Blockchains primarily enable credible accounting of digital events, e.g., money transfers in cryptocurrencies. However, beyond this original purpose, blockchains also irrevocably record arbitrary data, ranging from short messages to pictures. This does not come without risk for users as each participant has to locally replicate the complete blockchain, particularly including potentially harmful content. We provide the first systematic analysis of the benefits and threats of arbitrary blockchain content. Our analysis shows that certain content, e.g., illegal pornography, can render the mere possession of a blockchain illegal. Based on these insights, we conduct a thorough quantitative and qualitative analysis of unintended content on Bitcoin’s blockchain. Although most data originates from benign extensions to Bitcoin’s protocol, our analysis reveals more than 1600 files on the blockchain, over 99 % of which are texts or images. Among these files there is clearly objectionable content such as links to child pornography, which is distributed to all Bitcoin participants. With our analysis, we thus highlight the importance for future blockchain designs to address the possibility of unintended data insertion and protect blockchain users accordingly.
"illegal pornography, can render the mere possession of a blockchain illegal."
I see this as a very strong legal attack vector on full nodes and cryptocurrencies, probably a way around it, is to only allow meta information on a cryptographic form, even then the owner can publish the view key publicly.
A drastic solution is to just prune or don't even allow metadata.
You don't even need the ability to record metadata on-chain to encode arbitrary data. An agreed-upon method of encoding it into ordinary transactions is enough. Even if BTC-style transactions were just inputs/outputs (they're not), you could still encode information down into the satoshi-place of the inputs or outputs themselves. It's even worse for something like Ethereum: essentially the whole point of that blockchain is to encode abritrary (executable) metadata in the form of the contracts themselves.
You could do that with an ordinary bank account though, and call the cops on your bank. In fact, you could do it with any service provider who logs your activity. Simply invent an encoding scheme and encode something illegal in your actions.
Blockchains primarily enable credible accounting of digital events, e.g., money transfers in cryptocurrencies. However, beyond this original purpose, blockchains also irrevocably record arbitrary data, ranging from short messages to pictures. This does not come without risk for users as each participant has to locally replicate the complete blockchain, particularly including potentially harmful content. We provide the first systematic analysis of the benefits and threats of arbitrary blockchain content. Our analysis shows that certain content, e.g., illegal pornography, can render the mere possession of a blockchain illegal. Based on these insights, we conduct a thorough quantitative and qualitative analysis of unintended content on Bitcoin’s blockchain. Although most data originates from benign extensions to Bitcoin’s protocol, our analysis reveals more than 1600 files on the blockchain, over 99 % of which are texts or images. Among these files there is clearly objectionable content such as links to child pornography, which is distributed to all Bitcoin participants. With our analysis, we thus highlight the importance for future blockchain designs to address the possibility of unintended data insertion and protect blockchain users accordingly.