When is HMAC used?

HMAC is a computed "signature" often sent along with some data. The HMAC is used to verify (authenticate) that the data has not been altered or replaced. Here is a metaphor:

You are going to mail a package to Sarah which contains a photograph. You expect her to open the package and view the photograph. At some point in the near future you expect her to send you back the package with that photograph in it. It's vital that she put the same photograph back in the package. You need to be absolutely sure she doesn't send you back an altered photograph even a little bit, or replace it with a different one. You've got hundreds of these packages going out daily with different photos; you'd never remember the photo in such detail that you could tell if she changed a small bit of it (like if she airbrushed a small zit off her face).

Here's what you can do: Before you send her the package, place another copy of the photograph inside a small locked box. Keep the key. Place the small locked box inside the package along with the original photograph you are mailing her. Assume she knows she is not to remove the locked box from the package. When you receive the package back from her, open it, place the photo on the table. Open the locked box, remove the copy, compare the two. If they are the same, then she has not altered the photograph (it is "authentic"). If the locked box is not in the package or your key will not open it, then assume she has done something nefarious and throw the whole package in the trash. The beauty here is that you don't need to "remember" anything about what you originally sent her; everything you need to ensure legitimacy of the photo comes back inside the package.

In the example above, the small locked box represents an HMAC. Your key is the HMAC's key. The photograph is the data you are applying the HMAC to. The above is a round trip metaphor where only you have a key. In a different situation, let's say you often send packages to Tommy. You're worried that the nosey mail carriers might be opening your packages and replacing the photographs or changing them. You do the same thing with the locked box, except in this case, you let Tommy have a copy of the key, so that when he receives a package, he can open the locked box included and compare the photos himself. If upon receipt he finds the photos differ, his key doesn't open the box, or the box missing, he knows something is fishy.

The above metaphors describe why HMACs are needed but not so much how they work. Let's change the metaphor again to get closer to how they work: Let's keep the mental imagery of the package with the photograph: you want to mail it, then receive it back again like before, ensuring the photo was not altered or replaced by the receiver, or during the round trip. Before you close the package and mail it, you make a copy of the photograph. No locked box this time, instead you brush over the copy with a concoction of liquid chemicals. Only you know the recipe (key) for this mixture, and anytime you brush over a copy, you use the exact same brush strokes. The mixture will swirl and blur the copy of the photograph into something resembling modern art; let's call it an HMAC. You're not exactly sure what it will look like after it dries, but you know that if you brush any two identical photos with the same recipe and the same brush strokes, the resulting HMACs will look the same. So you place the dried HMAC into the package along with the original photograph and send it off to Sarah.

When you get the package back from Sarah, it contains what you hope is the un-altered original photograph along with what you expect is the HMAC you created and included with it. Take the photograph out of the package, copy it, and create another HMAC with that copy (apply your mixture/brush strokes). Compare the HMAC you just created with the HMAC that came back in the package. If they are identical, then you can be sure Sarah and the mail carriers did not alter the photograph. If Sarah had altered the photo, then the HMACs will not be identical. If Sarah had altered the HMAC, then the HMACs will not be identical. If Sarah had altered the photo, and tried to create a new HMAC, then the HMACs will not be identical (she doesn't know your recipe). Thus you know if the photo (data) is authentic or not, which is exactly what HMACs are used for.