Toward Generic method for Server-Aided Cryptography

Portable devices are very useful to access services from anywhere at any time. However, when the security underlying the service requires complex cryptography, implying the execution of several costly mathematical operations, these devices may become inadequate because of their limited capabilities. In this case, it is desirable to adapt the way to use cryptography. One possibility, which has been widely studied in many particular cases, is to propose a server-aided version of the executed cryptographic algorithm, where some well-chosen parts of the algorithm are delegated to a more powerful entity. As far as we know, nothing has been done to generically change a given well-known secure instance of a cryptographic primitive in its initial form to a secure server-aided version where the server (called the intermediary) may be corrupted by the adversary. In this paper, we propose an almost generic method to simplify the work of the operator who wants to construct this secure server-aided instance. In particular, we take into account the efficiency of the resulting server-aided instance by giving the best possible way to separate the different tasks of the instance so that the resulting time efficiency is optimal. Our methodology can be applied to most of public key cryptographic schemes.