A Zero-Agreement Protocol for Interstellar Technosignatures: Polarization Handedness on a Doppler-Cancelled Carrier

A transmitter that corrects its own motion for one receiver's line of sight produces a signal that looks like terrestrial interference along that line. This manuscript asks how such a signal could still be recognised and decoded with no prior agreement between sender and receiver, and no shared key, clock or coordinate frame. It combines special relativity with the Doppler-drift regimes of Sheikh et al. (2019) and Dixon's (1973) argument that a signal meant to be found should not be encrypted. The result is a keyless channel carried in the circular-polarization handedness of a Doppler-cancelled carrier. Robustness of that channel reduces to one measurable quantity, the cross-hand phase, which radio telescopes already calibrate routinely. The paper derives a ratio-valued differential encoding that does not depend on sign conventions, a closed-form symbol-capacity result with its timing-jitter statistics, and a parity rule for static perturbers. It also covers multi-station handover and keeps results forced by physics separate from choices made in the design. An archival check on Voyager 1 data from the Green Bank Telescope shows that the polarization sign can be recovered steadily from a real spacecraft carrier. This shows that sign recovery works. It does not test sensitivity to weak or switching polarization. The paper makes no claim that handedness is more robust than phase signalling; that comparison is left open.