Recognition: unknown
Regev's reduction as a candidate quantum algorithm for the discrete logarithm problem in finite abelian groups
Pith reviewed 2026-05-07 03:26 UTC · model grok-4.3
The pith
Regev's reduction applied to Cheng-Wan instances of discrete logarithm falls short of the required decoder threshold by a constant factor.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Regev's reduction does not yield an efficient quantum solver for discrete logarithm using known decoders on Cheng-Wan instances, as they fall short by a constant factor, and the required decoder performance is identified under an assumption on the instances; the obstruction is computational efficiency rather than fundamental solvability, since the Pretty Good Measurement solves the dual decoding problem on all instances including NP-hard ones but requires exponential resources.
What carries the argument
Regev's reduction, which turns a decoder for a linear code into a quantum solver for the corresponding dual decoding problem by preparing a superposition over codewords and measuring.
If this is right
- The Cheng-Wan reduction generalizes to show that solving discrete logarithm in any finite abelian group reduces to bounded distance decoding of Reed-Solomon codes.
- Bounded distance decoding for Reed-Solomon codes is NP-hard even at asymptotically zero rate.
- Known efficient decoders for Reed-Solomon codes fall short of the Cheng-Wan threshold by a constant factor when used inside Regev's reduction.
- The Pretty Good Measurement solves the dual decoding problem on every Cheng-Wan instance but requires exponential resources in general.
- A decoder reaching the identified QDP parameter would turn Regev's reduction into a quantum algorithm for discrete logarithm.
Where Pith is reading between the lines
- Quantum reductions like Regev's may be limited more by the classical task of building efficient decoders for hard instances than by quantum mechanics itself.
- This supplies a concrete performance target for future decoder research aimed at code-based quantum algorithms.
- Similar efficiency gaps could appear when applying Regev-style reductions to other problems that reduce to decoding.
- Testing the assumption on Cheng-Wan instance structure on small finite fields would clarify whether the identified threshold is robust.
Load-bearing premise
The assumption on the structure of the Cheng-Wan instances that allows identifying the precise QDP parameter a decoder must achieve to solve discrete logarithm.
What would settle it
Constructing or ruling out an efficient decoder that meets the identified QDP parameter on Cheng-Wan instances would determine whether Regev's reduction solves discrete logarithm, or showing an efficient implementation of the Pretty Good Measurement on these instances would confirm the efficiency barrier.
Figures
read the original abstract
We revisit the reduction of Cheng and Wan, which transforms instances of the discrete logarithm problem (DLOG) over finite fields into a decoding problem for Reed--Solomon codes, and study how Regev's reduction can be used to solve these instances. Regev's reduction turns a decoder for a code into a quantum solver for a decoding problem on the dual code. The quantum advantage depends on the dual problem being classically hard, which has proven difficult to establish. The Cheng--Wan reduction offers a natural source of such instances: solving them would solve discrete logarithm. Since Shor's algorithm already solves discrete logarithm, the goal is not a new quantum speedup but to understand whether Regev's reduction, applied to a problem we have independent reasons to believe is hard, can solve discrete logarithm, and if not, where it falls short. We generalize the hardness consequence of the Cheng--Wan reduction for Reed--Solomon bounded distance decoding -- from solving DLOG in $\mathbb{F}_{q^h}^\times$ to solving DLOG in finite abelian groups, and we prove that bounded distance decoding for Reed--Solomon codes is NP-hard even at asymptotically zero rate, though the known NP-hard radius lies well above the Cheng--Wan decoding radius. We then carry out Regev's reduction on the Cheng--Wan instances and evaluate it with known efficient decoders. All fall short of the Cheng--Wan threshold by a constant factor, and under an assumption on the Cheng--Wan instances we identify the QDP parameter a decoder would need to reach in order to solve discrete logarithm. The obstruction is one of efficiency rather than solvability: the Pretty Good Measurement solves the corresponding decoding problem on every instance, including NP-hard instances, but its implementation requires exponential resources in general.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript revisits the Cheng-Wan reduction mapping discrete logarithm instances over finite fields to bounded-distance decoding of Reed-Solomon codes. It generalizes the hardness implication to discrete logarithms in arbitrary finite abelian groups, proves that bounded-distance decoding of Reed-Solomon codes remains NP-hard even at asymptotically zero rate (though the hardness radius exceeds the Cheng-Wan radius), applies Regev's reduction to the resulting instances, and evaluates known efficient decoders, all of which fall short of the Cheng-Wan threshold by a constant factor. Under an assumption on the structure of the Cheng-Wan instances, the authors identify the precise QDP parameter a decoder must achieve to solve discrete logarithm and conclude that the obstruction is one of efficiency rather than solvability, since the Pretty Good Measurement solves the decoding problem on every instance (including NP-hard ones) but requires exponential resources in general.
