**The Optimal Error Resilience of Interactive Communication Over Binary Channels**

*Meghal Gupta and Rachel Yun Zhang*

**Abstract: **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 $f(x,y)$.

In this work, we study the optimal error resilience of such a protocol in the face of adversarial bit flip or erasures. While the optimal error resilience of such a protocol over a large alphabet is well understood, the situation over the binary alphabet has remained open. In this work, we resolve this problem of determining the optimal error resilience over binary channels. In particular, we construct protocols achieving $\frac16$ error resilience over the binary bit flip channel and $\frac12$ error resilience over the binary erasure channel, for both of which matching upper bounds are known. We remark that the communication complexity of our binary bit flip protocol is polynomial in the size of the inputs, and the communication complexity of our binary erasure protocol is linear in the size of the minimal noiseless protocol computing $f$.

**Category / Keywords: **cryptographic protocols / interactive communication

**Original Publication**** (in the same form): **Currently in submission to Symposium on Theory of Computing (STOC)

**Date: **received 7 Nov 2021

**Contact author: **meghal at mit edu, rachelyz at mit edu

**Available format(s): **PDF | BibTeX Citation

**Version: **20211108:135827 (All versions of this report)

**Short URL: **ia.cr/2021/1482

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