Encryption. You have been recruited by a network of spies known as APEX ("Agents with PHP and Encryption eXperience") and given the codename "agent99". The APEX website provides agents with a set of general purpose encryption tools which can encrypt or decrypt any message from any source. The APEX website also provides a directory of agents and allows agents to send encrypted messages to each other. Unfortunately, last week APEX was hacked by their nemesis, Dark Shadow. Dark Shadow agents hacked into APEX servers and destroyed all of the PHP cryptographic functions. APEX has determined that Dark Shadow was only able to modify three files: "private/crypto_functions.php", "public/agents/post_message.php", "public/agents/messages.php". APEX headquarters suspects that there is a double agent who helped Dark Shadow from the inside. Your primary objectives are to restore the website to full functionality and to identify the double agent.