Experimental clipboard sync client for Android

Synchronize clipboard between devices, focusing on security and privacy.

The code is not ready for average users, it's a proof of concept right now.

Protocol

Nonces are 24 bytes long and are generated using the NaCl CSPRNG.
NaCl Authenticated Encryption (crypto_box) requires such a nonce, the nonce and the box is sent by concatenating the two in the following order: nonce || box

An X25519 EC keypair is generated at both endpoints.
Upon registration, the server public key is presented in a QR code.
As a response to the QR code, the client sends a NaCl Sealed Box with a nonce and its own public key inside over UDP broadcast.
The server uses this public key and responds with this nonce, encrypted with crypto_box using UDP broadcast as well.

A nonce is generated.
The contents of the clipboard is converted to a Unicode string.
A validity Unix timestamp and the above Unicode string is combined as a two-element array and serialized using CBOR.
The CBOR output is encrypted using the nonce and a crypto_box and sent using UDP broadcast.

For key exchange, a network eavesdropper doesn't even know the public key of either participant, let alone the secret key. Even if the public key of the server became public knowledge, although a Sealed Box could be constructed with the attacker's own public key, the intended client couldn't open the crypto_box sent in the reply with its own key.
For clipboard sharing operations, the nonce can be used to filter replay attacks retransmitting packets verbatim. The Unix timestamp can be used to limit the validity of messages, and thus nonces can be purged from the cache after the validity has expired.

The whole project is licensed under MIT license, see LICENSE.txt