The present invention provides an authentication method and authorization key generation method in a wireless portable Internet system. In a wireless portable Internet system, the base station and the subscriber station share an authorization key when an authentication process is performed according to a predetermined authentication method negotiated therebetween. Particularly, the subscriber station and the base station perform an additional authentication process including an authorization key-related parameter and a security-related parameter and exchanges a security algorithm and SA (Security Association) information. In addition, an authorization key is derived from one or more basic key obtained through various authentication processes as an input key of an authorization key generation algorithm. Therefore, reliability of a security-related parameter received from the receiving node can be enhanced and an authorization key having a hierarchical and secure structure can be provided.