IACR News item: 30 June 2015
David Bernhard, Marc Fischlin, Bogdan Warinschi
ePrint Reportwhere the extractor has to recover witnesses for multiple, possibly adaptively chosen
statements and proofs. We also discuss extensions to simulation soundness, as typically
required for the ``encrypt-then-prove\'\' construction of strongly secure encryption
from IND-CPA schemes.
Utilizing our model we show three results:
(1) Simulation-sound adaptive proofs exist.
(2) The ``encrypt-then-prove\'\' construction with a simulation-sound
adaptive proof yields CCA security. This appears to be a ``folklore\'\' result
but which has never been proven in the random oracle model. As a corollary, we
obtain a new class of CCA-secure encryption schemes.
(3) We show that the
Fiat-Shamir transformed Schnorr protocol is _not_ adaptively secure and
discuss the implications of this limitation.
Our result not only separates
adaptive proofs from proofs of knowledge, but also gives a strong hint why
Signed ElGamal as the most prominent encrypt-then-prove example has not been
proven CCA-secure without making further assumptions.
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