Verifiable Delay Function (VDF) is a cryptographic concept that ensures a minimum delay before output through sequential processing, which is resistant to parallel computing. One of the significant VDF protocols academically reviewed is the VDF protocol proposed by Pietrzak. However, for the blockchain environment, the Pietrzak VDF has drawbacks including long proof size and recursive protocol computation. In this paper, we present an implementation study of Pietrzak VDF verification on Ethereum Virtual Machine (EVM). We found that the discussion in the Pietrzak's original paper can help a clear optimization in EVM where the costs of computation are predefined as the specific amounts of gas. In our results, the cost of VDF verification can be reduced from 4M to 2M gas, and the proof length can be generated under 8 KB with the 2048-bit RSA key length, which is much smaller than the previous expectation.

IOTA is a distributed ledger technology that uses a Directed Acyclic Graph (DAG) structure called the Tangle. It is known for its efficiency and is widely used in the Internet of Things (IoT) environment. Tangle can be configured by utilizing the tip selection process. Due to performance issues with light nodes, full nodes are being asked to perform the tip selections of light nodes. However, in this paper, we demonstrate that tip selection can be exploited to compromise users’ privacy. An adversary full node can associate a transaction with the identity of a light node by comparing the light node’s request with its ledger. We show that these types of attacks are not only viable in the current IOTA environment but also in IOTA 2.0 and the privacy improvement being studied. We also provide solutions to mitigate these attacks and propose ways to enhance anonymity in the IOTA network while maintaining efficiency and scalability.

Summary Bitcoin introduced blockchain which is the transparent and decentralized way of recording the lists of digital currency transactions. Bitcoin's blockchain uses Proof‐of‐Work as a Sybil control mechanism. However, PoW wastes energy since it uses hash computing competitions to find a block. Hence, various alternative mechanisms have been proposed. Among them, Proof‐of‐Stake, which is based on the deposit, has been spotlighted. As opposed to Proof‐of‐Work, Proof‐of‐Stake requires nodes to have a certain amount of tokens (stake) in order to qualify to validate blocks. The “one‐sentence philosophy” of proof of stake is not “security comes from burning energy,” but rather “security comes from putting up economic value‐at‐loss.” In this article, contrary to popular belief, we point out that this value‐at‐loss can be hedged by short selling or other financial products. We propose a “shorting attack,” which makes a profit by massive short selling and sabotage to a Proof‐of‐Stake‐based cryptocurrency. The shorting attack implies that the security of Proof‐of‐Stake‐based cryptocurrency can be vulnerable by a low stake ratio.

Proof-of-Work (PoW) is a Sybil control mechanism adopted in blockchain-based cryptocurrencies. It prevents the attempt of malicious actors to manipulate distributed ledgers. Bitcoin has successfully suppressed double-spending by accepting the longest PoW chain. Nevertheless, PoW encountered several major security issues surrounding mining competition. One of them is a Block WithHolding (BWH) attack that can exploit a widespread and cooperative environment called a mining pool. This attack takes advantage of untrustworthy relationships between mining pools and participating agents. Moreover, detecting or responding to attacks is challenging due to the nature of mining pools. In this paper, however, we suggest that BWH attacks also have a comparable trust problem. Because a BWH attacker cannot have complete control over BWH agents, they can betray the belonging mining pool and seek further benefits by trading with victims. We prove that this betrayal is not only valid in all attack parameters but also provides double benefits; finally, it is the best strategy for BWH agents. Furthermore, our study implies that BWH attacks may encounter self-destruction of their own revenue, contrary to their intention.