Sift and Cog

Sift went first, because ciphers were her craft. "A message like this is just letters wearing other letters," she said, tapping the wall. "Every A became some other letter, every B another — a costume, the same for the whole message. If I had the key — the shift, the swap — I'd read it in a heartbeat." She spun her wheel and shook her head. "But there's no key on the wall. And a wheel with no shift set is just two circles." That was Sift's craft, and Sift's wall: she could undo any cipher — once she knew the key — and here the key was missing.

Cog went next, laying out her grid. "My craft is what does not fit tells you what does," she said. "Give me a set of clues and I'll cross out every impossibility until one answer's left. But—" she tapped her empty paper "—I've got nothing to cross out yet. No clues. Just gibberish." That was Cog's craft, and Cog's wall: she could eliminate her way to any answer — once she had something to reason about — and here there was nothing to start on.

Then they turned to each other, and the wall cracked open between them. Sift ran her finger down the message, counting. "Look — this letter, J, shows up the most. Way more than any other." Cog's ears went up. "That's a clue," she said, and drew a grid — real letters down the side, code letters across the top. "The most common letter in English is E. So J is probably E. Mark it a maybe." Sift found more threads: a single-letter word — "that's A or I, nothing else" — and a doubled letter — "that's likely LL, SS, or EE." Cog took each thread onto her grid and eliminated: if J is E, then this mapping is impossible, cross it out; that one contradicts the doubled letters, cross it out. Guess by guess, cross-out by cross-out, the impossible mappings fell away — until the shift stood plain. 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 from the doorway. "You two are appointed together," he said. "One of you finds the loose thread. The other pulls it until the whole thing unravels."