// Implement modulo by replacing the negative operand 
// with an equivalent positive operand that has the same wrap-around effect
function mod(n, p) {
	if (n < 0)
		n = p - Math.abs(n) % p;

	return n % p;
}


// Function will implement Caesar Cipher to
// encrypt / decrypt the msg by shifting the letters
// of the message acording to the key
function encrypt(msg, key) {
	var encMsg = "";

	key -= 0;

	for (var i = 0; i < msg.length; i++) {
		var code = msg.charCodeAt(i);

		// Encrypt only letters in 'A' ... 'Z' interval
		if (code >= 97 && code <= 97 + 26 - 1) {
			code -= 97;
			code = mod(code + key, 26);
			code += 97;
		}
		if (code >= 65 && code <= 65 + 26 - 1) {
			code -= 65;
			code = mod(code + key, 26);
			code += 65;
		}

		encMsg += String.fromCharCode(code);
	}

	return encMsg;
}

function decrypt(msg, key) {
	return encrypt(msg, 26 - key);
}