Polylogarithmic Proofs for Multilinears over Binary Towers
Benjamin E. Diamond, Jim Posen
2024 · Full Version · eprint 2024/504
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Abstract
The use of small fields has come to typify the design of modern, production-oriented SNARKs. In this work, we treat multilinear polynomial commitment over tiny fields. A tiny-field polynomial—in the nomenclature of Diamond and Posen (EUROCRYPT ’25)—is defined over a field that has fewer elements than the polynomial itself has coefficients. We focus on multilinears over the field with just two elements.
In this work, we generically reduce the problem of tiny-field commitment to that of large-field commitment. We introduce a sumcheck-based compiler—called “ring-switching”—which, upon being fed a multilinear polynomial commitment scheme over some large extension field, yields a further scheme over that field’s ground field. The resulting scheme lacks embedding overhead, in that its commitment cost, on each input, equals that of the large-field scheme on each input of identical size (in bits). Its evaluation protocol’s overhead is linear for the prover and logarithmic for the verifier, and is essentially optimal.
Instantiating our ring-switching reduction on the BaseFold (CRYPTO ’24) large-field multilinear polynomial commitment scheme—or more precisely on a characteristic-2 adaptation of that scheme that we develop at length—we obtain an extremely fast polynomial commitment scheme for bit-valued multilinears. Our scheme outperforms its state-of-the-art peers, a fact we demonstrate experimentally.
1 Introduction
Today’s fastest, production-oriented SNARKs universally use small fields. Before 2022, virtually all SNARKs operated over just one—cryptographically large—field. The small-field revolution began in earnest with ethSTARK and Plonky2. Those systems decouple the respective fields used within their arithmetization and cryptography portions. That is, they use small fields—sized roughly like a 64-bit register—for their arithmetizations; each, during its security-critical portions, passes to a cryptographically large field extension of its arithmetization field. Subsequent production-oriented SNARKs like Plonky3 and RISC Zero have embraced similar designs, based on 32-bit prime fields; S-two has adopted a related architecture based on Haböck, Levit and Papini’s Circle STARK [HLP24]. By using small fields, these systems have managed to deliver industry-leading provers, which outperform those grounded in elliptic-curve-based SNARKs (like Sonic [MBKM19], PlonK [GWC19] and Marlin [Chi+20]).
These SNARKs all use arithmetization fields which, though relatively small, are nonetheless at least as large as the statements they’re capable of proving. This fact is not a coincidence. Indeed, all of them operate by, roughly, arranging their witness data into a trace table, Reed–Solomon-encoding that table’s columns, and finally invoking a low-degree test based on FRI [BBHR18a]. Reed–Solomon codes demand alphabets that are at least as large as their block lengths are.
A recent work of Diamond and Posen [DP25] breaks this trace-length barrier, in that it treats even polynomials over tiny fields—fields smaller than the statement’s trace length. Crucially, that work does so without embedding overhead. One might trivially attempt to commit to a tiny-field multilinear simply by embedding its coefficients into an extension, before blackbox-invoking a standard scheme on the resulting object. That approach would face at least two deficiencies. On the efficiency front, it would secure no gain from the tininess of its input’s coefficients. On the security front, it would fail to guarantee the tininess of the prover’s input, a security desideratum which, in practice, turns out to be essential.
In this work, we introduce a generic reduction from the problem of tiny-field multilinear polynomial commitment to that of large-field multilinear commitment. Our techniques are rather different from those of Diamond–Posen [DP25]. Our reduction, applied to any large-field scheme, yields a corresponding tiny-field scheme, which moreover lacks embedding overhead. It is agnostic to the large-field scheme used. In fact, our reduction works even on recently introduced, state-of-the-art large-field schemes like Blaze [Bre+25] and WHIR [ACFY25], and even on large-field schemes that haven’t been created yet. Its overhead over the underlying large-field scheme given to it is essentially optimal, and beats that associated with alternative constructions, like Hashcaster.
