A classic result in graph theory, due to Batson, Spielman, and Srivastava (STOC 2009) shows that every graph admits a \((1 \pm \varepsilon)\) cut (or spectral) sparsifier which preserves only \(O(n/\varepsilon^2)\) reweighted edges. However, when applying this result to Cayley graphs, the resulting sparsifier is no longer necessarily a Cayley graph — it can be an arbitrary subset of edges.

We describe a new parameterized family of symmetric error-correcting codes with low-density parity-check matrices (LDPC). Our codes can be described in two seemingly different ways. First, in relation to Reed-Muller codes: our codes are functions on a subset of the points in 𝔽ⁿ whose restrictions to a prescribed set of affine lines has low degree. Alternatively, they are Tanner codes on high dimensional expanders, where the coordinates of the codeword correspond to triangles of a 2-dimensional expander, such that around every edge the local view forms a Reed-Solomon codeword. For some range of parameters our codes are provably locally testable, and their dimension is some fixed power of the block length. For another range of parameters our codes have distance and dimension that are both linear in the block length, but we do not know if they are locally testable. The codes also have the multiplication property: the coordinate-wise product of two codewords is a codeword in a related code. The definition of the codes relies on the construction of a specific family of simplicial complexes which is a slight variant on the coset complexes of Kaufman and Oppenheim. We show a novel way to embed the triangles of these complexes into 𝔽ⁿ, with the property that links of edges embed as affine lines in 𝔽ⁿ. We rely on this embedding to lower bound the rate of these codes in a way that avoids constraint-counting and thereby achieves non-trivial rate even when the local codes themselves have arbitrarily small rate, and in particular below 1/2.

We give the first construction of explicit constantdegree lossless vertex expanders. Specifically, for any $\varepsilon\gt 0$ and sufficiently large d, we give an explicit construction of an infinite family of d-regular graphs where every small set S of vertices has $(1-\varepsilon) d|S|$ neighbors (which implies $(1-2 \varepsilon) d|S|$ unique-neighbors). Our results also extend naturally to construct biregular bipartite graphs of any constant imbalance, where small sets on each side have strong expansion guarantees. The graphs we construct admit a free group action, and hence realize new families of quantum LDPC codes of Lin and M. Hsieh [1] with a linear time decoding algorithm. Our construction is based on taking an appropriate product of a constant-sized lossless expander with a base graph constructed from Ramanujan Cayley cubical complexes.

In the setting of error correcting codes, Alice wants to send a message x ∈ {0,1}ⁿ to Bob via an encoding enc(x) that is resilient to error. In this work, we investigate the scenario where Bob is a low space decoder. More precisely, he receives Alice’s encoding enc(x) bit-by-bit and desires to compute some function f(x) in low space. A generic error-correcting code does not accomplish this because decoding is a very global process and requires at least linear space. Locally decodable codes partially solve this problem as they allow Bob to learn a given bit of x in low space, but not compute a generic function f. Our main result is an encoding and decoding procedure where Bob is still able to compute any such function f in low space when a constant fraction of the stream is corrupted. More precisely, we describe an encoding function enc(x) of length poly(n) so that for any decoder (streaming algorithm) A that on input x computes f(x) in space s, there is an explicit decoder B that computes f(x) in space s ⋅ polylog(n) as long as there were not more than 1/4 - ε fraction of (adversarial) errors in the input stream enc(x).

In an error-correcting code, a sender encodes a message x ∈ {0, 1}^k such that it is still decodable by a receiver on the other end of a noisy channel. In the setting of error-correcting codes with feedback, after sending each bit, the sender learns what was received at the other end and can tailor future messages accordingly. While the unique decoding radius of feedback codes has long been known to be 1/3, the list decoding capabilities of feedback codes is not well understood. In this paper, we provide the first nontrivial bounds on the list decoding radius of feedback codes for lists of size 𝓁. For 𝓁 = 2, we fully determine the 2-list decoding radius to be 3/7. For larger values of 𝓁, we show an upper bound of 1/2 - 1/{2^(𝓁+2) - 2}, and show that the same techniques for the 𝓁 = 2 case cannot match this upper bound in general.

