Visual cryptography, first introduced by Naor and Shamir, allows a secret (black and white) image to be encoded and distributed to a set of participants such that certain predefined sets of participants may reconstruct the image without any computation. In 2000, Blundo, De Santis, and Naor introduced a model for grey-level visual cryptography which is a generalization of visual cryptography for general access structures. Grey-level visual cryptography extends this model to include grey-scale images. Decoding is done by the human visual system. In this thesis we survey known results of grey-level visual cryptography and visual cryptography for general access structures. We extend several visual cryptography constructions to grey-level visual cryptography, and derive new results on the minimum possible pixel expansion for all possible access structures on at most four participants.

Visual cryptography, first introduced by Naor and Shamir, allows a secret (black and white) image to be encoded and distributed to a set of participants such that certain predefined sets of participants may reconstruct the image without any computation. In 2000, Blundo, De Santis, and Naor introduced a model for grey-level visual cryptography which is a generalization of visual cryptography for general access structures. Grey-level visual cryptography extends this model to include grey-scale images. Decoding is done by the human visual system. In this thesis we survey known results of grey-level visual cryptography and visual cryptography for general access structures. We extend several visual cryptography constructions to grey-level visual cryptography, and derive new results on the minimum possible pixel expansion for all possible access structures on at most four participants.

Secret Sharing refers to a method through which a secret key K can be shared among a group of authorized participants, such that when they come together later, they can figure out the secret key K to decrypt the encrypted message. Any group which is not authorized cannot determine the secret key K. Some of the important secret schemes are Shamir Threshold Scheme, Monotone Circuit Scheme, and Brickell Vector Space Scheme. Brikell’s vector space secret sharing construction requires the existence of a function from a set of participant P in to vector space Zdp, where p is a prime number and d is a positive number. There is no known algorithm to construct such a function in general. We developed an efficient algorithm to construct function for some special secret sharing scheme. We also give an algorithm to demonstrate how a secret sharing scheme can be used in visual cryptography.

Information security in the present era is becoming very important in communication and data storage. Data transferred from one party to another over an insecure channel (e.g., Internet) can be protected by cryptography. The encrypting technologies of traditional and modern cryptography are usually used to avoid the message from being disclosed. Public-key cryptography usually uses complex mathematical computations to scramble the message. / A digital signature is an important public-key primitive that performs the function of conventional handwritten signatures for entity authentication, data integrity, and non-repudiation, especially within the electronic commerce environment. Currently, most conventional digital signature schemes are based on mathematical hard problems. These mathematical algorithms require computers to perform the heavy and complex computations to generate and verify the keys and signatures. In 1995, Naor and Shamir proposed a visual cryptography (VC) for binary images. VC has high security and requires simple computations. The purpose of this thesis is to provide an alternative to the current digital signature technology. We introduce a new digital signature scheme based on the concept of a non-expansion visual cryptography. A visual digital signature scheme is a method to enable visual verification of the authenticity of an image in an insecure environment without the need to perform any complex computations. We proposed scheme generates visual shares and manipulates them using the simple Boolean operations OR rather than generating and computing large and long random integer values as in the conventional digital signature schemes currently in use.

