HMAC algoritham Explanation
HMAC(keyed-hash message authentication code ) 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 video explanation!!!
You are going to mail a package to jhon 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 he don'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 he 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 jhon 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.
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