Continuous After-the-fact Leakage-Resilient eCK-secure Key Exchange

Security models for two-party authenticated key exchange (AKE) protocols have developed over time to capture the security of AKE protocols even when the adversary learns certain secret values. Increased granularity of security can be modelled by considering partial leakage of secrets in the manner of models for leakage-resilient cryptography, designed to capture side-channel attacks. In this work, we use the strongest known partial-leakage-based security model for key exchange protocols, namely continuous after-the-fact leakage eCK (CAFL-eCK) model. We resolve an open problem by constructing the first concrete two-pass leakage-resilient key exchange protocol that is secure in the CAFL-eCK model

Review on Leakage Resilient Key Exchange Security Model

In leakage resilient cryptography, leakage resilient key exchange protocols are constructed to defend against leakage attacks. Then, the key exchange protocol is proved with leakage resilient security model to determine whether its security proof can provide the security properties it claimed or to find out any unexamined flaw during protocol building. It is an interesting work to review the meaningful security properties provided by these security models. This work review how a leakage resilient security model for a key exchange protocol has been evolved over years according to the increasing security requirement which covers a different range of attacks. The relationship on how an adversary capability in the leakage resilient security model can be related to real-world attack scenarios is studied. The analysis work for each leakage resilient security model here enables a better knowledge on how an adversary query addresses different leakage attacks setting, thereby understand the motive of design for a cryptographic primitive in the security model

On the Leakage-Resilient Key Exchange

Typically, secure channels are constructed from an authenticated key exchange (AKE) protocol, which authenticates the communicating parties based on long-term public keys and establishes secret session keys. In this paper we address the partial leakage of long-term secret keys of key exchange protocol participants due to various side-channel attacks. Security models for two-party authenticated key exchange protocols have developed over time to provide security even when the adversary learns certain secret values. This paper combines and extends the advances of security modelling for AKE protocols addressing more granular partial leakage of long-term secrets of protocol participants

Standard Model Leakage-Resilient Authenticated Key Exchange using Inner-product Extractors

With the development of side-channel attacks, a necessity arises to invent authenticated key exchange protocols in a leakage-resilient manner. Constructing authenticated key exchange protocols using existing cryptographic schemes is an effective method, as such construction can be instantiated with any appropriate scheme in a way that the formal security argument remains valid. In parallel, constructing authenticated key exchange protocols that are proven to be secure in the standard model is more preferred as they rely on real-world assumptions. In this paper, we present a Diffie-Hellman-style construction of a leakage-resilient authenticated key exchange protocol, that can be instantiated with any CCLA2-secure public-key encryption scheme and a function from the pseudo-random function family. Our protocol is proven to be secure in the standard model assuming the hardness of the decisional Diffie-Hellman problem. Furthermore, it is resilient to continuous partial leakage of long-term secret keys, that happens even after the session key is established, while satisfying the security features defined by the eCK security model

On Continuous After-the-Fact Leakage-Resilient Key Exchange

Side-channel attacks are severe type of attack against implementation of cryptographic primitives. Leakage-resilient cryptography is a new theoretical approach to formally address the problem of side-channel attacks. Recently, the Continuous After-the-Fact Leakage (CAFL) security model has been introduced for two-party authenticated key exchange (AKE) protocols. In the CAFL model, an adversary can adaptively request arbitrary leakage of long-term secrets even after the test session is activated. It supports continuous leakage even when the adversary learns certain ephemeral secrets or session keys. The amount of leakage is limited per query, but there is no bound on the total leakage. A generic leakage-resilient key exchange protocol $\pi$ has also been introduced that is formally proved to be secure in the CAFL model. In this paper, we comment on the CAFL model, and show that it does not capture its claimed security. Furthermore, we present an attack and counterproofs for the security of protocol $\pi$ which invalidates the formal security proofs of protocol $\pi$ in the CAFL model

New Approach to Practical Leakage-Resilient Public-Key Cryptography

We present a new approach to construct several leakage-resilient cryptographic primitives, including leakage-resilient public-key encryption (PKE) schemes, authenticated key exchange (AKE) protocols and low-latency key exchange (LLKE) protocols. To this end, we introduce a new primitive called leakage-resilient non-interactive key exchange (LR-NIKE) protocol. We introduce a generic security model for LR-NIKE protocols, which can be instantiated in both the bounded and continuous-memory leakage ((B/C)-ML) settings. We then show a secure construction of LR-NIKE protocol in the bounded- memory leakage (BML) setting, that achieves an optimal leakage rate, i.e., 1-o(1). Finally, we show how to construct the aforementioned leakage-resilient primitives from such a LR-NIKE protocol as summarized below. All the primitives also achieve the same (optimal) leakage rate as the underlying LR-NIKE protocol. We show how to construct a leakage-resilient IND-CCA-2-secure PKE scheme in the BML model generically from a LR-NIKE protocol. Our construction differs from the state-of-the-art constructions of leakage-resilient IND-CCA-2-secure PKE schemes, which use hash proof techniques to achieve leakage-resilience. Moreover, our transformation preserves the leakage-rate of the underlying LR- NIKE and admits more efficient construction than previous such PKE constructions. We introduce a new leakage model for AKE protocols, in the BML setting. We show how to construct a leakage-resilient AKE protocol starting from LR-NIKE protocol. We introduce the first-ever leakage model for LLKE protocols in the BML setting, and the first construction of such a leakage-resilient LLKE from LR-NIKE protocol

Efficient Compilers for After-the-Fact Leakage: from CPA to CCA-2 secure PKE to AKE

The goal of leakage-resilient cryptography is to construct cryptographic algorithms that are secure even if the adversary obtains side-channel information from the real world implementation of these algorithms. Most of the prior works on leakage-resilient cryptography consider leakage models where the adversary has access to the leakage oracle before the challenge-ciphertext is generated (before-the-fact leakage). In this model, there are generic compilers that transform any leakage-resilient CPA-secure public key encryption (PKE) scheme to its CCA-2 variant using Naor-Yung type of transformations. In this work, we give an efficient generic compiler for transforming a leakage-resilient CPA-secure PKE to leakage-resilient CCA-2 secure PKE in presence of after-the-fact split-state (bounded) memory leakage model, where the adversary has access to the leakage oracle even after the challenge phase. The salient feature of our transformation is that the leakage rate (defined as the ratio of the amount of leakage to the size of secret key) of the transformed after-the-fact CCA-2 secure PKE is same as the leakage rate of the underlying after-the-fact CPA-secure PKE, which is $1-o(1)$. We then present another generic compiler for transforming an after-the-fact leakage-resilient CCA-2 secure PKE to a leakage-resilient authenticated key exchange (AKE) protocol in the bounded after-the-fact leakage-resilient eCK (BAFL-eCK) model proposed by Alawatugoda et al. (ASIACCS\u2714). To the best of our knowledge, this gives the first compiler that transform any leakage-resilient CCA-2 secure PKE to an AKE protocol in the leakage variant of the eCK model