def rot13(plaintext)
	uppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	lowercase = "abcdefghijklmnopqrstuvwxyz"

	cypher = ""
	for char in plaintext
		if uppercase .contains. char
			cypher += uppercase[uppercase[char] - 13]
		else if lowercase .contains. char
			cypher += lowercase[lowercase[char] - 13]
		else
			cypher += char
		end
	end

	return cypher
end