Why we pivoted then
F4.1-F4.4 are closed. F4.5 is closed for the spelling difference and blocked for the shifted plate order, which needs Antipodes' own layout. F10 is seventh in the ranking, after F4; the ranking is unchanged.
What happened before
- 0184: every row, column, diagonal and spiral path over the copper as carved. The grids were K4's four lines starting after K3 and the ?, the lower plate, and the whole left side. With a repeating key before or after, the shortest key that fits is as long as on scrambled copies.
- 0185: trifid with any cube at all, groups of 16 to 97 letters, every start (4,633 settings): no cube fits K4 or any copy. With 0040, trifid is closed at every period.
- 0186: the places of the copper's oddities (the four ?, K2's dropped S, the raised YAR, the extra L, the odd lines) as the number half of a key: nothing fits.
- F4.4 closed by subsumption: whatever the extra L could select as a marker is a short periodic key (0005), a start in the tableau (0017) or a plate laid over K4 (0089).
The change we believe is needed
- F10 next, in its table's order: F10.1 Chaocipher (first count the checks the 24 known letters give), F10.2 the Wheatstone cryptograph, F10.3 a key stream from a deck of cards (state the bound).
- These alphabets change after every letter, so each test must first say whether the known stretches constrain the state at all.