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Unfortunately, decryption is currently not supported in Google Spreadsheet Integration. You have to access the form submissions page, upload the key to decrypt the data and manually export it to excel:
But allow me to forward this thread to our developers. We can't promise any timeframe for this, but if there are updates, you should know it via this thread.
Chief Technology Officer
With Encrypted Forms, our access to the data is completely blocked.
We can't decryption it without your private key, and if we gets your private key, then there is no sense to use Encryption Forms feature.
You can always disable Encryption feature to use your form with Google Spreadsheets.