ROT13_cipher.c

/*
WAP to read any string/message from the user irrespective of the case
Decode the string by shifting each alphabet 13 places.
If the alphabet is 'Z' then it would be 'g' but make it in a way so that it gives 'M' that means keep a note of the case
This is ROT13 Cipher Algorithm
*/

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

void decode_rot13(char *str)
{
    int i;
    for (i = 0; str[i] != '\0'; i++)
    {
        if (str[i] >= 'A' && str[i] <= 'Z')
        {
            str[i] = (((str[i] - 'A') + 13) % 26) + 'A';
        }
        else if (str[i] >= 'a' && str[i] <= 'z')
        {
            str[i] = (((str[i] - 'a') + 13) % 26) + 'a';
        }
    }
}

int main()
{
    char str[] = "CNFFJBEq";
    decode_rot13(str);
    printf("Decoded string: %s\n", str);
    return 0;
}
/*
OUTPUT
Decoded string: PASSWORD
*/