Polygraphic

The Tap Code is a manual cipher system used for encoding messages through a series of taps, knocks, or other simple signals, and was popularized for covert communication among prisoners of war in the 20th century, particularly during World War II and the Vietnam War. It is essentially a numeric substitution cipher based on a 5×5 Polybius square in which letters are assigned coordinates, with C and K sharing a single position.

The Nihilist Cipher is a classical cipher combining fractionation and polyalphabetic substitution, invented by the Russian-Jewish revolutionary and cryptographer F. K. Nihilist in the 1880s. It gained practical notoriety among Russian revolutionary groups for clandestine communication. The cipher operates by first converting plaintext letters into numbers using a Polybius square, then adding a numeric key sequence to these numbers to create the ciphertext.

The Bifid–Bacon Hybrid cipher is a composite classical cipher that combines the fractionation principles of the Bifid cipher, invented by Félix Delastelle in 1901, with the binary encoding approach of the Baconian cipher, developed by Francis Bacon in 1605. This hybrid system leverages both numerical fractionation and letter-to-binary substitution to increase diffusion and obscure the relationship between plaintext letters and ciphertext symbols.

The Straddling Checkerboard cipher is a classical cipher system invented by the American cryptographer George Washington Bazeries in the late 19th century and later popularized in manual cipher systems throughout the 20th century. It is a type of substitution cipher that converts letters into single or double-digit numbers using a configurable grid, combining both simplicity and efficiency.

The Base64 cipher is an encoding scheme developed in 1993 as part of the MIME (Multipurpose Internet Mail Extensions) standard by a group including Philippe J. Flajolet and other contributors to internet protocols. Unlike classical ciphers, Base64 is not used for cryptographic secrecy but for converting binary data into ASCII text to ensure safe transmission over systems that handle text poorly.