For authenticating messages, Chatter Net uses Linked Data Proofs (https://www.w3.org/TR/vc-data-integrity/#proofs). The JS client uses this implementation: https://github.com/digitalbazaar/crypto-ld. And the Rust server uses this one: https://github.com/spruceid/ssi.

I hadn't seen ucan yet, I think the space of JWT adjacent protocols is growing, and I'll be interested to see where it all goes!