This brings to mind Alan Turing's bicycle in Cryptonomicon:

  "So Turing can leave his bicycle anywhere and be confident that, if stolen, it won’t go more than a fifth of a kilometer before the chain falls off."

The whole passage (it relates eventually to Enigma and Cipher Text) is a good read: http://www.euskalnet.net/larraorma/crypto/slide18.html

Of course, the whole book is a good read, so skip the link and head to a bookstore.