How to open .DER files on Linux
To open .DER files on Linux, open a terminal.
Step-by-step instructions
- Open a terminal.
- Run: openssl x509 -in yourcert.der -inform DER -text -noout
Common issues
OpenSSL says it cannot read the certificate (wrong format)
This often happens when the file is PEM (Base64 text) but you told the tool it is DER, or when the file is not an X.509 certificate at all.
- Try reading it as DER explicitly: openssl x509 -in file.der -inform DER -text -noout
- If that fails and the file looks like text with BEGIN/END lines, try PEM: openssl x509 -in file.der -inform PEM -text -noout
The system/app expects PEM but you only have DER
Many configurations and libraries accept PEM text files, while .der is binary. PEM is Base64-encoded DER, so you can convert between them.
- Convert DER to PEM: openssl x509 -in cert.der -inform DER -out cert.pem -outform PEM
- Use the resulting .pem file where the software expects PEM
File extension confusion (.der vs .cer) and certificate import problems
Some environments treat .cer as either DER or PEM depending on content, while .der strongly implies binary DER. Import failures often come from using the wrong encoding for the import tool.
- Inspect the file with OpenSSL using the correct input type (DER vs PEM) to confirm what it contains
- If needed, convert to PEM or keep as DER depending on what the target importer expects
Security note
.der certificate files typically contain public certificate data, not executable content, but they can still be used in trust decisions; only install/import certificates from sources you trust.