Do I need to convert .CER to .CRT for Apache SSL certificates? If so, how?

Question

File extensions for cryptographic certificates aren't really as standardized as you'd expect. Windows by default treats double-clicking a .crt file as a request to import the certificate into the Windows Root Certificate store, but treats a .cer file as a request just to view the certificate. So, they're different in the sense that Windows has some inherent different meaning for what happens when you double click each type of file.

But the way that Windows handles them when you double-click them is about the only difference between the two. Both extensions just represent that it contains a public certificate. You can rename a certificate file to use one extension in place of the other in any system or configuration file that I've seen. And on non-Windows platforms (and even on Windows), people aren't particularly careful about which extension they use, and treat them both interchangeably, as there's no difference between them as long as the contents of the file are correct.

Making things more confusing is that there are two standard ways of storing certificate data in a file: One is a "binary" X.509 encoding, and the other is a "text" base64 encoding that usually starts with "-----BEGIN CERTIFICATE-----". These encode the same data but in different ways. Most systems accept both formats, but, if you need to, you can convert one to the other via openssl or other tools. The encoding within a certificate file is really independent of which extension somebody gave the file.

Basically there are two CER certificate encoding types, DER and Base64. When type DER returns an error loading certificate (asn1 encoding routines), try the PEM and it shall work.

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

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

According to documentation mod_ssl:

SSLCertificateFile: 
   Name: SSLCertificateFile
   Description: Server PEM-encoded X.509 certificate file

Certificate file should be PEM-encoded X.509 Certificate file:

openssl x509 -inform DER -in certificate.cer -out certificate.pem

CER is an X.509 certificate in binary form, DER encoded.
CRT is a binary X.509 certificate, encapsulated in text (base-64) encoding.

It is not the same encoding.