Patent ID: 8254580

Claim:
A method of acquiring encryption keys in a hierarchy of nodes, the method comprising the steps of: acquiring at a client node, at a time r−1, a first encryption key (EK r−1 ) and a first forward key (FK r−1 ); receiving at the client node, from a key server, at a time r, a backward key applicable at a time r+p(BK r+p ), the BK r+p being encrypted with the EK r−1 , p being a period defined for the hierarchy of nodes; applying at the client node a first one-way function p times to the BK r+p to obtain a backward key applicable at the time r(BK r ); applying at the client node a second one-way function to the FK r−1 to obtain a backward key applicable at the time r(FK r ); combining at the client node the FK r with the BK r to obtain a second encryption key applicable at the time r(EK r ); and receiving at the client node, at a time q which is later in time than the time r, a unicast rekey message comprising, for a parent node connecting the client node, a first parent forward key applicable at the time q, (P 1 -FK q ) and a first parent backward key applicable at a time q+p (P 1 -BK q+p ), the P 1 -FK q and the P 1 -BK q+p being encrypted with a time invariant key of the client node.