Paper 2021/1423

Encryption to the Future: A Paradigm for Sending Secret Messages to Future (Anonymous) Committees

Matteo Campanelli
Bernardo David
Hamidreza Khoshakhlagh
Anders Konring
Jesper Buus Nielsen

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 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 \emph{independent} of the length of plaintext messages being sent to the future.

Available format(s)
Cryptographic protocols
Publication info
A major revision of an IACR publication in ASIACRYPT 2022
applications blockchain witness encryption YOSO
Contact author(s)
matteo @ protocol ai
beda @ itu dk
hk @ concordium com
konr @ itu dk
jbn @ au dk
2022-09-12: last of 4 revisions
2021-10-24: received
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      author = {Matteo Campanelli and Bernardo David and Hamidreza Khoshakhlagh and Anders Konring and Jesper Buus Nielsen},
      title = {Encryption to the Future: A Paradigm for Sending Secret Messages to Future (Anonymous) Committees},
      howpublished = {Cryptology ePrint Archive, Paper 2021/1423},
      year = {2021},
      note = {\url{}},
      url = {}
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