# 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_assertion = JWT.encode(claims, params[:hmac_secret], "HS256")
        elsif params[:private_key]
          jwt_assertion = JWT.encode(claims, params[:private_key], "RS256")
        end
      end