A rich theory began to emerge, enabling the rigorous study of cryptography as a science and a mathematical discipline. This perspective has, in turn, influenced how researchers think about the broader field of computer security. Classical cryptography was concerned with designing and using codes that enable two parties to send messages while keeping those messages hidden from an eavesdropper who can monitor all communication between them. The hard drive serves as the communication channel on which an attacker might eavesdrop if it can gain access to the hard drive and read its contents. “Sharing” the key is trivial, though the user needs a secure and reliable way to remember/store the key for use at a later point in time. Writing a formal definition forces one to think about what is essential to the problem at hand and what properties are extraneous.