How to Create an HTTPS Proxy Like Burp Suite

How to Create an HTTPS Proxy Like Burp Suite

What is a Proxy?

To get started, let's clarify what a proxy is. According to Wikipedia:

"In computer networking, a proxy server is a server application that acts as an intermediary between a client requesting a resource and the server providing that resource. It improves privacy, security, and possibly performance in the process."

In simpler terms, a proxy intercepts traffic between a client (like your web browser) and another server (like a website).

What is HTTP?

HTTP (Hypertext Transfer Protocol) is the foundation of data communication for the web. It's an application layer protocol used to transfer hypermedia documents, such as HTML pages.

When we add the letter S to HTTP, making it HTTPS, it stands for "Secure." HTTPS uses SSL/TLS encryption to ensure that data transmitted between the client and server is secure.

How Does a Proxy Handle HTTPS?

When working with HTTPS traffic, proxies need to be able to decrypt the data. This is where things get a bit more complex. The key to handling HTTPS traffic lies in the CONNECT method of HTTP. This method allows a proxy to establish a tunnel between the client and the destination server, enabling secure communication.

To intercept HTTPS traffic, the proxy must be configured in the client's settings, typically in a web browser. The browser will then forward all requests through the proxy.

What Do You Need to Build an HTTPS Proxy?

To build a basic HTTPS proxy, you will need the following:

1. HTTP Parser: This helps read and interpret incoming HTTP requests and responses.
2. SSL Handler: This is necessary to manage the SSL/TLS encryption and decryption processes.

Once those are in place, the proxy must handle the CONNECT method. When a browser is set to use a proxy, it sends a CONNECT request to establish a connection with the proxy before making any actual requests.

Dealing with SSL Certificates

One of the biggest challenges in building an HTTPS proxy is dealing with SSL certificates. When intercepting HTTPS traffic, the proxy must be able to present a trusted certificate to the client; otherwise, the browser will show a warning.

To address this, you’ll need to:

• Create a CA (Certificate Authority) Root Certificate: This certificate acts as a "trusted" authority in the browser.
• Add the CA Root Certificate to the Browser: By adding the CA certificate to the browser's trust store, it will trust certificates signed by this CA.
• Sign Domain-Specific Certificates: For each website the user visits, the proxy can generate and sign a certificate using the trusted CA Root Certificate.

This allows the proxy to decrypt and re-encrypt HTTPS traffic without the browser raising security warnings.

Proxy Workflow

Here’s a simplified workflow for handling HTTPS traffic:

1. Intercept the CONNECT request: When the browser sends a CONNECT request, the proxy intercepts it.
2. Check for a certificate: The proxy checks if it already has a certificate for the domain. If not, it creates one on the fly.
3. Establish an SSL connection: The proxy sets up an SSL connection with the target server and forwards requests between the client and server, decrypting and re-encrypting the data as necessary.

Source Code

If you’re interested in a practical implementation, here’s a simple HTTPS proxy written in C: cyber-web HTTPS Proxy on GitHub.

It’s a basic example that may still have some issues, but it's a working prototype. Feel free to contribute or report any bugs you encounter!

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This guide gives you a starting point for understanding and creating your own HTTPS proxy, like Burp Suite. With the right components in place, you can intercept and analyze web traffic, ensuring both flexibility and security in your network interactions.