Add handling of Linked Data Signatures in payloads (#4687)

* Add handling of Linked Data Signatures in payloads

* Add a way to sign JSON, fix canonicalization of signature options

* Fix signatureValue encoding, send out signed JSON when distributing

* Add missing security context

app/lib/activitypub/linked_data_signature.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+class ActivityPub::LinkedDataSignature
+  include JsonLdHelper
+
+  CONTEXT = 'https://w3id.org/identity/v1'
+
+  def initialize(json)
+    @json = json
+  end
+
+  def verify_account!
+    return unless @json['signature'].is_a?(Hash)
+
+    type        = @json['signature']['type']
+    creator_uri = @json['signature']['creator']
+    signature   = @json['signature']['signatureValue']
+
+    return unless type == 'RsaSignature2017'
+
+    creator   = ActivityPub::TagManager.instance.uri_to_resource(creator_uri, Account)
+    creator ||= ActivityPub::FetchRemoteKeyService.new.call(creator_uri)
+
+    return if creator.nil?
+
+    options_hash   = hash(@json['signature'].without('type', 'id', 'signatureValue').merge('@context' => CONTEXT))
+    document_hash  = hash(@json.without('signature'))
+    to_be_verified = options_hash + document_hash
+
+    if creator.keypair.public_key.verify(OpenSSL::Digest::SHA256.new, Base64.decode64(signature), to_be_verified)
+      creator
+    end
+  end
+
+  def sign!(creator)
+    options = {
+      'type'    => 'RsaSignature2017',
+      'creator' => [ActivityPub::TagManager.instance.uri_for(creator), '#main-key'].join,
+      'created' => Time.now.utc.iso8601,
+    }
+
+    options_hash  = hash(options.without('type', 'id', 'signatureValue').merge('@context' => CONTEXT))
+    document_hash = hash(@json.without('signature'))
+    to_be_signed  = options_hash + document_hash
+
+    signature = Base64.strict_encode64(creator.keypair.sign(OpenSSL::Digest::SHA256.new, to_be_signed))
+
+    @json.merge('@context' => merge_context(@json['@context'], CONTEXT), 'signature' => options.merge('signatureValue' => signature))
+  end
+
+  private
+
+  def hash(obj)
+    Digest::SHA256.hexdigest(canonicalize(obj))
+  end
+end