User Leave. When an existing user Uold leaves, U0 discards its share (IDold , xold ). The set of users is updated as U = U ∩ IDold . U0 also refreshes its contribution to (ID0, x′ ) using a new value x′ . Then the shared secret is computed and distributed as described in steps 3 to 5 in key agreement. It can be noted that, as one group member joins or leaves, it’s corresponding contribution point is added/discarded. The U0’s contribution also changes. So, whenever there is a change in membership, atleast two points of the secure polynomial change and its value is refreshed. Now, from the property of polyno- mial interpolation, it is known that, if 1 out of (n + 1) points on a n degree polynomial is changed, the poly- nomial changes in an unpredictable way. This is in- formation theoretically true. Thus, secrecy of the pre- vious (new) key from new (former) group members is maintained. Figure 2 demonstrates one instance of the If this value is equal to xi, the user knows his/her contribution was used in key construction. Ac- cording to Xxxxxx’x rule, this computation can be written as a0 + a1IDi + a2ID2 + . . . + anIDn Key Agreement scheme of proposed protocol.
User Leave. Protocol LP1 Assume a scenario where a set of users L leaves a multicast group łGp. Then protocol LP1 is executed to provide each user of the new multicast group łGn = łGp \ L with a new session key. Any remaining user can act as the controller in the new multicast group łGn. LP1 requires only one communication round with a single broadcast and it proceeds as follows:
User Leave. Protocol LP1 MG MG MG \ L L MG Assume a scenario where a set of users leaves a multicast group p. Then protocol LP1 is executed to provide each user of the new multicast group n = p with a new session key. Any remaining user can act as the controller in the new multicast group n. LP1 requires only one communication round with a single broadcast and it proceeds as follows:
