A threshold key encapsulation mechanism (TKEM) facilitates the secure distribution of session keys among multiple participants, allowing key recovery through a threshold number of shares. TKEM has gained significant attention, especially for decentralized systems, including blockchains. However, existing constructions often rely on trusted setups, which pose security risks such as a single point of failure and are limited by fixed participant numbers and thresholds. To overcome this issue, we propose a dynamic TKEM with a transparent setup, allowing for a flexible selection of both recipients and thresholds without relying on trusted third parties in the setup phase. In addition, our construction does not rely on pairing operations, which are less efficient compared to exponentiation. We prove the selective chosen-ciphertext security of our construction under the decisional Diffie-Hellman assumption, zero-knowledge, and soundness of a non-interactive zero-knowledge (NIZK) proof system. We also show that our scheme satisfies decapsulation consistency when the underlying NIZK system is sound. Our proof-of-concept implementation highlights the practicality and efficiency of this approach, further advancing the field of threshold cryptography.

Implantable medical devices (IMDs) continuously monitor the condition of a patient and directly apply treatments if considered necessary. Because IMDs are highly effective for patients who frequently visit hospitals (e.g., because of chronic illnesses such as diabetes and heart disease), their use is increasing significantly. However, related security concerns have also come to the fore. It has been demonstrated that IMDs can be hacked—the IMD power can be turned off remotely, and abnormally large doses of drugs can be injected into the body. Thus, IMDs may ultimately threaten a patient’s life. In this paper, we propose an energy-aware key exchange protocol for securing IMDs. We utilize synchronous interpulse intervals (IPIs) as the source of a secret key. These IPIs enable IMDs to agree upon a secret key with an external programmer in an authenticated and transparent manner without any key material being exposed either before distribution or during initialization. We demonstrate that it is difficult for adversaries to guess the keys established using our method. In addition, we show that the reduced communication overhead of our method enhances battery life, making the proposed approach more energy-efficient than previous methods.