I want to encrypt my deisgn code. I am using the encrypt_1735 utility and able to encrypt the RTL. Now is there any way I can decrypt the file? Reason is I am unaware of how to input the key for encryption. It is using the public key of Quartus I suppose.

Fyi, only file encryption with a third-party tool requires the public encryption key. File encryption with the Intel® Quartus® Prime Pro Edition software does not require the public encryption key.

Which third-party tool are you using?

As far as I know, each tool might have different way in using the public key. I suggest to refer to the third-party tool guide.

The encrypted code by default is readable in Intel® Quartus® Prime Pro Edition and there is no private key to decrypt the code.

Hope it helps.