Patent ID: 8437476

Claim:
A method of broadcasting data in a system, said method implemented by a broadcast encryption apparatus, said method comprising: using a stateless scheme (A 1 ) that includes a binary tree T with a KEKs No 2 encryption key structure, wherein a key k i,j is associated with each difference of subsets S i,j =S i −S j , S i and S j are two subtrees, and subtree S i contains the subtree S j , and a root key k 0, - - - associated with the binary tree T; and using a stateful scheme (A 2 ) that includes a binary tree T with a KEKs No 1 encryption key structure, wherein a key k i is associated with each subtree S i , wherein the stateless scheme (A 1 ) is used for a current broadcast session, the keys known by denied users are updated with the stateful scheme (A 2 ) from time to time, combining the stateless scheme (A 1 ) and the stateful scheme (A 2 ) to determine a set {L ij } from the sets {k ij } and {k i } of the stateless scheme (A 1 ) and the stateful scheme (A 2 ), respectively, wherein the keys k i,j and k 0, - - - are static encryption keys defined by the stateless scheme (A 1 ) and have a length in bits marked N s , and the keys k i are dynamic encryption keys defined by the stateful scheme (A 2 ) and have a length in bits marked N d , using function F with inputs in {0,1} Ns ×{0,1} Nd and outputs in {0,1} Ns to calculate z=F(x,y) from any (x,y) belonging to {0,1} Ns ×{0,1} Nd and for any triplet (x,y,z) wherein z=F(x,y), it is impossible to find z if no information on y is possessed, even if x is known, for each index i of a node v i of the tree, and for each index j of a node v j belonging to the subtree S i , defining the following variable encryption keys: L i,j =F(k i,j , k i ) calculated from the static encryption key k i,j and the dynamic encryption key k i , L 0, - - - =F(k 0, - - - , k 0 ) calculated from the static key k 0, - - - and the dynamic key k 0 , each session t, using the variable encryption keys of the stateless scheme (A 1 ) to broadcast the data to the authorized users, wherein said data to be broadcasted is marked M t , and A 1 [M t , {L i,j }] is sent, each T session, where T is a parameter, using the stateful scheme (A 2 ) to renew the dynamic keys shared by the authorized and revoked users, wherein an item of information I(D) is sent by giving the date D of operational application of the set of renewed encryption keys, and A 2 [M, {k i }, {k′ i }] and I(D) is sent, and on the date D, the set of keys {k′ i }, replacing the set of keys {L ij } with a new set {L′ i,j } where L′ i,j =F(k ij , k′ i ).