Two friends pass blank public cards across a bridge watched by a small robot while face-down private cards stay at their desks.

Can friends make a secret on a public line?

tap the public bridge

Ana and Bo cannot whisper or meet. Every card they send crosses a public bridge where a listener copies it. Can they end with the same hidden number without sending that number?

the public machine

Multiply by 3, then keep the remainder

Everyone may know this rule. Nothing here is private.

try the forward machine

A hop count makes one public landing

unrelated practice count1 hop → landing 3
1 hop4 hops

These practice counts are not either friend's private card.

build on two sides

Private cards stay home

private cards face-down · result still covered
the listener's notebook

Everything that crossed is here

Is that transcript enough to recover the covered spot anyway?

Public: mod 17 · ×3 · 15 · 6. Private cards: covered.

On this tiny toy, what happens when the listener tries?