Preserving transparency and accountability in optimistic fair exchange of digital signatures

Optimistic fair exchange (OFE) protocols are useful tools for two participants to fairly exchange items with the aid of a third party who is only involved if needed. A widely accepted requirement is that the third party\u27s involvement in the exchange must be transparent, to protect privacy and avoid bad publicity. At the same time, a dishonest third party would compromise the fairness of the exchange and the third party thus must be responsible for its behaviors. This is achieved in OFE protocols with another property called accountability. It is unfortunate that the accountability has never been formally studied in OFE since its introduction ten years ago. In this paper, we fill these gaps by giving the first complete definition of accountability in OFE where one of the exchanged items is a digital signature and a generic (also the first) design of OFE where transparency and accountability coexist

Verifiably encrypted cascade-instantiable blank signatures to secure progressive decision management

National Research Foundation (NRF) Singapore under NC

Special Signature Schemes and Key Agreement Protocols

This thesis is divided into two distinct parts. The first part of the thesis explores various deniable signature schemes and their applications. Such schemes do not bind a unique public key to a message, but rather specify a set of entities that could have created the signature, so each entity involved in the signature can deny having generated it. The main deniable signature schemes we examine are ring signature schemes. Ring signatures can be used to construct designated verifier signature schemes, which are closely related to designated verifier proof systems. We provide previously lacking formal definitions and security models for designated verifier proofs and signatures and examine their relationship to undeniable signature schemes. Ring signature schemes also have applications in the context of fair exchange of signatures. We introduce the notion of concurrent signatures, which can be constructed using ring signatures, and which provide a "near solution" to the problem of fair exchange. Concurrent signatures are more efficient than traditional solutions for fair exchange at the cost of some of the security guaranteed by traditional solutions. The second part of the thesis is concerned with the security of two-party key agreement protocols. It has traditionally been difficult to prove that a key agreement protocol satisfies a formal definition of security. A modular approach to constructing provably secure key agreement protocols was proposed, but the approach generally results in less efficient protocols. We examine the relationships between various well-known models of security and introduce a modular approach to the construction of proofs of security for key agreement protocols in such security models. Our approach simplifies the proof process, enabling us to provide proofs of security for several efficient key agreement protocols in the literature that were previously unproven

The construction of ambiguous optimistic fair exchange from designated confirmer signature without random oracles

Ambiguous Optimistic Fair Exchange (AOFE), introduced by Huang et al. in ASIACRYPT 2008, is an extension of OFE that enhances the fairness of the two communicating parties in the exchange of signatures. The first scheme was proven secure without random oracles while its partial signature contains dozens of group elements. Recently, interactive AOFE was introduced and the construction is more practical, where one partial signature only contains three group elements. It is based on the existence of Designated Confirmer Signature (DCS) with a special property where one is able to sample a confirmer signature efficiently from a signer\u27s signature space. Nevertheless, we note that there are only a few DCS schemes that have this special property. Security of the interactive AOFE construction relies on the q-Computational and Decisional Hidden Strong Diffie-Hellman assumptions. In this paper, we propose a new construction of interactive AOFE from DCS, where the underlying DCS is standard and does not require any special property. We also propose a new DCS construction. By applying our transformation from DCS to interactive AOFE, we build a concrete interactive AOFE which is secure under more standard number-theoretic assumptions, namely Strong Diffie-Hellman and Decision Linear assumptions, without random oracles. A partial signature of the interactive AOFE contains six group elements, while a full signature contains two only

The construction of ambiguous optimistic fair exchange from designated confirmer signature without random oracles

Ambiguous Optimistic Fair Exchange (AOFE), introduced by Huang et al. in ASIACRYPT 2008, is an extension of OFE that enhances the fairness of the two communicating parties in the exchange of signatures. The first scheme was proven secure without random oracles while its partial signature contains dozens of group elements. Recently, interactive AOFE was introduced and the construction is more practical, where one partial signature only contains three group elements. It is based on the existence of Designated Confirmer Signature (DCS) with a special property where one is able to sample a confirmer signature efficiently from a signer\u27s signature space. Nevertheless, we note that there are only a few DCS schemes that have this special property. Security of the interactive AOFE construction relies on the q-Computational and Decisional Hidden Strong Diffie-Hellman assumptions. In this paper, we propose a new construction of interactive AOFE from DCS, where the underlying DCS is standard and does not require any special property. We also propose a new DCS construction. By applying our transformation from DCS to interactive AOFE, we build a concrete interactive AOFE which is secure under more standard number-theoretic assumptions, namely Strong Diffie-Hellman and Decision Linear assumptions, without random oracles. A partial signature of the interactive AOFE contains six group elements, while a full signature contains two only

O stars and Wolf-Rayet stars

Basic information is given about O and Wolf-Rayet stars indicating how these stars are defined and what their chief observable properties are. Part 2 of the volume discussed four related themes pertaining to the hottest and most luminous stars. Presented are: an observational overview of the spectroscopic classification and extrinsic properties of O and Wolf-Rayet stars; the intrinsic parameters of luminosity, effective temperature, mass, and composition of the stars, and a discussion of their viability; stellar wind properties; and the related issues concerning the efforts of stellar radiation and wind on the immediate interstellar environment are presented