1.1 Some Historical Remarks
Diamond and Posen [DP25] break the trace-length barrier by further decoupling two fields which, in all of the small-field schemes noted above, coincide: the arithmetization field and the alphabet field. All of those schemes use just one prime field—again, sized just under 32 or 64 bits—both as the coefficient field of the polynomials committed and as the alphabet of the Reed–Solomon code used to encode them. The scheme [DP25] makes possible the simultaneous use of a tiny arithmetization field and a small alphabet field. Separately, that work reintroduces the use of binary fields, fields of characteristic 2. Finally, that work treats exclusively multilinear polynomials; in this capacity, it extends an important line of work which includes Libra [Xie+19], Virgo [ZXZS20], Spartan [Set20], Brakedown [Gol+23], and HyperPlonk [CBBZ23].
To make their technique work, Diamond and Posen [DP25] introduce a data-casting operation—which they call packing, and which is based on field extensions—which serves to recast a witness defined over \mathbb{F}_2, say, into a shorter witness over the larger field \mathbb{F}_{2^{32}}. They then apply a Brakedown-like multilinear commitment procedure to the resulting witness, whose coefficient field, crucially, is large enough to be used as a Reed–Solomon alphabet.
As Diamond and Posen [DP25] implicitly observe, unlike DEEP-ALI [BGKS19], Brakedown [Gol+23]—like its predecessor works Bootle, Chiesa and Groth [BCG20] and Ron-Zewi and Rothblum [RR24]—decouples the coding-theoretic specifications of its code from the semantics of its code. This decoupling makes Diamond and Posen’s packing procedure coherent.
Zeilberger, Chen and Fisch’s BaseFold PCS [ZCF24, § 5] seems to be the first multilinear polynomial commitment scheme with a polylogarithmic verifier that doesn’t use quotienting. This work’s small-field PCS works in a drop-in way with the PIOP of [DP25]. By combining this work’s PCS with that work’s PIOP, one stands to obtain an efficient, full-blown SNARK for binary witnesses.
1.2 Our Contributions
We sketch our contributions here.
A reduction from tiny-field commitment to huge-field commitment. Ring-switching is a generic compiler, which, on input a multilinear polynomial commitment scheme over some large extension field L/K, yields a new multilinear polynomial commitment scheme over the ground field K. Here, L can be of any characteristic, including 2. Each scheme our compiler outputs lacks embedding overhead, in that its commitment cost on each K-multilinear equals the original scheme’s commitment cost on an L-multilinear of equal size in bits.
We write 2^\kappa for the degree of L over K, which we assume is a power of 2. Diamond and Posen’s [DP25, § 4] packing procedure takes in an \ell-variate multilinear t(X_0, \ldots, X_{\ell-1}) over K and produces the packed multilinear t'(X_0, \ldots, X_{\ell'-1}) over L, where \ell' := \ell - \kappa. Our K-scheme’s commitment procedure simply packs t(X_0,\ldots,X_{\ell-1}) and invokes the underlying L-scheme’s commitment procedure on t'(X_0,\ldots,X_{\ell'-1}), the result. The hard part is relating an evaluation claim on t to one on t'. To do this, we devise an unusual matrix-transposition trick, and write down an \ell - \kappa-round sumcheck between t' and a new sort of polynomial we call a ring-switch equality indicator.
An improved multilinear PCS for large binary fields, based on BaseFold. Zeilberger, Chen and Fisch’s BaseFold PCS [ZCF24, § 5] is an important multilinear polynomial commitment scheme for large prime fields. In order to spin up a target for our ring-switching reduction, we develop a characteristic-2 variant of BaseFold PCS, i.e. for large binary fields. We also improve that scheme by incorporating into it higher-arity folding, an optimization that reduces its proof sizes by more than half.
To make higher-arity folding work with BaseFold, we introduce oracle-skipping, a new higher-arity folding mechanism. Our security proof necessitates a different sort of proximity gap than FRI’s approach does, one pertaining not to low-degree curves but instead to tensor combinations. Precisely this kind of proximity gap is established in recent work of Diamond and Gruen [DG25], which we leverage.
