Leopold Samuel Marks (24 September 1920 – 15 January 2001) was an English cryptographer, screenwriter and playwright.
Born the son of an antiquarian bookseller in London, he was first introduced to cryptography when his father showed him a copy of Edgar Allan Poe's story, "The Gold-Bug". From this early interest, he demonstrated his skill at codebreaking at an early age by deciphering his father's secret price codes that he wrote inside the covers of books.
His father, Benjamin Marks, was joint owner of the Marks & Co bookshop at 84 Charing Cross Road, which achieved international fame with the 1970 book of that title by New York writer Helene Hanff and the later plays and movie.
As a teenager, he earned pocket money by setting the notoriously difficult Times cryptic crossword.
So begins his book, Between Silk and Cyanide, about his work in cryptography. Marks joined the Armed Services and went to Bedford to train as a cryptographer.
His original and unorthodox mode of thought led to him being the only one of his class judged not good enough to be sent to Bletchley Park; instead, he was sent to a rival organisation of the intelligence services, the recently formed Special