Abstract: There arc several uncertain factors in authentication procedure, so authentication services arc not absolutely credible. However existing researches focused on not whole but some aspects of authentication. Based on formalized description for authentication and logical analysis on security of authentication, uncertain factors in authentication procelure were divided into four categories. Meanwhile a common approach on trust degree of authentication was proposed through the introduction of trust classification,non-trust factor and security degree. Two examples were given to demonstratc how to use this approach.

Key words: Authentication, Authentication voucher, Non-trust factor, Security degree, Trust degree