An application programming interface (API) defines the contract enabling two parties to communicate to transmit data by creating a communication channel. An API key is a unique code used to identify the legitimacy of an application or a project that calls an API endpoint allowing APIs to control how they are being utilized. It is attached at the API caller’s end and used to prevent unauthorized access or abuse of an API, acting as a unique authentication token associated with API callers’ permission.
When an application sends an API request, it will attach the api key value to the request using the query string parameter in the request URL, request header, request body, and cookie. Then, when the application that consists of the API endpoint receives the API key, it will pass by an application and call the API to determine the callers’ identity or the application trying to access the website. However, even though the APIs can use API keys to identify the project, it is impossible to identify individual users or creators.
Generally, api keys are not secured as clients can access them, and there is no expiration set for it making them vulnerable to hacking or stealing . If any unauthorized party gets hold of the key, it is possible to use it indefinitely. Therefore, to enhance the security of an api key, users need to use it with a secure protocol such as HTTPS or SSL.
Popular websites like Google or Youtube provide API key generators for accessing their APIs. Some publicly available APIs also provide api-keys in header fields such as ‘X-authorization-key’ for testing purposes. API keys are often composed of Alphanumeric and special characters such as “etewt4e.fs535CGApwij-763DG,” making them unique across multiple applications.
The main uses of an API key are Project authorization and identification.
Usually, an API endpoint is accessed by specific projects or applications requiring or sending data through the API. An API needs to identify the project that calls it and rejects callers that are not identifiable. It can utilize an API key or an authentication token to identify the caller.
However, the security of an api key is not as good as an authentication token. But, the APIs can still use api keys to identify the calling project or the application. API keys can also associate user information and deny calls from unpermitted projects.
Authorization is the process of checking the permission of a specific project or an application. For example, APIs can check if the application that makes the API call has permission to call it and check the availability of the API.
To use an API key, you must first obtain or generate the key from the API provider.
Many companies provide accessible API endpoints for getting gata for project types, and some companies only offer API endpoints specific to a particular application. For example, ipstack.com offers a free IP geolocation API to get accurate location information from IP addresses. But, first, you can check the company developer guides or API documentation to know the API details, including acquiring or generating the API key.
To use an API, most companies require you to register with their product. You can use an API key generator to get your API key, or the company will provide you with an API key. If your API is publicly accessible or only used for testing purposes, most probably, it will be free of charge. However, some companies may require you to pay for the API key depending on its usage. Once you get your API endpoint with the key, you are ready to test and integrate it into your application.
Testing your API key is important as it can make or break your API call once it is integrated into your application. To try your API key, you can either check if the API provider has an API testing tool or use an open-source API testing application such as Postman that offers an easy interface for API testing.
First, identify the way you need to send the API key. Then, specify all the required details for the API endpoint, such as the endpoint URL, request body, and request header, along with the API key. Next, send the API call with the correct API key and check if the API key successfully grants access to the API and you receive the correct data. Then check if the API call fails if you alter your API key deliberately to ensure that the API key works as expected.
Once you test your API endpoints with the API Key, you are ready to use the API key by integrating the endpoints into your application. You can choose an appropriate API library to send and receive API requests configuring the API keys programmatically. Several programming languages like Node Js, Python, and PHP provide modules that you can utilize to construct the API requests.
However, when constructing the API requests, ensure that you never hard-code the API key in your code. Instead, please keep it in a separate file stored in a secure location and reference it from your code. 
There are several ways enterprises make use of API keys. The most popular use of an API key prevents malicious API requests as API keys can identify and authorize the caller application and deny anonymous application traffic.
Enterprises also use API Keys to filter the API calls using the API key, analyze the API usage, and find out API access patterns . Furthermore, they can use the keys to debug any issues found in the API requests from the API logs. Another way enterprises use the API keys is by limiting the number of API calls to control the API usage.