It is a common narrative that blockchains are immutable and so it is technically impossible to erase data stored on them. For legal and ethical reasons, however, individuals and organizations might be compelled to erase locally stored data, be it encoded on a blockchain or not. The common assumption for blockchain networks like Bitcoin is that forcing nodes to erase data contained on the blockchain is equal to permanently restricting them from participating in the system in a full-node role. Challenging this belief, in this paper, we propose and demonstrate a pragmatic approach towards functionality-preserving local erasure (FPLE). FPLE enables full nodes to erase infringing or undesirable data while continuing to store and validate most of the blockchain. We describe a general FPLE approach for UTXO-based (i.e., Bitcoin-like) cryptocurrencies and present a lightweight proof-of-concept tool for safely erasing transaction data from the local storage of Bitcoin Core nodes. Erasing nodes continue to operate in tune with the network even when erased transaction outputs become relevant for validating subsequent blocks. Using only our basic proof-of-concept implementation, we are already able to safely comply with a significantly larger range of erasure requests than, to the best of our knowledge, any other full node operator so far.
@cite_9 , discuss various approaches for preventing the insertion of arbitrary, potentially unwanted data onto cryptocurrency blockchains. Their proposals include content detectors, which filter transactions based on heuristics and knowledge about commonly used data insertion methods, as well as protocol modifications that would greatly increase the costs of including arbitrary data. Approaches along these lines have also surfaced in the non-academic cryptocurrency community See, e. ,g.: http: comments.gmane.org gmane.comp.bitcoin.devel 1996 . Approaches for avoiding the insertion of unwanted data depend on global adoption, for an effective filtering, and in some cases also on protocol changes when applied to existing networks. In contrast, FPLE requires only a node-local decision and is in this way both more practical and enables the incorporation of a wider range of individual preferences and constraints. Lastly, as can be seen in related application domains such as malware detection or digital rights protection via upload filtering, content-based filtering is never completely circumvention-proof. Once something "slips through", an erasure possibility again becomes necessary.
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Since the introduction of Bitcoin in 2008, blockchain systems have seen an enormous increase in adoption. By providing a persistent, distributed, and append-only ledger, blockchains enable numerous applications such as distributed consensus, robustness against equivocation, and smart contracts. However, recent studies show that blockchain systems such as Bitcoin can be (mis) used to store arbitrary content. This has already been used to store arguably objectionable content on Bitcoin's blockchain. Already single instances of clearly objectionable or even illegal content can put the whole system at risk by making its node operators culpable. To overcome this imminent risk, we survey and discuss the design space of countermeasures against the insertion of such objectionable content. Our analysis shows a wide spectrum of potential countermeasures, which are often combinable for increased efficiency. First, we investigate special-purpose content detectors as an ad hoc mitigation. As they turn out to be easily evadable, we also investigate content-agnostic countermeasures. We find that mandatory minimum fees as well as mitigation of transaction manipulability via identifier commitments significantly raise the bar for inserting harmful content into a blockchain.