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No, not possible for differences to cancel. The bitwise OR of a bunch of things is zero only if all of the individual things are zero. The difference between two things is zero only if they're equal. Therefore, the final value of x is zero only if all the elements of MAC_computed and MAC_received match.

(And -- the main point here -- how long it takes to do the calculation should be totally independent of what's actually in the arrays: no early termination, no operations that take different amounts of time depending on how many elements match, etc.)

ah, bitwise or, was thinking incorrectly for some reason. And early termination makes sense, thanks! Didn't think of that.

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