You guess the MAC tag value of the message and measure how long it takes the server to return "bad MAC" error or behave in a way that means the MAC was bad. In

You guess the MAC tag value of the message and measure how long it takes the server to return "bad MAC" error or behave in a way that means the MAC was bad. In 1/256 cases, it takes longer because the first byte was correct. You may need to send many queries to get the value because timing is noisy, but with statistics you'll find that value. Now you try all 256 possible values of the second byte and one of them will take longer because both 2 first bytes are correct. Repeat. For the normal way to safely compare MAC values, see for example: https://docs.python.org/3/library/hmac.html#hmac.compare_digest https://docs.python.org/3/library/hmac.html#hmac.compare_dig...