ISSN: 2250-3005 - ijcer

International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8and:K*KP K,PK,...,PKP (7)1*P K,PK,...,PK1*22*kkP . (8)Sets of values of a priori probabilities (6 - 8) form a priori knowledge of the opponent about the source of messages andthe source of keys, respectively. In fact, these sets characterize the a priori knowledge of the opponent on the possible"weakness" of steganographic system.Key selection K i determines specific mapping i of the set of mappings . With the help of mapping i correspondingto the selected key K i , following received message M j and the received container L u based on identified characteristics B u ofthe container L u , a steganogram (full container) is formed:Ewi K ,M , L ,i 1,2,...,k, j 1,2,...,m, u 1,2,...,l, 1,2,...,nijuw , n m .A steganogram E w is transferred to the receiving point by a certain channel and may be intercepted by the opponent. Atthe receiving end with the help of the reverse mapping -1 i (given by the key K * i ) of the steganogram E w restored the originalmessage and the empty container is restored:1M ,L K , E . juWhen transferring the steganogram E w through a communication channel and because of the opponent’s poss ibleimpact on E w , a transmitted steganogram may become distorted. In this case, the receiving side will get a mixture of adelivered filled container and of a feedback to the container during the transmission through the communication channel:E w +ε. Performing the operation of a reverse mapping -1 i (given by the key K * i ) will lead, in this case, to a certain evaluationof a transferred message and give an empty container, i.e. we get:* * 1M ,L K , E . ji u i i w*jFor fragile steganographic systems, an inequality M M should lead to a message rejection, i.e. at the slightestdistortion of the container ( 0 ), an extracted assessmentmessageM is destroyed when 0 ).jjiw*M j should not lead to the reading of embedded message (theRobust steganographic systems are resistant to the impact on a filled container. In the above notations, this means that*when 0 , an extracted assessment M j should be compared to one of the possible messages (ideally, with the message M j ).At the same time, the derived from a communication channel container E w can contain no embedded message at all, i.e. the*extracted from the container assessment M j should not be compared to any of the possible messages. A built-in messagedetection functions at a receiving side are assigned to messages’ detector, which by the received assessment*M j decide on thepresence or absence of an internal message in the received container E . Thus, the estimate of the detector S j can beinterpreted as a binary (yes/no) decision of an error-correcting decoder on the presence or absence of uncorrectable errors. Thedecoding itself is performed at a messages decoder, the main functions of which are to compare the extracted assessmentwith one of the possible messageswM j and to transform the latter to the informational message I j provided to the recipients ofinformation.The opponent may capture the steganogram E w . In this case, he can use it to try to calculate posteriori probabilities ofvarious possible messages:PME P M1Ew,PM2 Ew,...,PMm Ew(9)wand of a variety of possible keys:Pw P K E , P K E , ...,P K E , (10)K E 1wthat could be used in the formation of the steganogram E w .The sets of posterior probabilities (9 - 10) form a posteriori knowledge of the opponent about the keysK = {K 1 , K 2 , …, K k } and the messages M = {M 1 , M 2 , …, M m } after the interception of the steganogram E w . In fact, the setsP and P are sets of assumptions, which are assigned the corresponding probabilities.K E wM E w2wkw*M jIssn 2250-3005(online) December| 2012 Page 94