The Quagmire 3 cipher.
Quagmire 3 is a polyalphabetic cipher based on two keywords.
The first keyword is used to build a mixed alphabet. In the example below the keyword is 'STRONGCIPHER'. Repeated letters are removed and then the rest of the alphabet is added to make the mixed alphabet:
The second keyword determines the period. In this case I have chosen a period of 6 and the keyword 'SEARCH'.
Now an enciphering matrix is set up. The first line is the mixed alphabet representing plaintext, and thus is in lowercase. Then the second keyword is set up vertically beneath one of the plaintext letters (I have chosen 'a') to act as the anchor for filling in the mixed alphabets:
Cipher 1 JKLMQUVWXYZSTRONGCIPHEABDF
Cipher 2 ZSTRONGCIPHEABDFJKLMQUVWXY
Cipher 3 STRONGCIPHEABDFJKLMQUVWXYZ
Cipher 4 LMQUVWXYZSTRONGCIPHEABDFJK
Cipher 5 VWXYZSTRONGCIPHEABDFJKLMQU
Cipher 6 YZSTRONGCIPHEABDFJKLMQUVWX
To encipher the word 'beneath', one enciphers 'b' with the first cipher alphabet, 'e' with the second, 'n' with the third and so on, using the first cipher alphabet again for the 7th letter 'h': the ciphertext is thus THNTCZY.
Deciphering is the reverse process.
The Quagmire 3 cipher can usually be broken with the help of a probable word or 'crib'. initial steps are:
--determining the period by use of the Index of Coincidence.
-- placing the crib by reference to repeated letters. An example is given in the article on Symmetry. The placed crib provides information on the shifts of the cipher mixed alphabets.
Then you can proceed using one of the computer attacks below. Clicking on the highlights will, lead you to explanatory articles.
1. Dictionary. Trying every word in the dictionary as the keyword. This is clearly not effective when the keyword is actually a phrase.
2. Symmetry, expanding the partial decryption of the placed crib and enabling further plaintext to be guessed.
3. Keyspace algorithm. Finds the keyword when it is no longer than 7 letters, using Hillclimbing or Churn.
4. Algebraic approach, brute forcing the keyword. Requires the placed crib and the accompanying ciphertext to cover most of the letters in the alphabet.
[ These articles are currently being prepared. ]