Dr. Christoph Striecks

Authenticated Key Exchange (AKE) between any two entities is one of the most important security protocols available for securing our digital networks and infrastructures. In PQCrypto 2023, Bruckner, Ramacher and Striecks proposed a novel hybrid AKE (HAKE) protocol, dubbed Muckle+, that is particularly useful in large quantum-safe networks consisting of a large number of nodes. Their protocol is hybrid in the sense that it allows key material from conventional and post-quantum primitives, as well as from quantum key distribution, to be incorporated into a single end-to-end shared key. To achieve the desired authentication properties, Muckle+ utilizes post-quantum digital signatures. However, available instantiations of such signatures schemes are not yet efficient enough compared to their post-quantum key-encapsulation mechanism (KEM) counterparts, particularly in large networks with potentially several connections in a short period of time. To mitigate this gap, we propose Muckle# that pushes the efficiency boundaries of currently known HAKE constructions. Muckle# uses post-quantum key-encapsulating mechanisms for implicit authentication inspired by recent works done in the area of Transport Layer Security (TLS) protocols, particularly, in KEMTLS (CCS’20). We port those ideas to the HAKE framework and develop novel proof techniques on the way. Due to our novel KEM-based approach, the resulting protocol has a slightly different message flow compared to prior work that we carefully align with the HAKE framework and which makes our changes to the Muckle+ non-trivial.

Advancements in quantum computing pose a significant threat to most of the cryptography currently deployed. Fortunately, cryptographic building blocks to mitigate the threat are already available; mostly based on post-quantum and quantum cryptography, but also on symmetric cryptography techniques. Notably, quantum-safe building blocks must be deployed as soon as possible due to the “harvest-now decrypt-later” attack scenario, which is already challenging our sensitive and encrypted data today. Following an agile defense-in-depth approach, Hybrid Authenticated Key Exchange (HAKE) protocols have recently been gaining significant attention. Such protocols modularly combine conventional, post-quantum, and quantum cryptography to achieve confidentiality, authenticity, and integrity guarantees for network channels. Unfortunately, only a few protocols have yet been proposed (mainly Muckle and Muckle+) with different flexibility guarantees. Looking at available standards in the network domain (especially at the Media Access Control Security (MACsec) standard), we believe that HAKE protocols could already bring strong security benefits to MACsec today. MACsec is a standard designed to secure communication at the data link layer in Ethernet networks by providing security for all traffic between adjacent entities. In addition, MACsec establishes secure channels within a Local Area Network (LAN), ensuring that data remain protected from eavesdropping, tampering, and unauthorized access, while operating transparently to higher layer protocols. Currently, MACsec does not offer enough protection in the event of cryptographically relevant quantum computers. In this work, we tackle the challenge and propose a new versatile HAKE protocol, dubbed VMuckle, which is sufficiently flexible for the use in MACsec to provide LAN participants with hybrid key material ensuring secure communication.

End-to-end authenticity in public networks plays a significant role. Namely, without authenticity, the adversary might be able to retrieve even confidential information straight away by impersonating others. Proposed solutions to establish an authenticated channel cover pre-shared key-based, password-based, and certificate-based techniques. To add confidentiality to an authenticated channel, authenticated key exchange (AKE) protocols usually have one of the three solutions built in. As an amplification, hybrid AKE (HAKE) approaches are getting more popular nowadays and were presented in several flavors to incorporate classical, post-quantum, or quantum-key-distribution components. The main benefit is redundancy, i.e., if some of the components fail, the primitive still yields a confidential and authenticated channel. However, current HAKE instantiations either rely on pre-shared keys (which yields inefficient end-to-end authenticity) or only support one or two of the three above components (resulting in reduced redundancy and flexibility). In this work, we present an extension of a modular HAKE framework due to Dowling, Brandt Hansen, and Paterson (PQCrypto’20) that does not suffer from the above constraints. While their instantiation, dubbed Muckle, requires pre-shared keys (and hence yields inefficient end-to-end authenticity), our extended instantiation called Muckle+ utilizes post-quantum digital signatures. While replacing pre-shared keys with digital signatures is rather straightforward in general, this turned out to be surprisingly non-trivial when applied to HAKE frameworks (resulting in a significant model change with adapted proof techniques).

Forward secrecy is considered an essential design goal of modern key establishment (KE) protocols, such as TLS 1.3, for example. Furthermore, efficiency considerations such as zero round-trip time (0-RTT), where a client is able to send cryptographically protected pay- load data along with the very first KE message, are motivated by the practical demand for secure low-latency communication. For a long time, it was unclear whether protocols that simultaneously achieve 0-RTT and full forward secrecy exist. Only recently, the first forward-secret 0-RTT protocol was described by Günther et al. (Euro- crypt 2017). It is based on Puncturable Encryption. Forward secrecy is achieved by “puncturing” the secret key after each decryption operation, such that a given ciphertext can only be decrypted once (cf. also Green and Miers, S&P 2015). Unfortunately, their scheme is completely impractical, since one puncturing operation takes between 30 seconds and several minutes for reasonable security and deployment parameters, such that this solution is only a first feasibility result, but not efficient enough to be deployed in practice. In this paper, we introduce a new primitive that we term Bloom Filter Encryption (BFE), which is derived from the probabilistic Bloom filter data structure. We describe different constructions of BFE schemes, and show how these yield new puncturable encryption mechanisms with extremely efficient puncturing. Most importantly, a puncturing operation only involves a small number of very efficient computations, plus the deletion of certain parts of the secret key, which outperforms previous constructions by orders of magnitude. This gives rise to the first forward-secret 0-RTT protocols that are efficient enough to be deployed in practice. We believe that BFE will find applications beyond forward- secret 0-RTT protocols.