In most formal analysis methods including strand spaces method, action of penetrator is an emphasis to be studied, however, actions of legal principals are not be paid great attentions to. That is, actions of legal principals in the real world are not precisely abstracted in these methods, which causes these methods to hardly find attacks launched by dishonest legal principals. In this paper, a new conception, ldquoself-honestyrdquo, is proposed in strand spaces model to abstract legal principalspsila actions in the real world; some requirements of authentication based on ldquoself-honestyrdquo are proposed, and their formal expressions are present; finally, an improved scheme of protocol NSL-Lowe is proposed and it is proved secure against attacks of dishonest legal principals in the strand spaces model with ldquoself-honestyrdquo.