Cryptanalysis and improvement of a blind multi-document signcryption scheme

Blind signcryption is used to maintain the anonymity and privacy of the sender from other participants in an unsecured public network. It has vast applications for privacy related mechanisms such as electronic voting and electronic auction systems. Various blind signcryption schemes were introduced in the last two decades, depending upon the computational cost and the level of security. Some of these schemes were proven to be insecure and need further improvement. We analyzed a recently proposed blind signcryption scheme for multiple digital documents based on a hyperelliptic curve. Our cryptanalysis shows that the proposed blind signcryption scheme is not secure against the existing attacks. An adversary, with the knowledge of public parameters, can modify the signcrypted text of his choice. The successful cryptanalysis shows that the proposed scheme is unable to provide the security attributes of authentication and message integrity. To overcome the security flaws, we propose a modified version of this scheme.