Substitution

The Phillips Cipher is a classical polygraphic substitution cipher that encrypts plaintext by splitting it into fixed-size groups (typically digraphs or trigraphs) and then applying a series of transpositions and substitutions based on a keyword. It was designed to increase security over simple substitution ciphers by mixing the positions of letters within each group and leveraging the keyword to control letter mapping.

The Fractionated Morse Cipher is a classical encryption method that combines Morse code encoding with substitution techniques. It works by first converting plaintext into Morse code, then grouping the resulting symbols into fixed-length units, and finally substituting those groups using a keyed alphabet. This multi-stage process produces ciphertext that obscures both the original letters and the structure of Morse code.

The Cadenus Cipher is a polyalphabetic substitution cipher that uses a repeating keyword to alter the shift of each plaintext character. Named in reference to classical ciphering techniques, it is similar in principle to the Vigenère Cipher, but often employs a direct letter-to-letter mapping that adapts dynamically according to the keyword.

The Progressive Caesar Cipher is an evolution of the classic Caesar Cipher, introducing a dynamic, position-based shift rather than a fixed one. In this system, each successive plaintext letter is shifted by an incrementally increasing value, starting from an initial shift. This creates a ciphertext that is harder to analyze with frequency techniques than the standard Caesar method.

The Variant Beaufort Cipher is a classical polyalphabetic substitution cipher closely related to the Vigenère Cipher and the original Beaufort Cipher. It uses a repeating keyword to transform plaintext letters, but differs in the way the substitution is calculated. Instead of adding or subtracting shifts directly, the Variant Beaufort Cipher uses a reversed tabula recta relationship, producing a distinct encryption pattern.

The Three Square Cipher is a classical polygraphic substitution cipher that encrypts pairs of letters using three separate keyed squares. It is closely related to the Playfair Cipher and the Two-Square Cipher, but increases complexity by introducing a third square, which enhances diffusion and reduces recognizable patterns in the ciphertext.

The Digrafid Cipher is a classical digraphic substitution cipher that encrypts plaintext in pairs of letters, also known as digraphs. It uses a keyword to create a 5x5 (or 6x6 for extended alphabets) grid, similar in concept to the Playfair Cipher, but with modifications that enhance diffusion and obfuscation. Each digraph is mapped to another digraph according to the grid rules, producing ciphertext that appears less patterned than simple substitution ciphers.

The Gold-Bug Cipher is a substitution cipher made famous by Edgar Allan Poe in his short story The Gold-Bug. Unlike standard alphabetic ciphers, this system replaces letters with a mixture of symbols, numbers, and punctuation marks, creating a visually distinctive ciphertext. The cipher is monoalphabetic, meaning each plaintext letter always maps to the same symbol, but its unusual character set makes it appear more complex than it actually is.

The Albam Cipher is a classical substitution cipher dating back to the early modern period. It is a simple monoalphabetic cipher in which each letter of the plaintext is replaced by the letter 13 positions ahead in a specialized rearranged alphabet. Unlike the standard Caesar Cipher, which shifts letters uniformly in the standard A–Z order, the Albam Cipher uses a fixed substitution mapping based on two halves of the alphabet: the first half (A–M) is paired with the second half (N–Z), and vice versa.

The ADFGVX Cipher is an extension of the ADFGX Cipher, designed during World War I by the German army to secure telegraphic communications. It combines a 6×6 Polybius square mapping letters and digits to pairs of symbols A, D, F, G, V, X, followed by a columnar transposition using a keyword. Compared to ADFGX, it adds support for digits (0–9), providing a 36-character alphabet and stronger diffusion across the ciphertext.