CER vs. CRT File Extensions Explained - SSL Dragon (2024)

Installing and managing an SSL certificate can be tricky if you’re not familiar with different file formats like CER vs. CRT. These two extensions may have caught your attention, and you might be wondering what they are, how they differ, and, most importantly, how to convert one into the other.

It’s not as complicated as it seems, and with a bit of guidance, you’ll learn these concepts in no time. So, are you ready to discover the difference between .cer and .crt?

Table of Contents

1. What Is a CER File?
2. What Is a CRT File?
3. What Is the Difference Between .CRT and .CER?
4. How to Convert CRT to CER?
5. How to Convert CER to CRT?
6. Reasons to Convert a Certificate to Another

What Is a CER File?

A CER file, which stands for Certificate File, is a digital certificate file format. Digital certificates are used in public key cryptography to secure communication over networks such as the Internet. They are a key component of the SSL/TLS protocols, which ensure the confidentiality and integrity of data transmitted between a user’s web browser and a website.

The CER file extensions usually indicate that the file is in a binary format. However, it can also be in a base64 format. You’ll often find these extensions used interchangeably with CRT files, leading to some confusion. While CER and CRT files essentially serve the same purpose, they differ in the way they’re encoded.

A CER file contains a public key, information about the key owner, and the digital signature of the certificate authority (CA) that issued the certificate. The digital signature allows the recipient of the certificate to verify its authenticity and ensure that it has not been tampered with.

In practical terms, CER files are often used to distribute and install SSL certificates on various software applications and devices. For example, web servers use SSL/TLS certificates stored in CER files to establish secure connections with web browsers.

What Is a CRT File?

A CRT file, also known as a certificate file, contains only the public key and other details of the digital certificate, not the private key. The private key is kept secure and private in a file with a .key extension on the server or device that holds the certificate.

CRT files can be used in secure connections, allowing the receiver to authenticate the sender’s identity, ensuring that the sent information remains unchanged. Web servers often utilize SSL/TLS certificates stored in CRT files to establish secure connections with web browsers.

What Is the Difference Between .CRT and .CER?

Is .cer and .crt the same? Almost. The primary difference between CER and CRT file extensions is convention and naming. Technically, there is no inherent distinction in the content or format of the digital certificate files.

Using one file extension is often a matter of preference or system-specific conventions. Some systems may default to one extension, while others may use the other.

Microsoft platforms widely use the CER security file, while many Unix-based systems prefer the CRT format to SSL certificates for web servers.

Both CER and CRT files can be either binary or base64-encoded ASCII. The file extension does not dictate the encoding. Instead, it’s the content that does it. CER files are often base64-encoded binary files, and CRT files are typically base64-encoded ASCII files.

How to Convert CRT to CER?

You can convert a CRT file extension to a CER format in a few ways. One of the most common methods is through the command line, directly within your Operating System.

To begin, you’ll need to install OpenSSL, a robust toolkit for TLS protocols. Once OpenSSL is installed, you can use the .crt to .cer command line. Here’s how to convert .crt to .cer using OpenSSL:

openssl x509 -in input.crt -out output.cer

In the command, replace ‘input.crt’ with the name of your CRT file, and ‘output.cer’ with the desired name for your CER file.

To convert .crt to .cer on Mac, you can use the built-in Keychain Access tool. Simply import your CRT files into the Keychain, then export them with the CER file extensions.

How to Convert CER to CRT?

Just as you can convert CRT files to CER, flipping the process to convert CER files to CRT is also possible. You can convert .cer to .crt using OpenSSL with the following command:

openssl x509 -inform der -in certificate.cer -out certificate.crt

This command takes your CER file (‘certificate.cer’) and converts it into a CRT file (‘certificate.crt’).

If you’re using a Windows operating system and want to know how to convert .cer to .crt in Windows, the process is similar. Run the same command on the OpenSSL software. Make sure to replace ‘certificate.cer’ and ‘certificate.crt’ with the actual names of your files.

Remember that the conversion process depends on the encoding used in your CER file. Ensure that you’re using the correct command for your file’s encoding.

Reasons to Convert a Certificate to Another

Having understood the process of converting CER to CRT, let’s see why you’d need to change a certificate from one format to another.

The most common reasons to convert files, especially when dealing with CER and CRT, are compatibility and server requirements. Different systems accept certificate files in different file formats. For instance, Unix Operating Systems use CRT files, while Windows servers and the certificate wizard often use CER files.

Converting a certificate file, whether from CER to CRT or CRT to CER, ensures that your digital certificate file is in the correct format for your system. It’s crucial to remember that each format (CER, CRT, PFX, PEM, DER) represents the same certificate display but in a different container.

While CER and CRT are typically in a base-encoded X.509 format, PFX files contain the entire certificate chain and private keys, password-protected for security.

The conversion can involve different filename extensions, including CER file extension and CRT extension, and requires careful attention to the full file path. Moreover, the distinction between CER vs CRT lies primarily in how the certificate displays, either in binary form (CER) or distinguished encoding rules (CRT).

Bottom Line

In summary, the difference between .cer and .crt file extensions is mainly about conventions and preferences, not technical. Both are used for X.509 certificates, holding public key information. While .cer is common on Microsoft platforms and .crt on Unix-based systems, it’s not a strict rule.

The file format (binary or base64-encoded ASCII) depends on content, not extension. Despite naming variations, .cer vs .crt files are essentially the same. Check our complete guide on multiple certificate file formats to learn about other file extensions used in SSL installation.