Thesis (M. Soc. Sc.)--University of Hong Kong, 1993. / Includes bibliographical references (leaves 144-150).

Thesis (M. Sc.)--University of Hong Kong, 1996. / Includes bibliographical references (leaf [64-67]).

Thesis (M. Ed.)--University of Hong Kong, 1992. / Includes bibliographical references.

Thesis (M.Ed.)--University of Hong Kong, 1992. / Includes bibliographical references. Also available in print.

Thesis (M.P.A.)--University of Hong Kong, 1990. / Also available in print.

Thesis (M.Soc.Sc.)--University of Hong Kong, 1993. / Includes bibliographical references (leaves 144-150) Also available in print.

In the latest years, the public’s interest in blockchain technology has been growing since it was brought up in 2008, primarily because of its ability to create an immutable ledger, for storing information that never will or can be changed. As an expanding chain structure, the act of nodes adding blocks to the chain is called mining which is regulated by consensus mechanism. In the most widely used consensus mechanism Proof of work, this process is based on computationally heavy guessing of hashes of blocks. Today, there are several prominent ways developed of performing this guessing, thanks to the development of hardware technology, either using the regular all-rounded computer processing unit (CPU), or using the more specialized graphics processing unit (GPU), or using dedicated hardware. This thesis studied the working principles of blockchain, implemented the crucial hash function used in Proof of Work consensus mechanism and other blockchain structures with the popular programming language Java on various platforms. CPU implementation is done with Java’s built-in functions and for GPU I used OpenCL ’ s Java binding JOCL. This project gives a quantified measurement for hash rate on different devices, determines that all the GPUs tested advantage over CPUs in performance and memory consumption. Java’s built-in function is easier to use but both of the implementations are doing well in platform independent that the same code can easily be executed on different platforms. Furthermore, based on the measurements, I did in-depth exploration of the principles and proposed future work, analyzed their application values combined with future possibilities of blockchain based on implementation difficulties and performance.

The emerging topic, symmetric encryption technique for digital imagesusing chaotic function and DNA bases has recently drawn a lot ofattentions from the research community. The use of voice and visualmaterial as an addition to textual data has become more widespread.Multimedia content, the core of which is the digital image, has becomean earth-shaking element in prospective information transmission. Thismeans that multimedia contents such as audio, video and images haveto be transmitted over communication channels securely using newencryption techniques.Chaos theory is a widely studied topic in non-linear dynamics becauseof features like periodicity, sensitivity to initial conditions and controlparameters. The lower dimensional discrete chaotic functions are simpleand efficient to generate a pseudo-random sequence for encryption.However, DNA cryptography is an additional weapon in thecryptographer’s armoury. Characteristics such as of vast parallelism andlarge storage capacity make it a very promising field. The algorithmsbased on DNA and discrete chaotic functions use the advantages of bothfields to provide image protection in an effective way.Traditional digital cryptosystems like DES, AES and RSA are designedprimarily for textual data, which does not fit the multimedia streambecause of the essential structures of image data such as large size, solidpixel correlation and high redundancy, which incurs significantoverhead. Also, in conventional cryptographic techniques, thedecrypted data is exactly the same as plaintext data.To solve this problem, symmetric encryption aims to provide sufficientsecurity with an important gain in computational complexity anddelays, thereby making the cipher suitable for real time applications.This study uses an encryption approach by adding adjacent rows andcolumns of plain text after converting into DNA bases in order to reducethe encryption overhead. To improve the use of DNA complementary rules in a better and moredynamic way, and to incorporate DNA diffusion prior to the encryptionalgorithm, a complete set of DNA complementary rules is employed inthe encryption and decryption process. The selection of DNA rules foreach block is made depending on the chaotic sequence. The initialconditions and control parameters of the chaotic maps are madedepending on the 256-bits hash value which is generated from a plainimage to avoid chosen plaintext attacks.

This paper describes implementation of the whole cryptographic protocol TLS including control logic and used cryptographic systems. The goal is to implement an application in the FPGA technology, so it could be used in hardware accelerated network card. The reason for this is new supported higher transmission speeds that Ethernet is able to operate on, and the absence of implementation of this protocol on FPGA. In the first half of this paper is described theory of cryptography followed by description of TLS protocol, its development, structure and operating workflow. The second half describes the implementation on the chosen technology that is also described here. It is used already existing solutions of given cryptographic systems for the implementation, or at least their parts that are modified if needed for TLS. It was implemented just several parts of whole protocol, such are RSA, Diffie-Hellman, SHA and part of AES. Based on these implementations and continuing studying in this matter it was made conclusion, that FPGA technology is inappropriate for implementation of TLS protocol and its control logic. Recommendation was also made to use FPGA only for making calculations of given cryptographic systems that are controlled by control logic from software implemented on standard processors.

This thesis concerns with hash functions and their usage in authentication. It presents basics of hash functions theory and construction elements. In particular the thesis focuses on LMHash, MD4, MD5 and SHA family hash functions, which are compared from the security point of view. The thesis describes in general the most frequently used hash function attacks, points out the weaknesses of current construction and mentions the future perspective of hash functions. Furthermore the thesis outlines the area authentication and describes usage of hash functions in the area. Practical part of the thesis contains an implements of a general authentication framework implemented in programming language C#. The result is client and server applications, in which two selected authentication methods were successfully tested. The result implementation is flexible with respect to the possible future use of other authentication methods.