Paper 2026/034

Benchmarking of the Amortized Post Quantum Combiner for MLS

Abstract

Overhead costs associated with post quantum (PQ) algorithms, especially digital signatures, create a significant barrier to incorporation and adoption of post quantum cryptographic protocols in various settings. To counter this, the working group for the Messaging Layer Security (MLS) protocol under the Internet Engineering Task Force has proposed an approach where traditional and PQ sessions of the protocol are strategically combined in such a way as to amortize PQ-associated overhead, i.e., an Amortized Post Quantum (APQ) combiner. In this work, we implement and benchmark APQ using standardized NIST algorithms (ML-KEM and ML-DSA) integrated into OpenMLS with native Rust cryptographic libraries, presenting the first comprehensive performance evaluation of APQ to include PQ authenticity. Our evaluation encompasses execution run-time, message size, and memory consumption a cross various security levels and amortization ratios to compare and contrast MLS with traditional-only, APQ confidentiality-only, APQ confidentiality+authenticity, and an alternative hybrid ciphersuite. We demonstrate that APQ achieves exponential improvements in message size and memory efficiency as amortization traditional:PQ ratios decrease from 1:1 to 1:100, with optimal performance observed around 1:50 ratios. These findings establish APQ as a practical solution for deploying post quantum security in resource constrained settings.