DER Digital Certificate
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What is a DER file?
Web servers use digital certificates to establish that a website is authentic and legitimate. When a user visits a certified website in their web browser, the web server passes the site's digital certificate to the browser. After the browser verifies the certificate, it shows the associated website. (If a browser cannot verify a certificate, it may instead show a page that says "This website's certificate is expired.")
Some digital certificates are saved in the DER format. Web admins, responsible for maintaining websites and web servers, are most likely to encounter and use DER files. Web admins can create DER files using OpenSSL, a secure network communication toolkit.
NOTE: Though most DER files are used as website digital certificates, DER files can also be used for other identification purposes.
How to open a DER file
You can use OpenSSL (multiplatform) to view the contents of a DER file. To do so, use the following command:
openssl x509 -inform der -in YourFileName.der -text -noout