Another Mastodon / ActivityPub signature question.

Alice reposts Bob.
Bob updates their post.
Alice's instance syndicates that update to her followers.

The message Alice's instance sends has an Actor of Bob. But the headers are signed by Alice. So the signature doesn't match the actor.

Which means Bob's embedded RsaSignature2017 *also* needs to be checked.

Security - Mastodon documentation

Public key cryptography and supported signature schemes over HTTP and JSON-LD.

(docs.joinmastodon.org)

But that doesn't mean I can skip checking Alice's signature, right?

The documentation says:

> Strip type, id, and signatureValue from the signature, leaving only signature[creator] and signature[created].
> Base64-decode the signatureValue and verify it against the public key in signature[creator].

But it doesn't say *what* to validate the signature against!

Is it a JSON document just containing creator and created? Or is it something else?

@Edent oh.... it's against some wild RDF thingy that you obtain by putting the JSON document through some JSON-LD processing algorithms. My memory is fuzzy because I implemented that from scratch way back in 2019, but here's my known working implementation if that helps:
https://github.com/grishka/Smithereen/blob/master/src/main/java/smithereen/jsonld/LinkedDataSignatures.java

Smithereen/src/main/java/smithereen/jsonld/LinkedDataSignatures.java at master · grishka/Smithereen

Federated, ActivityPub-compatible social network server with friends, walls, and groups. - Smithereen/src/main/java/smithereen/jsonld/LinkedDataSignatures.java at master · grishka/Smithereen

GitHub (github.com)

@Edent oh.... it's against some wild RDF thingy that you obtain by putting the JSON document through some JSON-LD processing algorithms. My memory is fuzzy because I implemented that from scratch way back in 2019, but here's my known working implementation if that helps:
https://github.com/grishka/Smithereen/blob/master/src/main/java/smithereen/jsonld/LinkedDataSignatures.java

Smithereen/src/main/java/smithereen/jsonld/LinkedDataSignatures.java at master · grishka/Smithereen

Federated, ActivityPub-compatible social network server with friends, walls, and groups. - Smithereen/src/main/java/smithereen/jsonld/LinkedDataSignatures.java at master · grishka/Smithereen

GitHub (github.com)

A weird #ActivityPub message from #Frendica.

Signed on 2026-09-01
Published on 2026-03-02

That's a skew of six months! The message type is "Undo" - so they're undoing a like they sent in March.

Is there *really* a worry about accepting requests like this? Given the message has been signed, what risk is there to replay attacks?

@Edent I don’t treat the published date as authoritative of anything—it’s just a piece of user-supplied information 

The documentation says:

> Strip type, id, and signatureValue from the signature, leaving only signature[creator] and signature[created].
> Base64-decode the signatureValue and verify it against the public key in signature[creator].

But it doesn't say *what* to validate the signature against!

Is it a JSON document just containing creator and created? Or is it something else?

I've added an #OpenAPI / #Swagger specification to #ActivityBot.

https://gitlab.com/edent/activity-bot/-/blob/main/openapi.yaml

I've never done this before - so please do feel free to give me constructive criticism on what I've done

openapi.yaml · main · Terence Eden / ActivityBot · GitLab

A single PHP file which acts as a basic ActivityPub bot server.

GitLab (gitlab.com)

@Edent I used Swagger for something years and years ago, I really liked it. What did you think?

@denny Other than YAML (a cursed language) I like it!