Authenticated Key Exchange (AKE) is a cryptographic scheme with the aim to establish a high-entropy and secret session key over a insecure communications network. \emph{Password}-Authenticated Key Exchange (PAKE) assumes that the parties in play share a simple password, which is cheap and human-memorable and is used to achieve the authentication. PAKEs are practically relevant as these features are extremely appealing in an age where most people access sensitive personal data remotely from more-and-more pervasive hand-held devices. Theoretically, PAKEs allow the secure computation and authentication of a high-entropy piece of data using a low-entropy string as a starting point. In this paper, we apply the recently proposed technique introduced in~\cite{DXX2012} to construct two lattice-based PAKE protocols enjoying a very simple and elegant design that is an parallel extension of the class of Random Oracle Model (ROM)-based protocols \msf{PAK} and \msf{PPK}~\cite{BMP2000,M2002}, but in the lattice-based setting. The new protocol resembling \msf{PAK} is three-pass, and provides \emph{mutual explicit authentication}, while the protocol following the structure of \msf{PPK} is two-pass, and provides \emph{implicit authentication}. Our protocols rely on the Ring-Learning-with-Errors (RLWE) assumption, and exploit the additive structure of the underlying ring. They have a comparable level of efficiency to \msf{PAK} and \msf{PPK}, which makes them highly attractive. We present a preliminary implementation of our protocols to demonstrate that they are both efficient and practical. We believe they are suitable quantum safe replacements for \msf{PAK} and \msf{PPK}.

Key Exchange (KE) is, undoubtedly, one of the most used cryptographic primitives in practice. Its authenticated version, Authenticated Key Exchange (AKE), avoids man-in-the-middle-based attacks by providing authentication for both parties involved. It is widely used on the Internet, in protocols such as TLS or SSH. In this work, we provide new constructions for KE and AKE based on ideal lattices in the Random Oracle Model (ROM). The contributions of this work can be summarized as follows: 1) It is well-known that RLWE-based KE protocols are not robust for key reuses since the signal function leaks information about the secret key. We modify the design of previous RLWE-based KE schemes to allow key reuse in the ROM. Our construction makes use of a new technique called pasteurization which enforces a supposedly RLWE sample sent by the other party to be indeed indistinguishable from a uniform sample and, therefore, ensures no information leakage in the whole KE process. 2) We build a new AKE scheme based on the construction above. The scheme provides implicit authentication (that is, it does not require the use of any other authentication mechanism, like a signature scheme) and it is proven secure in the Bellare-Rogaway model with weak Perfect Forward Secrecy in the ROM. It improves previous designs for AKE schemes based on lattices in several aspects. Our construction just requires sampling from only one discrete Gaussian distribution and avoids rejection sampling and noise flooding techniques, unlike previous proposals (Zhang et al., EUROCRYPT 2015). Thus, the scheme is much more efficient than previous constructions in terms of computational and communication complexity. Since our constructions are provably secure assuming the hardness of the RLWE problem, they are considered to be robust against quantum adversaries and, thus, suitable for post-quantum applications.

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