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.DER file extension

To open .DER files on Windows, install OpenSSL (from a trusted source used in your environment).

To open a .der file, treat it as a binary X.509 certificate and inspect it with OpenSSL (for example, using the x509 command with DER input). If you need it for apps that expect PEM text, convert the DER file to PEM first.

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Last updated: June 12, 2026

Open on your device

Choose your operating system for a dedicated step-by-step opening guide.

How to open .DER files

Use these platform-specific instructions to open .DER files safely.

Windows

  1. Install OpenSSL (from a trusted source used in your environment).
  2. Open Command Prompt or PowerShell and run: openssl x509 -in yourcert.der -inform DER -text -noout
Full Windows guide

Mac

  1. Open Terminal.
  2. Run: openssl x509 -in yourcert.der -inform DER -text -noout
Full Mac guide

Linux

  1. Open a terminal.
  2. Run: openssl x509 -in yourcert.der -inform DER -text -noout
Full Linux guide

iOS

  1. There is no standard iOS built-in viewer for inspecting DER certificate fields; transfer the file to a desktop and inspect it with OpenSSL.
Full iOS guide

Android

  1. There is no standard Android built-in viewer for inspecting DER certificate fields; transfer the file to a desktop and inspect it with OpenSSL.
Full Android guide

Security notes

  • .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.
  • Be cautious about adding a certificate from a .der file to a trusted store: a malicious or incorrectly issued certificate could enable interception or impersonation if trusted.
  • DER is a binary encoding; prefer well-maintained tooling (such as OpenSSL) to parse it rather than unknown viewers, because malformed ASN.1/DER inputs can trigger parser bugs in some software.

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Can't open this file?

These are the most common causes and fixes when .DER files fail to open.

Common reasons

  • OpenSSL says it cannot read the certificate (wrong format)
  • The system/app expects PEM but you only have DER
  • File extension confusion (.der vs .cer) and certificate import problems

Fix steps

  1. Try reading it as DER explicitly: openssl x509 -in file.der -inform DER -text -noout
  2. If that fails and the file looks like text with BEGIN/END lines, try PEM: openssl x509 -in file.der -inform PEM -text -noout

OS-specific troubleshooting

What is a .DER file?

.der commonly refers to Distinguished Encoding Rules (DER), a binary encoding for ASN.1 structures used by X.509 certificates and related PKI objects. X.509 certificates can be stored as DER (binary) or as PEM, which is Base64-encoded DER. Many .der files therefore represent a single X.509 certificate encoded in DER.

Background

DER is a strict binary encoding of ASN.1 data and is widely used in public key infrastructure (PKI). The Internet X.509 PKI certificate profile is specified in RFC 5280 and relies on ASN.1 structures that are commonly encoded using DER.

In practice, a .der file is often interchangeable (in meaning) with files ending in .cer for certificates, with the key difference being the encoding: .der is typically binary DER, while PEM is a text form that wraps Base64-encoded DER. This is why many tools and guides describe PEM as “Base64-encoded DER” for X.509 objects.

Because DER is binary, it is not meant to be read in a text editor. Users typically open .der files to inspect certificate fields (subject, issuer, validity, extensions) or to convert them into PEM for use by servers, libraries, or configuration systems that expect PEM input.

Common MIME types: application/pkix-cert

Known aliases: .cer

Further reading

Authoritative resources for more details on the .DER format.

Common .DER 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.

  1. Try reading it as DER explicitly: openssl x509 -in file.der -inform DER -text -noout
  2. 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.

  1. Convert DER to PEM: openssl x509 -in cert.der -inform DER -out cert.pem -outform PEM
  2. 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.

  1. Inspect the file with OpenSSL using the correct input type (DER vs PEM) to confirm what it contains
  2. If needed, convert to PEM or keep as DER depending on what the target importer expects

FAQ

Is a .der file the same as a .pem file?

They usually represent the same kind of X.509 certificate data, but .der is binary DER encoding, while PEM is Base64-encoded DER in a text format.

What MIME type is used for DER-encoded X.509 certificates?

A commonly used registered media type for X.509 certificates is application/pkix-cert.

How do I view the certificate details inside a .der file?

Use OpenSSL: openssl x509 -in file.der -inform DER -text -noout

Why does my tool say “unable to load certificate” for a .der file?

Most often the tool is trying the wrong encoding (PEM vs DER) or the file is not an X.509 certificate. Verify by reading it with OpenSSL using -inform DER, and if the file is PEM text, use -inform PEM instead.

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