A DER-encoded certificate can be transformed into an ASCII (Base64) encoded certificate using OpenSSL. A DER-encoded certificate cannot be read as plain text (unlike a PEM-encoded certificate). DER-encoded certificates typically have the file extensions.DER,.CRT, or.CER.
In the event that a PEM-encoded certificate also has the.CRT or.CER file extension, you can simply copy the file to a new name using the.PEM extension:
$ cp hostname.cer and hostname.pem
Use OpenSSL to convert a DER-encoded certificate to PEM: