After the keyword, the message becomes its own key. The key never repeats, but it is ordinary English.
Build the key stream from a primer plus the plaintext.
Contrast the autokey stream with a repeating keyword.
Explain why a predictable key stream is still a weak one.
Primer of 7. Only the first 7 letters use a key you chose; after that the key is the plaintext.
| i | Pi | Ki | (Pi + Ki) mod 26 | Ci |
|---|---|---|---|---|
| 0 | W = 22 | A = 0 | (22 + 0) mod 26 = 22 | W |
| 1 | E = 4 | U = 20 | (4 + 20) mod 26 = 24 | Y |
| 2 | H = 7 | T = 19 | (7 + 19) mod 26 = 0 | A |
| 3 | A = 0 | O = 14 | (0 + 14) mod 26 = 14 | O |
| 4 | V = 21 | K = 10 | (21 + 10) mod 26 = 5 | F |
| 5 | E = 4 | E = 4 | (4 + 4) mod 26 = 8 | I |
| 6 | A = 0 | Y = 24 | (0 + 24) mod 26 = 24 | Y |
| 7 | M = 12 | W = 22 | (12 + 22) mod 26 = 8 | I |
| 8 | O = 14 | E = 4 | (14 + 4) mod 26 = 18 | S |
| 9 | L = 11 | H = 7 | (11 + 7) mod 26 = 18 | S |
| 10 | E = 4 | A = 0 | (4 + 0) mod 26 = 4 | E |
| 11 | A = 0 | V = 21 | (0 + 21) mod 26 = 21 | V |
The same board Vigenère uses. Only the K column differs: after the primer runs out the key letter is the plaintext, shifted along.