Ephemeral Spherical Modulation: Watermarking Where the Diffusion Channel Cannot Look

Diffusion watermarks written into the initial noise must keep it standard Gaussian while the message survives generation, image processing and inversion, and existing constructions are compared pipeline by pipeline under assumed noise models. We ask what the pipeline observes: under ideal secret frames, the same Gaussian latent for every prior-preserving watermark, so watermarks differ only in how their keys read one channel. The prior then reduces to a Gram invariant of the frame-sharing pattern. Without a noise model, the decoder reads the channel through a uniformly random consistent frame, and on acutely recovered blocks equal magnitudes minimize expected sign errors among profiles with no coordinate above a quarter of the block energy. Equal magnitudes and an exact prior force a separate frame for every block of every image: ephemeral spherical modulation (ESM), whose frames are regenerated from a key and per-image nonce, with half-normal modulation as its width-one case. On SD1.5 and SD2.1-base, ESM recovers 114/128 and 110/128 complete 512-bit messages under JPEG50, versus 69/128 and 74/128 for half-normal modulation. Channel samples from one watermark's images reproduce another's recovery rates without fitted parameters, and the tested crop leaves recovered blocks nearly orthogonal to their sources, putting every readout of the family at chance. These results concern payload recovery with the correct key and nonce, not calibrated detection, synchronization or cryptographic security.