A state-of-the-art PCS for multilinears over tiny binary fields. Putting the two parts above together, we obtain an extremely fast PCS for tiny-field multilinears. On a 28-variate multilinear over \mathbb{F}_2, our scheme achieves singlethreaded commitment and opening times of just 144 and 160 milliseconds, respectively; our multithreaded commitment and opening times are 44 and 71 milliseconds, respectively. Our proofs for polynomials of this size are 0.351 MiB, and our verifier runs in under a millisecond.
Our measurements beat Plonky3’s by between one and two orders of magnitude. Plonky3 takes 25 and 13 seconds, respectively, to commit a comparable amount of data, in the singlethreaded setting, and 4 and 2 seconds in multithreaded mode. Its proofs are also larger, and its verifier is around 20 milliseconds.
1.3 Ring-Switching
In this subsection, we gently introduce ring-switching, prioritizing technical simplicity and accuracy. We fix a field extension L/K of power-of-2 degree 2^\kappa. Though ring-switching works for any such field extension, of any characteristic, the special case K = \mathbb{F}_2 and L = \mathbb{F}_{2^{128}} is especially important and exemplary. We write \mathcal{B}_\kappa := \{0,1\}^\kappa for the \kappa-dimensional unit cube.
The problem. We assume access to a large-field multilinear polynomial commitment scheme for multilinears over L. How might we obtain a small-field commitment scheme for multilinears over K, assuming access to that large-field scheme?
We begin with a small-field multilinear, say t(X_0, \ldots, X_{\ell-1}) \in K[X_0, \ldots, X_{\ell-1}]^{\leq 1}, that we’d like to commit to. Following Diamond and Posen [DP25], we fix a basis (\beta_v)_{v \in \mathcal{B}_\kappa} of L over K, write \ell' := \ell - \kappa, and define the packed multilinear:
Our commitment procedure is simple: it just packs t(X_0, \ldots, X_{\ell-1}) and invokes the underlying large-field scheme’s commitment procedure on t'(X_0, \ldots, X_{\ell'-1}). This procedure lacks embedding overhead, since t and t' are of the same size in bits.
To check an evaluation claim on t, we must reduce it to one on t'. We fix a point (r_0, \ldots, r_{\ell-1}) and an evaluation claim s \stackrel{?}{=} t(r_0, \ldots, r_{\ell-1}).
A strawman approach. The prover sends values (\hat{s}_v)_{v \in \mathcal{B}_\kappa} which—it claims—respectively satisfy \hat{s}_v \stackrel{?}{=} t(v_0, \dots, v_{\kappa-1}, r_\kappa, \dots, r_{\ell-1}) for each v \in \mathcal{B}_\kappa. The verifier checks whether
holds. While this protocol is complete, it’s not secure. The basis (\beta_v)_{v \in \mathcal{B}_\kappa} is linearly independent over K, but not over L.
Our solution. Our idea is—very roughly—to further decompose the claims until they are defined over K. We may then apply the linear combination “slice-wise”. For each v \in \mathcal{B}_\kappa, the verifier can freely basis-decompose the prover’s quantity \hat{s}_v, writing
for appropriate K-elements. After further decomposition, batching via random scalars (r''_0, \ldots, r''_{\kappa-1}), and running a sumcheck, the verifier must finally evaluate the “ring-switch equality indicator”
where \varphi_0 and \varphi_1 denote the two canonical embeddings of L into the tensor algebra A := L \otimes_K L. The verifier may compute this quantity succinctly in just 2 \cdot 2^\kappa \cdot \ell' L-multiplications.
Hashcaster. We compare ring-switching in detail to Hashcaster. Soukhanov’s Hashcaster [Sou24] is a SNARK for binary witnesses. That work yields an alternative reduction from the problem of evaluating the K-multilinear to that of evaluating its packed L-multilinear. Hashcaster’s verifier must compute a matrix transformation that entails a quadratic number of L-multiplications in the extension degree 2^\kappa. Ring-switching’s verifier’s number of L-multiplications grows only linearly.
1.4 Binary BaseFold
In order to apply ring-switching, we need a large-field scheme to invoke it on. To this end, we adapt BaseFold PCS [ZCF24, § 5] to the characteristic 2 setting. To achieve this, we must re-examine the classic additive NTT of Lin, Chung and Han [LCH14]. We show that that NTT fits into a framework articulated by Haböck, Levit and Papini [HLP24], though it predates that latter work.
