Patent ID: 7778423

Claim:
A method for establishing a common key for a group of at least three subscriber devices, the method comprising: generating by each subscriber device Ti, i=1 to n, where n is the number of subscribers in the group of the at least three subscriber devices, a respective message Ni=(g zi mod p) from a publicly known element g of large order of a publicly known mathematical group G and a respective random number zi and electronically transmitting the respective message from the respective subscriber device to all other subscriber devices Tj of the at least three subscriber devices, each respective random number zi being selected or generated by the respective subscriber device Ti; generating by each subscriber device Ti a transmission key k ij from the messages Nj electronically received from the other subscriber devices Tj, j≠i, and the respective random number zi according to k ij :=Nj zi =(g zj ) zi ; electronically transmitting by each subscriber device Ti the respective random number zi in encrypted form to all other subscriber devices Tj by generating the message Mij according to Mij:=E(k ij , zi), E(k ij , zi) being a symmetrical encryption algorithm in which the random number zi is encrypted with the transmission key k ij ; electronically receiving, by each subscriber device Ti, messages Mji from the other subscriber devices Tj, j≠i, and decrypting the messages Mji to extract random numbers zj; and determining a common key k by each subscriber device Ti using the respective random number zi and the random numbers zj, j≠i, received from the other subscriber devices according to k:=f ( z 1 , . . . , zn ), f being a symmetrical function which is invariant under a permutation of its arguments.