ceasar.c

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

int main(){
    char plain[100], cipher[100];
    int key, i, length;
    printf("\nEnter the plain text: ");
    scanf("%s", plain);
    printf("\nEnter the key value: ");
    scanf("%d", &key);
    printf("\n\n\t Plain text: %s", plain);
    printf("\n\n\t Encrypyted text: ");
    for(i=0, length = strlen(plain); i< length; i++){
        cipher[i] = plain[i] +key;
        if(isupper(plain[i]) && (cipher[i] > 'Z'))
            cipher[i] = cipher[i] -26;
        if (islower(plain[i]) && (cipher[i] >'z'))
            cipher[i] = cipher[i] -26;
        printf("%c", cipher[i]);
    }
    printf("\n\n\t After decryption: ");
    for(i =0; i< length; i++){
        plain[i] = cipher[i] - key;
        if (isupper(cipher[i]) && (plain[i]< 'A'))
            plain[i] = plain[i] +26;
        if (islower(cipher[i]) && (plain[i] < 'a'))
            plain[i] = plain[i] =26;
        printf("%c", plain[i]);
    }
    getch();
}