Pigpen

The pigpen cipher, also called the Freemason's cipher, Napoleon cipher, and masonic cipher, is a substitution cipher used at least since the 18th century, employing a tic-tac-toe-like grid to represent letter symbolically.

History
While the exact origin of the pigpen cipher is unknown for certain, it is theorized to have an origin somewhere in the era of the christian crusades, dating it to around the 11th century. Additionally, use of something similar to the modern day cipher has been described as early as the 16th century, most notably by Cornelius Agrippa in his account of Jewish Kabbalah. His writing would imply that it was not actually used for cryptography, instead using a 9-celled grid system with the Hebrew alphabet as a form of religious symbolism. Because of this, distinct proof of the cipher being used for cryptography purposes only dates back to the 18th century.

The earliest confirmed use of a pigpen cipher is by the Freemasons, who encrypted many of their documents and correspondences using the system beginning in the early 1700s. This practice also extended to stone carvings, such as gravestones of deceased masons (the earliest being from 1785, with a carving reading "memento mori"); the practice worked well with the code, as the use of only straight lines and dots allowed for easy transferal to stone via chiselling.

The cipher was also popular with the Rosicrucian Order, although what's known today as the "Rosicrucian Cipher" is actually a variation on the traditional Masonic pigpen cipher.

The most recent historical use was by the Union Army during the American Civil War. A variant was developed that used an additional layer of alphabetic substitution, allowing for an extra level of encryption on Union messages. This ended up being a good call, as the cipher was primarily used by Union soldiers who had been taken prisoner by the confederates.

Since then, however, the cipher has fallen out of favor, primarily due to a lack of actual crytographical security. As the symbols are a 1:1 substitution for letters, even if one wasn't aware of the basic premise the code could be cracked via basic cryptanalysis. If one was aware of it, then it breaks down further, as anyone who gets ahold of the message can simply look up the key (assuming no additional layers are added).

Modern use is primarily in popular culture, with the code showing up in Dan Brown's The Lost Symbol, promotional material for BBC's Sherlock, and the video game Assassin's Creed II, with varying levels of mystery surrounding them.

Notable Examples

 * Dirty Laundry (MITMH 2005) - Similarly to a later puzzle from the 2020 MIT Mystery Hunt, the pigpen-based wordsearch requires solvers to
 * Safari Park (MITMH 2008) - While not involving a whole lot of actual decoding of pigpen, it does require solvers to
 * Vain Snowball (MITMH 2018) -