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 Science / M.Sc. (Computer Science)

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 Science / M.Sc. (Computer Science)

[pt] Cifragem e compressão de dados são funcionalidades essencias quando dados digitais são armazenados ou transmitidos através de canais inseguros. Geralmente, duas operações sequencias são aplicadas: primeiro, compressão de dados para economizar espaço de armazenamento e reduzir custos de transmissão, segundo, cifragem de dados para prover confidencialidade. Essa solução funciona bem para a maioria das aplicações, mas é necessário executar duas operações caras, e para acessar os dados, é necessário primeiro decifrar e depois descomprimir todo o texto cifrado para recuperar a informação. Neste trabalho são propostos algoritmos que realizam tanto compressão como cifragem de dados. A primeira contribuição desta tese é o algoritmo ADDNULLS - Inserção Seletiva de Nulos. Este algoritmo usa a técnica da esteganografia para esconder os símbolos codificados em símbolos falsos. É baseado na inserção seletiva de um número variável de símbolos nulos após os símbolos codificados. É mostrado que as perdas nas taxas de compressão são relativamente pequenas. A segunda contribuição desta tese é o algoritmo HHC - Huffman Homofônico-Canônico. Este algoritmo cria uma nova árvore homofônica baseada na árvore de Huffman canônica original para o texto de entrada. Os resultados dos experimentos são mostrados. A terceira contribuição desta tese é o algoritmo RHUFF - Huffman Randomizado. Este algoritmo é uma variante do algoritmo de Huffman que define um procedimento de cripto-compressão que aleatoriza a saída. O objetivo é gerar textos cifrados aleatórios como saída para obscurecer as redundâncias do texto original (confusão). O algoritmo possui uma função de permutação inicial, que dissipa a redundância do texto original pelo texto cifrado (difusão). A quarta contribuição desta tese é o algoritmo HSPC2 - Códigos de Prefixo baseados em Substituição Homofônica com 2 homofônicos. No processo de codificação, o algoritmo adiciona um bit de sufixo em alguns códigos. Uma chave secreta e uma taxa de homofônicos são parâmetros que controlam essa inserção. É mostrado que a quebra do HSPC2 é um problema NP- Completo. / [en] Data compression and encryption are essential features when digital data is stored or transmitted over insecure channels. Usually, we apply two sequential operations: first, we apply data compression to save disk space and to reduce transmission costs, and second, data encryption to provide confidentiality. This solution works fine for most applications, but we have to execute two expensive operations, and if we want to access data, we must first decipher and then decompress the ciphertext to restore information. In this work we propose algorithms that achieve both compressed and encrypted data. The first contribution of this thesis is the algorithm ADDNULLS - Selective Addition of Nulls. This algorithm uses steganographic technique to hide the real symbols of the encoded text within fake ones. It is based on selective insertion of a variable number of null symbols after the real ones. It is shown that coding and decoding rates loss are small. The disadvantage is ciphertext expansion. The second contribution of this thesis is the algorithm HHC - Homophonic- Canonic Huffman. This algorithm creates a new homophonic tree based upon the original canonical Huffman tree for the input text. It is shown the results of the experiments. Adding security has not significantly decreased performance. The third contribution of this thesis is the algorithm RHUFF - Randomized Huffman. This algorithm is a variant of Huffman codes that defines a crypto-compression algorithm that randomizes output. The goal is to generate random ciphertexts as output to obscure the redundancies in the plaintext (confusion). The algorithm uses homophonic substitution, canonical Huffman codes and a secret key for ciphering. The secret key is based on an initial permutation function, which dissipates the redundancy of the plaintext over the ciphertext (diffusion). The fourth contribution of this thesis is the algorithm HSPC2 - Homophonic Substitution Prefix Codes with 2 homophones. It is proposed a provably secure algorithm by using a homophonic substitution algorithm and a key. In the encoding process, the HSPC2 function appends a one bit suffx to some codes. A secret key and a homophonic rate parameters control this appending. It is shown that breaking HSPC2 is an NP-Complete problem.

