Efficient fair certified e-mail delivery based on RSA

Certified e-mail delivery (CEMD) has become one of the basic requirement in performing business transactions over the Internet securely. How to construct fair protocols for certified e-mail delivery based on the RSA cryptosystem is of great interest. Recently, Nenadic etc. proposed a novel RSA-based method for the verifiable and recoverable encrypted signature (VRES), anti utilized it to construct a security protocol for certified e-mail delivery, which are claimed to provide strong fairness to ensure that the recipient receives the e-mail if and only if the sender receives the receipt. However, as a building block, their RSA-based VRES is totally breakable. This papers shows that an adversary can generate a valid VRES which cannot be recovered by the designated TTP, and hence the proposed certified e-mail delivery protocol cannot guarantee the required fairness. Based on probabilistic signatures, we proposed a novel fair CEMD protocol which works with the RSA cryptosystem and guarantees strong fairness. Moreover, there is no need for a registration phase between a party and TTP, and the proposed protocol is more computation and communication efficient.