Hans Georg Schaathun

An Improved Decoding Algorithm for the Davey-MacKay Construction

The Deletion-Insertion Correcting Code construction proposed by Davey and MacKay consists of an inner code that recovers synchronization and an outer code that provides substitution error protection. The inner code uses low-weight codewords which are added (modulo two) to a pilot sequence. The receiver is able to synchronise on the pilot sequence in spite of the changes introduced by the added codeword. The original bit-level formulation of the inner decoder assumes that all bits in the sparse codebook are identically and independently distributed. Not only is this assumption inaccurate, but it also prevents the use of soft a- priori input to the decoder. We propose an alternative symbol-level inner decoding algorithm that takes the actual codebook into account. Simulation results show that the proposed algorithm has an improved performance with only a small penalty in complexity, and it allows other improvements using inner codes with larger minimum distance.

The Boneh-Shaw fingerprinting scheme is better than we thought

Digital fingerprinting is a forensic method against illegal copying. The distributor marks each individual copy with a unique fingerprint. If an illegal copy appears, it can be traced back to one or more guilty pirates due to this fingerprint. To work against a coalition of several pirates, the fingerprinting scheme must be based on a collusion-secure code. This paper addresses binary collusion-secure codes in the setting of Boneh and Shaw (1995/1998). We prove that the Boneh-Shaw scheme is more efficient than originally proven, and we propose adaptations to further improve the scheme. We also point out some differences between our model and others in the literature.

On error-correcting fingerprinting codes for use with watermarking

Digital fingerprinting has been suggested for copyright protection. Using a watermarking scheme, a fingerprint identifying the buyer is embedded in every copy sold. If an illegal copy appears, it can be traced back to the guilty user. By using collusion-secure codes, the fingerprinting system is made secure against cut-and-paste attacks. In this paper we study the interface between the collusion-secure fingerprinting codes and the underlying watermarking scheme, and we construct several codes which are both error-correcting and collusion-secure. Error-correction makes the system robust against successful attacks on the watermarking layer.

On Watermarking/Fingerprinting for Copyright Protection

Digital fingerprinting has been suggested for copyright protection. Using a watermarking scheme, a fingerprint identifying the buyer is embedded in every copy sold. If an illegal copy appears, it can be traced back to the guilty user. By using collusion-secure codes, the fingerprinting system is made secure against cut-and-paste attacks. In this paper we study the interface between the collusion-secure fingerprinting codes and the underlying watermarking scheme, and we construct several codes which are both error-correcting and collusion-secure. Error-correction makes the system robust against successful attacks on the watermarking layer

Improvement of the Davey-MacKay construction

The Davey-MacKay construction is a deletion-insertion correcting code scheme consisting of an inner code that functions as a pilot sequence to which the receiver seeks to synchronize, and an outer code that provides error protection. We analyse the performance of the inner code in isolation, arguing that these codes provide unequal protection, and demonstrate empirically that the error rate is dependent on the date symbol values. We also propose modifications to the code construction that alleviate this asymmetry. Simulation results show that these codes have an improved performance with no penalty.

Non-binary turbo codes and applications

In this paper we consider the use of q-ary turbo codes on abstract q-ary channels. Simulations show that our 16-ary codes perform well on the q-ary symmetric channel. This validates their suitability as outer codes in non-binary applications. We also compare with the performance of same codes on conventional PSK and QAM modulation, and demonstrate an application of these codes to the insertion/deletion channel.

Conditional Probability Based Steganalysis for JPEG Steganography

Inspired by works on the Markov process based steganalysis, we propose a new steganalysis technique based on the conditional probability statistics. Specifically we focus on its performance against the F5 software. In our experiment, we prove that the proposed technique works as well or better than the Markov process based technique in terms of classification accuracy on F5. Our main advantage is a much better computational efficiency. With different number of messages embedded, it can also be seen that the performance of steganalyis depends on the message size embedded. This paper includes the introduction to conditional probability features, how the experiment works, and the discussion of the results.

Novel Attacks on Spread-Spectrum Fingerprinting

Spread-spectrum watermarking is generally considered to be robust against collusion attacks, and thereby suitable for digital fingerprinting. We have previously introduced the minority extreme attack (IWDW 07), and showed that it is effective against orthogonal fingerprints. In this paper, we show that it is also effective against random Gaussian fingerprint. Furthermore, we develop new randomised attacks which counter the effect of the decoder preprocessing of Zhao et al.

Attack Analysis for He & Wu's Joint Watermarking/Fingerprinting Scheme

We introduce two novel collusion attacks against digital fingerprinting using additive spread-spectrum watermarks. These attacks demonstrate that the He-Wu fingerprinting system is considerably less secure against collusive attacks than suggested by the original paper. In addition to causing error rates above 85% at the decoder with as few as 8 colluders, one of our two attacks give copies with less distortion (measured by Euclidean distance) than the fingerprinted copies originally distributed.

On Higher Weights and Code Existence

Several open questions in coding theory relate to non- existence or construction of certain optimal codes. Many previous problems of this kind have been solved by studying possible weight enumerators. A couple of authors in this decade have proposed using higher weights (generalised Hamming weights) to a similar effect. In this paper we suggest one approach based on the weight hierarchy, and it allows us to conduct an extremely rapid computer search to prove that there are exactly two inequivalent [36,8,16] codes. The technique can also be used to gain new information about the weight hierarchy of the putative [72,36,16] code, but not yet enough to say if it exists or not.