Quoting from [1]
The HRF Bitcoin Bounty Challenge, with a 20 BTC total reward pool, has officially launched, with a focus on transactional privacy and Bitcoin self-sovereignty development.
The second bounty is a 2 BTC reward for a serverless implementation of a payjoin protocol. The bounty specifically calls for a “production-ready version 2 payjoin protocol which may send and receive Payjoin transactions without requiring a sender or recipient to operate a public server.” The Serverless Payjoin bounty asks for use of BIP-21 unified bitcoin URIs, and to remain independent from a trusted third party (hence serverless) which “could expose user privacy in a non-trivial way.”