The problem. BaseFold PCS identifies a connection between FRI and multilinear evaluation. Each honest FRI prover begins with the evaluation of some polynomial P(X) := \sum_{j=0}^{2^\ell-1} a_j \cdot X^j over its initial domain S^{(0)}. During the course of the protocol, the prover repeatedly “folds” its initial word. BaseFold PCS relies on the fact whereby a FRI execution which begins on the Reed–Solomon encoding will end on a constant polynomial whose value equals the multilinear evaluation at the verifier’s FRI challenges. For maps chosen generically, this fact will fail to hold in characteristic 2.
Our solution. We recover BaseFold PCS in characteristic 2 by introducing a specialization of binary FRI that works compatibly with Lin–Chung–Han’s additive NTT [LCH14]. We introduce a particular choice of folding maps q^{(0)}, \ldots, q^{(\ell-1)} which causes the equality between the final FRI constant and the multilinear evaluation to re-emerge. The right choice turns out to be that for which, for each i, the composition identity \widehat{W}_i = q^{(i-1)} \circ \dots \circ q^{(0)} holds, where \widehat{W}_i are Lin–Chung–Han’s normalized subspace vanishing polynomials.
Oracle-skipping. Standard FRI supports arbitrary-arity folding, controlled by a folding arity parameter \eta \geq 1. BaseFold PCS, however, does not support the use of univariate higher-arity folding. We introduce a new, multilinear style of many-to-one FRI folding. We parameterize our method by a constant \vartheta: the prover performs standard 2-to-1 folding \vartheta times in succession, committing only to the last among the oracles it obtains. Our folding technique makes necessary a tensor-folding proximity gap of the sort recently established by Diamond and Gruen [DG25].
1.5 Concurrent and Subsequent Works
Blaze. Blaze [Bre+25] is a polynomial commitment scheme for multilinears over large binary fields. Using a technique grounded in code-switching [RR24], Blaze obtains a strictly linear-time commitment procedure, a linear-time prover, and a polylogarithmic verifier. Operating over \mathbb{F}_{2^{128}} throughout, Blaze reports commitment and proving times that improve upon binary BaseFold’s by roughly threefold at the \ell = 28 problem size, though its proofs are larger (2.5 MiB compared to 1.4 MiB for binary BaseFold). Incorporating oracle-skipping and further concrete proof size optimizations, this work obtains a proof size of 0.436 MiB in the \ell = 28 case.
Blaze’s PCS is compatible with this work’s ring-switching compiler. Upon instantiating our ring-switching reduction on Blaze’s large-field PCS, one would obtain an alternative small-field scheme.
2 Background and Notation
We write \mathbb{N} for the nonnegative integers. All fields in this work are finite. We fix a binary field L. For each \ell \in \mathbb{N}, we write \mathcal{B}_\ell for the \ell-dimensional boolean hypercube \{0,1\}^\ell \subset L^\ell. For L a field and R \subset L^\vartheta a subset, we write \mu(R) := \frac{|R|}{|L|^\vartheta}.
2.1 Multilinear Polynomials
An \ell-variate polynomial in L[X_0,\ldots,X_{\ell-1}] is multilinear if each of its indeterminates appears with individual degree at most 1. We introduce the 2 \cdot \ell-variate polynomial
For each fixed (r_0, \ldots, r_{\ell-1}) \in L^\ell, the vector (\widetilde{\text{eq}}(r_0, \ldots, r_{\ell-1}, w_0, \ldots, w_{\ell-1}))_{w \in \mathcal{B}_\ell} takes the form of a tensor product expansion \bigotimes_{i=0}^{\ell-1} (1 - r_i, r_i), computable in 2^\ell L-additions and L-multiplications.
2.2 Error-Correcting Codes
A code of block length n over the alphabet \Sigma is a subset of \Sigma^n. A linear [n,k,d]-code over L is a k-dimensional linear subspace C \subset L^n for which d(v_0,v_1) \geq d holds for each unequal pair. The unique decoding radius is \lfloor \frac{d-1}{2} \rfloor.
