Pigpen is a cipher system of geometric shapes. Of ancient origin, its current usage is essentially confined to members of various secret societies to the extent that it is often called the Freemason's Code. A non-masonic organization which employed the pigpen cipher was The Knights of the Golden Circle, a white supremacist society which arose in America's postbellum South.
"Maggie Had A Little Pigpen,"
Word Ways: Vol. 24
, Article 13.
Available at: https://digitalcommons.butler.edu/wordways/vol24/iss2/13