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Andrew Lewis – PhD student, security group, Cambridge University Computer Laboratory
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Andrew Lewis

Andrew Lewis

University of Cambridge
Computer Laboratory
15 JJ Thomson Avenue
Cambridge CB3 0FD
UK

Email:
first name.last name @ cl.cam.ac.uk

Telephone:
+44 (1223) 7-63568

GE20

Personal home page

Andrew Lewis

I started a PhD in multimedia security in the security group of the computer laboratory in October 2007.

My interests include forensics of compressed images and video, tampering detection and watermarking. My multimedia forensics bibliography contains information and links for papers in multimedia security and digital forensics.

I took over running the computer lab's security seminars in November 2007. Please contact me if you would like to be added to the mailing list, or would like to suggest a talk.

Projects

July 2008

Exact JPEG recompression and forensics using interval arithmetic (slides | poster)

Presented at MM&Sec08 rump session and Microsoft Research Summer School 2008

Andrew B. Lewis and Markus G. Kuhn

The standard JPEG compression algorithm was only designed to process images as they were acquired by camera sensors. In practice, however, photographic images are often compressed and decompressed multiple times. Here, image quality can be preserved better with special recompression algorithms designed for input that has been compressed and decompressed before. Our new JPEG recompressor represents the exact arithmetic operations of the preceding decompression as an overdetermined system of equations. It then uses interval arithmetic and iterative refinement to invert decompression steps in a way that takes into consideration all possible rounding effects. This results in a set of possible compression results, which will include the actual result of the last compression. We have also extended our recompression algorithm into a forensic tool that identifies those regions of a raw image that cannot possibly have been the result of a previous JPEG decompression step.

August 2008

Multimedia forensics bibliography

View multimedia forensics bibliography

Supervisions

I supervise some courses in the undergraduate computer science tripos.

Here are some Part II project ideas.