Patent ID: 8363841

Claim:
A method for managing keys for a user to access one or more given services S in a communication system, the user not being able to be continuously connected to this service, comprising the following steps: generating a key K(t) providing access to the service of day [t] for all the t<t fin by using a one-way function in the following manner: using a root key K(t fin ) and generating the key K(t fin-1 ) for the day [t fin-1 ] prior to the day t fin , by using a function f such that K(t fin-1 )=f(K(t fin )), using the new value of key K(t fin-1 ) in order to generate the key for the previous day K(t fin-2 ) and reiterating this step over the limited time period of day [t] to day [t+d] in order to obtain the chain K( ) equal to K(t+d−1), K(t+d−2), . . . , K(t) and sending each day t the key K(t+d) to the users or user group having a trust level d; toward the end of the period of validity of the current chain for the traffic key, the receivers or users having a trust level d for a first period and a trust level d* for a second period store two types of traffic keys: current traffic key K(t+d) allowing access for the last days under the control of the current chain K( ), with the trust level d, and second traffic key K*(t+d*) allowing access for the future days under the control of a new chain K*( ) with a trust level d* which is different from the trust level d, wherein the new chain K*( ) is equal to K*(t+d*−1), K*(t+d*−2), . . . , K*(t), wherein, if the period of transition between the chain K( ) and the chain K*( ) includes r consecutive days from day [T 0 ] to day [T 0 +r−1] inclusive, then for each user group G of trust level d for the chain K( ) and d* for the chain K*( ) a function OTAR associated with the group G broadcasts the following information: before the transition period: K(t+d) each day [t] with t<T 0 , in order to give potential access to the service during the sliding time window [t, t+d], during the transition period: K(t′) each day [t] with T 0 <=t<T 0 +r, where t′ is the minimum between (t+d) and (T 0 +r−1), in order to retain access to the service up to the last day [T 0 +r−1] of activation of the chain K( ), K*(t+d*) for each day [t] with T 0 +m<=t<T 0 +r, where m is the maximum between 0 and (r−d*), in order to give a valid future access key of the new chain K*( ), d, if the new trust level d* is different from the old level d, the date [T 0 +r] of activation of the new chain K*( ), after the transition period: K*(t+d*) for each day [t] with T 0 +r<=t.