C++ Cypher

#include <iostream>
#include <string>

void encrypt( std::string &iostr,int key){
	key %= 26;
	int ch;
	for(auto &it:iostr){
		ch = tolower(it) + key;
		if( ch > 'z' )
			ch -= 26;
		it = ch;}}

int main(){
	std::string source = "zebra";
	int key = 15;
	std::cout << source << std::endl;
	encrypt( source , key );
	std::cout << source << std::endl;
	return 0;}