On the Synergy Between Certificate Verification Trees and PayTree-like Micropayments

A substantial number of micropayment schemes in the literature are based on distributing the cost of a digital signature over several successive micropayments (e.g. Payword). Thus, a stable relationship between user and merchant is assumed: the micropayments validated by the same signature must take place between the same user and merchant. This stability is ill-suited for surfing on the web, a situation in which successive micropayments are made to different merchants. Thus coin-based micropayments, in which successive micropayments can be unrelated to one another, are far more interesting. One practical coinbased micropayment system is PayTree, which is amazingly similar to PKIs based on certificate verification trees (CVTs). We propose in this paper a synthesis of a CVT-based PKI with a PayTree-like micropayment system. The proposed system achieves a threefold reduction of the cost of public key certification and coin processing through: 1) sharing certificates by two applications (public key and multi-coin certification); 2) including certificates in the CVT on a batch basis (costs decrease as the batch size increases); 3) accelerating certificate (and thus public key and coin) verification through the verifier/merchant caching feature of the tree structure used. Furthermore, certificate and coin verification costs also become smaller as the number of users increases.