Heterogeneous Internet of Things (IoTs) harboring resource-limited devices like wearable sensors are essential for next-generation networks. Ensuring the authentication and integrity of security-sensitive telemetry in these applications is vital. Digital signatures provide scalable authentication with non-repudiation and public verifiability, making them essential tools for IoTs. However, emerging quantum computers necessitate post-quantum (PQ) secure solutions, yet existing NIST-PQC standards are costlier than their conventional counterparts and unsuitable for resource-limited IoTs. There is a significant need for lightweight PQ-secure digital signatures that respect the resource constraints of low-end IoTs. We propose a new multiple-time hash-based signature called Maximum Utilization Multiple HORS (MUM-HORS) that offers PQ security, short signatures, fast signing, and high key utilization for an extended lifespan. MUM-HORS addresses the inefficiency and key loss issues of HORS in offline/online settings by introducing compact key management data structures and optimized resistance to weak-message attacks. We tested MUM-HORS on two embedded platforms (ARM Cortex A-72 and 8-bit AVR ATmega2560) and commodity hardware. Our experiments confirm up to 40x better utilization with the same signing capacity (2^20 messages, 128-bit security) compared to multiple-time HORS while achieving 2x and 156-2463x faster signing than conventional-secure and NIST PQ-secure schemes, respectively, on an ARM Cortex. These features make MUM-HORS ideal multiple-time PQ-secure signature for heterogeneous IoTs.
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