Variable-to-Fixed Length Homophonic Coding Suitable for Asymmetric Channel Coding

In communication through asymmetric channels the capacity-achieving input distribution is not uniform in general. Homophonic coding is a framework to invertibly convert a (usually uniform) message into a sequence with some target distribution, and is a promising candidate to generate codewords with the nonuniform target distribution for asymmetric channels. In particular, a Variable-to-Fixed length (VF) homophonic code can be used as a suitable component for channel codes to avoid decoding error propagation. However, the existing VF homophonic code requires the knowledge of the maximum relative gap of probabilities between two adjacent sequences beforehand, which is an unrealistic assumption for long block codes. In this paper we propose a new VF homophonic code without such a requirement by allowing one-symbol decoding delay. We evaluate this code theoretically and experimentally to verify its asymptotic optimality.Comment: Full version of the paper to appear in 2017 IEEE International Symposium on Information Theory (ISIT2017

Digital rights management (DRM) - watermark encoding scheme for JPEG images

The aim of this dissertation is to develop a new algorithm to embed a watermark in JPEG compressed images, using encoding methods. This encompasses the embedding of proprietary information, such as identity and authentication bitstrings, into the compressed material. This watermark encoding scheme involves combining entropy coding with homophonic coding, in order to embed a watermark in a JPEG image. Arithmetic coding was used as the entropy encoder for this scheme. It is often desired to obtain a robust digital watermarking method that does not distort the digital image, even if this implies that the image is slightly expanded in size before final compression. In this dissertation an algorithm that combines homophonic and arithmetic coding for JPEG images was developed and implemented in software. A detailed analysis of this algorithm is given and the compression (in number of bits) obtained when using the newly developed algorithm (homophonic and arithmetic coding). This research shows that homophonic coding can be used to embed a watermark in a JPEG image by using the watermark information for the selection of the homophones. The proposed algorithm can thus be viewed as a ‘key-less’ encryption technique, where an external bitstring is used as a ‘key’ and is embedded intrinsically into the message stream. The algorithm has achieved to create JPEG images with minimal distortion, with Peak Signal to Noise Ratios (PSNR) of above 35dB. The resulting increase in the entropy of the file is within the expected 2 bits per symbol. This research endeavor consequently provides a unique watermarking technique for images compressed using the JPEG standard.Dissertation (MEng)--University of Pretoria, 2008.Electrical, Electronic and Computer Engineeringunrestricte

Adaptive homophonic coding techniques for enhanced e-commerce security

This dissertation considers a method to convert an ordinary cipher system, as used to secure e-commerce transactions, into an unconditionally secure cipher system, i.e. one that generates ciphertext that does not contain enough statistical information to break the cipher, irrespective of how much ciphertext is available. Shannon showed that this can be achieved by maximizing the entropy of the message sequence to be encrypted. This, in turn, achieved by means of homophonic coding. Homophonic coding substitutes characters in the message source with randomly chosen codewords. It offers the advantage that it enables protection against known- and chosen plaintext attacks on cipher algorithms since source statistics are randomly changed before encryption. The disadvantage of homophonic substitution is that it will in general increase the length of the message sequence. To compensate for this, homophonic coding is combined with the data compression algorithm known as arithmetic coding. It is shown that the arithmetic coding algorithm can be adapted to perform homophonic coding by dyadically decomposing the character probabilities in its probability estimation phase. By doing this, a faster version of arithmetic coding, known as shift-and-add arithmetic coding can be implemented. A new method of statistical modelling, based on an Infinite Impulse Response filtering method is presented. A method to adapt the well-known Lempel-Ziv- Welch compression algorithm to perform homophonic coding is also presented. The procedure involves a bit-wise exclusive-or randomization operation during encoding. The results show that the adapted algorithms do indeed increase the entropy of the source sequences by no more than 2 bits/symbol, and even offers compression in some cases.Dissertation (MEng (Data Security))--University of Pretoria, 2006.Electrical, Electronic and Computer Engineeringunrestricte

Universal homophonic coding

Redundancy in plaintext is a fertile source of attack in any encryption system. Compression before encryption reduces the redundancy in the plaintext, but this does not make a cipher more secure. The cipher text is still susceptible to known-plaintext and chosen-plaintext attacks. The aim of homophonic coding is to convert a plaintext source into a random sequence by randomly mapping each source symbol into one of a set of homophones. Each homophone is then encoded by a source coder after which it can be encrypted with a cryptographic system. The security of homophonic coding falls into the class of unconditionally secure ciphers. The main advantage of homophonic coding over pure source coding is that it provides security both against known-plaintext and chosen-plaintext attacks, whereas source coding merely protects against a ciphertext-only attack. The aim of this dissertation is to investigate the implementation of an adaptive homophonic coder based on an arithmetic coder. This type of homophonic coding is termed universal, as it is not dependent on the source statistics.Computer ScienceM.Sc. (Computer Science

Auditory interference and phonological encoding in reading for meaning

A thesis Submitted to the Faculty of Arts, University of the, Witwatersrand, Johannesburg,, in fulfilment of the requirements for the degree of Master of Arts. Johannesburg 1983The main aim of the present research was zo investigate the effects of auditory interfering stimuli and an articulatory suppression task on pre- and postlexical phonological encoding during reading. Sixty undergraduate students performed a Prose Comprehension task (Experiment 1) and a Nonword-rhyming task (Experiment 2) under conditions of INTERFERENCE and NO INPUT. An analysis of covariance and post-hoc t-tests revealed that semanticslly and syntactically complex verbal auditory input had the greatest interfering effect on the speed of performance of the Prose Comprehension task, No other results were statistically significant, Twenty undergx-aduate students (Experiment 3) and twenty children (ten dyslexics, ten normal readers - Experiment 4) performed a Magnitude Judgment task under conditions of INTERFERENCE and NO INPUT. Prose auditory interference and an articulatory suppression task did not significantly slow down the performance of skilled readers while prose input did slow down the performance of both dyslexic and normal children. Magnitude Judgment accuracy data was not analysed due to the low error rate. The results of. the present research ware interpreted within the framework of a neuro-cognitive model of reading based largely on Luria's neuropsychological model of the "working brain" and Morton's "Logogen" model of word recognition

“Something that just hovers”: Charting Feldman’s Neither

Senior Project submitted to The Division of Arts of Bard College