Patent ID: 8488794

Claim:
A method for access control to a content supplied to a reception terminal by an operator, with which an access management unit is associated, the method comprising: a conditioning phase of the content by the operator and a usage phase of the content by the terminal; wherein the terminal is configured to store three doublets composed of a current key K c and data D c related to the key, a previous key K p and data D p related to the previous key, and a next key K s and data D s related to the next key respectively, wherein the conditioning phase comprises: a) breaking down a duration of the content into N time segments each being associated with one identifier S j , one key K j and one item of data D j related to the key, each segment S j including an integer number M j of crypto-periods CP i ,i=1 to M j and j=1 to N, b) scrambling the content using a sequence of control words CW i,j , c) encrypting each control word CW i,j using the K j key, and d) transmitting the scrambled content to the terminal during each crypto-period CP i , with an Entitlement Control Message (ECM) message comprising at least the encrypted control word CW i,j , the data D j related to the current key K j , the data D j−1 related to the previous key K j−1 and the data D j+1 related to the next key K j+1 , to decrypt the content during a given time segment S j ; and the usage phase comprises, on reception of the ECM message: e) the terminal analyzing data D j contained in the message; and f) if data D j contained in the ECM message corresponds to the data D c previously stored in the terminal, the terminal decrypting the control word CW i,j with the key K c associated with the data D c , g) if the data D j contained in the ECM message corresponds to the data D p previously stored in the terminal, the terminal decrypting the control word CW i,j using the key K p , and if the data D j corresponds to the data D s , the terminal decrypts the control word CW i,j using the key K s , and h) if the data D j contained in the ECM message does not correspond to the data D c previously stored in the terminal, the terminal sending the received ECM message to the access management unit that determines the current key K j from the data D j , the previous key K j−1 from the data D j−1 and the next key K j+1 from the data D j+1 , and sending the keys and their related data to the terminal that stores their corresponding values as new values of the keys K c , K p and K s and data D c , D p and D s related to these keys, and wherein the data D j , D j−1 and D j+1 transmitted in the ECM include at least the cryptograms corresponding to the keys K j , K j−1 and K j+1 that can be decrypted by a management key known exclusively to the management unit.