A certified e-mail protocol suitable for mobile environments

We describe a novel certified e-mail protocol that is particularly suitable for mobile environments. Our protocol uses an off-line trusted third party (TTP). Protocols with an off-line TTP—also known as optimistic protocols—have numerous practical advantages over protocols with an on-line TTP. Nonetheless, many protocols adopt an on-line TTP primarily because optimistic protocols often entail intricate cryptographic primitives that incur considerable overhead. By using a novel signature paradigm, which we call gradational signatures, we show that it is possible to construct optimistic protocols that are comparable to on-line protocols in terms of computation and communication overhead. This makes our scheme especially desirable in the mobile setting. Keywords—certified e-mail; fair exchange; multisignatures