Polygraphic

The Double Transposition Cipher is a classical manual cipher that applies two consecutive columnar transpositions to a plaintext message. Unlike a simple columnar transposition, which rearranges letters using a single key, the Double Transposition Cipher uses two separate keys sequentially, providing significantly stronger diffusion and making frequency analysis more difficult. Each key determines the column order for its respective transposition stage.

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.

The ADFGX Cipher is a fractionating transposition cipher created by the German army during World War I. It converts each letter of plaintext into a pair of symbols chosen from A, D, F, G, X using a 5×5 Polybius square with a custom alphabet (merging I and J). After this substitution, the resulting pairs are rearranged using a columnar transposition keyed by a chosen word. This combination of fractionation and transposition makes the cipher significantly more secure than a simple substitution or basic columnar cipher.

The Tap Code is a substitution cipher that encodes letters based on their position in a 5x5 Polybius square, typically combining the letters I and J. Each letter is represented by two numbers: the row and column where it appears in the square. These numbers are then expressed as a series of taps, with the number of taps indicating the row and column.

The Nihilist Cipher is a polygraphic substitution cipher that combines the principles of the Polybius Square with a numeric key to produce a series of ciphertext numbers. It was developed in the 19th century by Russian nihilists to securely transmit messages, hence its name. Each letter is first converted into a pair of digits using a Polybius grid, then combined with corresponding digits from a numeric key by addition, resulting in ciphertext that appears as a sequence of numbers.

The Bifid–Bacon Hybrid Cipher is a creative fusion of the Bifid Cipher and the Baconian Cipher. It combines polygraphic transposition with dual-character substitution to create a cipher that spreads each plaintext letter across multiple symbols while encoding them into A and B sequences. This hybrid approach increases diffusion and adds a layer of steganography, making it more resistant to frequency analysis than either cipher alone.

The Straddling Checkerboard Cipher is a substitution cipher that encodes letters into digits using a numeric grid with main rows and offset rows. Common letters are assigned a single digit in the main rows, while less frequent letters use one of the offset rows as a prefix to create a two-digit code. This method compresses messages and provides a simple layer of obfuscation.