#include <iostream>    // C++ header file for input/output
#include <string>      // C++ header file for strings

std::string key = "FCTUNL";
int n = 4;

bool inKey(char s){
	return key.find(s) != std::string::npos;
}

int mod(int a, int b){
	if(a >= b)
		return a%b;
	else if(a < 0) return 26 + a;
	else return a;
}

std::string Encode(const std::string &message) {
	std::string encoded = "";
	int m = 0;
	int i;
	int size = message.size();
	for(i = 0; i < size; i++){
		char s = message[i];
		if(inKey(s)){
			encoded += key[m];
			m = mod(m + 1, key.size());
		}
		int s2 = (s - 'A') + n;
		int newS = mod(s2, 26);
		encoded += 'A' + newS;
		if(inKey(s))
			encoded += key[m];
	}
	return encoded ;
}

std::string Decode(const std::string &message) {

	std::string decoded = "";
	int m = 0;
	int i;
	int size = message.size();
	for(i = 0; i < size; i++){
		char s = message[i];
		if(s == key[m] && i + 2 < size && message[i + 2] == key[m+1]){
			m++;
			int s2 = message[i+1] - 'A';
			int newS = mod(s2 - n, 26);
			decoded += 'A' + newS;
			i = i+2;
		}
		else {
			int s2 = message[i] - 'A';
			int newS = mod(s2 - n, 26);
			decoded += 'A' + newS;
		}
	}
	return decoded;
}

int main()
{
	std::string original = "ATACAMOSAOAMANHECER" ;
	std::string decode = "EFXCECGTEQSWESEQETRULIUGNIV" ;

	std::cout << "Escreva a chave:\n";
	std::cin >> key;

	std::cout << "Escreva o valor de n:\n";
	std::cin >> n;

	std::cout << "Escreva a palavra a cifrar:\n";
	std::cin >> original;

	std::string encoded = Encode(original) ;
	std::cout << encoded << std::endl ;

	std::cout << "Escreva a palavra a decifrar:\n";
	std::cin >> decode;

	std::string decoded = Decode(decode) ;
	std::cout << decoded << std::endl ;

	return 0;
}