Affine Cipher Implementation in C

Overview

The Affine cipher is a type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its numeric equivalent (a=0, b=1, c=2, ..., z=25), encrypted using a simple mathematical function, and converted back to a letter.

Choose two integers a and b such that gcd(a, 26) == 1. The affine encryption function is

E(x) = a*x + b (mod 26)

The affine decryption function is

D(x) = a^-1 * (x - b) (mod 26)

where a^-1 is the modular multiplicative inverse of 'a' modulo 26 such that a * a^-1 mod 26 == 1.

For example, let a = 5 and b = 7. Take a plain text letter x = 'H' (value is 7).

It is encrypted as a*x + b => 5*7 + 7 => 42 => 16 which is letter 'Q'.

So, the text "CODE VIBES" is encrypted as "RZWB IVMBT".

More about Affine Cipher

Aim

To write a C program to implement Affine cipher cryptography encryption and decryption algorithm.

Program Code

#include <stdio.h>

void encrypt(char msg[], int a, int b, char encrypted[])

{

    int i;

    for (i = 0; msg[i] != '\0'; i++) {

        if (msg[i] != ' ')

            encrypted[i] = (char)(((a * (msg[i] - 'A') + b) % 26) + 'A');

        else

            encrypted[i] = msg[i];

    }

    encrypted[i] = '\0';

}

void decrypt(char encrypted[], int a, int b, char decrypted[])

{

    int aInv = 0, flag = 0, i;

    for (i = 0; i < 26; i++) {

        flag = (a * i) % 26;

        if (flag == 1)

            aInv = i;

    }

    for (i = 0; encrypted[i] != '\0'; i++) {

        if (encrypted[i] != ' ')

            decrypted[i] = (char)(((aInv * ((encrypted[i] + 'A') - b)) % 26) + 'A');

        else

            decrypted[i] = encrypted[i];

    }

    decrypted[i] = '\0';

}

int main()

{

    char msg[100], encrypted[100], decrypted[100];

    int a = 5, b = 7;

    printf("Enter the message in upper case: ");

    scanf("%[^\n]s", msg);

    getchar();

    encrypt(msg, a, b, encrypted);

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

    decrypt(encrypted, a, b, decrypted);

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

    return 0;

}

Output