A suit of security protocols for sensor networks was presented by A.Perrig.The security of the protocols was analyzed

and then one attack had been found in the two-party key agreement protocol that a false request message from an intruder.In that case

the intruder impersonates the node and creates false requests.An improved protocol to this attack was proposed.The security of improved protocol was analyzed by the extended strand spaces

where two main security properties

authenticity and confidentiality

were checked.And the analysis proves the correctness of the protocol.The extended strand spaces also gives a practical base for further application of strand spaces in analyzing complicated protocols.