Member Join Phase— Sample Clauses

Member Join Phase—. During a group session, it is possible that a new member requests to join the group occasionally. The new member chooses a joined group member as his partner, who locates at the leaf node nearest the root and is responsible for his joining as well as departure. The new member sends his partner the join-request message. If the new member is permitted to join, the partner performs the key agreement procedure with the new member. For providing perfect backward privacy, the joining member cannot get any old keys in the key tree. K1234 = g3.f(r12,K12).f(r34,K34) K12 = g3.f(r1,K1).f(r2,K2) K34 = g3.f(r3,K3).f(r4,K4) K1 K2 K3 K4 M4 M1 M2 M3 Figure 1. The group key tree hierarchy Figure 2. Key tree after the new member m4 joining Hence all the keys from the leaf node of the new member up to the root need to be changed. As an example in Figure 2, the new member m4 joins at node 7, and member m3 moves one level down to node 6 for accommodating m4. The key values of nodes 1, 3, 6 and 7 must be renewed. Here, two issues are not addressed that how the new member finds the partner who is at the leaf node nearest the root, and how the keys are renewed. We will discuss the two issues in the next section.

Member Join Phase—. In case of a new participant is authorized to join the group of which size is n, the new participant Un+1 becomes the successor of participant Un and the participant U1 becomes the successor of participant Un+1. First of all, Un needs to send a new message {Cn, Trn(x), MACn} to its new successor Un+1 while Un+1 sends message { Cn+1, Trn+1(x), MACn+1} to its new successor U1. Then Un+1 verifies the validity of the message {Cn, Trn(x), MACn} and computes the new secret SKn,n+1 shared between Un+1 and its new predecessor Un. Similarly, the first participant U1 updates its new secret with SKn+1,1. Finally, all the (n + 1) participants in the group can get a new group session key by recomputing the protocol of Section II.C.