Statistical Tools for Digital Forensics

A digitally altered photograph, often leaving no visual clues of having been tampered with, can be indistinguishable from an authentic photograph. As a result, photographs no longer hold the unique stature as a definitive recording of events. We describe several statistical techniques for detecting traces of digital tampering in the absence of any digital watermark or signature. In particular, we quantify statistical correlations that result from specific forms of digital tampering, and devise detection schemes to reveal these correlations

Towards Making Broadcast Encryption Practical

The problem we address is how to communicate securely with a set of users (the target set) over an insecure broadcast channel. In order to solve this problem, several broadcast encryption schemes have been proposed. In these systems, the parameters of major concern are the length of transmission and number of keys held by each user's set top terminal (STT). Due to the need to withstand hardware tampering, the amount of secure memory available in the STTs is quite small, severely limiting the number of keys each user holds. In such cases, known theoretical bounds seem to indicate that non-trivial broadcast encryption schemes are only feasible when the number of users is small. In order to break away from these theoretical bounds, our approach is to allow a controlled number of users outside the target set to occasionally receive the multicast. This relaxation is appropriate for low-cost transmissions such as multicasting electronic coupons. For this purpose, we introduce f-redundant est..