ActivityPub and HTTP Signatures recommends double-knocking to those who want to produce RFC 9421 signatures.

This is ridiculous because it means making ~2x more POST requests to deliver an activity. And eventually there will be another upgrade. Then what, triple-knocking?

I think capabilities like RFC-9421 support can advertised via actor properties.

It can look like this:

{
  "id": "https://social.example/actor",
  "type": "Person",
  "generator": {
    "type": "Application",
    "implements": [
      {
        "name": "RFC-9421: HTTP Message Signatures",
        "href": "https://datatracker.ietf.org/doc/html/rfc9421"
      }
    ]
  }
}

Here is a FEP draft:

https://codeberg.org/silverpill/feps/src/branch/main/844e/fep-844e.md

@rfc9421