We recall Reed–Solomon codes. We fix nonnegative message length and rate parameters \ell and \mathcal{R}, as well as a subset S \subset L of size 2^{\ell+\mathcal{R}}. We write C for the Reed–Solomon code \mathsf{RS}_{L,S}[2^{\ell+\mathcal{R}},2^\ell], defined to be the set of evaluation vectors of polynomials of degree less than 2^\ell over S.
Lemma 2.1
If d(f,C) \geq \frac{d}{2}, then Algorithm 1 (Berlekamp–Welch) outputs \bot.
2.3 The Novel Polynomial Basis
We recall in detail the novel polynomial basis of Lin, Chung and Han [LCH14, § II. C.]. We fix a binary field L of degree r over \mathbb{F}_2. We fix an \mathbb{F}_2-basis (\beta_0, \dots, \beta_{r-1}) of L. For each i \in \{0, \dots, \ell\}, we define the subspace vanishing polynomial W_i(X) := \prod_{u \in U_i}(X-u) and its normalized variant \widehat{W}_i(X) := \frac{W_i(X)}{W_i(\beta_i)}. The novel polynomial basis elements are X_j(X) := \prod_{i=0}^{\ell-1}\widehat{W}_i(X)^{j_i}.
2.4 FRI
We recall Ben-Sasson, Bentov, Horesh and Riabzev’s [BBHR18a] Fast Reed–Solomon Interactive Oracle Proof of Proximity (FRI). For L a binary field, FRI yields an IOP of proximity for the Reed–Solomon code. Internally, FRI makes use of a fixed, global sequence of subspaces and maps:
For each i, q^{(i)} is a subspace polynomial of degree 2, whose kernel is 1-dimensional and contained in S^{(i)}.
2.5 Tensor Products of Fields
We define the tensor product A := L \otimes_K L of L with itself over K. Each A-element is, concretely, a 2^\kappa \times 2^\kappa array of K-elements. For each a \in A, there is a unique tuple of L-elements (a_v)_{v \in \mathcal{B}_\kappa} for which a = \sum_{v \in \mathcal{B}_\kappa} a_v \otimes \beta_v holds (the column representation). Similarly, there is a unique row representation.
The maps \varphi_0 and \varphi_1 respectively embed L into A’s left-hand column and top row. Concretely, \varphi_0(\alpha) \cdot a differs from a by column-wise multiplication by \alpha; \varphi_1(\alpha) \cdot a differs from a by row-wise multiplication by \alpha.
Definition 2.2 (Packed Polynomial)
For each extension L/K, with K-basis (\beta_v)_{v \in \mathcal{B}_\kappa}, and each multilinear t(X_0, \ldots, X_{\ell-1}) over K, we write \ell' := \ell - \kappa and define the packed polynomial t'(X_0,\ldots,X_{\ell'-1}) := \sum_{v \in \mathcal{B}_\kappa} t(v_0,\ldots,v_{\kappa-1},X_0,\ldots,X_{\ell'-1}) \cdot \beta_v.
2.6 Proximity Gaps
Theorem 2.3 (Ben-Sasson et al. [Ben+23, Thm. 4.1])
For each proximity parameter e \in \{0, \ldots, \lfloor \frac{d-1}{2} \rfloor\} and each pair of words u_0 and u_1 in L^{2^{\ell+\mathcal{R}}}, if \Pr_{r \in L}[d((1-r) \cdot u_0 + r \cdot u_1, C) \le e] > \frac{n}{|L|}, then d^2((u_i)_{i=0}^1, C^2) \le e.
Theorem 2.4 (Diamond–Gruen [DG25, Cor. 1])
Reed–Solomon codes exhibit tensor-style proximity gaps within the unique decoding radius. For each proximity parameter e, tensor arity \vartheta \geq 1, and list of words, if the probability that the tensor combination is within distance e to C exceeds \vartheta \cdot \frac{n}{|L|}, then the interleaved distance is at most e.
