Blockchain has been used in many applications: financial services including virtual currency like Bitcoin, asset management, insurance services, government, and health care. We believe that a blockchain is resistant to modification of the data where each block uses a cryptographic hash. SHA256 is a hashing function used in the blockchain. We also believe that SHA256 cannot be reversed because it's a one-way function. However, as long as two keywords used in the blockchain-based system, it is reversible by using classic database approach from a hash string to two keywords without cracking SHA256 algorithm. This paper demonstrates how to reverse a hash string to keyword(s). The proposed database approach simply defeats its design goal of the blockchain without cracking the algorithm as long as the database can be created. This paper recommends that three or more keywords of the hash function SHA256 should be used for user account access in the blockchain-based systems.
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