Fix Potential Overflow Problem within NamecheapPushDomainVerifier

[Koukyosyumei]

Overview

NamecheapPushDomainVerifier template currently validates fromEmailIndex < emailHeaderLength, namecheapBuyerIdIndex < emailBodyLength, and namecheapDomainNameIndex < emailBodyLength with LessThan template.

However, the LessThan(N) template has a known over-flow issue: If the bit-length of the input exceeds $N$, LessThan may produce unintended results. For example, suppose maxHeadersLength = 768, emailHeaderLength = 512, and fromEmailIndex = 21888242871839275222246405745257275088548364400416034343698204186575808495588. In this case, the output of LessThan(log2Ceil(maxHeadersLength))([fromEmailIndex, emailHeaderLength]) is 1, meaning that this malicious input satisfies the constraints of NamecheapPushDomainVerifier.

Fix

To address this problem, I implemented a check on the bit-length of inputs with Num2Bits from circomlib.

Reference

For more details on this vulnerability, refer to:

• https://github.com/BlakeMScurr/comparator-overflow
• Optional Rigorous Range Check to Prevent Potential Overflow Vulnerability in LessThan(8) Usage zkemail/zk-regex#83

Note

FromRegex, BodyHashRegex, and VenmoTimestampRegex also contains LessThan without bit-length check. However, this seems fine since they are used for utf-8 body.