In the late 1400s, King Ferdinand and Queen Isabella of Spain established the Tribunal of the Holy Office of the Inquisition, better known simply as the "Spanish Inquisition." The tribunal, in general terms, was designed to ensure that Catholicism would be the prevailing and official religion of their nation by investigating the practices of new converts to the religion -- inquisitors would visit former Jews and Muslims to ensure that these new Catholics were actually practicing Catholicism, and not faking it in hopes to simply fit in with broader society. Over time, the Inquisition became increasingly nefarious; many Jews and Muslims did not want to renounce their birth religion. The Inquisition wouldn't stand for that, and failure to convert became grounds for expulsion from Spain, incarceration, torture, and even execution in some cases. 

For Italian scholar Giambattista della Porta, the Inquisition wasn't a deep concern. As an Italian, he wasn't subject to the Spanish laws in the first place, and as a Roman Catholic by birth, he wouldn't have had to convert anyway. But Della Porta wasn't a fan of the Catholic church and certainly not of the practices of the Inquisition. And, on a personal level, many of his fellow scientists and friends were non-Catholics living in Spain. Some of them were swept up by the inquisitors and incarcerated -- and cut off from the outside world. Della Porta went to visit them but was turned away at the gate. No visitors were allowed, with few exceptions.

But running a prison is expensive, so the Inquisition did allow for care packages of sorts. Everything was inspected, of course -- you don't want people smuggling in weapons or passing secret messages to their friends on the inside. The only thing allowed was food, and that was easy to search through -- or so the authorities thought. Della Porta wasn't just some average Joe visiting his friends; he was, as discussed by scholars in the American Philosophical Society centuries later, "the outstanding cryptographer of the Renaissance." For Della Porta, the "no messages allowed" rule wasn't an edict, but a challenge. In one of his books, titled Magia Naturalis ("Natural Magic"), he revealed the basis for his master plan: "eggs are not stopped by the Papal Inquisition and no fraud is suspected to be in them."

Eggs, Della Porta knew, were actually the perfect food for fraud. In the 10th century, a Greek whose identity has been lost time put together a 20-book long agricultural guide titled Geoponika. In that book is a recipe titled "To Make Eggs Bear and Inscription." Della Porta was familiar with Geoponika and the egg trick, a translated version of which (via AncientMedicine) is below. And he got to work.

To make inscriptions on eggs. From Africanus. Grind up oak gall and alum with vinegar until it reaches the thickness of black ink. Use it to write whatever you want on the egg. Once the writing has dried in the sun, place the egg into a sharp brine. Once it has dried, boil it, and when you have peeled it, you will find the inscription.

To summarize, he says one should first boil the egg, coat it in wax, and then inscribe the message in the wax through to the shell (as in etching); next, he says to put the egg in a solution of alum and gall (for how long is unclear), followed by a solution of sharp vinegar, after which the egg is dried and the shell removed to find the message in saffron-colored writing.