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CRT vs DER
CRT is a common certificate filename extension; DER is a binary encoding for ASN.1 data such as certificates. They describe different things.
Which format should you choose?
Follow the receiving system's required certificate encoding. A .crt extension alone does not tell you whether the content is PEM or DER.
Good fit for CRT
Transfer or deploy X.509 certificates
- Servers and clients requesting certificate files
Good fit for DER
Certificate deployment
- Supplying an ASN.1 certificate to software that requires binary input
Basic information
| Field | CRT | DER |
|---|---|---|
| Extension | .crt, .cer | .der |
| MIME type | application/pkix-cert | application/pkix-cert |
| Common uses | Transfer or deploy X.509 certificates | Certificate deployment |
| How to open | Use a certificate viewer to inspect subject names, issuer, validity dates, and chain. | Inspect with a certificate tool and confirm the object type. |
| Export or convert | Before converting, determine encoding, whether the file includes a chain, and whether a private key is stored separately. | When converting to PEM, verify the content and destination requirements. |
Choose by use case
Servers and clients requesting certificate files
Transfer or deploy X.509 certificates
CRTSupplying an ASN.1 certificate to software that requires binary input
Certificate deployment
DERKey point 1
CRT/CER is a filename extension, not a single encoding guarantee.
More on the differencesKey point 2
DER is a binary encoding rule used for X.509 certificates and other ASN.1 structures.
More on the differencesMore on the differences
Inspect and validate the actual file. Confirm whether a private key is included and whether the service requires a certificate chain.
- A CRT file may contain PEM text or DER binary data.
- PEM is a textual representation that can carry DER-encoded structures.
Frequently asked questions
Is every .crt file DER encoded?
No. The extension alone does not determine encoding. Check the file contents and the deployment requirements.