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Blockchain technologies recently received a considerable amount of attention. While the initial focus was mainly on the use of blockchains in the context of cryptocurrencies such as Bitcoin, application scenarios now go far beyond this. Most blockchains have the property that once some object, e.g., a block or a transaction, has been registered to be included into the blockchain, it is persisted and there are no means to modify it again. While this is an essential feature of most blockchain scenarios, it is still often desirable-at times it may be even legally required-to allow for breaking this immutability in a controlled way. Only recently, Ateniese et al. (EUROS&P 2017) proposed an elegant solution to this problem on the block level. Thereby, the authors replace standard hash functions with so-called chameleon-hashes (Krawczyk and Rabin, NDSS 2000). While their work seems to offer a suitable solution to the problem of controlled rewriting of blockchains, their approach is too coarse-grained in that it only offers an all-or-nothing solution. We revisit this idea and introduce the novel concept of policybased chameleon-hashes (PCH). PCHs generalize the notion of chameleon-hashes by giving the party computing a hash the ability to associate access policies to the generated hashes. Anyone who possesses enough privileges to satisfy the policy can then find arbitrary collisions for a given hash. We then apply this concept to transaction-level rewriting within blockchains, and thus support fine-grained and controlled modifiability of blockchain objects. Besides modeling PCHs, we present a generic construction of PCHs (using a strengthened version of chameleon-hashes with ephemeral trapdoors which we also introduce), rigorously prove its security, and instantiate it with efficient building blocks. We report first implementation results.

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A redactable signature scheme (RSS) allows removing parts of a signed message by any party without invalidating the respective signature. State-of-the-art constructions thereby focus on messages represented by one specific data-structure, e.g., lists, sets or trees, and adjust the security model accordingly. To overcome the necessity for this myriad of models, we present a general framework covering arbitrary data-structures and even more sophisticated possibilities. For example, we cover fixed elements which must not be redactable and dependencies between elements. Moreover, we introduce the notion of designated redactors, i.e., the signer can give some extra information to selected entities which become redactors. In practice, this often allows to obtain more efficient schemes. We then present two RSSs; one for sets and one for lists, both constructed from any EUF-CMA secure signature scheme and indistinguishable cryptographic accumulators in a black-box way and show how the concept of designated redactors can be used to increase the efficiency of these schemes. Finally, we present a black-box construction of a designated redactor RSS by combining an RSS for sets with non-interactive zero-knowledge proof systems. All the three constructions presented in this paper provide transparency, which is an important property, but quite hard to achieve, as we also conceal the length of the original message and the positions of the redactions.

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