2.7 Security Definitions
Definition 2.8 (IOPCS)
An interactive oracle polynomial commitment scheme (IOPCS) is a tuple of algorithms \Pi = (\mathsf{Setup}, \mathsf{Commit}, \mathcal{P}, \mathcal{V}) with Setup, Commit, and an evaluation IOP b \leftarrow \langle \mathcal{P}([f], s, r; t), \mathcal{V}([f], s, r) \rangle.
Definition 2.9 (Security of IOPCS)
The IOPCS \Pi is secure if, for each PPT adversary \mathcal{A}, there exists a PPT emulator \mathcal{E} and a negligible function \mathsf{negl} such that \Pr[\mathsf{Real}_{\mathcal{A}}^{\Pi,\ell}(\lambda) \neq \mathsf{Ideal}_{\mathcal{E},\mathcal{A}}^{\Pi,\ell}(\lambda)] \leq \mathsf{negl}(\lambda).
Definition 2.10 (Small-Field IOPCS)
A small-field IOPCS is defined identically to Definition 2.8, except that the polynomial t(X_0,\ldots,X_{\ell-1}) is required to have coefficients in a subfield K \subset L, and the emulator must extract a polynomial over K.
3 Ring-Switching
In this section, we formally present ring-switching. The key identity underlying the protocol is tensor-algebraic:
For each pair of L-elements \alpha_0 and \alpha_1, the exterior product satisfies \alpha_0 \star \alpha_1 = \varphi_0(\alpha_0) \cdot \varphi_1(\alpha_1), where the right-hand product is the ambient multiplication in the tensor algebra.
3.1 Ring-Switching Protocol
Construction 3.1 (Ring-Switching Compiler)
A large-field scheme \Pi' = (\mathsf{Setup}', \mathsf{Commit}', \mathcal{P}', \mathcal{V}') is given as input. The small-field scheme \Pi is defined as follows:
- Setup: Run \Pi'.\mathsf{Setup}'(1^\lambda, \ell'), where \ell' = \ell - \kappa.
- Commit: Pack t(X_0,\ldots,X_{\ell-1}) into t'(X_0,\ldots,X_{\ell'-1}) via Definition 2.2; output \Pi'.\mathsf{Commit}'(\mathsf{params}, t').
- Evaluate: \mathcal{P} computes \hat{s} := \varphi_1(t')(\varphi_0(r_\kappa), \dots, \varphi_0(r_{\ell-1})) and sends the A-element \hat{s} to \mathcal{V}. \mathcal{V} checks the evaluation claim via the column decomposition, samples batching scalars, then both parties execute an \ell'-round sumcheck. At the end, \mathcal{V} computes e := \widetilde{\text{eq}}(\varphi_0(r_\kappa), \dots, \varphi_0(r_{\ell-1}), \varphi_1(r'_0), \dots, \varphi_1(r'_{\ell'-1})) and verifies the final sumcheck check using the underlying large-field scheme.
Theorem 3.2 (Completeness)
If \Pi' is complete, then \Pi also is.
Theorem 3.5 (Security)
If \Pi' is secure, then \Pi also is. The soundness error contributed by ring-switching is at most \frac{2 \cdot \ell' + \kappa}{|L|}.
3.2 Efficiency
Prover cost. The prover’s main cost is that of computing the tensor expansion \bigotimes_{i=\kappa}^{\ell-1} (1 - r_i, r_i), taking 2^{\ell'} L-multiplications. The total prover cost is O(2^\ell) \cdot \lambda K-operations—linear in the input length and the security parameter.
Verifier cost. To compute e, the verifier must perform 2 \cdot 2^\kappa \cdot \ell' L-multiplications (via Remark 3.4: alternating column-scaling and row-scaling with transpositions). The verifier is logarithmic in the statement size and quadratic in \lambda (up to polylogarithmic factors): O(\ell) \cdot \widetilde{O}(\lambda^2) K-operations.
Comparison to Hashcaster. Hashcaster’s verifier’s dependence on \lambda is cubic (\widetilde{O}(\lambda^3) K-operations), as opposed to our quadratic. Hashcaster’s prover, if implemented naïvely, is about 2^\kappa-fold more costly than ours.
