A group authenticated key exchange (GAKE) protocol allows a set of parties belonging to a certain designated group to agree upon a common secret key through an insecure communication network. In the last few years, many new cryptographic tools have been specifically designed to thwart attacks from adversaries which may have access to (different kinds of) quantum computation resources. However, few constructions for group key exchange have been put forward. Here, we propose a four-round GAKE which can be proven secure under widely accepted assumptions in the Quantum Random Oracle Model. Specifically, we integrate several primitives from the so-called Kyber suite of post-quantum tools in a (slightly modified) compiler from Abdalla et al. (TCC 2007). More precisely, taking as a starting point an IND-CPA encryption scheme from the Kyber portfolio, we derive, using results from Hövelmanns et al. (PKC 2020), a two-party key exchange protocol and an IND-CCA encryption scheme and prove them fit as building blocks for our compiled construction. The resulting GAKE protocol is secure under the Module-LWE assumption, and furthermore achieves authentication without the use of (expensive) post-quantum signatures.

Group authenticated key exchange protocols (GAKE) are cryptographic tools enabling a group of several users communicating through an insecure channel to securely establish a common shared highentropy key. In the last years, the need to design cryptographic tools which provide security in the presence of attackers with access to quantum resources has become unquestionable; the field dealing with these types of protocols is usually referred to as Post-Quantum Cryptography. The U.S. National Institute for Standards and Technology (NIST) launched in 2017 an open call to find suitable post-quantum public-key algorithms for standardization. In this work, we design a GAKE that can be instantiated with any key encapsulation mechanism (KEM) that satisfies the strong security notion IND-CCA, matching NIST's requirements for this primitive. We have implemented our GAKE with the four finalist KEMs from the NIST process: Classic McEliece, Kyber, NTRU, and Saber, making use of the open-source library LibOQS where these algorithms are provided. We have conducted a detailed comparative performance analysis of the resulting GAKE protocols, taking into account all the parameter sets proposed in the submissions. We have also made a performance analysis of all the involved building pieces, including the four finalist KEMs. Finally, we also compare our GAKE with a previous proposal implemented with Kyber.

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This paper reports on the Walnut Digital Signature Algorithm (WalnutDSA), which is an asymmetric signature scheme recently presented for standardization at the NIST call for post-quantum cryptographic constructions. WalnutDSA is a group theoretical construction, the security of which relies on the hardness of certain problems related to an action of a braid group on a finite set. In spite of originally resisting the typical attacks succeeding against this kind of construction, soon different loopholes were identified rendering the proposal insecure (and finally, resulting in it being excluded from Round 2 of the NIST competition). Some of these attacks are related to the well-structured and symmetric masking of certain secret elements during the signing process. We explain the design principles behind this proposal and survey the main attack strategies that have succeeded, contradicting its claimed security properties, as well as the recently-proposed ideas aimed at overcoming these issues.

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