Sift and Cog

Sift went first, because ciphers were her craft. "A message like this is letters wearing other letters," she said, tapping the wall. "Every plaintext letter became some cipher letter under one consistent rule — a costume worn identically across the whole message. Given the key, I'd read it in a heartbeat." She spun her wheel and shook her head. "But no key is posted. And a wheel with no shift set is just two idle circles." That was Sift's craft and Sift's wall: she could undo any substitution once she knew the key — and here the key was withheld.

Cog went next, laying out her grid. "My craft is that what does not fit tells you what does," she said. "Give me constraints and I'll cross out every impossibility until one mapping survives." She tapped her empty paper. "But I have nothing to eliminate on — no clues, only gibberish." That was Cog's craft and Cog's wall: she could deduce her way to any answer once she had a constraint to start from — and here there was none.

Then they turned to each other, and the keyless wall cracked open between them — because Sift could manufacture the constraints Cog lacked. Sift ran a finger down the message, counting. "This letter, J, recurs far more than any other." Cog's ears went up. "That's a constraint. English text is most often E, so J is probably E — enter it as a hypothesis." Sift surfaced more threads: a one-letter word ("that's A or I, nothing else") and a doubled letter ("likely LL, SS, or EE"). Cog took each onto her grid and eliminated: if J is E, this shift is impossible — cross it out; that candidate contradicts the doubled letters — cross it out. Hypothesis by hypothesis, elimination by elimination, the impossible mappings fell away until a single consistent shift remained. Sift set her wheel to it and read the wall aloud: THE KEY IS HIDDEN. "We found the key," Sift breathed, "by counting and crossing-out." Latch nodded. "You two are appointed together. One finds the loose thread; the other pulls it until the whole thing unravels."