julian: If the activity is the thing being forwarded, then an additional complication could arise in that I cannot simply re-sign the activity, as my instance key does not belong to the originating actor. this is indeed an issue that arises due to the following: ActivityPub does not specify any authentication/verification mechanism HTTP Signatures, which the current fediverse uses, are not replayable or relayable. If you're concluding that this makes inbox forwarding impossible with HTTP Signatures, then congratulations, you understand why LD Signatures ended up being used for this (and why FEP-8b32 proposes using Data Integrity Proofs at the LD level as well). It's either that, or find a way to replay entire HTTP messages (so that the HTTP Signature can be validated against the original HTTP interaction). Or otherwise rethink the fundamentals of the fediverse's entire security model.