The Bitcoin protocol allows to save arbitrary data on the blockchain througha special instruction of the scripting language, called OP_RETURN. A growingnumber of protocols exploit this feature to extend the range of applications ofthe Bitcoin blockchain beyond transfer of currency. A point of debate in theBitcoin community is whether loading data through OP_RETURN can negativelyaffect the performance of the Bitcoin network with respect to its primary goal.This paper is an empirical study of the usage of OP_RETURN over the years. Weidentify several protocols based on OP_RETURN, which we classify by theirapplication domain. We measure the evolution in time of the usage of eachprotocol, the distribution of OP_RETURN transactions by application domain, andtheir space consumption.
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