International Association
for Cryptologic Research

In this work we study the feasibility of knowledge extraction for succinct non-interactive arguments of knowledge (SNARKs) in a scenario that, to the best of our knowledge, has not been analyzed before. While prior work focuses on the case of adversarial provers that may receive (statically generated) {\em auxiliary information}, here we consider the scenario where adversarial provers are given {\em access to an oracle}. For this setting we study if and under what assumptions such provers can admit an extractor. Our contribution is mainly threefold.

First, we formalize the question of extraction in the presence of oracles by proposing a suitable proof of knowledge definition for this setting. We call SNARKs satisfying this definition O-SNARKs.

Second, we show how to use O-SNARKs to obtain formal and intuitive security proofs for three applications (homomorphic signatures, succinct functional signatures, and SNARKs on authenticated data) where we recognize an issue while doing the proof under the standard proof of knowledge definition of SNARKs.

Third -- and this is the most prominent part of our work -- we study whether O-SNARKs exist, providing both negative and positive results. On the negative side, we show that, assuming collision-resistant hash functions, there do not exist O-SNARKs in the standard model for every oracle family. Next, we study the interesting case of {\em signing} oracles. We give a negative result showing that even O-SNARKs for every signing oracle family do not exist. On the positive side, instead, we show that, when considering signature schemes with appropriate restrictions on the message length, O-SNARKs for the corresponding signing oracles exist, based on classical SNARKs and assuming extraction with respect to specific distributions of auxiliary input.