This is in the same spirit as #39518, but adding support for a PQC algorithm. However, RFC9421 does not specify the use of ML-DSA-44, and it is not implemented in Linzer, the library we use.

This proposal describes how ActivityPub servers and clients could create self-authenticating activities and objects. HTTP signatures are often used for authentication during server-to-server interactions. However, this ties authentication to activity delivery, and limits the flexibility of the protocol. Integrity proofs are sets of attributes that represent digital signatures and parameters required to verify them. These proofs can be added to any activity or object, allowing recipients to verify the identity of the actor and integrity of the data. That decouples authentication from the transport, and enables various protocol improvements such as activity relaying, embedded objects and client-side signing.