So Kali Linux lost access to the signing key of its repository, and it had to create a new one. Further updating previously installed Kali requires users to download and install the new repo signing key, as instructed by the blog post https://www.kali.org/blog/new-kali-archive-signing-key/
That's all okay, these things happen. But... Am I the only one who feels a bit uneasy telling people to sudo wget or curl like this? I'd rather instruct people to download the file as regular users, verify its authenticity and *only then* sudo cp it in place.
