Some similarities we can find in our two examples are the use of a key, the unlocking instructions, to decode the message. Our ciphers used only one key to encrypt and decrypt the message and this creates security issues. The single key itself must be kept secret while somehow being transmitted to the person receiving encoded messages. Even if the key is transmitted safely (which you can never know for certain) the recipient can never be sure the received message hasn't been intercepted by the enemy, altered, or passed along to create havoc. This major fault of the one key system made it very vulnerable to attack. An answer to the problem was found in 1976. Up until 1976 nobody outside the government, or at least outside the government's control, performed any serious work in cryptography. The National Security Agency (NSA) was in charge of all advancements in cryptography, but that changed when a 31-year old computer wizard named Whitfield Diffie discovered a new system, called "public-key" cryptography.
Diffie profession in 1976 was tending a complicated multi-user computer system at MIT. He became troubled with the problem of how to make the system, which held a person's work and sometimes his or her intimate secrets, truly secure. The traditional, top-down approach to the problem " protecting the files by user passwords, which in turn were stored in the electronic equivalent of vaults tended by trusted system administrators - was not satisfying. The weakness of the system was clear: a user's privacy depended on the degree to which the administrators were willing to protect it. Diffie recognized the solution rested in a decentralized system in which each person held the literal key to his or her own privacy. He tried to get people interested in taking on the mathematical challenge of discovering such a system, but there were no takers. It wasn't until the early 1970s, when those running the ARPAnet were exploring security options for their members, that Diffie decided to take on the issue himself.