A tecnologia atual não oferece prevenção contra cópia, adulteração ou plágio de uma imagem estática em meio digital sem autorização do verdadeiro autor. Dado que tais mal feitos não podem ser evitados, resta ao criador da obra original lutar a posteriori por seus direitos nos fóruns adequados (no tribunal, por exemplo). Na época da fotografia analógica com filme, o negativo poderia ser utilizado como prova. Hoje este recurso raramente está disponível e se faz necessária uma solução alternativa. A técnica de Marca d´Água é uma das possibilidades criptográficas existentes para apoiar o autor em sua defesa. O principio da Marca d´Água é o encapsulamento de informações relevantes, preferencialmente de forma imperceptível, na imagem a ser protegida. Tais informações, quando extraídas da imagem marcada, devem revelar o verdadeiro autor num processo de disputa. Soluções de Marca d´Água combinada com Criptografia Visual são encontradas na literatura. A principal vantagem deste caminho é a propriedade Imperceptível por segurança perfeita que a Marca d´Água assume quando tratada por Criptografia Visual. O segredo (neste caso, a Marca d´Água) é segmentado via Criptografia Visual em 2 transparências: uma delas é encapsulada na imagem a ser protegida e a outra é mantida pelo verdadeiro autor. Basta a sobreposição de tais transparências para que a Marca d´Água seja revelada. Nesta pesquisa propomos um novo método, denominado MACV, que combina Marca d´Água, Criptografia Visual e um algoritmo de hashing. O MACV apresenta, entre outras, as seguintes propriedades desejáveis de Marca d´Água: imperceptível por segurança perfeita, alta entropia, armazenamento na própria imagem e sem ambiguidade. Veremos em nossa pesquisa bibliográfica que há uma lacuna de soluções que apresentem, simultaneamente, todas estas propriedades. Esta lacuna torna o MACV único em sua categoria. / Current technology does not prevent copy, tamper or plagiarism of a still image in digital media without authorization of the true author. Since such misdeeds can not be avoided, the creator of the original work has to defend his rights a posteriori in the appropriate forum (in court, for example). A negative could be used as evidence in the era of analog photography. Currently this feature is rarely available and an alternative solution is needed. Watermark is one of the existing cryptographic possibilities to support the author in his defense. A Watermark is embedded into the image to be protected, preferably imperceptibly. Such information when extracted from the watermarked image should reveal the true author in a dispute process. Solutions using Watermark with Visual Cryptography are found in the literature. The main advantage of this way is the property Imperceptible by perfect secrecy that the Water- mark assumes when treated by Visual Cryptography. The key (in this case, the Watermark) is partitioned by Visual Cryptography in 2 transparencies: one is embedded in the image to be protected and the other is kept by the true author. The Watermark is revealed by overlapping such transparencies. In this research we propose a new method, called MACV, combining Watermark, Vi- sual Cryptography and a hashing algorithm. The MACV has, among others, the following desirable properties of Watermark: invisible by perfect secrecy, high entropy, stored into the image itself and unambiguously. We will see in our research that there is no solution that presents, simultaneously, all these properties. This fact makes the MACV the first one to show all these properties

A tecnologia atual não oferece prevenção contra cópia, adulteração ou plágio de uma imagem estática em meio digital sem autorização do verdadeiro autor. Dado que tais mal feitos não podem ser evitados, resta ao criador da obra original lutar a posteriori por seus direitos nos fóruns adequados (no tribunal, por exemplo). Na época da fotografia analógica com filme, o negativo poderia ser utilizado como prova. Hoje este recurso raramente está disponível e se faz necessária uma solução alternativa. A técnica de Marca d´Água é uma das possibilidades criptográficas existentes para apoiar o autor em sua defesa. O principio da Marca d´Água é o encapsulamento de informações relevantes, preferencialmente de forma imperceptível, na imagem a ser protegida. Tais informações, quando extraídas da imagem marcada, devem revelar o verdadeiro autor num processo de disputa. Soluções de Marca d´Água combinada com Criptografia Visual são encontradas na literatura. A principal vantagem deste caminho é a propriedade Imperceptível por segurança perfeita que a Marca d´Água assume quando tratada por Criptografia Visual. O segredo (neste caso, a Marca d´Água) é segmentado via Criptografia Visual em 2 transparências: uma delas é encapsulada na imagem a ser protegida e a outra é mantida pelo verdadeiro autor. Basta a sobreposição de tais transparências para que a Marca d´Água seja revelada. Nesta pesquisa propomos um novo método, denominado MACV, que combina Marca d´Água, Criptografia Visual e um algoritmo de hashing. O MACV apresenta, entre outras, as seguintes propriedades desejáveis de Marca d´Água: imperceptível por segurança perfeita, alta entropia, armazenamento na própria imagem e sem ambiguidade. Veremos em nossa pesquisa bibliográfica que há uma lacuna de soluções que apresentem, simultaneamente, todas estas propriedades. Esta lacuna torna o MACV único em sua categoria. / Current technology does not prevent copy, tamper or plagiarism of a still image in digital media without authorization of the true author. Since such misdeeds can not be avoided, the creator of the original work has to defend his rights a posteriori in the appropriate forum (in court, for example). A negative could be used as evidence in the era of analog photography. Currently this feature is rarely available and an alternative solution is needed. Watermark is one of the existing cryptographic possibilities to support the author in his defense. A Watermark is embedded into the image to be protected, preferably imperceptibly. Such information when extracted from the watermarked image should reveal the true author in a dispute process. Solutions using Watermark with Visual Cryptography are found in the literature. The main advantage of this way is the property Imperceptible by perfect secrecy that the Water- mark assumes when treated by Visual Cryptography. The key (in this case, the Watermark) is partitioned by Visual Cryptography in 2 transparencies: one is embedded in the image to be protected and the other is kept by the true author. The Watermark is revealed by overlapping such transparencies. In this research we propose a new method, called MACV, combining Watermark, Vi- sual Cryptography and a hashing algorithm. The MACV has, among others, the following desirable properties of Watermark: invisible by perfect secrecy, high entropy, stored into the image itself and unambiguously. We will see in our research that there is no solution that presents, simultaneously, all these properties. This fact makes the MACV the first one to show all these properties