4 Binary BaseFold
In this section, we present our large-field PCS. We tie together Lin, Chung and Han [LCH14]’s additive NTT and FRI [BBHR18a].
4.1 Using FRI in Novel Polynomial Basis
Definition 4.1 (Domains and Folding Maps)
For each i \in \{0, ..., \ell\}, we define the domain S^{(i)} := \widehat{W}_i(\langle \beta_0, \dots, \beta_{\ell+\mathcal{R}-1} \rangle). For each i \in \{0, \dots, \ell - 1\}, we define q^{(i)}(X) := \frac{W_i(\beta_i)^2}{W_{i+1}(\beta_{i+1})} \cdot X \cdot (X+1).
Lemma 4.2
For each i \in \{0, ..., \ell-1\}, we have the equality q^{(i)} \circ \widehat{W}_i = \widehat{W}_{i+1} of polynomials.
Theorem 4.3
For each i \in \{0, ..., \ell - 1\}, q^{(i)}(S^{(i)}) = S^{(i+1)}.
Corollary 4.4
For each i \in \{0, \dots, \ell\}, \widehat{W}_i = q^{(i-1)} \circ \dots \circ q^{(0)} holds.
4.2 FRI Folding, Revisited
Definition 4.6 (Lagrange-style Folding)
We fix an index i and a map f^{(i)}: S^{(i)} \to L. For each r \in L, we define \mathsf{fold}(f^{(i)}, r): S^{(i+1)} \to L by setting, for each y \in S^{(i+1)}:
where (x_0, x_1) := q^{(i)}^{-1}(\{y\}) is the fiber over y.
Definition 4.8 (Iterated Folding)
For a positive folding factor \vartheta, we abbreviate \mathsf{fold}(f^{(i)}, r_0, \dots, r_{\vartheta-1}) := \mathsf{fold}(\dots \mathsf{fold}(f^{(i)}, r_0), \dots, r_{\vartheta-1}).
Lemma 4.9
For each folding factor \vartheta, each y \in S^{(i+\vartheta)}, there is a 2^\vartheta \times 2^\vartheta invertible matrix M_y such that \mathsf{fold}(f^{(i)},r_0,\ldots,r_{\vartheta-1})(y) = [\bigotimes_{j=0}^{\vartheta-1}(1-r_j,r_j)] \cdot M_y \cdot [f^{(i)}(x_0), \ldots, f^{(i)}(x_{2^\vartheta-1})]^T.
4.3 Our Large-Field IOPCS
Construction 4.12 (Binary BaseFold IOPCS)
We define \Pi = (\mathsf{Setup}, \mathsf{Commit}, \mathcal{P}, \mathcal{V}):
- Setup: Choose a binary field L/\mathbb{F}_2 of degree r \geq \ell + \mathcal{R}. Fix a folding factor \vartheta \mid \ell and a repetition parameter \gamma = \omega(\log \lambda).
- Commit: Use t’s Lagrange coefficients as the coefficients, in the novel polynomial basis, of P(X). Using Algorithm 2 (additive NTT), compute the Reed–Solomon codeword f: S^{(0)} \to L.
- Evaluate: Interleave a sumcheck with FRI folding. For each round, the prover sends a sumcheck polynomial and folds its oracle. At multiples of \vartheta, it commits the folded oracle (oracle-skipping). The verifier checks via \gamma query repetitions.
Theorem 4.13 (Completeness)
The IOPCS of Construction 4.12 is complete.
Lemma 4.14
If f^{(i)} is the evaluation of P^{(i)}(X) = \sum_{j=0}^{2^{\ell-i}-1} a_j \cdot X_j^{(i)}(X), then under honest prover behavior, f^{(i+1)} is the evaluation of P^{(i+1)}(X) = \sum_{j=0}^{2^{\ell-i-1}-1} ((1-r_i') \cdot a_{2j} + r_i' \cdot a_{2j+1}) \cdot X_j^{(i+1)}(X).
Theorem 4.17 (Security)
The IOPCS of Construction 4.12 is secure.
