Various Aspects of Digital Cash

In this thesis, we study various aspects of digital cash systems. In particular, we concentrate on two schemes, one due to Okamoto-Ohta and the other due to Eng-Okamoto. The main part of the thesis is devoted to providing more detailed explanations of the issues of divisibility and prevention of double-spending. We then study a few additional systems to explain other aspects of electronic cash

SHAREDWEALTH: A CRYPTOCURRENCY TO REWARD MINERS EVENLY

Bitcoin [19] is a decentralized cryptocurrency that has recently gained popularity and has emerged as a popular medium of exchange. The total market capitalization is around 1.5 billion US dollars as of October 2013 [28]. All the operations of Bitcoin are maintained in a distributed public global ledger known as a block chain which consists of all the successful transactions that have ever taken place. The security of a block chain is maintained by a chain of cryptographic puzzles solved by participants called miners, who in return are rewarded with bitcoins. To be successful, the miner has to put in his resources to solve the cryptographic puzzle (also known as a proof of work). The reward structure is an incentive for miners to contribute their computational resources and is also essential to the currency\u27s decentralized nature. One disadvantage of the reward structure is that the payment system is uneven. The reward is always given to one person. Hence people form mining pools where every member of the pool solves the same cryptographic puzzle and irrespective of the person who solved it, the reward is shared evenly among all the members of the pool. The Bitcoin protocol assumes that the miners are honest and they follow the Bitcoin protocol as prescribed. If group of selfish miners comes to lead by forming pools, the currency stops being decentralized and comes under the control of the selfish miners. Such miners can control the whole Bitcoin network [29]. Our goal is to address this problem by creating a distinct peer-to-peer protocol that reduces the incentives for the miners to join large mining pools. The central idea is to pay the “runners-up” who come close to finding a proof, thereby creating a less volatile payout situation. The work done by the “runners-up” can be used by other miners to find the solution of proof of work by building upon their work. Once they find the actual solution they have to include the solution of the other miner in order to get rewarded. The benefit of this protocol is that not only the miners save their computational resources but also the reward is distributed among the miners

Improvement of a convertible undeniable partially blind signature scheme

Undeniable signatures are the digital signatures that should be verified with the help of the signer. A signer may disavow a genuine document, if the signature is only verifiable with the aid of the signer under the condition that the signer is not honest. Undeniable signatures solve this problem by adding a new feature called the disavowal protocol in addition to the normal components of signature and verification. Disavowal protocol is able to prevent a dishonest signer from disavowing a valid signature. In some situations, an undeniable signature should be converted into a normal digital signature in order that the signature can be universally verified. Blind signatures are the digital signatures that help a user to get a signature on a message without revealing the content of the message to a signer. For the blind signatures, if the signer is able to make an agreement with the user, then the underlying signer may include some common information that is known to the user, then such signatures are partially blind signatures. Convertible undeniable partially blind signatures are of the features of undeniable signatures, blind signatures, convertible undeniable signatures, and partially blind signatures. Recently, a convertible undeniable partially blind signature scheme was presented. In this paper, we first analyse a security flaw of the convertible undeniable partially blind signature scheme. To address the security flaw, we present an improvement on the disavowal protocol. The improved scheme can prevent the signer from either proving that a given valid signature as invalid, or cheating the verifier

Year 2010 Issues on Cryptographic Algorithms

In the financial sector, cryptographic algorithms are used as fundamental techniques for assuring confidentiality and integrity of data used in financial transactions and for authenticating entities involved in the transactions. Currently, the most widely used algorithms appear to be two-key triple DES and RC4 for symmetric ciphers, RSA with a 1024-bit key for an asymmetric cipher and a digital signature, and SHA-1 for a hash function according to international standards and guidelines related to the financial transactions. However, according to academic papers and reports regarding the security evaluation for such algorithms, it is difficult to ensure enough security by using the algorithms for a long time period, such as 10 or 15 years, due to advances in cryptanalysis techniques, improvement of computing power, and so on. To enhance the transition to more secure ones, National Institute of Standards and Technology (NIST) of the United States describes in various guidelines that NIST will no longer approve two-key triple DES, RSA with a 1024-bit key, and SHA-1 as the algorithms suitable for IT systems of the U.S. Federal Government after 2010. It is an important issue how to advance the transition of the algorithms in the financial sector. This paper refers to issues regarding the transition as Year 2010 issues in cryptographic algorithms. To successfully complete the transition by 2010, the deadline set by NIST, it is necessary for financial institutions to begin discussing the issues at the earliest possible date. This paper summarizes security evaluation results of the current algorithms, and describes Year 2010 issues, their impact on the financial industry, and the transition plan announced by NIST. This paper also shows several points to be discussed when dealing with Year 2010 issues.Cryptographic algorithm; Symmetric cipher; Asymmetric cipher; Security; Year 2010 issues; Hash function