Ler à primeira vista ao órgão é considerada uma tarefa complexa, visto que na execução do instrumento é necessário conciliar a leitura de três pentagramas e a utilização de pés e mãos simultaneamente. O objetivo desta pesquisa é investigar as abordagens apresentadas por organistas na tarefa de leitura à primeira vista com dois trechos contrastantes (texturas homofônica e polifônica). Estudos apontam que a diferença de textura pode influenciar o resultado na leitura à primeira vista. Outros objetivos incluem: investigar se aspectos de análise musical são utilizados conscientemente na prática da leitura à primeira vista; observar o desempenho em leitura à primeira vista com sujeitos de diferentes níveis de formação e/ou experiências musicais; identificar os aspectos observados pelos participantes referente à leitura e à autorreflexão do processo. Surgem as seguintes questões de pesquisa: o resultado da leitura à primeira vista ao órgão é mais satisfatório no trecho homofônico ou no polifônico? A autoavaliação após assistir ao vídeo modifica a percepção de sua interpretação? Dez sujeitos de idades e níveis de formação musical distintos, vinculados à Universidade Federal do Rio Grande do Sul (UFRGS) foram selecionados. Utilizou-se questionário, filmagem das execuções dos dois trechos, duas entrevistas semiestruturadas, mostra dos vídeos das execuções para os participantes e avaliadores externos. Concluímos que a autoavaliação posterior à mostra do vídeo modificou a percepção da execução com a maioria dos participantes, mostrando-se uma ferramenta útil neste processo. Os níveis de formação não foram determinantes na textura homofônica, e a textura contrapontística foi considerada a mais difícil pela maioria. / Sight-reading on the organ is a complex task since this instrument requires one to reconcile reading in three staves while playing with feet and hands simultaneously. The purpose of this research is to investigate the approaches used by organists in a sight-reading exercise with two contrasting excerpts, one of homophonic and the other of polyphonic texture. Studies indicate that the difference in texture can influence the result of sight-reading tasks. Others goals include: to determine if aspects of musical analysis are utilized consciously during sight-reading; observe the performance in sight-reading with subjects of different levels of training and/or musical experiences; identify the aspects, observed by the participants relative to sight-reading and self-reflection. Thus, the research questions arise: Is the result of sight-reading on the organ more satisfactory in homophonic or polyphonic excerpts? Did the self-evaluation after watching the video modify the perception of their interpretations? Ten subjects of distinct ages and music backgrounds were selected, all linked to the Universidade Federal do Rio Grande do Sul (UFRGS). A questionnaire, videos of the performance of both excerpts, two semi-structured interviews and the exhibit of the videos to the participants were used. The audios of the performances were submitted to seven external evaluators. We concluded that self-evaluation after watching their videos modified the perception of the majority of the participants, proving to be a useful tool in this process. The musical background was not determinant in the homophonic texture, and the contrapuntal texture was considered more difficult among most participants.

Ler à primeira vista ao órgão é considerada uma tarefa complexa, visto que na execução do instrumento é necessário conciliar a leitura de três pentagramas e a utilização de pés e mãos simultaneamente. O objetivo desta pesquisa é investigar as abordagens apresentadas por organistas na tarefa de leitura à primeira vista com dois trechos contrastantes (texturas homofônica e polifônica). Estudos apontam que a diferença de textura pode influenciar o resultado na leitura à primeira vista. Outros objetivos incluem: investigar se aspectos de análise musical são utilizados conscientemente na prática da leitura à primeira vista; observar o desempenho em leitura à primeira vista com sujeitos de diferentes níveis de formação e/ou experiências musicais; identificar os aspectos observados pelos participantes referente à leitura e à autorreflexão do processo. Surgem as seguintes questões de pesquisa: o resultado da leitura à primeira vista ao órgão é mais satisfatório no trecho homofônico ou no polifônico? A autoavaliação após assistir ao vídeo modifica a percepção de sua interpretação? Dez sujeitos de idades e níveis de formação musical distintos, vinculados à Universidade Federal do Rio Grande do Sul (UFRGS) foram selecionados. Utilizou-se questionário, filmagem das execuções dos dois trechos, duas entrevistas semiestruturadas, mostra dos vídeos das execuções para os participantes e avaliadores externos. Concluímos que a autoavaliação posterior à mostra do vídeo modificou a percepção da execução com a maioria dos participantes, mostrando-se uma ferramenta útil neste processo. Os níveis de formação não foram determinantes na textura homofônica, e a textura contrapontística foi considerada a mais difícil pela maioria. / Sight-reading on the organ is a complex task since this instrument requires one to reconcile reading in three staves while playing with feet and hands simultaneously. The purpose of this research is to investigate the approaches used by organists in a sight-reading exercise with two contrasting excerpts, one of homophonic and the other of polyphonic texture. Studies indicate that the difference in texture can influence the result of sight-reading tasks. Others goals include: to determine if aspects of musical analysis are utilized consciously during sight-reading; observe the performance in sight-reading with subjects of different levels of training and/or musical experiences; identify the aspects, observed by the participants relative to sight-reading and self-reflection. Thus, the research questions arise: Is the result of sight-reading on the organ more satisfactory in homophonic or polyphonic excerpts? Did the self-evaluation after watching the video modify the perception of their interpretations? Ten subjects of distinct ages and music backgrounds were selected, all linked to the Universidade Federal do Rio Grande do Sul (UFRGS). A questionnaire, videos of the performance of both excerpts, two semi-structured interviews and the exhibit of the videos to the participants were used. The audios of the performances were submitted to seven external evaluators. We concluded that self-evaluation after watching their videos modified the perception of the majority of the participants, proving to be a useful tool in this process. The musical background was not determinant in the homophonic texture, and the contrapuntal texture was considered more difficult among most participants.

