CCFII
Double Autokey
Step (1):
Plain text autokey with following alphabet:
| F | A | P | Z | L | B | Q | O | C | S | U | G | T | N | H | V | D | I | W | E | K | X | R | M | Y |
| A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y |
Key letter in upper alphabet is positioned above letter "A" in lower alphabet. Then, intermediary text letter is found in lower alphabet under plain text letter in upper alphabet.
Key letter for the first plain text letter is "Z". Key letter for the n-th plain text letter is plain text letter (n-1).
Word space is "Z".
If the key letter is equal plain text letter, then the plain text letter is substituted by "Q", e.g. "FF" is substituted by "FQ", "TT" by "TQ".
Step (2):
Cipher text autokey with following alphabet:
| F | A | M | Y | L | B | P | Z | N | H | T | O | C | Q | R | K | X | U | G | S | D | I | V | E | J | W |
| A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
Key letter in upper alphabet is positioned above letter "A" in lower alphabet. Then, cipher text letter is found in upper alphabet above intermediary text letter (of step 1) in lower alphabet.
The key letter for the first letter is given as double letter in the beginning of the cipher text. Key letter for the n-th letter is cipher text letter (n-1).
The two alphabets of step (1) and step (2) are constructed in the following way:
(1)
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| F | L | O | U | N | D | E | R |
| A | B | C | G | H | I | K | M |
| P | Q | S | T | V | W | X | Y |
| Z | | | | | | | |
(2)
| 1 | 2 | 4 | 6 | 3 | 7 | 8 | 5 |
| F | L | O | U | N | D | E | R |
| A | B | C | G | H | I | J | K |
| M | P | Q | S | T | V | W | X |
| Y | Z | | | | | | |
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