@0x0 Have you implemented Linked Data Signature or Verifiable Credentials before? IIUC they are based on RDF (and hence JSON-LD when applied to AP) and they are required for e.g. relays to work efficiently (without the signature, you wouldn't be able to to verify the authenticity of activities forwarded by relays without pulling the data from the originating server every time. HTTP Message Signatures aren't applicable to this kind of forwarding IIUC)

@0x0 I'm not an expert of it either, but I think the W3C Recommendation (<https://www.w3.org/TR/json-ld/>) actually serves as a good introductory material. Most of it comprises non-normative explanatory sections and one of the main editors has said that it's explicitly meant to be so (<https://web.archive.org/web/20190405035404/manu.sporny.org/2014/json-ld-origins-2/>)

@0x0 Ah, and I forgot to mention that you'll likely not need to understand it so much. The main point of JSON-LD is to be just a plain JSON from normal Web developer's perspective (maybe that's why you haven't need to understand it at all), plus a `@context` to make it interpretable as Linked Data if you need to, and Activity Stream only permits the use of the compacted form of it (the most plain-JSON-looking form)

@0x0 Also, I think you'd want to use a thirdparty library (like `sophia_rs`) if you'd ever need to use a JSON-LD algorithm, instead of implementing it by yourself since the algorithms are really (I mean, really) complicated for implementing in-house (making Linked Data look like a plain JSON necessarily involves pretty much amount of dirty work, I guess…)