International Association for Cryptologic Research

International Association
for Cryptologic Research


Christian Schallhart


Enforcing Semantic Integrity on Untrusted Clients in Networked Virtual Environments
During the last years, large-scale simulations of realistic physical environments which support the interaction of multiple participants over the Internet have become increasingly available and economically viable, most notably in the computer gaming industry. Such systems, commonly called networked virtual environments (NVEs), are usually based on a client-server architecture where for performance reasons and bandwidth restrictions, the simulation is partially delegated to the clients. This inevitable architectural choice renders the simulation vulnerable to attacks against the semantic integrity of the simulation: malicious clients may attempt to compromise the physical and logical rules governing the simulation, or to alter the causality of events a posteriori. In this paper, we initiate the systematic study of semantic integrity in NVEs from a security point of view. We argue that naive policies to enforce semantic integrity involve intolerable network load, and are therefore not practically feasible. We present a new provably secure semantic integrity protocol based on cryptographic primitives which enables the server system to audit the local computations of the clients on demand. Our approach facilitates low network and CPU load, incurs reasonable engineering overhead, and maximally decouples the auditing process from the soft real time constraints of the simulation.
Provably Secure Authentication of Digital Media Through Invertible Watermarks
The recent advances in multimedia technology have made the manipulation of digital images, videos or audio files easy. On the one hand the broad availability of these new capabilities enabled numerous new applications. On the other hand, for the same reasons, digital media can easily be forged by almost anyone. To counteract this risk, fragile watermarks were proposed to protect the integrity and authenticity of digital multimedia objects. Traditional watermarking schemes employ non-cryptographic and signal processing oriented techniques, which fail to provide any provable security guarantee against malicious modification attempts. In this paper, we give for the first time a provably secure authentication mechanism for digital multimedia files that is based on both cryptographic signatures and invertible watermarks. While traditional watermarking schemes introduce some small irreversible distortion in the digital content, invertible watermarks can be completely removed from a watermarked work.