valid_authenticity_token?

# File actionpack/lib/action_controller/metal/request_forgery_protection.rb, line 500
      def valid_authenticity_token?(session, encoded_masked_token) # :doc:
        if encoded_masked_token.nil? || encoded_masked_token.empty? || !encoded_masked_token.is_a?(String)
          return false
        end

        begin
          masked_token = decode_csrf_token(encoded_masked_token)
        rescue ArgumentError # encoded_masked_token is invalid Base64
          return false
        end

        # See if it's actually a masked token or not. In order to deploy this code, we
        # should be able to handle any unmasked tokens that we've issued without error.

        if masked_token.length == AUTHENTICITY_TOKEN_LENGTH
          # This is actually an unmasked token. This is expected if you have just upgraded
          # to masked tokens, but should stop happening shortly after installing this gem.
          compare_with_real_token masked_token

        elsif masked_token.length == AUTHENTICITY_TOKEN_LENGTH * 2
          csrf_token = unmask_token(masked_token)

          compare_with_global_token(csrf_token) ||
            compare_with_real_token(csrf_token) ||
            valid_per_form_csrf_token?(csrf_token)
        else
          false # Token is malformed.
        end
      end