In 1685, the year when Charles II died and his brother James II succeeded to the throne, John Falconer's Cryptomenysis Patefacta was published in London. The Latin title means something like "disclosed secret communication." The English subtitle "the art of secret information disclosed without a key, containing plain and demonstrative rules, for decyphering all manner of secret writing" suggests its main subject: codebreaking. The book explains various kinds of cipher and techniques to break them. Notably, it demonstrates deciphering of Vigenere cipher, then reputed to be "undecypherable", by use of a probable word.
Falconer's demonstrations of codebreaking seem to assume that the decipherer knows which scheme is used. While this may not be the case in actual situations, a decipherer faced with an unknown cipher would try every one of the schemes explained especially when the letter might contain serious intrigues against the nation.
While influence of John Wilkins' Mercury (1641) is apparent from arrangement of chapters etc., the schemes are critically reviewed by the author, who was actually involved in codebreaking. In the last chapter, he makes critical remarks on Trithemius' works.
Falconer tells how he got interested in cryptography in "To the Reader": "A few years ago, having had some discourse with a Gentleman, concerning the Possibility of Resolving any Writing in Secret Character, and the means to perform it; I was taken with the Novelty of the Thing, and after some few serious thoughts, ventured upon the Tryal, which succeeding at first, I still went further: And you have here the account of my Discoveries in this progress."
While some had believed that disclosure of techniques for secret communication was prejudicious to the security of the nation, Falconer decided that knowledge of cipher should also be beneficial to the authority.
The publication of this work was further motivated by political turmoil at the time. In particular, the Rye House Plot in the spring of 1683, when the life of King Charles II and his brother James was attempted, led to discovery "last year" of treasonable acts of the Earl of Argyll (Wikipedia), a powerful peer in Scotland. Falconer felt alarmed to know that the authority took considerable time in deciphering Argyll's ciphers (see another article). The "open Rebellion now on foot" (probably referring to the Monmouth Rebellion occasioned by James II's accession to the thrown in February 1685, in which the Earl of Argyll joined and was executed) added new motives for speedy publication of the work. Such political backgrounds are reflected in many specific references to Argyll's ciphers.
Little is known about the life of Falconer. Even Oxford DNB does not have an entry for him.
While David Kahn, an authority on cryptography, says the book was published after his death, many references to Argyll and, apparently, the Monmouth Rebellion in 1685 suggest that the author was active at least in 1685 when the book came out. The description of Biographie universelle cited by Kahn can also be read to indicate that the exile of Falconer with his master James was not before the accession as Kahn says, but after the Glorious Revolution (1688).
While Kahn also refers to what he considers to be the source of Biographie universelle (probably, Thomas Falconer, Bibliography of the Falconer Family, 1866), the latter (which the present author has not seen) may be based on The Annual Biography and Obituary for the Year 1825, in particular its annex "Biographical Index of Deaths, for 1824" (Google). From this, it appears that Falconer published the work in life and, after the Glorious Revolution, followed James II to exile in France, where he died.
(By the way, James had three periods of life in exile. When young, James was in exile on the continent due to the English Civil War (in France from 1648 to 1656 then in the Low Countries until 1660). During his brother's reign, he had to be in temporary exile in Brussels in 1679 for political considerations. After the Glorious Revolution, he held a court in exile in France until his death. One author says, citing Kahn, Falconer wrote the book in France while serving James during his exile there (1652-1660) but this seems to be incorrect in many ways.)
Falconer's attachment to the Stuarts is also apparent in "The Epistle Dedicatory." He dedicated the book to the second Earl of Middleton (Wikipedia) not only because Middleton was secretary of state (August 1684 to February 1689) but also because of the adherence of the family to the crown. Falconer specifically refers to the career of his father, the first Earl of Middleton (Wikipedia). Charles II trusted the first earl with a command of a rising in 1654 in Scotland to regain the crown and, after the Restoration in 1660, chose the first earl as Lord High Commissioner to represent the king in Scotland.
According to The Annual Biography and Obituary, John Falconer had a son, William Falconer, recorder of Chester. The latter had two surviving sons: Thomas Falconer (1738-1792) (Wikipedia) and William Falconer (1744-1824) (Wikipedia).
Text of Cryptomenysis Patefacta