Interactive　TV　is　a　promising　technique　to　realize　individualized　services　besides　traditional　video　broadcasting,　such　as　VOD　(Video　on　Demand),　Email　on　DTV　(Digital　TV),　E-banking.　Utilizing　return　channel　(like　Internet,　mobile　communication),　many　new　applications　can　be　established,　which　promote　the　value-added　services　of　DTV.　For　more　profound　applications,　security　mechanisms　must　be　deployed　to　protect　the　profits　of　both　the　content　providers　and　subscribers.　This　paper　presents　investigations　on　the　smart　card-based　secure　protocol　for　user-STB　authentication　and　session　key　establishment　in　DTV　system.　The　proposed　secure　protocol　has　features　of　bidirectional　authentication,　session　key　secrecy,　session　key　confirmation,　forward　security,　and　efficient　interval　restart,　etc.　Therefore,　the　protocol　is　appropriate　for　use　on　secure　conditional　access　and　can　be　referenced　easily　in　future　CA　systems　for　value-added　applications.