This paper proposes a novel cryptographic authentication protocol in which physical geographic coordinates serve as the cryptographic key. Unlike conventional password-based systems, the Coordinate-as-Key (CaK) protocol requires physical presence at a specific GPS location. The key cannot be replicated remotely — it is bound to existence in space, not to knowledge alone. This creates an entirely new category of authentication: Presence-Bound Cryptography (PBC). The concept was conceived and defined at Fukui Prefecture, Japan (35.9606° N, 136.2215° E) on 2026-05-06.
Yoshimitsu Katayama (2026) studied this question.