Abstract
A number of recent works have constructed cryptographic protocols with flavors of adaptive security by having a randomly-chosen anonymous committee run at each round. Since most of these protocols are stateful, transferring secret states from past committees to future, but still unknown, committees is a crucial challenge. Previous works have tackled this problem with approaches tailor-made for their specific setting, which mostly rely on using a blockchain to orchestrate auxiliary committees that aid in the state hand-over process. In this work, we look at this challenge as an important problem on its own and initiate the study of Encryption to the Future (EtF) as a cryptographic primitive. First, we define a notion of an EtF scheme where time is determined with respect to an underlying blockchain and a lottery selects parties to receive a secret message at some point in the future. While this notion seems overly restrictive, we establish two important facts: 1. if used to encrypt towards parties selected in the “far future”, EtF implies witness encryption for NP over a blockchain; 2. if used to encrypt only towards parties selected in the “near future”, EtF is not only sufficient for transferring state among committees as required by previous works, but also captures previous tailor-made solutions. To corroborate these results, we provide a novel construction of EtF based on witness encryption over commitments (cWE), which we instantiate from a number of standard assumptions via a construction based on generic cryptographic primitives. Finally, we show how to use “near future” EtF to obtain “far future” EtF with a protocol based on an auxiliary committee whose communication complexity is independent of the length of plaintext messages being sent to the future.
M. Campanelli—Work done in part while the author was affiliated to Aarhus University.
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Notes
- 1.
The word “winner” here refers to the party who is selected to perform a role according to the underlying lottery of the PoS blockchain (see remainder of introduction).
- 2.
The family of protocols we consider actually has two role-related aspects to solve. The first—and the focus of this paper—is the aforementioned role assignment (RA) which deals with the sending of messages to parties selected to perform future roles of a protocol while hiding the identities of such parties. The other aspect is role execution (RX) which focuses on the execution of the specific protocol that runs on top of the RA mechanism, i.e., what messages are sent to which roles and what specification the protocol implements. In [15] the so-called You Only Speak Once (YOSO) model is introduced for studying RX. In the YOSO model the protocol execution is between abstract roles which can each speak only once. Later these can then be mapped to physical machines using an RA mechanism. The work of [15] shows that given RA in a synchronous model, any well-formed ideal functionality can be implemented in the YOSO model with security against malicious adaptive corruption of a minority of machines. Concretely, [15] gives an ideal functionality for RA and shows that a YOSO protocol for abstract roles can be compiled into the RA-hybrid model to give a protocol secure against adaptive attacks.
- 3.
While one might define EtF in more general settings, namely without a blockchain, it is unclear how to obtain interesting instantiations, that is from standard primitives.
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Acknowledgements
Bernardo David is supported by the Concordium Foundation and by the Independent Research Fund Denmark (IRFD) grants number 9040-00399B (TrA2C), 9131-00075B (PUMA) and 0165-00079B. Hamidreza Khoshakhlagh has been funded by the Concordium Foundation under Concordium Blockchain Research Center, Aarhus. Anders Konring is supported by the IRFD grant number 9040-00399B (TrA2C). Jesper Buus Nielsen is partially funded by the Concordium Foundation; The Danish Independent Research Council under Grant-ID DFF-8021-00366B (BETHE); The Carlsberg Foundation under the Semper Ardens Research Project CF18-112 (BCM).
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Campanelli, M., David, B., Khoshakhlagh, H., Konring, A., Nielsen, J.B. (2022). Encryption to the Future. In: Agrawal, S., Lin, D. (eds) Advances in Cryptology – ASIACRYPT 2022. ASIACRYPT 2022. Lecture Notes in Computer Science, vol 13793. Springer, Cham. https://doi.org/10.1007/978-3-031-22969-5_6
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