ZyXEL Communications USG-300 - V2.20 ED 2 Manual page 784

Chapter 46 Certificates
Make sure that the certificate has a ".cer" or ".crt" file name extension.
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Figure 516 Remote Host Certificates
Double-click the certificate's icon to open the Certificate window. Click the
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Details tab and scroll down to the Thumbprint Algorithm and Thumbprint
fields.
Figure 517 Certificate Details
Use a secure method to verify that the certificate owner has the same information
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in the Thumbprint Algorithm and Thumbprint fields. The secure method may
very based on your situation. Possible examples would be over the telephone or
through an HTTPS connection.
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ZyWALL USG 300 User's Guide