Internet of Things (IoT) applications are composed of massive quantities of resource-limited devices that collect sensitive data with long-term operational and security requirements. With the threat of emerging quantum computers, Post-Quantum Cryptography (PQC) is a critical requirement for IoTs. In particular, digital signatures offer scalable authentication with non-repudiation and are an essential tool for IoTs. However, as seen in NIST PQC standardization, post-quantum signatures are extremely costly for resource-limited IoTs. Hence, there is a significant need for quantum-safe signatures that respect the processing, memory, and bandwidth limitations of IoTs. In this paper, we created a new lightweight quantum-safe digital signature referred to as INFinity-HORS (INF-HORS), which is (to the best of our knowledge) the first signer-optimal hash-based signature with (polynomially) unbounded signing capability. INF-HORS enables a verifier to non-interactively construct one-time public keys from a master public key via encrypted function evaluations. This strategy avoids the performance bottleneck of hash-based standards (e.g., SPHINCS+) by eliminating hyper-tree structures. It also does not require a trusted party or non-colliding servers to distribute public keys. Our performance analysis confirms that INF-HORS is magnitudes of times more signer computation efficient than selected NIST PQC schemes (e.g., SPHINCS+, Dilithium, Falcon) with a small memory footprint.

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