  Blockchain-based systems have gained immense popularity as enablers of
independent asset transfers and smart contract functionality. They have also,
since as early as the first Bitcoin blocks, been used for storing arbitrary
contents such as texts and images. On-chain data storage functionality is
useful for a variety of legitimate use cases. It does, however, also pose a
systematic risk. If abused, for example by posting illegal contents on a public
blockchain, data storage functionality can lead to legal consequences for
operators and users that need to store and distribute the blockchain, thereby
threatening the operational availability of entire blockchain ecosystems. In
this paper, we develop and apply a cloud-based approach for quickly discovering
and classifying content on public blockchains. Our method can be adapted to
different blockchain systems and offers insights into content-related usage
patterns and potential cases of abuse. We apply our method on the two most
prominent public blockchain systems - Bitcoin and Ethereum - and discuss our
results. To the best of our knowledge, the presented study is the first to
systematically analyze non-financial content stored on the Ethereum blockchain
and the first to present a side-by-side comparison between different
blockchains in terms of the quality and quantity of stored data.