La cryptographie visuelle consiste à partager entre plusieurs « Shadow Images » (SIs) un secret qui ne se révèlera à l'oeil de l'observateur qu'à leur superposition. Depuis les travaux de Naor et Shamir, ce procédé cryptographique a été étendu à Des schémas numériques variés, adaptés à diverses problématiques. En revanche, les travaux concernant son implémentation physique sont peu nombreux à ce jour. Cette thèse est consacrée à l'implémentation de la cryptographie visuelle sur des SIs imprimés en demi-tons en vue de l'authentification de documents. Le SI associé au document peut être imprimé sur support opaque ou transparent, les autres SIs étant imprimés sur films transparents. Nous avons résolu la difficulté de leur superposition par une méthode de Fourier permettant le recalage de la structure quasi-périodique d'un SI. La précision de cette méthode nous a permis de développer un système optique de superposition par projection. On verra que les phénomènes physiques responsables du rendu visuel de SIs superposés sont propices à une protection contre la copie illicite du SI associé à un document. La complexité de ces phénomènes et leur dépendance au type d'impression imposent une modélisation physique pour obtenir un rendu précis. Cette approche nous a conduit à considérer la problématique de la reproduction des couleurs et à développer des modèles spectraux adaptés à la superposition de supports imprimés non diffusants et/ou diffusants, en réflexion et en transmission. La précision de ces modèles prédictifs est tout à fait satisfaisante au regard de celle habituellement obtenue dans le domaine de la reproduction des couleurs. Cela nous a permis d'introduire une approche originale de la cryptographie visuelle par ajustement de couleur (color matching) : une même couleur, à une tolérance près basée sur la vision humaine, est générée par différents demi-tons imprimés sur les supports à superposer. La couleur du message peut ainsi constituer un secret partagé entre les SIs de la même façon que l'est le contenu du message. Chaque SI pris individuellement ne laisse fuir aucune information sur la couleur du message, qui ne sera révélée qu'à leur superposition. Cela peut permettre de prévenir une attaque par falsification du SI associé au document (cheating attack ). De plus, le rendu des couleurs étant très dépendant du système d'impression utilisé, une reproduction fidèle à partir d'un système d'impression différent est difficile. La difficulté peut être encore accrue par l'utilisation de caractéristiques d'impression non standard / In this thesis, we will focus on the physical implementation of visual cryptography, which consists in sharing a secret message between several unmeaning images, so-called shadow images, at least one of them being printed. By the principle of the method, no information leaks about the message until the images are properly stacked together. As the alignment of the shadow images hampers the deployment of the visual cryptography in practice, we develop a dedicated image registration method. In contrast with existing methods, ours is not intrusive. We make use of the particular shape of the elementary constituents of the shadow images, the shares, to extract in the Fourier domain the main parameters of the geometrical transformations occurring between the superposed images. We prove that this method allows subpixel accuracy in shadow images registration. We benefit from such ability by implementing visual cryptography in an image projection configuration : the digital shadow image is projected onto the printed one. In this way, the registration is performed automatically by using a digital camera (the resulting superposition being observable by the eye). For the purpose of authentication, one has to deal with specific attacks: the shadow image attached to a given document could be tampered with or copied. In order to prevent such attacks, we have increased the di_culty to reproduce the shadow image by considering color. This approach requires a complete management of colors. Thanks to recent advances in color reproduction, we are able to predict the reflectance and transmittance spectra of supports printed in color. In this thesis, we develop new spectral prediction models namely for piles of printed transparencies as well as for transparencies stacked onto papers, all printed in color. Thus, we are able to predict the color of each share in a shadow image to be printed and to achieve color matching i.e. we are able to reach a color by various combinations of superposed colors. Such a prediction allowed us to introduce a new approach in visual cryptography: color matching when revealing the secret message to be shared into two (or more) shadow images, in order to authenticate the shadow images provider. As the prediction models are sensitive to the calibration of the printing system (printer, inks, supports, halftoning and geometry measurement conditions), the use of special materials will increase the di_culty to generate visually acceptable fake pairs of shadow images

