Starting with version 3.17.0, Mitra can verify HTTP signatures generated using Ed25519 keys. Both publicKey and FEP-521a key representations are supported.
I know only one service capable of generating such signatures, it's FedBOX, and we have done successful federation tests with its developer,
@mariusormarius.
RFC 9421 support is planned too.
(Another nice feature of FedBOX is implementation of ActivityPub C2S API. I had the opportunity to use it, and it actually works!)
RE: https://mitra.social/objects/019500d1-4529-5726-faed-e84138bbff9d