OK, I just tried decrypting the save data, selecting the option to use save on any system, and encrypting it again, but every time, my PS3 says it's corrupted. Did I do something wrong or is there another way?
You don't need to decrypt unless you're adding Cheats to the save.
Just use PARAM.sfo to make a template first. This is used to overwrite the PARAM.sfo of any other saves you will resign. To do this Set Profile and use your save's PARAM.SFO. Profile name can be anything.
IF successful, you will have your profile name with account ID in brackets.
Right click the save you want to resign and "Rebuild with current profile (Full)"
If all goes well, the save will be resigned. To check this, just scroll right. It'll have the same Profile, np_account, PSID, etc.
Does it show your profile/template on the resigned save?
The way I got BF Save to work for me was I activated my PS3 system for PSN then saved a game and used that as the template for everything else.
The difference for me was PSID, np_account , and everything else showing up all of a sudden. (Before nothing showed up, didn't work for me)
Edit: Here's a pic of working BruteForce