[Resource Topic] 2020/909: When is a test not a proof?

Welcome to the resource topic for 2020/909

Title:
When is a test not a proof?

Authors: Eleanor McMurtry, Olivier Pereira, Vanessa Teague

Abstract:

A common primitive in election and auction protocols is plaintext equivalence test (PET) in which two ciphertexts are tested for equality of their plaintexts, and a verifiable proof of the test’s outcome is provided. The most commonly-cited PETs require at least one honest party, but many applications claim universal verifiability, at odds with this requirement. If a test that relies on at least one honest participant is mistakenly used in a place where universally verifiable proof is needed, then a collusion by all participants can insert a forged proof of equality into the tallying transcript. We show this breaks universal verifiability for the JCJ/Civitas scheme among others, because the only PETs they reference are not universally verifiable. We then demonstrate how to fix the problem.

ePrint: https://eprint.iacr.org/2020/909

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