Ler à primeira vista ao órgão é considerada uma tarefa complexa, visto que na execução do instrumento é necessário conciliar a leitura de três pentagramas e a utilização de pés e mãos simultaneamente. O objetivo desta pesquisa é investigar as abordagens apresentadas por organistas na tarefa de leitura à primeira vista com dois trechos contrastantes (texturas homofônica e polifônica). Estudos apontam que a diferença de textura pode influenciar o resultado na leitura à primeira vista. Outros objetivos incluem: investigar se aspectos de análise musical são utilizados conscientemente na prática da leitura à primeira vista; observar o desempenho em leitura à primeira vista com sujeitos de diferentes níveis de formação e/ou experiências musicais; identificar os aspectos observados pelos participantes referente à leitura e à autorreflexão do processo. Surgem as seguintes questões de pesquisa: o resultado da leitura à primeira vista ao órgão é mais satisfatório no trecho homofônico ou no polifônico? A autoavaliação após assistir ao vídeo modifica a percepção de sua interpretação? Dez sujeitos de idades e níveis de formação musical distintos, vinculados à Universidade Federal do Rio Grande do Sul (UFRGS) foram selecionados. Utilizou-se questionário, filmagem das execuções dos dois trechos, duas entrevistas semiestruturadas, mostra dos vídeos das execuções para os participantes e avaliadores externos. Concluímos que a autoavaliação posterior à mostra do vídeo modificou a percepção da execução com a maioria dos participantes, mostrando-se uma ferramenta útil neste processo. Os níveis de formação não foram determinantes na textura homofônica, e a textura contrapontística foi considerada a mais difícil pela maioria. / Sight-reading on the organ is a complex task since this instrument requires one to reconcile reading in three staves while playing with feet and hands simultaneously. The purpose of this research is to investigate the approaches used by organists in a sight-reading exercise with two contrasting excerpts, one of homophonic and the other of polyphonic texture. Studies indicate that the difference in texture can influence the result of sight-reading tasks. Others goals include: to determine if aspects of musical analysis are utilized consciously during sight-reading; observe the performance in sight-reading with subjects of different levels of training and/or musical experiences; identify the aspects, observed by the participants relative to sight-reading and self-reflection. Thus, the research questions arise: Is the result of sight-reading on the organ more satisfactory in homophonic or polyphonic excerpts? Did the self-evaluation after watching the video modify the perception of their interpretations? Ten subjects of distinct ages and music backgrounds were selected, all linked to the Universidade Federal do Rio Grande do Sul (UFRGS). A questionnaire, videos of the performance of both excerpts, two semi-structured interviews and the exhibit of the videos to the participants were used. The audios of the performances were submitted to seven external evaluators. We concluded that self-evaluation after watching their videos modified the perception of the majority of the participants, proving to be a useful tool in this process. The musical background was not determinant in the homophonic texture, and the contrapuntal texture was considered more difficult among most participants.

