You can encrypt the data before passing it to the K2 datafields. You can use any of the components available or you can create your own. You will just have to decrypt them on your page before showing them to the user.

A couple of considerations however:
1. If these datafields are being used to evaluate rules, they will have to be decrypted before the rules can use them. Modifying the existing rules and decrypting should save some time there.
2. The same goes for all events that use encrypted fields. The code will have to be modified and testing well.
3. Instead of encrypting all the data, consider creating all the fields as hidden. This way unauthorised users will not see any data.