Substitution

The Zodiac Cipher is a symbolic substitution cipher that encodes plaintext letters into a set of distinct Zodiac-themed symbols. Unlike historical cryptograms associated with the Zodiac Killer, this version is purely thematic, using visual symbols inspired by astrological signs to represent letters of the alphabet. The cipher is typically monoalphabetic, meaning each plaintext letter maps to a single symbol, but it can be adapted with a keyword to shuffle the symbol order for added security.

The Patristic Cipher, also known as a Patristocrat, is a form of monoalphabetic substitution cipher specifically designed to conceal word boundaries and sentence structure. Unlike a standard substitution cipher where spaces are preserved, the Patristic Cipher removes all spaces and punctuation from the plaintext and then regroups the resulting ciphertext into uniform blocks, traditionally of five letters. This visual flattening makes frequency analysis more difficult and forces the solver to reconstruct word breaks mentally.

The Letter Number Substitution cipher is a classical substitution method in which each letter of the alphabet is replaced by a corresponding number. Its origins are difficult to trace precisely, as variations of letter-to-number mapping have existed since ancient times, but it became widely referenced in cryptographic manuals during the 19th century for educational and puzzle purposes. The method provides a simple way to transform text into a numeric sequence while remaining reversible, making it suitable for manual encryption and early secret messaging.

The D’Agapeyeff Polybius Cipher is a classical cipher introduced by British cryptographer Alexander D’Agapeyeff in his 1939 book Codes and Ciphers. It combines the principles of a Polybius square with a keyword to create a keyed numeric substitution system. Each letter of the plaintext is converted into numeric coordinates within a square grid based on the keyword, and this keyed approach modifies the standard Polybius mapping, adding complexity and obfuscation.

The Cadenus–Gronsfeld cipher is a hybrid classical cipher that combines the key-based numeric shifts of the Gronsfeld cipher, a variant of the Vigenère cipher popularized in the 19th century, with an autokey-like sequence inspired by Cadenus, an early cryptographic system emphasizing dynamic key extension. While the exact origins of the Cadenus–Gronsfeld cipher are less documented, it emerged as a method to improve the security weaknesses of simple repeating-key polyalphabetic ciphers.

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 Autokey Vigenère cipher is an extension of the classical Vigenère cipher, designed to eliminate the repeating key weakness and increase security. It was popularized by the British cryptographer Giovanni Battista Bellaso in 1553 and later refined in practice by various cryptographers, though its autokey variant became well known in the 19th century.

The Wheatstone cipher is a classical cipher system developed by the English scientist Charles Wheatstone in 1854. It is a type of polyalphabetic transposition cipher, often implemented using a mechanical device called the Playfair cipher square, which Wheatstone popularized and patented. The system was designed to encrypt pairs of letters rather than individual letters, introducing a degree of diffusion that makes simple frequency analysis much less effective.

The Trithemius cipher is a polyalphabetic substitution cipher invented by the German abbot and cryptographer Johannes Trithemius in 1508. It is considered one of the earliest examples of systematic polyalphabetic encryption, preceding the more widely known Vigenère cipher by several decades. The key innovation of the Trithemius cipher is that it uses a progressive shift based on the position of each letter in the plaintext rather than a repeating keyword.

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.