Development of Time-Stamped Signcryption Scheme and its Application in E-Cash System

A signcryption scheme combining public key encryptions and digital signatures in one logical step can simultaneously satisfy the security requirements of confidentiality, integrity, authenticity and non-repudiation and with a cost significantly lower than that required by the traditional "signature followed by encryption" approach. This thesis presents a new generic concept of time-stamped signcryption scheme with designated verifiability. Here an authenticated time-stamp is associated with the signcrypted text which can only be verifiable by a specific person, known as the designated verifier. The time-stamp is provided by a trusted third party, namely, Time Stamping System (TSS). The scheme is proved to be secure, as, no one, not even the signcrypter or TSS can produce a valid signcrypted text on behalf of them. We analyzed the security of the proposed scheme and found that it can withstand some active attacks. This scheme is resistant against both inside and outside attacks. The security of our scheme is based upon the hardness of solving Computational Diffie Hellman Problem (CDH), Discrete Logarithm Problem (DLP) and Integer Factorization Problem (IFP). The proposed scheme is suitable in scenarios such as, on-line patent submission, on-line lottery, e-cash, e-bidding and other e-commerce applications. Also we propose an e-cash system based on our proposed time-stamped signcryption scheme which confirms the notion of e-cash securities like anonymity of the spender, unforgeablity of the digital coin, prevention of double spending