The only time CRT and CER can be safely interchanged is when the encoding type can be the same. (i.e. PEM-encoded CRT = PEM-encoded CER)

There are four basic types of certificate operations. View, convert, combine and export

View a PEM encoded certificate using the command with the certificate extension, replacing cert.xxx with the certificate name

If you receive the following error, it means You are trying to view a DER encoded certificate and need to use the following command in View DER Encoded Certificate

View a DER Encoded Certificate

If you receive the following error, then it means you are trying to view a PEM encoded certificate using the command for DER encoded certificates. Use the commands in "View PEM encoded certificates above"

PEM to DER

DER to PEM

In some cases, multiple It is advantageous to combine the .509 infrastructure into one file. A common example is combining both private and public keys into the same certificate.

The simplest way to combine certificate keys and chains is to convert each key to a PEM-encoded certificate and simply copy the contents of each file into a new file. This is suitable for combining files for use in Apache applications

Some certificates will appear in combined form. One of the files can contain any of the following: a certificate, private key, public key, signing certificate, certification authority (CA), or authority chain.