Ancient Greece – write message on shaved head of
slave, allow hair to grow back
Ancient China – write message on fine silk, roll into ball,
cover in wax, swallow
century AD. – Invisible ink from variety of organic
Century Italy – Write on shell of egg in alum
solution. Message appears on egg inside when boiled.
World war – Microfilm, text shrunk to full stop size.
Now: Hide inside music file/image ?
Histaiaeus sent message on head of slave
Pliny described ‘invisible ink’ in 100AD