4.4 Efficiency
Prover cost. In its commitment phase, the prover uses Lin, Chung and Han’s additive NTT to encode its length-2^\ell vector, using 2^{\ell+\mathcal{R}-1} \cdot \ell L-multiplications and 2^{\ell+\mathcal{R}} \cdot \ell L-additions. The prover is linear-time.
Verifier cost. The verifier expends O(\ell) L-operations for the sumcheck, and O(\lambda \cdot \ell) total L-operations for \gamma query repetitions.
5 Unrolled Small-Field IOPCS
In this section, we describe a one-shot small-field IOPCS construction. This construction inlines the large-field IOPCS of Section 4 into the ring-switching reduction of Section 3, and streamlines the combination by unifying the two sumchecks.
5.1 Combined Small-Field Protocol
Construction 5.1 (Combined Small-Field IOPCS)
We define \Pi = (\mathsf{Setup}, \mathsf{Commit}, \mathcal{P}, \mathcal{V}). Setup chooses a rate parameter \mathcal{R} and an extension field L/K of degree 2^\kappa. Commit packs t into t', flattens t' into a univariate polynomial P(X) via the novel basis, and Reed–Solomon-encodes it via Algorithm 2. The evaluation IOP interleaves ring-switching’s sumcheck with binary BaseFold’s FRI folding: \mathcal{P} sends \hat{s}, the parties run an \ell'-round sumcheck combined with oracle-skipping FRI, and \mathcal{V} performs \gamma query repetitions.
5.2 Efficiency
Concrete soundness. Construction 5.1’s concrete soundness error is bounded from above by
Proof sizes. In Table 1 below, we compare [DP25, Cons. 3.11] and Construction 5.1 on input polynomials over \mathbb{F}_2, processing evaluation claims over \mathbb{F}_{2^{128}}, attaining 100 bits of provable security.
| Num. Variables \ell | Log Inv. Rate \mathcal{R} | [DP25, Cons. 3.11] | Construction 5.1 |
|---|---|---|---|
| 24 (2 MiB) | 1 | 0.922 MiB | 0.202 MiB |
| 2 | 0.758 MiB | 0.143 MiB | |
| 3 | 0.681 MiB | 0.129 MiB | |
| 28 (32 MiB) | 1 | 3.562 MiB | 0.342 MiB |
| 2 | 2.935 MiB | 0.237 MiB | |
| 3 | 2.629 MiB | 0.210 MiB | |
| 32 (512 MiB) | 1 | 14.050 MiB | 0.514 MiB |
| 2 | 11.598 MiB | 0.351 MiB | |
| 3 | 14.376 MiB | 0.336 MiB |
Table 1: Proof sizes of polynomial commitment schemes for \mathbb{F}_2-multilinears. Folding factor \vartheta := 4, Merkle cap height j := 8.
Concrete performance. We benchmark Construction 5.1 against Plonky3 using Binius, an open-source SNARK. Both schemes use a 128-bit field. This work uses the GHASH field \mathbb{F}_2[X]/(X^{128} + X^7 + X^2 + X + 1) \cong \mathbb{F}_{2^{128}}. Plonky3 uses the quartic extension of the Baby Bear prime field. Throughout, we use rate \rho = \frac{1}{2} and attain 100 bits of provable security.
| Protocol | Proof Size | Commit (ST) | Commit (MT) | Prove (ST) | Prove (MT) | Verify (ST) |
|---|---|---|---|---|---|---|
| Plonky3 | 1.931 MiB | 24,893 ms | 3,991 ms | 13,257 ms | 1,794 ms | 18.2 ms |
| Cons. 5.1 | 0.351 MiB | 144 ms | 44 ms | 160 ms | 71 ms | 0.7 ms |
| Cons. 5.1* | 0.930 MiB | 143 ms | 44 ms | 181 ms | 75 ms | 1.8 ms |
Table 2: Concrete performance benchmarks on 28-variate multilinears over \mathbb{F}_2 (this work) and batches of 16 degree-2^{24} polynomials over Baby Bear (Plonky3). AWS c7i.8xlarge, SHA-256 Merklization.
Our scheme outperforms Plonky3 by between one and two orders of magnitude across all computational performance metrics. Our proofs are also smaller by more than fivefold.
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