International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Determining the Core Primitive for Optimally Secure Ratcheting

Fatih Balli
Paul Rösler
Serge Vaudenay
DOI: 10.1007/978-3-030-64840-4_21
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Abstract: After ratcheting attracted attention mostly due to practical real-world protocols, recently a line of work studied ratcheting as a primitive from a theoretic point of view. Literature in this line, pursuing the strongest security of ratcheting one can hope for, utilized for constructions strong, yet inefficient key-updatable primitives – based on hierarchical identity based encryption (HIBE). As none of these works formally justified utilizing these building blocks, we answer the yet open question under which conditions their use is actually necessary. We revisit these strong notions of ratcheted key exchange (RKE), and propose a more realistic (slightly stronger) security definition. In this security definition, both exposure of participants' local secrets and attacks against executions' randomness are considered. While these two attacks were partially considered in previous work, we are the first to unify them cleanly in a natural game based notion. Our definitions are based on the systematic RKE notion by Poettering and Rösler (CRYPTO 2018). Due to slight (but meaningful) changes to regard attacks against randomness, we are ultimately able to show that, in order to fulfill strong security for RKE, public key cryptography with (independently) updatable key pairs is a necessary building block. Surprisingly, this implication already holds for the simplest RKE variant. Hence, (1) we model optimally secure RKE under randomness manipulation to cover realistic attacks, (2) we (provably) extract the core primitive that is necessary to realize strongly secure RKE, and (3) our results indicate which relaxations in security allow for constructions that only rely on standard public key cryptography.
Video from ASIACRYPT 2020
  title={Determining the Core Primitive for Optimally Secure Ratcheting},
  booktitle={Advances in Cryptology - ASIACRYPT 2020},
  author={Fatih Balli and Paul Rösler and Serge Vaudenay},