CER - Internet Security Certificate

What is a CER file

CER files are security files provided and generated by an Certificate Authority. These files help a browser to verify if a website is secure and save to enter, verifying its authenticity. These CER security certificates are usually installed on a web server. If a user enters a URL to a website hosted on the same server, the browser checks the validity of the website by calling the certificate attached to it.

The public certificate inside the CER file maps a public key with a certain identity using a digital signature, thus verifying a website. Different encoding algorithms like Base64 and DER can be used to encode the contents of the CER file. When browsing a validated site, the according CER file can be accessed by clicking on the lock icon on the browser window (usually located near the edge). This is available for websites with SSL encryption and can easily determined by the "https://" prefix of the URL. Double-clicking on said lock icon allows you to view the information contained in the CER file, at least in IE, Firefox and Safari.

Here's a small, but not exhaustive list of programs that can open CER documents:

  • Adobe Acrobat DC (Windows & Mac)
  • Adobe Reader (Windows, Mac & Linux)
  • Apple Keychain Access (Mac)
  • Apple Safari (Mac)
  • Microsoft Internet Explorer (Windows)
  • Mozilla Firefox (Windows)
  • Other Web Browsers
Extension CER
MIME type application/x-x509-ca-cert, application/pkix-cert, application/keychain_access
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