Submitted by JÚLIO HEBER SILVA (julioheber@yahoo.com.br) on 2017-03-24T17:48:37Z No. of bitstreams: 2 Dissertação - Alan Henrique Ferreira Silva - 2016.pdf: 10881029 bytes, checksum: 1c411277f8b103cc8a55709053ed7f9b (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2017-03-27T15:13:01Z (GMT) No. of bitstreams: 2 Dissertação - Alan Henrique Ferreira Silva - 2016.pdf: 10881029 bytes, checksum: 1c411277f8b103cc8a55709053ed7f9b (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2017-03-27T15:13:01Z (GMT). No. of bitstreams: 2 Dissertação - Alan Henrique Ferreira Silva - 2016.pdf: 10881029 bytes, checksum: 1c411277f8b103cc8a55709053ed7f9b (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2016-03-31 / This work proposes method to encode and decode imas using conformal mapping. Conformal mapping modifies domains without modifyung physical characteristics between them. Real images are processed between these domains using encoding keys, also called transforming functions. The advantage of this methodology is the ability to carry the message as an encoded image in printed media for posterior-decoding. / Este trabalho propõe método que utiliza transformações conformes para codificar e decodificar imagens. As transformações conformes modificam os domínios em estudos sem modificar as características físicas entre eles. As imagens reais são transformadas entre estes domínios utilizando chaves, que são funções transformadoras. o diferencial desta metodologia é a capacidade de transportar a mensagem contida na imagem em meio impresso codificado e depois, decodificá-la.

Orientador: Julio Cesar López Hernández / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Computação / Made available in DSpace on 2018-08-27T12:14:05Z (GMT). No. of bitstreams: 1 Pietz_Franz_M.pdf: 27442530 bytes, checksum: 1648252389eb63cf26ca0525be124bda (MD5) Previous issue date: 2014 / Resumo: A criptografia visual é um método de compartilhamento de segredos proposto por Naor em Shamir no artigo ''Criptografia Visual'' de 1994. Nele, uma imagem secreta é dividida em um conjunto de parcelas, sendo necessário sobrepor um número mínimo de parcelas para decodificarmos o segredo visualmente, sem nenhum tipo de dispositivo ou cálculo criptográfico; e analisando as parcelas isoladamente, não é possível recuperar nenhuma informação sobre a imagem secreta original. O esquema é considerado seguro e pode ser comparado com as cifras de one-time-pad, também chamadas de cifras perfeitas, devido à dificuldade do atacante obter o segredo ou parte dele. Existem propostas para a utilização de criptografia visual em protocolos de autenticação, como autenticação de transações bancárias e verificação de legitimidade de produtos. Entretanto, esse método possui problemas como definição do segredo recuperado, baixo contraste e desvios de alinhamento, que é o problema mais sensível. Nossa proposta mostra como utilizar um dispositivo móvel, como smartphone ou tablet, para realizar o alinhamento automático de parcelas e auxiliar o usuário no processo de recuperação de segredos encriptados utilizando criptografia visual. Para isso, utilizamos a câmera do dispositivo móvel para torná-lo uma ''transparência'' e técnicas de análise de imagens para localizar uma parcela exibida em um monitor ou impressa na embalagem de um produto, e sobrepô-la com uma parcela presente no dispositivo móvel, permitindo a visualização do segredo recuperado na tela do dispositivo. A utilização de um dispositivo móvel traz vantagens imediatas, como facilidade para a entrega de parcelas no momento da transação, sem necessidade de guardar informação previamente / Abstract: Visual cryptography is a secret sharing method proposed by Naor and Shamir in the paper ''Visual Cryptography'', in 1994. It split a secret image into a set of shares, so that we need to stack a minimum number of shares to visually decode the secret image without the help of hardware or computation, and analyzing the shares alone is not possible to obtain any information about the secret image. The scheme is considered safe and can be compared to the one-time-pad cyphers, also called perfect cyphers, due to the difficulty of an attacker to obtain the secret or part of it. There are proposals to use visual cryptography in authentication protocols, such as in bank transactions and product's legitimacy verification. But these methods have problems with recovered secret's definition, low contrast and misalignment of the shares, which is the most sensitive. Our proposal shows how to use a mobile device, such as smartphone or tablet, to perform automatic alignment of the shares and to assist a user to recover a secret encrypted using visual cryptography. For this, we use the device camera to turn it into a ''transparency'' and image analysis techniques to locate a share that can be displayed on a monitor or printed on the packaging of a product, and overlay it with a second share present on the mobile device, allowing the visualization of the recovered secret on the device's display. Using a mobile device brings immediate advantages, such as easy delivery of shares at the transaction's time, without having to store information in advance / Mestrado / Ciência da Computação / Mestre em Ciência da Computação