# File lib/oauth2/strategy/assertion.rb, line 60
      def build_assertion(params)
        claims = {
          :iss => params[:iss],
          :aud => params[:aud],
          :prn => params[:prn],
          :exp => params[:exp],
        }
        if params[:hmac_secret]
          JWT.encode(claims, params[:hmac_secret], 'HS256')
        elsif params[:private_key]
          JWT.encode(claims, params[:private_key], 'RS256')
        end
      end