Kaesa

"Pick a number — say three," Kaesa said. "Now every letter in my message moves three steps forward along the alphabet. A becomes D. B becomes E. C becomes F. The word CAT — C slides to F, A slides to D, T slides to W — becomes FDW. Anyone who reads FDW sees nonsense. But a friend who knows the secret number just slides every letter three steps back — F to C, D to A, W to T — and CAT walks right out again." Kaesa smiled. "Same slide, one way to hide, the other way to find."

"What I love about the shift," Kaesa said, "is that the secret isn't the method — everybody can know I 'slid the alphabet.' The secret is the number. Three, or seven, or twenty. That one little number is the whole key. And when the alphabet runs off the end — Y shifted by three — you just wrap around to the front: Y, Z, then A, B. It loops like a circle. So no letter is ever stuck. That's why I trust it for a hunt: it's simple enough that I never make a mistake building it, but a solver still has to do something — figure out or be told the number — before the message opens. A good clue should make you work a little, not lock you out forever."

The newcomer tried it, shifting the word GO by five: "G slides to... H, I, J, K, L. And O slides to... P, Q, R, S, T. So GO becomes LT." They wrote LT on a card. "And to solve it, my friend slides L and T back five — L, K, J, I, H, G, and T, S, R, Q, P, O — GO." They grinned. "It came back perfectly. And if they don't know it's five, they just have to try — there's only twenty-six ways." Kaesa nodded. "Exactly. Hidden, but always findable. That's the balance."