The feature uses the same mechanism as auto unlock for Mac. The guide states that a mutually authenticated station-to-station (STS) protocol that uses long-term keys that are established when the feature is enabled. Once two devices are paired, unique and ephemeral keys are negotiated for each unlock request. Apple says the tunnel is negotiated directly between the Secure Enclave is both devices. That means all cryptographic material is kept within that secured box. The distance between the two devices must also be two to three meters or less. An iPhone must also have been unlocked at least once since the Apple Watch was placed on a user's wrist and unlocked. The Apple watch must not be in bedtime mode, and sensors must be able to determine whether the nose and mask of the user are covered.