This work proposes a lightweight biometric key-binding scheme that adapts a PUF-style challenge–response mechanism to face geometry: a two-factor password and session nonce generate random challenge points, Gray-coded Euclidean distances to facial landmarks form responses, and a random key is bound by discarding selected positions so only a reduced subset, the nonce, and a key hash are stored. At authentication, a fresh response set is compared to the subset with a Hamming-distance tolerance, and bounded local search corrects residual errors; each successful session rotates the nonce and refreshes the ephemeral key. We frame this as a conceptual exploration of an interpretable, on-device, controlled-capture design niche—a per-session nonce-driven cancelable biometric key-binding mechanism—and we quantify the resulting security–usability trade-offs. Empirically, the scheme works under stable capture conditions with carefully tuned thresholds, and it is naturally suited to tightly controlled deployments (e.g., access kiosks) where it can also incorporate user-driven micro-gestures as an extra behavioral factor. While the construction is fragile under broader variability and leans on the second factor for security, it offers an alternative to existing mechanisms and a clear niche, and we present it as a conceptual exploration showing how CRP mechanisms can inform cancelable biometrics with per-session revocability.