With the advent of cloud computing, it is not difficult to think of applications that integrate with many others available in the cloud. Right now, it's easy to see how applications interact with Flickr, Facebook, Twitter, Tumblr, and so on.

To enable these integrations, a new authentication mechanism has been developed using API keys. This authentication method is used primarily when we need to authenticate from another application but we do not want to access the private user data hosted in another application. On the contrary, if you want to access this information, you must use OAuth. If you are interested in this, don't worry, we will study this wonderful technology later in this book.

We want to understand how the API keys work, so let's take the case of Flickr. The important thing here is to understand how the API keys work because the same concept can be applied to companies like Google, Facebook, and so on. For those unfamiliar with Flickr, it is an application in the cloud...