A double-spend attack is one of the major security issues in most blockchain systems, but it is difficult to successfully launch unless an adversary has massive computing power. In this paper, we introduce a new attack model that combines a double-spend attack with a Sybil attack in the Bitcoin network. We present analysis results that a double spending attacker can make a block propagation delay by conducting Sybil attacks and increase the probability of winning the mining race, thus successfully launching the double-spend attack. We develop the probability of success of this new attack in mathematics forms and analyze this attack model from the perspective of economics. We present the attacker's break-even point in various situations and demonstrate the effect of the proposed attack.

Blockchain technology has been known as the underlying technology of cryptocurrencies, but nowadays it is further considered as a functional technology for improving existing technologies and creating new applications previously never practical. In this paper, we are focused on utilizing blockchain technology to introduce a new ID as a service (IDaaS) for digital identity management. The proposed blockchain-based ID as a service (BIDaaS) is explained with one practical example that shows how the proposed BIDaaS works as an identity and authentication management infrastructure for mobile users of a mobile telecommunication company.

Blockchain-based mobile edge computing (BMEC) is an innovative architecture, which is essential in the future fifth-generation (5G) network era. BMEC solves the problem of limited computing resources of devices in the mobile blockchain environment while ensuring the distributed deployment of computing resources and the traceable of transaction data. However, the blockchain technology has many security vulnerabilities, among which attacks against consensus algorithms are particularly serious, i.e., double-spend attacks, long-range attacks, selfish mining. All of these attacks can break the integrity of BMEC, allowing the correct block record to be overwritten with a false one. In this paper, we propose a group signature scheme designed for validating blocks of blockchain to address such issues. Each new block will be regarded as a valid block if it obtains a valid group aggregate signature of the group which the block creator belongs to. We describe in detail the process of authentication and key changes when mobile devices join and leave BMEC. We also provide a more efficient authentication scheme of authenticating mobile devices compared to traditional schemes in BMEC. Lastly, the security analysis is presented to prove that our proposed group signature scheme is effective.