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Key agreement protocol is a fundamental protocol in cryptography whereby two or more participants can agree on a common conference key in order to communicate securely among themselves. In this situation, the participants can securely send and receive messages with each other. An adversary not having access to the conference key will not be able to decrypt the messages. In this paper, we propose a novel identity-based authenticated multi user key agreement protocol employing a symmetric balanced incomplete block design. Our protocol is built on elliptic curve cryptography and takes advantage of a kind of bilinear map called Weil pairing. The protocol presented can provide an identification (ID)-based authentication service and resist different key attacks. Furthermore, our protocol is efficient and needs only two rounds for generating a common conference key. It is worth noting that the com munication cost for generating a conference key in our protocol is only O(n√n) and the computation cost is only O(nm2), where n implies the number of participants and m denotes the extension degree of the finite field Fpm. In addition, in order to resist the different key attack from malicious participants, our protocol can be further extended to provide the fault tolerant property.