The modified version of Yahalom protocol improved by Burrows, Abradi, and Needham （BAN） still has security drawbacks. This study analyzed such flaws in a detailed way from the point of strand spaces, which is a novel method of analyzing protoeol＇s security. First, a mathematical model of BAN-Yahalom protocol is constructed. Second, penetrators＇ abilities are restricted with a rigorous and formalized definition. Moreover, to increase the security of this protocol against potential attackers in practice, a further improvement is made to the protocol. Future application of this re-improved protocol is also discussed.
