Challenges in Proving Post-Quantum Key Exchanges Based on Key Encapsulation Mechanisms
Bibliographic record
Abstract
Modern key exchange protocols are usually based on the Diffie-Hellman (DH) primitive.The beauty of this primitive, among other things, is its potential reusage of key shares: DH shares can be either used a single time or in multiple runs.Since DH-based protocols are insecure against quantum adversaries, alternative solutions have to be found when moving to the post-quantum setting.However, most postquantum candidates, including schemes based on lattices and even supersingular isogeny DH, are not known to be secure under key reuse.In particular, this means that they cannot be necessarily deployed as an immediate DH substitute in protocols.In this paper, we introduce the notion of a split key encapsulation mechanism (split KEM) to translate the desired key-reusability of a DH-based protocol to a KEM-based flow.We provide the relevant security notions of split KEMs and show how the formalism lends itself to lifting Signal's X3DH handshake to the post-quantum KEM setting without additional message flows.Although the proposed framework conceptually solves the raised issues, instantiating it securely from post-quantum assumptions proved to be non-trivial.We give passively secure instantiations from (R)LWE, yet overcoming the above-mentioned insecurities under key reuse in the presence of active adversaries remains an open problem.Approaching one-sided key reuse, we provide a split KEM instantiation that allows such reuse based on the KEM introduced by Kiltz (PKC 2007), which may serve as a post-quantum blueprint if the underlying hardness assumption (gap hashed Diffie-Hellman) holds for the commutative group action of CSIDH (Asiacrypt 2018).The intention of this paper hence is to raise awareness of the challenges arising when moving to KEM-based key exchange protocols with key-reusability, and to propose split KEMs as a specific target for instantiation in future research.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.030 | 0.096 |
| Meta-epidemiology (narrow) | 0.001 | 0.002 |
| Meta-epidemiology (broad) | 0.004 | 0.003 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.004 | 0.015 |
| Scholarly communication | 0.012 | 0.035 |
| Open science | 0.006 | 0.011 |
| Research integrity | 0.008 | 0.013 |
| Insufficient payload (model declined to judge) | 0.005 | 0.002 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".