Why the earliest forms of encryption are still used for super-secrets

Have you ever listened to short wave radio and heard something strange?  A station with no identification has a voice reading a long list of letters or numbers.  We call them number stations and they’ve been around since before World War II.  The funny thing is that they are still transmitting secret messages open and in the clear for anyone to intercept -but not decode.

Why?  Because this is unbeatable encryption that has been used for thousands of years and no one yet has found a way to defeat it without the key and, most likely, never will.

Earliest forms of encryption used what we call substitution ciphers.  This is where each letter in a message is replaced by a different letter to make a written message unreadable.  This is, at its core, how encryption still works today.  The first uses of substitution ciphers used what is called shifting.  This is accomplished with a encoding/decoding wheel.  A well documented one is the Caesar Disk:

The Caesar disk has two or more concentric rings that can rotate.  Each ring has a copy of the alphabet on it.  The user rotates the ring and effectively shifts the alphabet by lining up different letters as seen below. The message is then sent by replacing the message text with the shifted letter below it.  This is not the unbeatable encryption that I was talking about, but it is a part of it.

The problem with this method is that one only needs to try to decode this message in 26 possible ways in order to crack the code.  In today’s world this is NOT SECURE. You may ask, “How then can it become unbreakable?”  Well, what if we were to randomly rotate the inner ring of the disk between every substitution?  If the rotations are truly random and unpredictable, no one could decode the message, because the message could then be anything and everything.  This is the unbeatable encryption.  Of course, if you don’t keep track of the position of the wheel when you encoded the message, even you can’t decrypt the message.  This is where the concept of a key comes from.  An encryption key keeps track of the wheel position so that it may be duplicated by the decoder.

This brings us to the one-time pad; the unbeatable encryption technique.  The concept is simple: Two copies of a key are made.  One goes to the person that will receive the message and one to the sender.

The key may be split up into sections to make it easier not to lose one’s place.  The sender then uses each letter in the key to tell them where to rotate their encoding wheel.  Each letter in the key is used only once, so the message can only be as long as the key.

Once the message is encoded, the sender destroys her copy of the key, leaving only one copy with the receiver.  The sender then sends the secure message via radio, pigeon, signal light, or whatever means.  When the receiver gets the message, he uses the key to decode the message in the same way it was encoded and then destroy the key as well.

The security in this method comes from these two factors:

1. The keys are destroyed immediately after use.  This prevents the key from falling into the wrong hands or from being used a second time to encode another message.  As soon as a key is used to encode more than one message, the security is compromized.
2. The key is completely random and therefore unpredictable.  This prevents someone without the key from decoding it using cryptanalysis.  It is effectively saying that the message could be any combination of letters.

So that’s it.  It’s been in well documented use since about 1917, but was almost certainly used by secret keepers for thousands of years before that.  The beautiful thing is that it still works today.  It is every bit as secure as it was back then as long as the key is truly random.  If you really want to send a message that is an order of magnitude more difficult to crack than modern forms of digital encryption, use a one-time-pad.  Happy encrypting!

For fun, Here is a link to a little program that I wrote for my kids to play with one-time pad encoding and decoding.  It uses a webcam to help make the numbers nice and random.