What is an API?

An API (Application Programming Interface) is a software intermediary that lets two applications communicate with each other in a defined format both understand. It handles requests and responses between a client and a server, so applications can exchange data and functionality without exposing their internal workings.

An API, or Application Programming Interface, is a software intermediary that allows two separate applications to communicate. Even if they are built in different languages, they exchange information in a format both understand. When you stream music or pay online, an API is carrying the request to a server and returning the response. Because APIs expose only defined data and functions, they also form a boundary that can be secured.

Key Takeaways

• An API (Application Programming Interface) is a software intermediary that lets two applications exchange data and functionality in a defined format.
• APIs handle a request from a client and a response from a server, without exposing either system’s internal workings.
• The core parts of an API are endpoints (where requests go), methods (GET, POST, PUT, DELETE), and the request and response themselves.
• Common API styles and protocols include REST (the modern default, usually with JSON), plus SOAP, XML-RPC, and JSON-RPC.
• API security relies on encryption in transit (TLS/HTTPS), authentication (API keys, tokens), and controls like rate limiting, since APIs are a common attack surface.

What Is an API?

An Application Programming Interface (API) is a software intermediary that allows two separate applications to communicate with each other. The two applications might be completely different, built in different languages, yet they can exchange information in a defined format that both understand. Every time you play a video on YouTube or stream music, you are using an API behind the scenes.

When a client communicates with a server or another application to retrieve information to show a user, that communication typically happens through an API. The API makes it possible for an application and a server, or two separate applications, to exchange information in a controlled, predictable way.

How Does an API Work?

A simple example is paying online with a third-party system like Apple Pay. When you click ‘Pay with Apple Pay’ on an online store, it makes a request through an API:

1. The request: Information about your purchase travels from the website to the external payment system through the API. In effect, the website is asking the external system to do something for it.
2. Processing: The external system processes the request and prepares the information the website needs.
3. The response: The external system sends the necessary information back to the website through the API, and the website completes the payment.

Even though you never see it, this exchange happens behind the scenes, so the whole process feels smooth and seamless to you as the user. That request-and-response pattern is the heart of how every API works.

The Important Parts of an API

1. Endpoints: An API endpoint is like a specific web address where you go to get something specific. A weather service, for example, has an endpoint dedicated to returning weather for a given location.
2. Methods: Methods define what you can do with the data, like a menu of actions: GET to retrieve data, POST to create something new, PUT to update it, and DELETE to remove it.
3. Request and response: When one program wants information or an action from another through an API, it sends a request; the receiving program processes it and sends back a response. This request-response communication is what APIs are all about.

Common API Protocols and Styles

API protocols are the rules and conventions that dictate how systems communicate through an API, defining the methods and standards for data exchange so the client and server stay consistent and interoperable. The main ones:

• REST (Representational State Transfer): Not a strict protocol but a set of principles for web APIs; those that follow them are called RESTful. REST is the modern default, typically using JSON over HTTP, and is easy to use from languages like Java, Python, and Ruby.
• SOAP (Simple Object Access Protocol): Uses XML to exchange information over HTTP and SMTP, making it easier for apps written in different languages or on different platforms to share data. More rigid and heavyweight than REST.
• XML-RPC: An older protocol using a specific XML format to move data. Simpler and lighter than SOAP.
• JSON-RPC: Like XML-RPC, but uses JSON instead of XML to move data.

Historically, ‘API’ often meant a way for parts of a single program to talk to each other in low-level code. Modern APIs are typically web-based, follow REST principles, and use JSON, and GraphQL has also become popular as a flexible alternative to REST. This makes today’s APIs easy for developers to use across many languages.

