Summary

abi.encodePacked() should not be used with dynamic types when passing the result to a hash function such as keccak256()

Impact

Hash collisions (e.g. abi.encodePacked(0x123,0x456) => 0x123456 => abi.encodePacked(0x1,0x23456)

But with abi.encode(0x123,0x456) => 0x0...1230...456).

Recommendation

Unless there is a compelling reason, abi.encode should be used.

If there is only one argument to abi.encodePacked() it can often be cast to bytes() or bytes32() instead.

If all arguments are strings and or bytes, bytes.concat() should be used instead.