Impersonation　attack　was　proposed　to　show　that　the　references　scheme　was　not　free　from　the　key　escrow　problem.　Then　an　improvement　was　proposed.　The　improved　scheme　verified　a　user＇s　identity　by　KGC　(key　generation　center)　and　PRA　(private　key　remove　agencies)　issuing　identity　key,　which　resolves　user＇s　key　escrow　problem　of　references.　Otherwise,　the　improved　scheme　can　make　the　secret　information　transmit　between　KGC　and　PRA　without　a　secure　channel　based　on　a　simple　blinding　technique,　which　resolves　the　burden　of　the　additional　secure　channel.