Overview
The pigpen cipher (alternatively referred to as the masonic cipher, Freemason's cipher, Rosicrucian cipher, Napoleon cipher, and tic-tac-toe cipher) is a geometric simple substitution cipher, which exchanges letters for symbols which are fragments of a grid. The example key shows one way the letters can be assigned to the grid.
Insecurity
The Pigpen cipher offers little cryptographic security. It differentiates itself from other simple monoalphabetic substitution ciphers solely by its use of symbols rather than letters, the use of which fails to assist in curbing cryptanalysis. Additionally, the prominence and recognizability of the Pigpen leads to it being arguably worthless from a security standpoint. Knowledge of Pigpen is so ubiquitous that an interceptor might not need to actually break this cipher at all, but merely decipher it, in the same way that the intended recipient would.
Due to Pigpen's simplicity, it is very often included in children's books on ciphers and secret writing.
History
The cipher is believed to be an ancient cipher and is said to have originated with the Hebrew rabbis. Thompson writes that, “there is evidence that suggests that the Knights Templar utilized a pig-pen cipher” during the Christian Crusades.
From Wikipedia (CC BY-SA 4.0).