Support more variations of ActivityPub keyId in signature (#4630)

- Tries to avoid performing HTTP request if the keyId is an actor URI
- Likewise if the URI is a fragment URI on top of actor URI
- Resolves public key, returns owner if the owner links back to the key

1 files changed, 47 insertions, 0 deletions

diff --git a/app/services/activitypub/fetch_remote_key_service.rb b/app/services/activitypub/fetch_remote_key_service.rb
new file mode 100644
index 000000000..ebd64071e
--- /dev/null
+++ b/app/services/activitypub/fetch_remote_key_service.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+class ActivityPub::FetchRemoteKeyService < BaseService
+  include JsonLdHelper
+
+  # Returns account that owns the key
+  def call(uri, prefetched_json = nil)
+    @json = body_to_json(prefetched_json) || fetch_resource(uri)
+
+    return unless supported_context?(@json) && expected_type?
+    return find_account(uri, @json) if person?
+
+    @owner = fetch_resource(owner_uri)
+
+    return unless supported_context?(@owner) && confirmed_owner?
+
+    find_account(owner_uri, @owner)
+  end
+
+  private
+
+  def find_account(uri, prefetched_json)
+    account   = ActivityPub::TagManager.instance.uri_to_resource(uri, Account)
+    account ||= ActivityPub::FetchRemoteAccountService.new.call(uri, prefetched_json)
+    account
+  end
+
+  def expected_type?
+    person? || public_key?
+  end
+
+  def person?
+    @json['type'] == 'Person'
+  end
+
+  def public_key?
+    @json['publicKeyPem'].present? && @json['owner'].present?
+  end
+
+  def owner_uri
+    @owner_uri ||= value_or_id(@json['owner'])
+  end
+
+  def confirmed_owner?
+    @owner['type'] == 'Person' && value_or_id(@owner['publicKey']) == @json['id']
+  end
+end