Abstract:

Blockchain technology is a kind of emerging information technology model. It is widely regarded as a promising concept because of its advantages such as decentralization, high efficiency, and transparency. The breadth of application scenarios and the underlying layer of application determine that the security of the blockchain must be guaranteed. Hash functions are one of the most important foundations for providing the blockchain’s usability and security. Starting from Hash functions in the blockchain and based on the principle of cryptanalysis, this paper presents a type of second preimage attack on the existing blocks by employing the structure and workflow of the blockchain. Specially, the attack constructed in this paper uses the fact that the Hash values in the leaf nodes of a Merkle tree have the same status. After theoretical analysis of proving that the complexity of such an attack is lower than that of trivial brute-force, the attack’s concrete steps based on Hellman’s time-memory tradeoff principle are also described. The conclusion of the attack shows that both the mathematical structure of the Hash function itself and data format of blockchain transaction records are important to the security of the blockchain. This should be considered in the future when we design blockchain systems.

Key words: blockchain, Merkle tree, second-preimage attack, Hellman’s time-memory tradeoff;