Virtual money, virtual control?: electronic money, electronic cash and governance

The modern monetary system is comprised of a number of different types of money, many of which are in forms connected with developing information and communications technologies. This category of money is generally referred to as electronic money. This thesis explores whether these new forms of money are in part responsible for the apparently changing abilities of central banks to govern monetary policy. Lastly, I seek to determine whether theorized trends of money toward electronic cash are likely, and if so, what sort of impact they will have on central banks\u27 monetary policy efficacy

Theoretical examination and practical implementation on cryptography algorithms, digital money protocols and related applications.

by Shek Wong.Thesis submitted in: December 1997.Thesis (M.Phil.)--Chinese University of Hong Kong, 1998.Includes bibliographical references (leaves 90-[94]).Abstract also in Chinese.Chapter 1 --- Introduction --- p.1Chapter 1.1 --- Electronic Commerce --- p.3Chapter 1.2 --- Electronic Cash --- p.7Chapter 1.3 --- What This Report Contains --- p.9Chapter 2 --- Cryptographic Background --- p.11Chapter 2.1 --- Euler Totient Function --- p.12Chapter 2.2 --- Fermat's Little Theorem --- p.12Chapter 2.3 --- Quadratic Residues --- p.12Chapter 2.4 --- Legendre Symbol --- p.13Chapter 2.5 --- Jacobi Symbol --- p.14Chapter 2.6 --- Blum Integer --- p.16Chapter 2.7 --- Williams Integer --- p.18Chapter 2.8 --- The Quadratic Residuosity Problem --- p.19Chapter 2.9 --- The Factorization Problem --- p.20Chapter 2.10 --- The Discrete Logarithm Problem --- p.20Chapter 2.11 --- One-way Functions --- p.21Chapter 2.12 --- Blind Signature --- p.22Chapter 2.13 --- Cut-and-choose Methodology --- p.24Chapter 3 --- Anatomy and Panorama of Electronic Cash --- p.26Chapter 3.1 --- Anatomy of Electronic Cash --- p.26Chapter 3.1.1 --- Three Functions and Six Criteria --- p.28Chapter 3.1.2 --- Untraceable --- p.29Chapter 3.1.3 --- Online and Off-line --- p.30Chapter 3.1.4 --- Security --- p.32Chapter 3.1.5 --- Transferability --- p.33Chapter 3.2 --- Panorama of Electronic Cash --- p.34Chapter 3.2.1 --- First Model of Off-line Electronic Cash --- p.34Chapter 3.2.2 --- Successors --- p.35Chapter 3.2.3 --- Binary Tree Based Divisible Electronic Cash --- p.36Chapter 4 --- Spending Limit Enforced Electronic Cash --- p.37Chapter 4.1 --- Introduction to Spending Limit Enforced Electronic Cash --- p.37Chapter 4.2 --- The Scheme --- p.41Chapter 4.3 --- An Example --- p.44Chapter 4.4 --- Techniques --- p.47Chapter 4.5 --- Security and Efficiency --- p.51Chapter 5 --- Interest-bearing Electronic Cash --- p.53Chapter 5.1 --- Introduction to Interest-bearing Electronic Cash --- p.53Chapter 5.2 --- An Example --- p.55Chapter 5.3 --- The Scheme --- p.55Chapter 5.4 --- Security --- p.57Chapter 5.5 --- An Integrated Scheme --- p.58Chapter 5.6 --- Applications --- p.59Chapter 6 --- Abacus Type Electronic Cash --- p.61Chapter 6.1 --- Introduction --- p.61Chapter 6.2 --- Abacus Model --- p.63Chapter 6.3 --- Divisible Abacus Electronic Coins --- p.66Chapter 6.3.1 --- Binary Tree Abacus Approach --- p.66Chapter 6.3.2 --- Multi-tree Approach --- p.57Chapter 6.3.3 --- Analysis --- p.69Chapter 6.4 --- Abacus Electronic Cash System --- p.71Chapter 6.4.1 --- Opening Protocol --- p.71Chapter 6.4.2 --- Withdrawal Protocol --- p.74Chapter 6.4.3 --- Payment and Deposit Protocol --- p.75Chapter 6.5 --- Anonymity and System Efficiency --- p.78Chapter 7 --- Conclusions --- p.80Chapter A --- Internet Payment Systems --- p.82Chapter A.1 --- Bare Web FORM --- p.82Chapter A.2 --- Secure Web FORM Payment System --- p.85Chapter A.3 --- Membership Type Payment System --- p.86Chapter A.4 --- Agent Based Payment System --- p.87Chapter A.5 --- Internet-based POS --- p.87B Papers derived from this thesis --- p.89Bibliography --- p.9