CER vs. CRT: What's the Difference and How to Convert
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The .cer and .crt extensions are two names for the same thing: an SSL certificate. Here's why both exist, what actually matters when a server rejects your file, and how to switch from one extension to the other.
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CER and CRT Are the Same Thing
Both extensions are used for X.509 certificate files, the standard format for SSL certificates. A .cer or .crt file contains the certificate's public key, the domain or organization it was issued to, its validity period, and the certificate authority's signature. Neither contains the private key.
Which extension you get depends only on habit: some certificate authorities, servers, and tools use .crt, others use .cer. The content inside can be exactly the same.
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What Actually Matters: PEM or DER
What does make a difference is how the certificate is encoded inside the file. There are two options, and either one can have a .cer or .crt extension:
- PEM (Base64) is plain text. This is the most common format, and it's what Apache, NGINX, and most control panels expect.
- DER is binary. It's used mainly by Java applications and some Windows tools.
To check which one you have, open the file in Notepad or any text editor. A PEM file starts and ends with these lines, with a block of letters and numbers between them:
-----BEGIN CERTIFICATE----- MIIGkTCCBXmgAwIBAgIQ... -----END CERTIFICATE-----
If you see unreadable symbols instead, the file is in DER format.
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How to Change .cer to .crt (or Back)
If your file is in PEM format, changing the extension is all the conversion it needs. There are two easy ways:
- Rename the file. In File Explorer, turn on file extensions (View > Show > File name extensions in Windows 11, or View > File name extensions in Windows 10), then rename certificate.cer to certificate.crt, or the other way around.
- Save it with a new name. Open the file in Notepad, choose File > Save As, and type the new name with the extension you need.
Tip: In Notepad's Save As window, set Save as type to All files. Otherwise, Notepad may add .txt to the end, and you'll get certificate.crt.txt.
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If the File Is in DER Format
Renaming a DER file doesn't change what's inside it, so a server that expects PEM still won't accept it. In that case, the file needs a real conversion to PEM. See How to Convert SSL Certificates: PEM, DER, PFX, and P7B .
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