The belt of the messenger was composed of a stretch of leather which had characters written on it on the backside. It seemed like a meaningless sequence of characters but when the stretch of leather is winded around a piece of wood with the correct diameter, an encoded message can be read.
Fig. 1: Scytale¹
The Scytale is a cipher that is based on a transposition method. The diameter of the Scytale can be regarded as the key of the cipher.
(1) http://commons.wikimedia.org/wiki/File:Skytale.png, 2009-03-15
The Scytale has already been used more than 2500 years ago and therefore it is the oldest known military ciphering method. In the year 404 B.C. only one of five messengers survived the grueling march from Persia back to the Spartan general Lysander. The messenger gave Lysander his belt and Lysander winded his belt around the so-called Scytale. Thus he was informed that the Persians planned an attack, he prepared for this attack and was able to fend it successfully.¹
(1) Singh, Simon: „Geheime Botschaften“, Carl Hanser Verlag, 1999, p.23
The principle of the Scytale was very secure for its time. After the demise of the Greek culture, comparably secure ciphers were not used until medieval times.