The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography

by Simon Singh

Returning to padlock analogies, asymmetric cryptography can be thought of in the following way. Anybody can close a padlock simply by clicking it shut, but only the person who has the key can open it. Locking (encryption) is easy, something everybody can do, but unlocking (decryption) can be done only by the owner of the key. The trivial knowledge of knowing how to click the padlock shut does not tell you how to unlock it. Taking the analogy further, imagine that Alice designs a padlock and key. She guards the key, but she manufactures thousands of replica padlocks and distributes them to post offices all over the world. If Bob wants to send a message, he puts it in a box, goes to the local post office, asks for an “Alice padlock” and padlocks the box. Now he is unable to unlock the box, but when Alice receives it she can open it with her unique key. The padlock and the process of clicking it shut is equivalent to the public encryption key, because everyone has access to the padlocks, and everyone can use a padlock to seal a message in a box. The padlock’s key is equivalent to the private decryption key, because only Alice has it, only she can open the padlock, and only she can gain access to the message in the box.