What secret alchemical knowledge could be so important it required sophisticated encryption? The setting was Amsterdam, 2019. A conference organized by the Society for the History of Alchemy and Chemistry had just concluded at the Embassy of the Free Mind, in a lecture hall opened by historical fiction author Dan Brown. At the conference, Science History Institute postdoctoral researcher Megan Piorko presented a curious manuscript belonging to English alchemists John Dee (1527–1608) and his son Arthur Dee (1579–1651). In the pre-modern world, alchemy was a means to understand nature through ancient secret knowledge and chemical experiment. Within Dee’s alchemical manuscript was a cipher table, followed by encrypted ciphertext under the heading “Hermeticae Philosophiae medulla”—or Marrow of the Hermetic Philosophy. The table would end up being a valuable tool in decrypting the cipher, but could only be interpreted correctly once the hidden “key” was found. It was during post-conference drinks in a dimly lit bar that Megan decided to investigate the mysterious alchemical cipher—with the help of her colleague, University of Graz postdoctoral researcher Sarah Lang. A Recipe for the Elixir of Life Megan and Sarah shared their initial analysis on a history of chemistry blog and presented the historical discovery to cryptology experts from around the world at the 2021 HistoCrypt conference. Based on the rest of the notebook’s contents, they believed the ciphertext contained a recipe for the fabled Philosophers’ Stone—an elixir that supposedly prolongs the owner’s life and grants the ability to produce gold from base metals. The mysterious cipher received much interest, and Sarah and Megan were soon inundated with emails from would-be code-breakers. That’s when Richard Bean entered the picture. Less than a week after the HistoCrypt proceedings went live, Richard contacted Lang and Piorko with exciting news: he’d cracked the code. Megan and Sarah’s initial hypothesis was confirmed; the encrypted ciphertext was indeed an alchemical recipe for the Philosophers’ Stone. Together, the trio began to translate and analyze the 177-word passage. The Alchemist Behind the Cipher But who wrote this alchemical cipher in the first place, and why encrypt it? Alchemical knowledge was shrouded in secrecy, as practitioners believed it could only be understood by true adepts. Encrypting the most valuable trade secret, the Philosophers’ Stone, would have provided an added layer of protection against alchemical fraud and the unenlightened. Alchemists spent their lives searching for this vital substance, with many believing they had the key to successfully unlocking the secret recipe. Arthur Dee was an English alchemist and spent most of his career as royal physician to Tsar Michael I of Russia. He continued to add to the alchemical manuscript after his father’s death—and the cipher appears to be in Arthur’s handwriting. We don’t know the exact date John Dee, Arthur’s father, started writing in this manuscript, or when Arthur added the cipher table and encrypted text he titled “The Marrow of Hermetic Philosophy.” However, we do know Arthur wrote another manuscript in 1634 titled Arca Arcanorum—or Secret of Secrets—where he celebrates his alchemical success with the Philosophers’ Stone, claiming he discovered the true recipe. He decorated Arca Arcanorum with an emblem copied from a medieval alchemical scroll, illustrating the allegorical process of alchemical transmutation necessary for the Philosophers’ Stone. Cracking the Code What clues led to decrypting the mysterious Marrow of the Hermetic Philosophy passage? Adjacent to the encrypted text is a table resembling one used in a traditional style of cipher called a Bellaso/Della Porta cipher—invented in 1553 by Italian cryptologist Giovan Battista Bellaso, and written about in 1563 by Giambattista della Porta. This was the first clue. The Latin title indicated the text itself was also in Latin. This was ...