What protocol does Apple airdrop use?

Could you elaborate on the underlying protocol that Apple's Airdrop feature utilizes? I'm curious to understand the technical aspects behind this convenient file sharing functionality. Specifically, I'm interested in knowing if it relies on Bluetooth, Wi-Fi Direct, or perhaps a combination of both? Additionally, does it employ any encryption measures to ensure the secure transmission of files between devices? Understanding the protocol and its security features would help me assess the overall safety and reliability of using Airdrop for file sharing.