Trying to install VPN and these are the instructions Mullvad is giving me. This is ridiculous. There must be a more simple way. I know how to follow the instructions but I have no idea what I'm doing here. Can't I just download a file and install it? I'm on Ubuntu.
You can verify the signature of the manual download as well. Either way, you are trusting the files you download over HTTPS from mullvad.net. There's no real difference, except that when you use the repo, you are trusting it indefinitely, whereas if you download the deb directly, you are only trusting it once.
Using the repo is less secure, because it opens you to future attacks against the repo itself.
That's why you download the key from multiple distinct domains from multiple distinct locations using multiple distinct devices and veryify their fingerprints match. If the key/fingerprint is only available on one domain, open a bug report with the maintainer.
There's nothing wrong with installing a .deb manually.
Personally, I'd hesitate to add any third-party repos unless there is a very good reason. In this case, the only real difference is that you won't get the updates automatically with sudo apt update; sudo apt upgrade without the repo. Either way, the desktop app will notify you when updates are available. There's very little advantage to using the repo.
Adding a repo is very rarely required. It has deeper consequences than simply installing an app, and requires a higher level of trust. If you don't understand the security implications of adding a repo (and its associated key), then my advice is: just don't.
What are you on about? If you are using the 3rd party repo, you are just as likely to get malware than if you download the deb directly from the wbsite. Its literally the same thing, just adding the repo means that the malware could get installed automatically and without you knowing where it came from.
No, you're confusing two vectors of attack. I'm saying that if you fan trust the vendor, then you're still at risk from downloading malicious software that was manipulated between the vendor and you (man in the middle attack), unless you verified a signature using a key stores offline (note https is still vulnerable because the keys are stored online)
Not untrue, and I don't think that the possibility should be glossed over, but honestly, what do you think is more likely: this specific person getting specifically MitM'ed by a bad actor, or a bad actor taking control of a repo that hundreds of people blindly trust. I have a sneaking suspicion that OP's threat model isn't sophisticated enough to need to really, truly, be worrying about that.
This sort of thing happens dragnet. And mullvad users are definitely a group to be targeted. Dont assume OP isnt a refugee or journalist and give them bad advice that could get them killed