Wormhole William is an end-to-end encrypted file transfer app that uses the Magic Wormhole secure file transfer protocol.
With Wormhole William you can send and receive files between your phone and a computer or another phone by simply sharing a short, one time passcode.
Wormhole William is an open source. You can find the source code here: https://github.com/psanford/wormhole-william-mobile.
You can read more about the Magic Wormhole protocol at https://magic-wormhole.readthedocs.io/en/latest/.