Devil's Night

In parallel with the development of steganography, there was the evolution of cryptography, derived from Greek word krypto, which means hidding. Despite of hiding the existence of a message by steganography. Cryptography chooses to hide its meaning by a process called encryption.

To render a message unintelligible, it is scrambled according to a particular protocol which is agreed beforehand between the sender and the intended recipient. Thus the recipient can reverse the scrambling protocol and make the message comprehensible. Compared with steganography, the advantage of cryptography is that if the enemy intercepts an encrypted message, then the message is unreadable. Without knowing knowing the scrambling protocol, the enemy should find it difficult, if not impossible, to recreate the original message from the encrypted text.

In fact, cryptography itself can be divided into 2 branches, one is transposition another is substitution. In transposition the letters of the messages of a message a simply rearranged, forming an anagram. the first implementation of cryptography is transposition, known as Spartan Scytale. Dating back to 5 century B.C. The scytale is a wooden staff around which a strip of leather or cloth is wound, as show on the right. The render wants to send a message like this:

send more troops to southern flank at eight am tomorrow

so he writes message along the length of the staff

|S|E|N|D| |M|O|R|E| |T|R|O|O|
|P|S| |T|O| |S|O|U|T|H|E|R|N|
| |F|L|A|N|K| |A|T| |E|I|G|H|
|T| |A|M| |T|O|M|O|R|R|O|W| |

then unwind the strip, now the content on the leather or cloth turns into a list of meaningless letters.

SP T ESF N LA DTAM ON M KT OS OROAMEUTO T RTHERREIOORGWONH

The message now has been scrambled, this is what the messenger would take. To recover a message, the receiver simply finds a scytale of the same diameter and wraps the strip onto it.

In 404 B.C. Lysander of Sparta was confronted by a messenger , bloody and battered, one of only five have to survived the arduous journey from Persia. The messenger handed his belt to Lysander, who wound the belt around his scytale to learn that Pharnabazus of Persia was planning to attack him. Thanks to the scytale, Lysander was prepared for the attack and repulsed it.

The disadvantage of Spartan Scytale is obvious, in this cryptography system: key is diameter of scytale and protocol is wrapping. If enemy knows the protocol, guessing key is quite a simple job. These days, Internet gives spartan scytale a second life. For example, in mainland of China, people writes message vertically in forums to bypass the censor content checking performed by government.