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 Engineering / unrestricted

L'orthographe du français : usages et représentations d'adultes socio-différenciés. Approche pluridisciplinaire.Dans une perspective linguistique, psycholinguistique, sociolinguistique et didactique, qui intègre le plan cognitif et les facteurs sociaux, ce travail de thèse porte sur l'étude de la variation orthographique dans les pratiques et les représentations de l'orthographe du français d'usagers de la langue qui, par obligation professionnelle ou scolaire, écrivent quotidiennement : des scripteurs adultes en cours d'apprentissage (élèves de lycée professionnel) et des scripteurs experts (secrétaires, DRH et futurs enseignants du secondaire).Le questionnement de cette recherche sur l'enjeu social de l'orthographe se centre plus précisément sur la manière dont les usagers de la langue écrite, en difficulté ou en relative sécurité, se construisent des savoirs orthographiques et se représentent l'orthographe ; il pourrait se formuler en ces termes : Face au système complexe de l'écriture du français, quels sont les secteurs de fragilité ou d'insécurité orthographique des scripteurs experts et des scripteurs en difficulté ? En d'autres termes, ces secteurs recouvrent-ils les zones d'instabilité du code graphique ? Quels jugements les scripteurs portent-ils sur l'orthographe et sur leurs pratiques orthographiques, ainsi que sur les propositions d'une nouvelle réforme ?Les objectifs que recouvrent ces questions sont nombreux, cette recherche essaie d'apporter des éléments de réponse sur 1/ la variation orthographique dans les écrits formels afin de déterminer les secteurs de fragilité orthographique des usagers de la langue écrite ; 2/ les patrons ou schémas intériorisés, responsables des variantes orthographiques ; 3/ l'existence d'une demande sociale ou non, de la part des « professionnels » de l'orthographe, en matière d'une réforme orthographique.Somme toute, cette recherche permet de comprendre les difficultés orthographiques d'un certain nombre d'adolescents et d'adultes dans une société où l'on produit de plus en plus d'écrits et où la compétence de lecture et d'écriture est déterminante pour l'insertion sociale et professionnelle. Les analyses tant qualitatives et quantitatives menées ont permis de dégager un profil général de chacun des publics observés au regard des données croisées de leurs compétences orthographiques et de leurs représentations.Enfin, ce travail offre, en ouverture, une réflexion renouvelée sur l'orthographe actuelle et propose des suggestions dans une perspective de formation des enseignants à la variation orthographique - informer des tendances fortes repérées, apporter des outils théoriques pour favoriser les apprentissages. / Uses and Representations of French Spelling in Socially Differentiated Adults.A Multidisciplinary ApproachThis thesis examines orthographic variation from a linguistic, psycholinguistic, sociolinguistic and didactic perspective, integrating both cognitive and social factors. More specifically, it focuses on this variation in the French-spelling practices and representations of language users who use the written medium daily, due to either professional or educational obligations. The users in question are adult writers in the process of learning (sixth form students at a vocational secondary school) and expert writers (secretaries, human resources managers and future secondary school teachers). This research questions the social stakes of spelling, focusing more specifically on 1/ how users of written language, whether they have difficulties or are relatively secure in their uses, build orthographical knowledge and 2/ the representations they have of spelling. The key question could be expressed in the following terms: in the face of the complex French writing system, what are the areas of fragility or orthographical insecurity among expert writers and writers with difficulties? In other words, do these areas overlap with the areas of instability that exist within the French writing system? What judgments do these writers make regarding spelling and their own orthographic practice, and regarding suggestions for a new reform of the spelling system?These questions address a number of aims and this research attempts to provide some answers concerning 1/ spelling variation in formal writing, in order to determine areas of spelling fragility among users of written language; 2/ the internalised patterns or schemas that are responsible for spelling variants; 3/ the existence of social demand for spelling reform on the part of spelling “professionals”. In sum, this study allows an understanding of the spelling difficulties faced by a certain number of adults and adolescents in a society in which more and more written texts are produced and in which competence in reading and writing is a determining factor for social and professional integration. The qualitative and quantitative analyses conducted allow a general profile of each of the groups of writers observed to be drawn up, on the basis of cross-referenced data about their orthographic competence and representations. Finally, this work opens up the perspective of renewed reflection about current spelling and puts forward suggestions for training teachers in orthographic variation i.e. informing them about the strong tendencies identified and providing theoretical tools to foster learning.

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