better signature validation

app/controllers/api/attachments_controller.rb

@@ -10,12 +10,23 @@ module Api
     def create
       submitter = Submitter.find_by!(slug: params[:submitter_slug])
 
-      if params[:type].in?(%w[initials signature]) && ImageUtils.blank?(Vips::Image.new_from_file(params[:file].path))
+      if params[:type].in?(%w[initials signature])
+        image = Vips::Image.new_from_file(params[:file].path)
+
+        if ImageUtils.blank?(image)
           Rollbar.error("Empty signature: #{submitter.id}") if defined?(Rollbar)
 
           return render json: { error: "#{params[:type]} is empty" }, status: :unprocessable_entity
         end
 
+        if ImageUtils.error?(image)
+          Rollbar.error("Error signature: #{submitter.id}") if defined?(Rollbar)
+
+          return render json: { error: "#{params[:type]} error, try to sign on another device" },
+                        status: :unprocessable_entity
+        end
+      end
+
       attachment = Submitters.create_attachment!(submitter, params)
 
       if params[:remember_signature] == 'true' && submitter.email.present?

lib/image_utils.rb

@@ -14,4 +14,12 @@ module ImageUtils
 
     false
   end
+
+  def error?(image)
+    image = image.crop(0, 0, image.width / 4, 2)
+
+    row1, row2 = image.to_a
+
+    row1[3..] == row2[..-4] && row1.each_cons(2).none? { |a, b| a == b }
+  end
 end