The spies had a few special tricks up their sleeves. “They practiced secret inks,” explains Alford. “Quite a lot of use of code and cypher, which to our eyes looks relatively unsophisticated, although it develops an increasing sophistication.”
Cyphers became particularly important during the infamous Babington Plot, when Walsingham’s agents decrypted letters to and from Mary Queen of Scots. This provided evidence that Mary was conspiring against Elizabeth, leading to Mary’s trial and execution.
By the 1580s, ciphers were extremely complex – they could incorporate substitute letters, Arabic numerals, nulls, letters with a dot before or after, substitute names for locations, and numbers, signs of the zodiac or days of the week for individuals.
If you think that sounds innovative, consider how French and English secrecy methods seem to have roots elsewhere:
Muhammad ibn Abbad al-Mu’tamid (المعتمد بن عباد), King of Seville from 1069-1092, used birds in poetry for secret correspondence.