Arithmetic Rounding

Submitted by (11) scaraven, also found by 0x52, auditor0517, Deivitto, hansfriese, Lambda, rbserver, simon135, smiling_heretic, sseefried, and TrungOre

Summary:

https://code4rena.com/reports/2022-08-rigor/#h-02-builder-can-halve-the-interest-paid-to-a-community-owner-due-to-arithmetic-rounding

This issue occurs in the calculation of noOfDays in returnToLender() which calculates the number of days since interest has last been calculated. If a builder repays a very small amount of tokens every 1.9999 days, then the noOfDays will be rounded down to 1 days however lastTimestamp is updated to the current timestamp anyway, so the builder essentially accumulates only 1 day of interest after 2 days.

Tip: Calculate interest by seconds and not by days

Mitigation:

1) Add a scalar to noOfDays so that any rounding which occurs is negligible or 2) Remove the noOfDays calculation and calculate interest in one equation which reduces arithmetic rounding

TAGS: #math

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