Make sure the data fed to your MAC is unambiguous. Or rather, make sure the data fed to your MAC is done in such a way that you cannot have different messages appear the same to the MAC encoder.
For instance, say you sort and concatenate your options without a delimiter. Then ["ab", "cd"] will have the same MAC as ["a", "bcd"], as in both cases the actual data fed to the MAC will be "abcd". This is a very bad thing.
Make sure the data fed to your MAC is unambiguous. Or rather, make sure the data fed to your MAC is done in such a way that you cannot have different messages appear the same to the MAC encoder.
For instance, say you sort and concatenate your options without a delimiter. Then ["ab", "cd"] will have the same MAC as ["a", "bcd"], as in both cases the actual data fed to the MAC will be "abcd". This is a very bad thing.