API Security

Because APIs expose data and functionality across a network, they are a significant and frequently targeted attack surface, and securing them is where encryption and identity come in. An API is designed so that it never exposes the server to the client, or vice versa: it carries only the request, and the server returns only the information that fits the defined format. If a request lacks the required information or format, the API can refuse it, which limits data exposure. But that structural boundary is only part of security. The key protections:

• Encryption in transit (TLS/HTTPS): API traffic should always travel over HTTPS, so requests and responses (which often carry credentials or sensitive data) are encrypted with TLS and cannot be read or altered in transit. An API served over plain HTTP exposes everything it carries.
• Authentication and authorization: APIs use API keys, tokens (such as OAuth access tokens), or certificates to verify who is calling and what they are allowed to do. Companies like Stripe, Google, eBay, and Amazon expose APIs protected this way, so developers can build on them without the provider taking on undue risk.
• Governance and rate limiting: APIs are monitored, managed, versioned, and scaled based on observed performance, and rate limiting protects against abuse and denial-of-service attempts.

In short, an API’s own design reduces exposure, but real API security depends on strong encryption in transit and sound authentication, the same building blocks that secure HTTPS and other web communication.

Features of an API

• Standards-based: APIs follow established standards (HTTP, REST, GraphQL), which makes them developer-friendly and widely understood.
• Treated as products: APIs are built like mini-applications for web and mobile developers, well documented, versioned, and maintained across their lifecycle, with providers responding to the developer community.
• Secured and governed: APIs carry access controls (such as API keys), and are monitored, managed, and scaled according to performance.

How Encryption Consulting Helps

APIs are only as secure as the encryption and authentication protecting them, and misconfigured TLS or weak API authentication is a common source of exposure. Encryption Consulting’s Encryption Advisory Services help organizations assess how their APIs and web services use encryption, ensure TLS is configured correctly, strengthen authentication and key handling, and align to standards such as NIST and PCI DSS. Backed by ISO/IEC 27001:2022 and SOC 2 certified practices.

Frequently Asked Questions

What is an API in simple terms?

An API (Application Programming Interface) is a software intermediary that lets two applications talk to each other. It takes a request from one application, passes it to another, and returns the response, all in a format both sides understand. A good analogy is a waiter in a restaurant: you (one application) give your order to the waiter (the API), who takes it to the kitchen (another application) and brings back your food (the response).

How does an API work?

An API works through a request-and-response cycle. One application sends a request through the API, for example asking a payment system to process a transaction. The receiving system processes that request and sends back a response with the result or data. This happens behind the scenes, so the experience feels seamless to the user. The request specifies an endpoint (where to go) and a method (what action to take, such as GET or POST).

What are the main parts of an API?

The main parts are endpoints, methods, and the request and response. An endpoint is a specific address where a particular kind of request is sent, like a weather service’s location endpoint. Methods define the action: GET retrieves data, POST creates, PUT updates, and DELETE removes. The request is what one program sends to ask for data or an action, and the response is what the other program returns.

What are the different types of API protocols?

Common API protocols and styles include REST, SOAP, XML-RPC, and JSON-RPC. REST is the modern default, a set of principles for web APIs that usually use JSON over HTTP. SOAP is a more rigid, XML-based protocol. XML-RPC is an older, lighter XML-based option, and JSON-RPC is similar but uses JSON. GraphQL has also become popular as a flexible alternative to REST for querying exactly the data a client needs.

Are APIs secure?

APIs can be secure, but security is not automatic. An API’s design already limits exposure by revealing only defined data and rejecting malformed requests. Real protection, though, depends on encrypting traffic with TLS/HTTPS so data cannot be intercepted, authenticating callers with API keys or tokens, and applying controls like rate limiting. Because APIs are a common attack surface, weak TLS configuration or poor authentication is a frequent cause of breaches.

Why does API security matter for encryption?

API security matters for encryption because APIs constantly move data, often sensitive data like credentials or payment details, between systems over networks. If that traffic is not encrypted with TLS/HTTPS, it can be intercepted and read. Encryption protects the data in transit, while authentication confirms who is calling the API. Together they are the core of API security, which is why sound encryption and key management practices are essential to protecting APIs.

Secure the APIs That Power Your Business

APIs move your data between systems all day, and encryption plus authentication are what keep that data safe. Explore Encryption Consulting’s Encryption Advisory Services to assess how your APIs and web services use encryption and strengthen their protection.