Spy rings (groups of spies or ‘intelligencers’) used a range of tools to keep their correspondence safe. Secret codes, or ciphers were widely used. Substitution ciphers, when , for instance, a number was substituted for a letter were used to code and decode messages.
Exclusion ciphers, where a piece of paper with cut out windows was placed over a nonsense letter to reveal key words of the real message were also used.
Invisible ink was made from lemon or artichoke juice. Words written with this liquid were revealed only when a letter was heated or when a chemical was painted over the top.
Fake envelopes with cover addresses were used to forward mail to a spy, who would open the real letter inside and deliver it to a spy by hand. Personal, puzzle-like paper locks, also kept spy correspondence safe. As well as sending and receiving messages, spies eavesdropped on enemy conversations, intercepted written communication, and bribed informants