An improved certified e-mail protocol for mobile environments

As e-mail services get wide use in business transactions, the privacy and fairness properties of these services become more important. A certified e-mail protocol focuses mainly on the fairness of e-mail exchanges: it ensures that the receiver get access to the e-mail content if and only if the sender obtains a non-refutable proof of receipt, which is usually a signature of the receiver. This paper identifies a security flaw in a certified e-mail protocol for mobile environments based on the Guillou-Quisquater signature scheme, proposed at GLOBECOM 2003. A dishonest sender can exploit this flaw to gain advantage over the message receiver with high probability within a few rounds of interactions. We then propose an improved protocol, which is based on a verifiably encrypted RSA signature scheme, to fix this security flaw. The new protocol obtains a trustable security (fairness) level and is as efficient as the original insecure protocol.