Make use of a bitmap for claims to save gas in Converter.sol

[code423n4]

Handle

TomFrench

Vulnerability details

Impact

Gas consumption

Proof of Concept

https://github.com/code-423n4/2021-11-vader/blob/3a43059e33d549f03b021d6b417b7eeba66cf62e/contracts/tokens/converter/Converter.sol#L52

Each claim performs a SSTORE to a fresh slot in order to mark a leaf as claimed. This results in each user paying 20k gas in order to invalidate a leaf whereas if many users shared the same slot they would pay 5k gas instead.

See Uniswaps implementation for inspiration:

https://github.com/Uniswap/merkle-distributor/blob/master/contracts/MerkleDistributor.sol

Recommended Mitigation Steps

Use bitmap for leaf invalidation