JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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The .key file generated when you enabled Form Encryption is not something you will open. It's something you will upload in order to decrypt your Submission Data.
Before you continue using the Form Encryption feature though, I highly recommend you take your time reading this very detailed guide so you have an idea what this does: What-Are-Encrypted-Forms-and-How-to-Use-Them-as-Expert
To give you an idea on how to use the jotform.key file you downloaded to decrypt your submission data, both from your Submissions page and from Email, refer to the following clips:
1. Via Submission page:
2. Via email notification:
If you need more info, just click the link I gave above.