Quagmire Cipher

A Quagmire is a periodic polyalphabetic substitution cipher, a direct cousin of the Vigenere cipher, in which one or both substitution alphabets is a mixed alphabet built from a keyword.

The only difference lies in the creation of the cipher table.

The table displays the plaintext alphabet (the letters of the plaintext message) in the column headers, and $ n $ rows representing the $ n $ alphabets (the letters of the ciphertext message).

Below are the various tables generated using the keyword CLE (with A as the indicator letter).

For a classic Vigenère cipher, the table is:

For Quagmire, a mixed alphabet is used—in this case, based on the word WORD.

In Quagmire 1, it modifies the plain alphabet (at the top of the columns)

In Quagmire 2, he modifies the cipher alphabet (within the grid)

In Quagmire 3, he modifies both alphabets simultaneously.

For Quagmire IV, the two alphabets are mixed using two distinct keywords; in this case, the second word is PASS.

The encryption mechanism is common to all four variants:

— create an encipherment tableau (from a plaintext alphabet and a ciphertext alphabet, straight or mixed depending on the variant);

— choose an indicator keyword whose length $ n $ sets the period;

— split the message into groups of $ n $ letters

— encipher the i-th letter of each group with the i-th substitution alphabet

The English word quagmire means swamp, bog, mire

The four variants differ only by the nature of the two alphabets; everything else (period, indicator, splitting) is identical.

— Quagmire I combines a mixed plaintext alphabet and a straight ciphertext alphabet (key type K1)

— Quagmire II reverses the layout: straight plaintext alphabet and mixed ciphertext alphabet (key type K2).

— Quagmire III uses the same mixed alphabet on both sides, for plaintext and ciphertext (key type K3)

— Quagmire IV, the most complex, uses two different mixed alphabets (key type K4) and requires two alphabet keywords.

This is the K1 to K4 classification of keyed alphabets by the ACA (American Cryptogram Association).