secret writing

He was allowed to mail two letters with "secret writings" to his loved ones per month, he said.

Examples he cites are "from the chemistry that enabled us to produce secret writing technologies in the early days, to the wireless and secure speech technologies we developed during the Second World War".

Prosecutors have video and audio proving he provided classified information including CIA sources and methods, communication systems and secret writing, Sorenson said.

Holmes is an expert in practical chemistry, as well as an authority on newspaper typography, cigarette ash, the sonnets of Petrarch, secret writing, bicycle tyres, music and the varieties of London mud.

There are secret doors, secret passages, secret codes, secret writing.