Cipher

Plotters used ciphers to disguise their messages, so intercepting a letter was only part of the task: its meaning also had to be deciphered.

Walsingham learned about the conspiracy through a double agent: someone who appeared to help the conspirators but secretly worked for him. He then allowed a secret correspondence route to operate between Babington and Mary, who was held at Chartley Hall in Staffordshire. Their letters were written in cipher and hidden in beer barrels.