#include <stdio.h>
#include <ctype.h>
/*
ROT13 is a simple substitution cipher that replaces a letter with the 13th letter
after it in the alphabet. Because the English alphabet has 26 letters (13 * 2),
applying ROT13 twice restores the original text (it is its own inverse).
*/
/**
* Encrypts/decrypts a null-terminated string in-place using the ROT13 cipher.
*
* @param str Pointer to the null-terminated string to transform.
*/
void rot13(char *str) {
if (str == NULL) {
return; // Guard against NULL pointer dereferences
}
for (int i = 0; str[i] != '\0'; i++) {
char c = str[i];
if (isupper(c)) {
// Offset from 'A', shift by 13, wrap around with modulo 26
str[i] = (c - 'A' + 13) % 26 + 'A';
} else if (islower(c)) {
// Offset from 'a', shift by 13, wrap around with modulo 26
str[i] = (c - 'a' + 13) % 26 + 'a';
}
// Non-alphabetic characters (digits, punctuation, spaces) remain unchanged
}
}
int main(void) {
// Note: Must be a mutable character array, not a string literal (const char*)
char message[] = "Hello, World! 123";
printf("Original: %s\n", message);
// First pass: Encrypt
rot13(message);
printf("ROT13: %s\n", message);
// Second pass: Decrypt (ROT13 is self-inverse)
rot13(message);
printf("Decrypted: %s\n", message);
return 0;
}
/*
run:
Original: Hello, World! 123
ROT13: Uryyb, Jbeyq! 123
Decrypted: Hello, World! 123
*/
