5th century BC – Spartan generals exchanged secret messages by wrapping narrow ribbons of parchment around a cylindrical staff known as a scytale, then transcribing their messages on the papyrus. The messages could only be read when the papyrus was re-wound around a scytale of the same thickness.

The diameter of the cylinder , therefore is the key

Substitution Method

The scytale method was used by the government officials of Sprta to communicate with their military officer.

It was not until a couple of centuries later that another popular encryption method arises: The ceasar cipher, named after Julius Caesar.

CODE BOOK

Each Letter is replaced by only one cipher symbol

MONO-ALPHABETIC

There is no distinguish between cryptographic system and key because system were so simple (code book is the key, and the system is mapping plain text to cipher)

All the expense is making the code book and making the secret piece.

Polyalphabetic

All the expense is on the public piece

Symmetric Encryption