In the setting of error-correcting codes with feedback, Alice wishes to communicate a k-bit message x to Bob by sending a sequence of bits over a channel while noiselessly receiving feedback from Bob. It has been long known (Berlekamp, 1964) that in this model, Bob can still correctly determine x even if$\approx \frac {1}{3}$of Alice’s bits are flipped adversarially. This improves upon the classical setting without feedback, where recovery is not possible for error fractions exceeding$\frac {1}{4}$. In the corresponding setting of erasures rather than bit flips, feedback improves the error resilience from$\frac {1}{2}-\epsilon $to$1-\epsilon $for any$\epsilon \gt 0$. The original feedback setting assumes that after transmitting each bit, Alice knows (via feedback) what bit Bob received. In this work, our focus in on the limited feedback model, where Bob is only allowed to send a few bits at a small number of pre-designated points in the protocol. For any desired$\epsilon \gt 0$, we construct a coding scheme that tolerates a$ 1/3-\epsilon $fraction of bit flips (respectively a$1-\epsilon $fraction of erasures) relying only on$O_{\epsilon } (\log k)$bits of feedback from Bob sent in a fixed$O_{\epsilon } (1)$number of rounds. We complement this with a matching lower bound showing that$\Omega (\log k)$bits of feedback are necessary to recover from an error fraction exceeding$1/4$(respectively$1/2$for erasures), and for schemes resilient to a$1/3-\epsilon $fraction of bit flips (respectively a$1-\epsilon $fraction of erasures), the number of rounds must grow as$\epsilon \to 0$.

An interactive error correcting code (iECC) is an interactive protocol with the guarantee that the receiver can correctly determine the sender’s message, even in the presence of noise. It was shown in works by Gupta, Kalai, and Zhang (STOC 2022) and by Efremenko, Kol, Saxena, and Zhang (FOCS 2022) that there exist iECC’s that are resilient to a larger fraction of errors than is possible in standard error-correcting codes without interaction. In this work, we improve upon these existing works in two ways: - First, we improve upon the erasure iECC of Kalai, Gupta, and Zhang, which has communication complexity quadratic in the message size. In our work, we construct the first iECC resilient to > 1/2 adversarial erasures that is also positive rate. For any ε > 0, our iECC is resilient to 6/11 - ε adversarial erasures and has size O_ε(k). - Second, we prove a better upper bound on the maximal possible error resilience of any iECC in the case of bit flip errors. It is known that an iECC can achieve 1/4 + 10^{-5} error resilience (Efremenko, Kol, Saxena, and Zhang), while the best known upper bound was 2/7 ≈ 0.2857 (Gupta, Kalai, and Zhang). We improve upon the upper bound, showing that no iECC can be resilient to more than 13/47 ≈ 0.2766 fraction of errors.

In the setting of error-correcting codes with feedback, Alice wishes to communicate a k-bit message x to Bob by sending a sequence of bits over a channel while noiselessly receiving feedback from Bob. It has been long known (Berlekamp, 1964) that in this model, Bob can still correctly determine x even if ≈ 1/3 of Alice’s bits are flipped adversarially. This improves upon the classical setting without feedback, where recovery is not possible for error fractions exceeding 1/4.

Given a noiseless protocol π0 computing a function f(x, y) of Alice and Bob’s private inputs x, y, the goal of interactive coding is to construct an error-resilient protocol π computing f such that even if some fraction of the communication is adversarially corrupted, both parties still learn f(x, y). Ideally, the resulting scheme π should be positive rate, computationally efficient, and achieve optimal error resilience.

An error correcting code (ECC) allows a sender to send a message to a receiver such that even if a constant fraction of the communicated bits are corrupted, the receiver can still learn the message correctly. Due to their importance and fundamental nature, ECC’s have been extensively studied, one of the main goals being to maximize the fraction of errors that the ECC is resilient to.

In interactive coding, Alice and Bob wish to compute some function f of their individual private inputs x and y. They do this by engaging in a non-adaptive (fixed order, fixed length) interactive protocol to jointly compute f(x,y). The goal is to do this in an error-resilient way, such that even given some fraction of adversarial corruptions to the protocol, both parties still learn f(x,y).

We construct a succinct non-interactive publicly-verifiable delegation scheme for any log-space uniform circuit under the sub-exponential Learning With Errors (LWE) assumption. For a circuit C:{0,1}N→{0,1} of size S and depth D, the prover runs in time poly(S), the communication complexity is D · polylog(S), and the verifier runs in time (D+N) ·polylog(S). To obtain this result, we introduce a new cryptographic primitive: a lossy correlation-intractable hash function family. We use this primitive to soundly instantiate the Fiat-Shamir transform for a large class of interactive proofs, including the interactive sum-check protocol and the GKR protocol, assuming the sub-exponential hardness of LWE.