Random　Oracle　Model　(ROM)　is　an　effective　method　for　measuring　the　practical　security　of　cryptograph.　In　this　paper,　we　try　to　use　it　into　information　hiding　system　(IHS).　Because　IHS　has　its　own　properties,　the　ROM　must　be　modified　if　it　is　used　into　IHS.　Firstly,　we　fully　discuss　why　and　how　to　modify　each　part　of　ROM　respectively.　The　main　changes　include:　1)　Divide　the　attacks　that　IHS　may　be　suffered　into　two　phases　and　divide　the　attacks　of　each　phase　into　several　kinds.　2)　Distinguish　Oracles　and　Black-boxes　clearly.　3)　Define　Oracle　and　four　Black-boxes　that　IHS　used.　4)　Propose　the　formalized　adversary　model.　And　5)　Give　the　definition　of　judge.　Secondly,　based　on　ROM　of　IHS,　the　security　against　known　original　cover　attack　(KOCA-KOCA-security)　is　defined.　Then,　we　give　an　actual　information　hiding　scheme　and　prove　that　it　is　KOCA-KOCA-secure.　Finally,　we　conclude　the　paper　and　propose　the　open　problems　of　further　research,