Hill Cipher Implementation in C

Overview

Hill cipher is developed by a mathematician named Lester S. Hill in 1929. The encryption algorithm takes m successive plain text letters an substitutes for them, m cipher text letters. The substitution is determined by m linear equations in which each character is assignd a numerical value (a=0, b=1, c=3, ..., z=25). In general terms, the Hill system can be expressed as

C = E(K, P) = P*K mod 26

P = D(K, C) = C*K^-1 mod 26 = P*K*K^-1 = P

More about Hill Cipher

Aim

To write a C program to implement Hill Cipher cryptography encryption and decryption algorithm.

Program Code

#include <stdio.h>

#include <ctype.h>

void encrypt(int msgVector[][1], int keyMat[][3], int cipherMat[][1])

{

    for (int i = 0; i < 3; i++) {

        cipherMat[i][0] = 0;

        for (int j = 0; j < 3; j++) {

            cipherMat[i][0] = cipherMat[i][0] + keyMat[i][j] * msgVector[j][0];

        }

        cipherMat[i][0] = cipherMat[i][0] % 26;

    }

}

void decrypt(int cipherMat[][1], int keyInv[][3], int decryptMat[][1])

{

    for (int i = 0; i < 3; i++) {

        decryptMat[i][0] = 0;

        for (int j = 0; j < 3; j++) {

            decryptMat[i][0] = decryptMat[i][0] + keyInv[i][j] * cipherMat[j][0];

        }

        decryptMat[i][0] = decryptMat[i][0] % 26;

    }

}

int main()

{

    char msg[4], encrypted[4], decrypted[4];

    int keyMat[3][3], keyInv[3][3], msgVector[3][1], cipherMat[3][1], decryptMat[3][1];

    printf("Enter 3 letter message: ");

    for (int i = 0; i < 3; i++) {

        char c;

        scanf("%c", &c);

        msg[i] = toupper(c);

    }

    msg[3] = '\0';

    for (int i = 0; i < 3; i++) {

        msgVector[i][0] = msg[i] - 'A';

    }

    printf("Enter 3x3 key matrix...\n");

    for (int i = 0; i < 3; i++) {

        for (int j = 0; j < 3; j++) {

            scanf("%d", &keyMat[i][j]);

        }

    }

    encrypt(msgVector, keyMat, cipherMat);

    for (int i = 0; i < 3; i++) {

        encrypted[i] = (char)(cipherMat[i][0] + 'A');

    }

    encrypted[3] = '\0';

    printf("\nEncrypted text: %s\n", encrypted);

    printf("\nEnter 3x3 inverse of key matrix...\n");

    for (int i = 0; i < 3; i++) {

        for (int j = 0; j < 3; j++) {

            scanf("%d", &keyInv[i][j]);

        }

    }

    decrypt(cipherMat, keyInv, decryptMat);

    for (int i = 0; i < 3; i++) {

        decrypted[i] = (char)(decryptMat[i][0] + 'A');

    }

    decrypted[3] = '\0';

    printf("\nDecrypted text: %s", decrypted);

    return 0;

}

Output