0; i < 3; i++)
   ms[i][0] = me[i] - 65;
}
void encryption() {
   int i, j, k;
   for(i = 0; i < 3; i++)
   for(j = 0; j < 1; j++)
   for(k = 0; k < 3; k++)
   e[i][j] = e[i][j] + a[i][k] * ms[k][j];
   cout<<"\nEncrypted string is: ";
   for(i = 0; i < 3; i++)
   cout<<(char)(fmod(e[i][0], 26) + 65);
}
void inverse() {
   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 decryption() {
   int i, j, k;
   inverse();
   for(i = 0; i < 3; i++)
   for(j = 0; j < 1; j++)
   for(k = 0; k < 3; k++)
   d[i][j] = d[i][j] + b[i][k] * e[k][j];
   cout<<"\nDecrypted string is: ";
   for(i = 0; i < 3; i++)
   cout<<(char)(fmod(d[i][0], 26) + 65);
   cout<<"\n";
}
int main() {
   getKey();
   encryption();
   decryption();
}

Output

Enter 3x3 matrix for key (should have inverse):
1 0 1 2 4 0 3 5 6
Enter a string of 3 letter(use A through Z): ACT

Encrypted string is: TIU

Inverse Matrix is:
1.09091 0.227273 -0.181818 
-0.545455 0.136364 0.0909091 
-0.0909091 -0.227273 0.181818 

Decrypted string is: ACT

Coders Packet