Cyberspace of Shujun LI


What multimedia security do you mean?

As a broad research topic, multimedia security can be loosely split into digital watermarking, steganography and steganalysis, and multimedia encryption. This web page is mainly about multimedia encryption, although it also overlaps with digital watermarking. For encryption of digital images and video sequences, several ideas have been widely used in the literature: The above ideas can be applied alone or combined (the latter is more common).

Why does discrete optimization matter?

Cryptanalysis of many multimedia encryption problems can be conducted by modelling the image/video recovery problem as a discrete optimization model and then solving it using standard discrete optimization tools or developing bespoke discrete optimization algorithms. This is a less studied topic in multimedia encryption, and previous methods are often not based on discrete optimization or based on some local optimization approaches which can not find global optimum solutions. Shujun Li has been working in this area since 2010 and produced two important milestone papers on cryptanalysis of selective encryption of blockwise DCT coefficients: The basic idea behind the ICIP 2011 work is to represent the recovery problem as a linear programming (LP) model which can be solved in polynomial time. Shujun is currently generalizing the idea to other types of multimedia encryption methods. The above work outperforms the previous work and extends to recover not only DC coefficients: A unique feature of the approach is that it provide a general framework of ciphertext-only attacks (COA) on multimedia encryption, which is the least studied area of cryptanalysis of multimedia encryption.

Other Related Topics

A highly related line of research is puzzle solving or image defragmenting such as the following work:

Extension to Digital Watermarking

The use of discrete optimization to cryptanalysis of multimedia encryption is effectively about image recovery where unknown information has a known special structure. This idea has also been applied to image authentication and self-restoration watermarking by Shujun Li and his co-authors:

Are there other ways to cryptanalyze multimedia encryption schemes?

Yes, there are some other approaches some of which are briefed below.

Error-Concealment Attacks (ECA)

A more straightforward and "naive" method is the so-called Error-Concealment Attacks (ECA) which tries to replace encrypted elements by some values to reveal sufficient visual information that is concealed by encryption. A common practice is to replace all encrypted elements by a fixed value (the most likely values). Such methods can be also more intelligent in that the replacement can be driven by statistical rules (see one example in the following paper).

Sketch Attacks

This attack aims at a different goal: rather than trying to recover all pixels in the plaintext image, one can switch to recover as much as edge information to reveal the shapes of key objects in an encrypted image or video sequence.

Known-Plaintext Attacks (KPA) and Chosen-Plaintext Attacks (CPA)

Some multimedia encryption schemes are not secure when some plaintexts are known or can be chosen by the attacker. The most well-studied multimedia encryption schemes in this context is permutation-only image ciphers which just shuffle positions of all pixels or blocks within a plaintext image. The idea can be generalized to video encryption easily. The following key papers have proved quantitative results of such attacks on digital images.

You are interested?

Please contact Shujun Li for possible collaborations! There are plenty of ideas but short of hands! :-)

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