Quagmire Ciphers

Quagmire I through IV - a Vigenere tableau built from a keyed plaintext alphabet, a keyed ciphertext alphabet, or both.

The Quagmires, as the American Cryptogram Association numbers them, are a Vigenere cipher whose two alphabets may each be keyed. A passphrase - the indicator key - selects a column of the tableau for each letter in turn, exactly as an ordinary Vigenere does.

Which alphabet carries a keyword is the whole difference between them:

	Plain alphabet	Cipher alphabet
Vigenere	normal	normal
Quagmire I	keyed	normal
Quagmire II	normal	keyed
Quagmire III	keyed	the same key
Quagmire IV	keyed	a different key

Quagmire III is the one you may have met as the Keyed Vigenere.

The indicator key is written under one letter of the plaintext alphabet, and which letter that is shifts the whole tableau. Set it below.

Examples, all four of them the ACA’s own worked examples from its cipher sheets:

Leave both keys blank and this is an ordinary Vigenere cipher. Leave the indicator key blank as well and the tableau shows all twenty-six rows, which is the table itself rather than any one message’s worth of it.