.der · application/x-x509-ca-cert
What is a .der file?
DER-Encoded Certificate
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Open the Certificate Decoder →About the format
A .der file is binary ASN.1 — most commonly a certificate stored in its raw encoded form rather than the Base64 PEM wrapper. DER (Distinguished Encoding Rules) is the canonical byte encoding that everything else derives from: a PEM file is literally DER passed through Base64 with fence lines. The practical consequences are mundane but constant: text editors show binary soup, some tools demand PEM while others demand DER, and conversion is a one-liner in both directions (openssl x509 -inform der -in cert.der -out cert.pem, and -outform der for the reverse). Windows is comfortable with DER natively — double-click opens the certificate viewer — while much Unix tooling defaults to PEM. Files starting with the bytes 30 82 are the giveaway (an ASN.1 SEQUENCE with a two-byte length): that prefix appears on DER certificates, DER keys, CSRs and PKCS#12 bundles alike, so a decoder that inspects the actual structure beats guessing from the extension.
How to open a .der file
Windows
- WhyCrashed Certificate Decoder (browser)
- double-click (certificate viewer)
macOS
- openssl x509 -inform der
Linux
- openssl x509 -inform der
Is a .der file dangerous?
No — same public material as .cer/.crt, in binary form.
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