The 51% attack is a technique which intends to fork a blockchain in order to conduct double-spending. Adversaries controlling more than half of the total hashing power of a network can perform this attack. In a similar way, n confirmation and selfish mining are two attack techniques that comprise a similar strategy to the 51% attack. Due to the immense attacking cost to perform the 51% attack, it was considered very unlikely for a long period. However, in recent times, the attack has befallen at a frequent pace, costing millions of dollars to various cryptocurrencies. The 51% attack strategy varies based upon the adopted consensus mechanism by a particular cryptocurrency, and it enables attackers to double-spend the same crypto-coin, restrict transactions, cancel blocks, and even have full control over the price of a cryptocurrency. A crypto-coin with a low hashing power is always jeopardized by the 51% attack due to the easily attainable hashing. In this paper, we analyze the real impact of the 51% attack, revealing serious weaknesses in consensus protocols that made this attack possible. We discuss the five most advanced protection techniques to prevent this attack and their main limitations. We conclude that in most cases, security techniques fail to provide real protection against the 51% attack because the weaknesses are inherited from the consensus protocols.

Smart contracts are programs that reside within decentralized blockchains and are executed pursuant to triggered instructions. A smart contract acts in a similar way to a traditional agreement but negates the necessity for the involvement of a third party. Smart contracts are capable of initiating their commands automatically, thus eliminating the involvement of a regulatory body. As a consequence of blockchain’s immutable feature, smart contracts are developed in a manner that is distinct from traditional software. Once deployed to the blockchain, a smart contract cannot be modified or updated for security patches, thus encouraging developers to implement strong security strategies before deployment in order to avoid potential exploitation at a later time. However, the most recent dreadful attacks and the multifarious existing vulnerabilities which result as a consequence of the absence of security patches have challenged the sustainability of this technology. Attacks such as the Decentralized Autonomous Organization (DAO) attack and the Parity Wallet hack have cost millions of dollars simply as a consequence of naïve bugs in the smart contract code. In this paper, we classify blockchain exploitation techniques into 4 categories based on the attack rationale; attacking consensus protocols, bugs in the smart contract, malware running in the operating system, and fraudulent users. We then focus on smart contract vulnerabilities, analyzing the 7 most important attack techniques to determine the real impact on smart contract technology. We reveal that even adopting the 10 most widely used tools to detect smart contract vulnerabilities, these still contain known vulnerabilities, providing a dangerously false sense of security. We conclude the paper with a discussion about recommendations and future research lines to progress towards a secure smart contract solution.

Identity management (IdM) is a method used to determine user identities. The centralized aspect of IdM introduces a serious concern with the growing value of personal information, as well as with the General Data Protection Regulation (GDPR). The problem with currently-deployed systems and their dominating approach, with identity providers (IdP) and single-point services, is that a third party is in charge of maintaining and controlling the personal data. The main challenge to manage data securely lies in trusting humans and institutes who are responsible for controlling the entire activity. Identities are not owned by the rightful owners or the user him/herself, but by the mentioned providers. With the rise of blockchain technology, self-sovereign identities are in place utilizing decentralization; unfortunately, the flaws still exist. In this research, we propose DNS-IdM, a smart contract-based identity management system that enables users to maintain their identities associated with certain attributes, accomplishing the self-sovereign concept. DNS-IdM has promising outcomes in terms of security and privacy. Due to the decentralized nature, DNS-IdM is able to avoid not only the conventional security threats, but also the limitations of the current decentralized identity management systems.

E-governance is a process that aims to enhance a government’s ability to simplify all the processes that may involve government, citizens, businesses, and so on. The rapid evolution of digital technologies has often created the necessity for the establishment of an e-Governance model. There is often a need for an inclusive e-governance model with integrated multiactor governance services and where a single market approach can be adopted. e-Governance often aims to minimise bureaucratic processes, while at the same time including a digital-by-default approach to public services. This aims at administrative efficiency and the reduction of bureaucratic processes. It can also improve government capabilities, and enhances trust and security, which brings confidence in governmental transactions. However, solid implementations of a distributed data sharing model within an e-governance architecture is far from a reality; hence, citizens of European countries often go through the tedious process of having their confidential information verified. This paper focuses on the sinGLe sign-on e-GovernAnce Paradigm based on a distributed file-exchange network for security, transparency, cost-effectiveness and trust (GLASS) model, which aims to ensure that a citizen can control their relationship with governmental agencies. The paper thus proposes an approach that integrates a permissioned blockchain with the InterPlanetary File System (IPFS). This method demonstrates how we may encrypt and store verifiable credentials of the GLASS ecosystem, such as academic awards, ID documents and so on, within IPFS in a secure manner and thus only allow trusted users to read a blockchain record, and obtain the encryption key. This allows for the decryption of a given verifiable credential that stored on IPFS. This paper outlines the creation of a demonstrator that proves the principles of the GLASS approach.

The blockchain is a distributed ledger technology that is growing in importance since inception. Besides cryptocurrencies, it has also crossed its boundary inspiring various organizations, enterprises, or business establishments to adopt this technology benefiting from the most innovative security features. The decentralized and immutable aspects have been the key points that endorse blockchain as one of the most secure technologies at the present time. However, in recent times such features seemed to be faded due to new attacking techniques. One of the biggest challenges remains within the consensus protocol itself, which is an essential component to bring all network participants to an agreed state. Cryptocurrencies adopt suitable consensus protocols based on their mining requirement, and Proof of Work (PoW) is the consensus protocol that is being predominated in major cryptocurrencies. Recent consensus protocol-based attacks, such as the 51% attack, Selfish Mining, Miner Bribe Attack, Zero Confirmation Attack, and One Confirmation Attack have been demonstrated feasible. To overcome these attacks, we propose Proof of Adjourn (PoAj), a novel consensus protocol that provides strong protection regardless of attackers hashing capability. After analyzing the 5 major attacks, and current protection techniques indicating the causes of their failure, we compared the PoAj against the most widely used PoW, showing that PoAj is not only able to mitigate the 5 attacks but also attacks relying on having a large amount of hashing power. In addition, the proposed PoAj showed to be an effective approach to mitigate the processing time issue of large-sized transactions. PoAj is not tailored to any particular attack; therefore, it is effective against malicious powerful players. The proposed approach provides a strong barrier not only to current and known attacks but also to future unknown attacks based on different strategies that rely on controlling the majority of the hashing power.

Despite the intense efforts to prevent programmers from writing code with memory errors, memory corruption vulnerabilities are still a major security threat. Consequently, control-flow integrity has received significant attention in the research community, and software developers to combat control code execution attacks in the presence of type of faults. Control-flow Integrity (CFI) is a large family of techniques that aims to eradicate memory error exploitation by ensuring that the instruction pointer (IP) of a running process cannot be controlled by a malicious attacker. In this paper, we assess the effectiveness of 14 CFI techniques against the most popular exploitation techniques, including code reuse attacks, return-to-user, return-to-libc, and replay attacks. We also classify these techniques based on their security, robustness, and implementation complexity. Our study indicates that the majority of the CFI techniques are primarily focused on restricting indirect branch instructions and cannot prevent all forms of vulnerability exploitation. We conclude that the performance overhead introduced, jointly with the partial attack coverage, is discouraging the industry from adopting most of them.