What is a .crt file in Ubuntu?

In the world of Linux and web security, the CRT file is a cornerstone for establishing secure, encrypted connections.

For Ubuntu users, from system administrators to developers, understanding how to work with these certificate files is a critical skill.

This article provides a deep dive into what CRT files are, how they are used in Ubuntu, and the practical steps for managing them for various applications.

What is a CRT File?

A CRT file, short for certificate file, is a digital file that contains a public key and other identifying information for a website or server.

These files are used in the SSL/TLS (Secure Sockets Layer/Transport Layer Security) protocol to encrypt communication between a web server and a client, such as a web browser.

When you visit a website with “https://” in its URL, you are interacting with a system that uses a certificate, often in the form of a CRT file, to secure your connection.

Unix-based systems, like Ubuntu, frequently use the .crt extension for these certificates.

Understanding CRT File Formats: PEM vs. DER

CRT files typically come in two main encoding formats:

• PEM (Privacy-Enhanced Mail): This is the most common format for certificate files in Ubuntu. PEM files are Base64 ASCII encoded, meaning they are plain text files. You can open a PEM-formatted CRT file in a text editor and see blocks of text starting with ----BEGIN CERTIFICATE----- and ending with ----END CERTIFICATE-----. This format is favored for its readability and ease of use in text-based configurations.
• DER (Distinguished Encoding Rules): This is a binary format for certificates. Unlike PEM files, DER-encoded files are not human-readable in a standard text editor. While less common for manual configuration on Ubuntu, some applications may require certificates in this format.

It’s important to know the format of your CRT file, as the tools and configuration directives you use may depend on it.

See also: Mastering the Linux Command Line — Your Complete Free Training Guide

Generating a CRT File in Ubuntu with OpenSSL

For development, testing, or internal services, you might need to create your own self-signed CRT file. The go-to tool for this in Ubuntu is OpenSSL, a powerful and versatile command-line cryptography toolkit.

Here’s a step-by-step guide to generating a self-signed CRT file:

1. Generate a Private Key: The first step is to create a private key. This key should be kept secure and is used to decrypt information encrypted with the corresponding public key.
   

   openssl genrsa -out private.key 2048
   

   
   
2. Create a Certificate Signing Request (CSR): A CSR is a file that contains information about the entity the certificate is for, such as the domain name, organization, and location.
   

   openssl req -new -key private.key -out mydomain.csr
   
   

   
   

   You will be prompted to enter information for the certificate. For a self-signed certificate, the “Common Name” should typically be the domain name you want to secure.

3. Generate the Self-Signed CRT File: Now, you will use your private key and CSR to generate the CRT file.
   

   openssl x509 -req -days 365 -in mydomain.csr -signkey private.key -out mydomain.crt
   

   
   

   This command creates a certificate (mydomain.crt) that is valid for 365 days.

Installing a CRT File as a Trusted Root Certificate

Sometimes, you need to make your Ubuntu system trust a custom Certificate Authority (CA), for instance, in a corporate environment or when using a self-signed certificate for a local development server. To do this, you need to add the CA’s CRT file to the system’s trusted certificate store.

1. Copy the CRT File: The CRT file (it must have a .crt extension) needs to be placed in the /usr/local/share/ca-certificates/ directory. You might need to create this directory if it doesn’t exist.
   

   sudo mkdir -p /usr/local/share/ca-certificates/extra
   sudo cp your-ca.crt /usr/local/share/ca-certificates/extra/
   
   

   
   
2. Update the Certificate Store: After copying the certificate, run the update-ca-certificates command. This command will update the system’s list of trusted CAs.
   

   sudo update-ca-certificates
   
   

   
   

   Your system will now trust certificates signed by this CA.

Viewing the Contents of a CRT File

sudo systemctl reload nginx

To inspect the details of a CRT file, such as the issuer, subject, and validity period, you can use OpenSSL.

openssl x509 -in your_certificate.crt -text -noout

This command will print the certificate’s information in a human-readable format.

Converting CRT Files to Other Formats

While CRT is common, you might need to convert it to other formats like PEM (if it’s not already in that format) or other certificate types.

• Convert a DER-encoded CRT to PEM:
  

  openssl x509 -inform DER -in your_certificate.crt -out your_certificate.pem
  
  

  
  
• Convert a PEM-encoded CRT (often just a change of extension): If your .crt file is already in PEM format, you can often just rename it to .pem. However, to be certain, you can run:
  

  openssl x509 -in your_certificate.crt -out your_certificate.pem
  
  

  
  

Conclusion

CRT files are a fundamental component of securing network communications in Ubuntu.

From generating self-signed certificates for local development to configuring production web servers and managing system-wide trust, a solid understanding of how to handle these files is indispensable.

By leveraging the power of OpenSSL and following the standard practices for installation and configuration, you can effectively manage SSL/TLS certificates and ensure the security and integrity of your applications on Ubuntu.