Unfortunately, there's no good way to do what you want. There are a number of ways do data integrity and authentication, but all the reliable ones require adding to the data. The addition can be of small fixed size, but you must add to the size of the data. There are, however, other schemes that allow data to shrink -- if your data is large enough, you could compress it and get back more than the size of the MAC. But this isn't guaranteed.
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