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

app/controllers/concerns/signature_verification.rb

@@ -98,7 +98,9 @@ module SignatureVerification
     if key_id.start_with?('acct:')
       ResolveRemoteAccountService.new.call(key_id.gsub(/\Aacct:/, ''))
     elsif !ActivityPub::TagManager.instance.local_uri?(key_id)
-      ActivityPub::FetchRemoteAccountService.new.call(key_id)
+      account   = ActivityPub::TagManager.instance.uri_to_resource(key_id, Account)
+      account ||= ActivityPub::FetchRemoteKeyService.new.call(key_id)
+      account
     end
   end
 end

app/helpers/jsonld_helper.rb

@@ -9,6 +9,10 @@ module JsonLdHelper
     value.is_a?(Array) ? value.first : value
   end
 
+  def value_or_id(value)
+    value.is_a?(String) ? value : value['id']
+  end
+
   def supported_context?(json)
     equals_or_includes?(json['@context'], ActivityPub::TagManager::CONTEXT)
   end
@@ -20,7 +24,7 @@ module JsonLdHelper
   end
 
   def body_to_json(body)
-    body.nil? ? nil : Oj.load(body, mode: :strict)
+    body.is_a?(String) ? Oj.load(body, mode: :strict) : body
   rescue Oj::ParseError
     nil
   end

app/lib/activitypub/activity.rb

@@ -58,7 +58,7 @@ class ActivityPub::Activity
   end
 
   def object_uri
-    @object_uri ||= @object.is_a?(String) ? @object : @object['id']
+    @object_uri ||= value_or_id(@object)
   end
 
   def redis

app/lib/activitypub/activity/accept.rb

@@ -20,6 +20,6 @@ class ActivityPub::Activity::Accept < ActivityPub::Activity
   end
 
   def target_uri
-    @target_uri ||= @object['actor']
+    @target_uri ||= value_or_id(@object['actor'])
   end
 end

app/lib/activitypub/activity/reject.rb

@@ -20,6 +20,6 @@ class ActivityPub::Activity::Reject < ActivityPub::Activity
   end
 
   def target_uri
-    @target_uri ||= @object['actor']
+    @target_uri ||= value_or_id(@object['actor'])
   end
 end

app/lib/activitypub/activity/undo.rb

@@ -64,6 +64,6 @@ class ActivityPub::Activity::Undo < ActivityPub::Activity
   end
 
   def target_uri
-    @target_uri ||= @object['object'].is_a?(String) ? @object['object'] : @object['object']['id']
+    @target_uri ||= value_or_id(@object['object'])
   end
 end

app/lib/activitypub/tag_manager.rb

@@ -93,7 +93,7 @@ class ActivityPub::TagManager
     elsif ::TagManager.instance.local_id?(uri)
       klass.find_by(id: ::TagManager.instance.unique_tag_to_local_id(uri, klass.to_s))
     else
-      klass.find_by(uri: uri)
+      klass.find_by(uri: uri.split('#').first)
     end
   end
 end

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