Research on the Security Criteria of Hash Functions in the Blockchain.

The blockchain is an emerging technology. Because of its efficiency and functionality, it is widely considered to have revolutionary application prospects. As a supporting part of the data structure, the hash function is important for ensuring the availability and security of the blockchain. To evaluate the security of blockchain technology it is important to analyze several security criteria of the hash functions used in the blockchain. This paper focuses on two security criteria-hiding and puzzle-friendliness-that the hash function should satisfy in the blockchain proposed by Narayanan et al. (Narayanan A, Bonneau J, Felten E, et al. Bitcoin and Cryptocurrency Technologies: A Comprehensive Introduction. Princeton University Press, 2016). Under the framework of Rogaway?Shrimpton's theory, their definitions are mathematically described and their relation is proved. Also, comparisons between the two criteria and traditional ones in hash functions are made. The conclusions of this paper point out that for hash functions, it is harder to break hiding and puzzle-friendliness than to break preimage resistance, which shows that hash functions proven to be preimage-resistant or tested by preimage cryptanalysis like SHA256 tend to be enough to be used in the corresponding parts of the blockchain design. At the same time, from the attackers' view the conclusions can provide a theoretical basis for studying the potential attacks on and defects that may exist in the structure of the hash functions applied in the blockchain.