Basic Public/Private key example in C

public_key.c

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

const int publicKey  = 42;
const int privateKey = 256 - publicKey;
const int m = publicKey + privateKey;

void scramble(char* s, int key) {
	for (size_t i = 0; i < strlen(s);i++)
		s[i] = (s[i] + key) % m;
}

int main() {
	char in[] = "Hello World!";

	scramble(in, publicKey);
	printf("%s\n", in);
	scramble(in, privateKey);
	printf("%s\n", in);

	return 0;
}