The usual way is to setup the form so that some of the submitted form fields are unencrypted and some are encrypted.
The unencrypted fields are things that aren't secret. This depends on your requirements, but might include, name, address, phone number, etc.
The encrypted fields are best attached as a file to the email.
The receiving user can then open the attachment to begin the decryption process.
After being prompted for, and entering, the password that protects the decryption key, the user is presented with the decrypted fields.
We can offer you a free trial license if you'd like to test FormMailEncoder/Decoder to see if it meets your requirements.