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

Abstract

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 coin-based 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.