How do I get advanced decryption for Rahool now?

I've searched it up in a few different ways on Google but everything is outdated. I want to get Hazardous Propulsion from him instead of Legend campaign, but it says I need everything from the engram unlocked already.

Do I really need to unlock them first now, or is there something I'm missing?

To be more clear, I do not own Hazardous Propulsion yet, but I was told I can use advanced decryption to get it.