International Association for Cryptologic Research

International Association
for Cryptologic Research


Gareth T. Davies


Symmetric Key Exchange with Full Forward Security and Robust Synchronization 📺
We construct lightweight authenticated key exchange protocols based on pre-shared keys, which achieve full forward security and rely only on simple and efficient symmetric-key primitives. All of our protocols have rigorous security proofs in a strong security model, all have low communication complexity, and are particularly suitable for resource-constrained devices. We describe three protocols that apply linear key evolution to provide different performance and security properties. Correctness in parallel and concurrent protocol sessions is difficult to achieve for linearly key-evolving protocols, emphasizing the need for assurance of availability alongside the usual confidentiality and authentication security goals. We introduce synchronization robustness as a new formal security goal, which essentially guarantees that parties can re-synchronize efficiently. All of our new protocols achieve this property. Since protocols based on linear key evolution cannot guarantee that all concurrently initiated sessions successfully derive a key, we also propose two constructions with non-linear key evolution based on puncturable PRFs. These are instantiable from standard hash functions and require O( C log(|CTR|)) memory, where C is the number of concurrent sessions and |CTR| is an upper bound on the total number of sessions per party. These are the first protocols to simultaneously achieve full forward security, synchronization robustness, and concurrent correctness.
Fast and Secure Updatable Encryption 📺
Updatable encryption allows a client to outsource ciphertexts to some untrusted server and periodically rotate the encryption key. The server can update ciphertexts from an old key to a new key with the help of an update token, received from the client, which should not reveal anything about plaintexts to an adversary. We provide a new and highly efficient suite of updatable encryption schemes that we collectively call SHINE. In the variant designed for short messages, ciphertext generation consists of applying one permutation and one exponentiation (per message block), while updating ciphertexts requires just one exponentiation. Variants for longer messages provide much stronger security guarantees than prior work that has comparable efficiency. We present a new confidentiality notion for updatable encryption schemes that implies prior notions. We prove that SHINE is secure under our new confidentiality definition while also providing ciphertext integrity.