caesarCipher.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

int main(int argc, char *argv[]){

    char plaintext[] = "DEFEND THE EAST WALL OF THE CASTLE";
    int L = strlen(plaintext);
    char *ciphertext = malloc(sizeof(char)*(L+1));
    int i, key = 3;
    
    //*****encipher******
    for(i=0;i<L;i++){
        if(isalpha(plaintext[i])) ciphertext[i] = (toupper(plaintext[i]) - 'A' + key)%26 + 'A';
        else ciphertext[i] = plaintext[i];
    }
    ciphertext[i] = 0;
    
    char *plaintext2 = malloc(sizeof(char)*(L+1));
    //*****decipher******
    for(i=0;i<L;i++){
        if(isalpha(plaintext[i])) plaintext2[i] = (toupper(ciphertext[i]) - 'A' + 26 - key)%26 + 'A';
        else plaintext2[i] = ciphertext[i];
    }
    plaintext2[i] = 0;
    
    printf("%s\n%s\n%s\n",plaintext,ciphertext,plaintext2);
    return 0;
}