A sanitizable signcryption scheme with public verifiability via chameleon hash function

Abstract

Sanitizable signcryption adds sanitization functionality to signcryption, such that a delegated sanitizer can modify a signcryptext and still derive a valid signcryptext without cooperation of the original sender. Sanitizable signcryption is useful for data sharing with authentication and access control. Existing sanitizable signcryption scheme cannot achieve public verifiability, which can prevent malicious senders or sanitizers from cheating the receivers. In order to realize public verifiability, we propose a new composition method called “Encrypt-then-Commit-then-Sign (EtCtS)”. In particular, our method carefully embeds a key-exposure free chameleon hash function (also known as trapdoor commitment) between encryption and signing operations. Accordingly, we convert the sanitization operation into finding collisions in the key-exposure free chameleon hash function using the trapdoor key, and after sanitization the plaintext is still confidential to the sanitizer. Based on the EtCtS method, we construct the first sanitizable signcryption scheme that is public verifiable. We give a rigorous security proof of our scheme in the random oracle model. We also provide an implementation of our scheme for performance analysis.