Paper 2026/2391 Original: eprint.iacr.org/2026/2391 ↗

Retention Choices for Quantum CHAM Key Search under a Global Qubit Budget

Abstract

Retaining intermediate values can shorten a quantum cipher oracle but increase the logical qubits required by each parallel search worker. We propose a joint selection procedure for 80-round and revised 112-round CHAM-128/128 under a fixed global logical-qubit budget. It selects retained round addends, feasible integer allocations of search workers, and execution plans to minimize maximum scheduled logical depth subject to a target probability of recovering and accepting the original key. We evaluate libraries of 83 and 99 retention sets against a baseline whose retention choices are restricted to no retention and full retention. Both comparison groups use the same implementation and execution rules. Across 654 budget, success-target, and Toffoli-decomposition conditions per round count, the mean depth reductions are 11.12% and 8.39%, with strict improvements in 606 and 588 conditions, respectively. The selected retention count need not increase with the budget, and a fixed-circuit capacity discontinuity can disappear after reselection. The findings concern the measured libraries under an ideal-cipher success model and logical-layer costs.