—The role of internet in our life is increasing rapidly. Anytime and anywhere access to internet service is a main desire of users. Sometimes and due to various reasons such as heavy traffic or technical problems, a primary Wireless Internet Service Provider (i.e. primary WISP) might not be able to provide internet service for new customers. If another WISP (i.e. secondary WISP) can provide the requester with internet service at the same time, a proposal is to transfer the user to the new WISP. In this thesis we propose a novel secure scheme for handoff between independent WI in multi operator networks. These WI belong to different network operators. Designed protocols for the proposed scheme include: "Token purchase protocol", "Handoff and payment protocol", "Clearance and deposit protocol" and " Dispute resolution protocol". In addition to traditional internet service, our approach includes emerging real time and interactive services. The proposed scheme uses a Financial Intermediary (FI) to handle offline financial transactions such as token purchase, clearance and deposit. Payment method of our scheme is a token-based method in which tokens can be purchased by primary WISP from FI for the user to pay the secondary WISP for providing internet service. The secondary WISP, can cash the collected tokens in the FI. The micropayment method of our payment system is well extendable to macropayment method through submitting more than one token in each payment. Since we apply the hash chain technique to generate tokens, there is no risk of credit loss in the clearance and deposit phase. Furthermore, any dispute among entities is solvable in this scheme. We applied symmetric key cryptography for secure communication between user and new WISP. The shared key between the user and new WISP is generated according to Diffie-Hellman Key exchange algorithm. Second handoff occurs when the new WISP is unable to continue providing the user with the internet service. For such a situation, the "second handoff and payment" protocol is developed. Furthermore, the proposed scheme provides mutual authentication, user anonymity, user untraceability, confidentiality, non repudiation, double spending prevention and joint key control services and is secure against "deposit-case" attack that some previous roaming schemes are known vulnerable to it. Keywords: Handoff, WISP, Payment system, Token, Security
