What commands (exactly) should replace the deprecated apt-key?

You need to know why apt-key add is deprecated

All of the answers so far work around the symptom ("Don't use apt-key add") but fail to address the actual problem that led to apt-key add being deprecated. The problem is not a question of appending a key to one big keyring file etc/apt/trusted.gpg vs manually putting single-key keyring files into the directory /etc/apt/trusted.gpg.d/. These two things are equivalent, and doing either one is a huge security risk.

The problem is that any key you add to either of the above is completely and unconditionally trusted by apt. This means that any third-party repo whose apt signing key is in there is allowed to replace any package on your entire system, no questions asked. Even if we assume none of the repo owners are malicious, an attacker might compromise a repo, which has the same end result.

The instructions given in Ugo Delle Donne's answer for converting the key to the (legacy) keyring v4 format that apt will accept are correct and helpful, but that's only half of the solution. I'll reiterate them here (cleaned up slightly) so all the steps are consolidated in one place:

• Download the key:
  • wget https://host.domain.tld/path/to/<keyfile>.<ext>
    (No need for -O or >; wget defaults to saving the file in your current directory with the same filename it has on the server.)
• Verify that the filetype is "PGP public key block Public-Key (old)":
  • file <keyfile>.<ext>
• Convert the key by importing it into a temp keyring, then exporting it again:
  • gpg --no-default-keyring --keyring ./temp-keyring.gpg --import <keyfile>.<ext>
  • gpg --no-default-keyring --keyring ./temp-keyring.pgp --export --output <your-keyfile-name>.gpg
  • rm temp-keyring.gpg

Now that you have your converted key, do not add it to apt's trusted keystore by copying it into /etc/apt/trusted.gpg.d/. Instead, put it somewhere like /usr/local/share/keyrings/. (You'll need to create that keyrings directory first.) There's nothing special about that location, it's just convention that /usr/local is for stuff that's specific to this machine, share because it's not a binary or a library or specific to any given user, and keyrings is just a descriptive name.

At this point, nothing has changed and apt doesn't know the key exists. The last step is to modify the specific .list file for the repository to tell apt where to find the key for that specific repo.

• Edit the file /etc/apt/sources.list.d/<example>.list, and in between deb and the url, add [signed-by=/usr/local/share/keyrings/<your-keyfile-name>.gpg]

Now apt will accept that key's signature for all packages in that repo and only that repo.

Notes:

• If you already have keyring files in /etc/apt/trusted.gpg.d/, you can copy move them to /usr/local/share/keyrings/ as-is, and then update all the corresponding .list files so each one has a signed-by field pointing to its own key.
• If you already have keys in the /etc/apt/trusted.gpg keyring file beyond the official repo keys, this answer details the steps to locate and remove them. You can then follow all the same steps above to set them up the safer way. (Exporting them from that keyring is also possible, but the exact steps are left as an exercise for the reader.)
• To import a repo's key from a keyserver to a standalone file:
  • gpg --no-default-keyring --keyring <output-file-name>.gpg --keyserver <some.keyserver.uri> --recv-keys <fingerprint>
  • This should give you a key that apt will accept without conversion.
• Apt is still very trusting, and a malicious or compromised repo can bypass this measure easily because packages currently can run arbitrary shell code as root in their setup scripts. Closing off one attack vector doesn't hurt, though, and progress is (slowly) being made on other fronts.
• Optionally, you can switch to the newer, more verbose Deb822 format using individual .sources files instead of .list files. It's more work, but personally I find the result far more readable.

Sources:

• Debian Wiki
• Excellent and detailed answer on Unix&Linux StackExchange

I stumbled on the same problem and luckily some other question lighted the way. In my example I was trying to add a teamviewer repository to a recent Kali linux and I was being blocked by the key verification.

I'm quite sure there's a more elegant way to do this but the following steps helped me fix the problem:

1. Download the relevant key

   wget -O - https://download.teamviewer.com/download/linux/signature/TeamViewer2017.asc > ~/teamviewer.key

2. Verify the type of file

   file ~/teamviewer.key

   it should be PGP public key block Public-Key (old)

3. Create a keyring

   gpg --no-default-keyring --keyring ./teamviewer_keyring.gpg --import teamviewer.key

4. This file is still not a valid key that can be added to /etc/apt/trusted.gpg.d/ since it's a keyring, but from the keyring we can extract the key with

   gpg --no-default-keyring --keyring ./teamviewer_keyring.gpg --export > ./teamviewer.gpg

5. This file is the key you want to move to the trusted key folder

   sudo mv ./teamviewer.gpg /etc/apt/trusted.gpg.d/

happy sudo apt update!!!

The reason for this deprecation is because using apt-key add simply appends the gpg key to the trusted global APT keyring. It's similar to the preferred method of adding local_repo.list to /etc/apt/sources.list.d/ instead of using add-apt-repository dep /link/to/repo version, which appends the message to the global sources.list file.

I think it's a bit more awkward to understand than using the .d folder, but essentially we want to get the gpg key into a standalone keyring file, then point to this keyring file in the source listing. The default keyring file location is /usr/share/keyrings, and it can be a .asc or .gpg file. I'm not sure the difference but I do know the global keyring files are binary files, not plaintext.

For example:

Using generic names can be a bit hard to understand sometimes, so here is an example of installing mongoDB:

Get the MongoDB gpg key and add it to a new keyring file

curl https://www.mongodb.org/static/pgp/server-4.2.asc | sudo tee -a /usr/share/keyrings/buster-mongodb-org-4_2.asc

Add a source entry for apt, pointed to this new keyring

echo "deb [signed-by=/usr/share/keyrings/buster-mongodb-org-4_2.asc] https://repo.mongodb.org/apt/debian buster/mongodb-org/4.2 main

Install mongodb from this newly added repo

sudo apt install -y mongodb-org

Reference

This is still new to me, but most of what I know came from this excellent answer in the unix SE