Concurrently Non-Malleable Zero Knowledge in the Authenticated Public-Key Model

We consider a type of zero-knowledge protocols that are of interest for their practical applications within networks like the Internet: efficient zero-knowledge arguments of knowledge that remain secure against concurrent man-in-the-middle attacks. As negative results in the area of concurrent non-malleable zero-knowledge imply that protocols in the standard setting (i.e., under no setup assumptions) can only be given for trivial languages, researchers have studied such protocols in models with setup assumptions, such as the common reference string (CRS) model. This model assumes that a reference string is honestly created at the beginning of all interactions and later available to all parties (an assumption that is satisfied, for instance, in the presence of a trusted party). A growing area of research in Cryptography is that of reducing the setup assumptions under which certain cryptographic protocols can be realized. In an effort to reduce the setup assumptions required for efficient zeroknowledge arguments of knowledge that remain secure against concurrent man-in-the-middle attacks, we consider a model, which we call the Authenticated Public-Key (APK) model. The APK model seems to significantly reduce the setup assumptions made by the CRS model (as no trusted party or honest execution of a centralized algorithm are required), and can be seen as a slightly stronger variation of the Bare Public-Key (BPK) model from [8, 30], and a weaker variation of the registered public-key model used in [3]. We then define and study man-in-the-middle attacks in the APK model. Our main result is a constant-round concurrent non-malleable zero-knowledge argument of knowledge for any polynomial-time relation (associated to a language in NP), under the (minimal) assumption of the existence of a one-way function family. We also show time-efficient instantiations of our protocol, in which the transformation from a 3-round honest-verifier zero-knowledge argument of knowledge to a 4-round concurrently non-malleable zero-knowledge argument of knowledge for the same relation incurs onlyO(1) (precisely, a small constant) additional modular exponentiations, based on known number-theoretic assumptions. Furthermore, the APK model is motivated by the consideration of some man-inthe-middle attacks in models with setup assumptions that had not been considered previously and might be of independent interest. We also note a negative result with respect to further reducing the setup assumptions of our protocol to those in the (unauthenticated) BPK model, by showing that concurrently non-malleable zero-knowledge arguments of knowledge in the BPK model are only possible for trivial languages.