Like hourly buses on a wet winter morning, here’s a pair of Chaocipher pages that arrived at my stop one after the other, both discussing how to break John Byrne’s Exhibit 1, and both strongly recommended reading for those interested in the Chaocipher.
First to arrive was Carl Scheffler’s page on Exhibit 1 (but you might perhaps want to read his introductory page on the Chaocipher first, complete with nice coloured disk diagrams). By looking for long sequences of repeated symbols, he managed to reduce the staggeringly-large search space down to a mere ~457,000 permutations to check: in fact, he further managed to reduce the space to only 444 permutations, which would probably be achievable even without the aid of computers. Furthermore, once he had discovered the initial ring state, Carl went on to reverse engineer the keyphrase used to set the disks up (‘THINKTHINK’, with the sequence of letters applied to the disks with the pattern LLRLLRLRRLR). He has a further page planned on Exhibit 4 – I’ll let you know when he posts this.
Subsequently, Moshe Rubin’s near-definitive update on Exhibit 1 turned up. As usual, Moshe’s 12-page PDF manages to answer more or less every question you find yourself asking along the way (though admittedly he doesn’t yet know to whom Byrne’s enciphered “CORDIALTHANKSTOLO” was referring). From this, you can also see that Byrne used ‘Q’ and ‘W’ for ‘,’ and ‘.’ (plus ‘Z’ for ‘end-of-line’), hence the plaintext begins “ALLGOODQQUICKBROWNFOXESJUMPOVERLAZYDOGTOSAVETHEIRPARTYW“.
Incidentally, though the idea of encoding punctuation as rarely-used letters is a well-known cipher trick, I find the historical question of when this mechanism was first used particularly intriguing. This is because I’ve long wondered whether the “am” letterpair frequently found at Voynich line-ends might also encipher a rare letter (such as ‘X’). True, there are some Milanese ciphers with letters for scribal abbreviations and contractions (the 1450 cipher for Tristano Sforza enciphers ‘-9’, while the 1455 cipher for Ludovico Petronio Senen has a cipher for ‘subscriptio’), but these seem to belong to a quite different family. I can’t see this in Kahn or al-Qalqashandi, so… what was the earliest cipher to replace punctuation with rarely used letters?