We all carry around with us a clumsy arrangement of keys and cards that give us access to things like houses, cars, and bank accounts. The computer equivalent of a house key is a randomly chosen sequence of digits, or bits, that often resides inside a dongle--a software-protection device about the size of a credit card. Some dongles also contain a microprocessor that manipulates the digital key. To gain access to my computer system at work, for example, I use a dongle that has both a unique 58-bit key and a processor that uses the key to respond to challenges from the computer I log on to.
Dongles, which rely on encryption to do their work, are just the modern expression of an age-old effort to safeguard information. Even the ancients used encryption--though generally not for business but in wartime. All classic encryption methods--past and present--are based on one of two fundamental principles: a seemingly random transposition of the letters in a message or a similarly random substitution of one letter for another.
The oldest known encryption device based on the transposition principle is the scytale, which was first mentioned by Thucydides in his History, in the fifth century B.C. The Greek biographer Plutarch described the scytale this way: "When the ephors send out an admiral or a general, they make two round pieces of wood exactly alike in length and thickness, so that each corresponds to the other in its dimensions, and keep one themselves, while they give the other to their envoy." To encode a message, the sender winds a strip of parchment around the scytale in a long spiral and writes the message along the length of the wooden cylinder, across the windings. Then the strip is removed from the scytale and transported. On its own, the wording on the strip looks like a random sequence of letters, but it is easily read when the strip is rewound over the matching scytale.
A good example of the substitution model of encryption is the wheel cypher, a device invented by Thomas Jefferson in the 1790s. The wheel cypher is a spindle about six inches long strung with 36 discs, each about two inches in diameter. To encode a message, the sender takes a block of 36 letters from the message, lines up the 36 discs to spell out the block, and reads off any one of the other 25 positions of the wheels for the encoded text. The sender then repeats the process until the entire message is encoded. The encryption is easily reversed, but only by someone who has a wheel cypher whose disc alignment matches the sender's exactly.
How good is the system? There are 25 x 24 x 23 x...x 3 x 2 x 1 ways to label the first disc, which works out to a 26-digit number. And there are as many ways to label each of the remaining 35 discs. In all, the number of ways to label the 36 discs of the wheel cypher and mount them on the spindle works out to a number with more than 900 digits.
Both the scytale and the wheel cypher are small and portable and give remarkably good protection. Imagine what their inventors could have done with a dongle, a modem, and a modern PC.
Gerard J. Holzmann is a member of the technical staff at Bell Laboratories, in Murray Hill, N.J.