| Summary: | The immutability of the blockchain technology facilitates it to establish a general consensus in a trustless environment, enabling a wide range of new applications, including distributed general-purpose data management and digital data sharing marketplace. This immutability, however, presents disadvantages for the blockchain technology, when it is used in other areas where the modification of data in blockchain is demanded. In this study, we propose a method for building modifiable blockchains in decentralized public network. To be specific, in computing the hash value of the block, the proposed method uses truncated hash values (these are called `target values' in this paper) of the transactions that are modifiable upon future requests, instead of transactions themselves. By doing so, the proposed method provides an opportunity to modify those transactions by making truncated hash values of modified versions equal to their original target values. The proposed method uses several cryptographic techniques to prevent the modification of the transaction from being performed for malicious purposes, and a multichain structure to improve the efficiency in transaction modification. By accommodating the modification feature to the blockchain, proposed architecture complies to key demands of the data protection regulations such as `right to rectification', `right to withdraw consent', and `right to be forgotten', et cetera. In addition, detailed threat analysis demonstrates that the proposed truncated hash-based modification is sufficiently secure to open up a wide range of new blockchain based services through added modifiability feature.
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