Abstract
Commitments to key-value maps (or, authenticated dictionaries) are an important building block in cryptographic applications, including cryptocurrencies and distributed file systems.
In this work we study short commitments to key-value maps with two additional properties: double-hiding (both keys and values should be hidden) and homomorphism (we should be able to combine two commitments to obtain one that is the “sum” of their key-value openings). Furthermore, we require these commitments to be short and to support efficient transparent zero-knowledge arguments (i.e., without a trusted setup).
As our main contribution, we show how to construct commitments with the properties above as well as efficient zero-knowledge arguments over them. We additionally discuss a range of practical optimizations that can be carried out depending on the application domain. Finally, we formally describe a specific application of commitments to key-value maps to scalable anonymous ledgers. We show how to extend QuisQuis (Fauzi et al. ASIACRYPT 2019). This results in an efficient, confidential multi-type system with a state whose size is independent of the number of transactions.
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Notes
- 1.
This is a way to describe our setting asymptotically. We stress, however, that is not necessary: an interesting setting for our constructions is just one where the universe of keys is concretely large.
- 2.
In a transparent argument system the setup does not need to be produced by a trusted party. This property is interesting in the case of non-interactive argument systems, which are the focus of this work.
- 3.
This is true for the aforementioned approach with sigma-protocols as well as for other straightforward applications of NIZKs.
- 4.
I.e., a commitment which can be opened using a secret key in place of the randomness.
- 5.
Here we consider the case where leaking the density of the key-value map is not a problem. We will also adapt our construction where this leakage does not occur if an upper bound on this density is known.
- 6.
An accumulator is a cryptographic data structure that allows to commit to a set in a binding manner and to prove membership of an element efficiently. NB: we can compute accumulators deterministically from a set, i.e., without a trusted authority.
- 7.
We refer the reader to [35] for a survey of this rapidly growing field.
- 8.
Plausible deniability: no one can tell if a user meant to be involved in a transaction.
- 9.
We will use this when defining the homomorphic property of commitments.
- 10.
If malleability is a concern, Bulletproofs are proven to be non-malleable in the AGM [24].
- 11.
For some applications this can be huge—for comparison, the ZCash circuit [26] has approximately 100k constraints.
- 12.
- 13.
Since there is no public circuit implementation for Ristretto operations for this, we use arkworks [1] BL12-381 implementation for this estimate. We expect this to be an upper bound on Ristretto points given their smaller field size—\(\frac{255}{381}\)x smaller, more precisely. We measure this number using the implementation in [1].
- 14.
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Acknowledgements
Research supported by: the Concordium Blockhain Research Center, Aarhus University, Denmark; the Carlsberg Foundation under the Semper Ardens Research Project CF18-112 (BCM); the European Research Council (ERC) under the European Unions’s Horizon 2020 research and innovation programme under grant agreement No 803096 (SPEC). This work was partly produced while Matteo Campanelli and Felix Engelmann were affiliated with Aarhus University.
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Campanelli, M., Engelmann, F., Orlandi, C. (2022). Zero-Knowledge for Homomorphic Key-Value Commitments with Applications to Privacy-Preserving Ledgers. In: Galdi, C., Jarecki, S. (eds) Security and Cryptography for Networks. SCN 2022. Lecture Notes in Computer Science, vol 13409. Springer, Cham. https://doi.org/10.1007/978-3-031-14791-3_33
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