Significance. If the results hold, the work supplies a concrete case study of Regev's reduction applied to instances that have independent hardness evidence (discrete logarithm). It isolates an efficiency barrier rather than an information-theoretic one and supplies a new NP-hardness statement for Reed-Solomon decoding at vanishing rate. These contributions are useful for understanding the scope and limitations of quantum decoding reductions for number-theoretic problems.
major comments (2)
- The assumption on the structure of the Cheng-Wan instances (used to convert the observed constant-factor gap into a precise QDP-parameter threshold) is load-bearing for the central negative claim about known decoders. The manuscript should either prove the assumption or state it with sufficient precision that its failure modes can be checked; without this, the conclusion that the gap blocks an efficient solver rests on an unverified property of the reduced instances rather than on the reduction itself.
- The NP-hardness result for bounded-distance Reed-Solomon decoding at asymptotically zero rate is stated, yet the paper explicitly notes that the known hardness radius lies well above the Cheng-Wan decoding radius. This limits the direct relevance of the hardness statement to the instances actually fed into Regev's reduction; the evaluation with concrete decoders therefore carries more weight than the complexity-theoretic claim for the paper's main argument.
minor comments (1)
- The abstract and introduction would benefit from a single-sentence statement of the precise assumption on Cheng-Wan instance structure immediately after the sentence that invokes it.
Simulated Author's Rebuttal
We thank the referee for the careful reading and constructive suggestions. The comments correctly identify two points where the manuscript's emphasis and precision can be improved. We address each below and will revise the manuscript to clarify the assumption on Cheng-Wan instances and to better foreground the concrete decoder evaluations as the primary evidence.
read point-by-point responses
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Referee: The assumption on the structure of the Cheng-Wan instances (used to convert the observed constant-factor gap into a precise QDP-parameter threshold) is load-bearing for the central negative claim about known decoders. The manuscript should either prove the assumption or state it with sufficient precision that its failure modes can be checked; without this, the conclusion that the gap blocks an efficient solver rests on an unverified property of the reduced instances rather than on the reduction itself.
Authors: We agree that the assumption is central to converting the observed gap into a precise threshold for the QDP parameter. In the revised manuscript we will restate the assumption with greater precision, explicitly enumerating the structural properties of the Cheng-Wan instances that are used (the algebraic form of the error vectors and the preservation of the discrete-log relation under the reduction). We will also add a short discussion of plausible failure modes and how they could be checked computationally on small instances or ruled out by further analysis of the reduction map. While a complete proof of the assumption lies beyond the scope of the present work, making the statement fully explicit and falsifiable directly ties the negative conclusion to the reduction itself rather than to an opaque property of the instances. revision: partial
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Referee: The NP-hardness result for bounded-distance Reed-Solomon decoding at asymptotically zero rate is stated, yet the paper explicitly notes that the known hardness radius lies well above the Cheng-Wan decoding radius. This limits the direct relevance of the hardness statement to the instances actually fed into Regev's reduction; the evaluation with concrete decoders therefore carries more weight than the complexity-theoretic claim for the paper's main argument.
Authors: We concur. The manuscript already records that the NP-hardness radius exceeds the Cheng-Wan radius, and we agree that this limits the direct applicability of the hardness result to the instances actually supplied to Regev's reduction. The central argument of the paper rests on the concrete decoder evaluations and the efficiency barrier for the Pretty Good Measurement. In the revision we will reorganize the exposition to place the decoder analysis and Regev reduction in the foreground, moving the NP-hardness theorem to an appendix so that it does not compete with the main narrative. revision: yes
Circularity Check
No circularity: external reductions evaluated against known decoders with explicit assumption only for threshold
full rationale
The derivation composes the Cheng-Wan reduction (transforming DLOG to Reed-Solomon decoding) with Regev's reduction (decoder to quantum dual decoder) and evaluates the result using independently known efficient decoders. All steps rely on prior external work and standard decoders without parameter fitting to the target DLOG instances. The single explicit assumption on Cheng-Wan instance structure is stated only to compute a precise QDP threshold and is not used to derive the central claim that known decoders fall short by a constant factor or that PGM solves the problem (albeit inefficiently). The NP-hardness extension for Reed-Solomon codes at zero rate is proved directly and does not reduce to self-citations or fitted inputs. No self-definitional, fitted-prediction, or load-bearing self-citation patterns appear.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption The Cheng-Wan reduction correctly maps discrete-logarithm instances in finite fields to bounded-distance decoding instances for Reed-Solomon codes.
- domain assumption Regev's reduction converts a classical decoder for a code into a quantum procedure for the dual decoding problem.
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