Polyalphabetic

The Two Square Cipher is a polygraphic substitution cipher that uses two separate 5x5 squares to encrypt pairs of letters. It is a variant of the Playfair Cipher but uses two grids instead of one, providing a more complex substitution for digraphs (pairs of letters). This cipher increases security by avoiding simple frequency analysis that single-letter substitution ciphers are vulnerable to.

The Cadenus–Gronsfeld Cipher is a variation of the Gronsfeld Cipher, itself a numeric version of the Caesar Cipher. It operates on alphabetic text by shifting each letter according to a repeating numeric key, but with the added twist that the key may include a reversible sequence or "cadenus" pattern, providing irregularity in the shifts. This makes it a hybrid between classical polyalphabetic ciphers and simple numeric substitution, increasing resistance to frequency analysis.

The Autokey Vigenère Cipher is an advanced polyalphabetic cipher that builds upon the classic Vigenère Cipher by incorporating an autokey mechanism. Instead of repeating a short keyword cyclically, the key is extended by appending the plaintext itself after the initial keyword. This produces a variable-length key, reducing repeating patterns in ciphertext and making frequency analysis considerably more difficult than in the standard Vigenère system.

The Wheatstone Cipher, also known as the Playfair Cipher, is a digraphic substitution cipher that encrypts pairs of letters (digraphs) rather than single letters. It was described by Charles Wheatstone in 1854 and later popularized by Lord Playfair. Its main advantage over simple substitution ciphers is that frequency analysis is more difficult because the unit of encryption is two letters instead of one.

The Trithemius Cipher is a polyalphabetic substitution cipher that shifts the alphabet progressively for each letter of the message. Instead of using a fixed shift like the Caesar Cipher, the shift increases step by step as the message is encoded.

The cipher was introduced by Johannes Trithemius in 1508. It works by applying a sequence of Caesar shifts to each letter of the plaintext. The first letter is shifted by 0, the second by 1, the third by 2, and so on. This changing shift produces a new substitution alphabet for every letter in the message.

The Porta Cipher is a classical polyalphabetic substitution cipher named after the Italian scholar Giambattista della Porta, who described it in the 16th century. It is a variant of the polyalphabetic cipher family, similar in principle to the Vigenère Cipher, but with a reciprocal structure that makes encoding and decoding symmetrical—using the same process in reverse produces the original text.

The Vigenère Cipher is a classical polyalphabetic substitution cipher that uses a repeating keyword to determine shifting values for each letter in a message. Unlike the Caesar Cipher, which applies a single fixed shift, the Vigenère Cipher changes the shift for every letter based on the key. This shifting pattern significantly reduces simple frequency analysis.

It can be viewed as a systematic expansion of the Trithemius Cipher, which uses a progressive shifting pattern instead of a repeating keyword.

The Trifid Cipher is a classical polygraphic cipher that extends the principles of the Bifid Cipher by using three-dimensional coordinates. It combines substitution and transposition to encrypt messages in a way that mixes letters across multiple positions, providing higher security than simple monoalphabetic or basic polygraphic ciphers.

The Running Key Cipher is a polyalphabetic substitution cipher that uses a long piece of text as the key instead of a short repeating keyword. The key must be at least as long as the plaintext. Each letter of the plaintext is combined with the corresponding letter of the key using modular arithmetic based on the alphabet.

The Polygraphia Cipher originates from the work Polygraphia, a 16th-century cryptographic treatise written by the German abbot and polymath Johannes Trithemius and first published in 1518. The book is considered the first printed work devoted entirely to cryptography. Among its many systems, Trithemius introduced a progressive substitution method in which the encryption alphabet changes with each letter of the message.