Invalid signature from repo.skype.com: how can I clear this?

Question

Today I did sudo apt upgrade and then, as a double-check, did sudo apt update. The latter terminated with:

Hit:5 http://dl.google.com/linux/chrome/deb stable InRelease                                       
Get:6 https://repo.skype.com/deb stable InRelease [4,502 B]                                        
Err:6 https://repo.skype.com/deb stable InRelease
  The following signatures were invalid: EXPKEYSIG 1F3045A5DF7587C3 Skype Linux Client Repository <se-um@microsoft.com>
Fetched 4,502 B in 5s (894 B/s)
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up to date.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://repo.skype.com/deb stable InRelease: The following signatures were invalid: EXPKEYSIG 1F3045A5DF7587C3 Skype Linux Client Repository <se-um@microsoft.com>
W: Failed to fetch https://repo.skype.com/deb/dists/stable/InRelease  The following signatures were invalid: EXPKEYSIG 1F3045A5DF7587C3 Skype Linux Client Repository <se-um@microsoft.com>
W: Some index files failed to download. They have been ignored, or old ones used instead.

In an attempt to clear this problem, I called sudo apt remove skypeforlinux and then did another sudo apt update. The update again terminated with the message about the invalid signature.

How can I clear up this invalid signature problem?

$ uname -mrs
Linux 5.4.0-77-generic x86_64
$ lsb_release -d
Description:    Ubuntu 20.04.2 LTS

Answer 1

The validity of the signature key has expired, and only after a few days Microsoft generated a new key. The key replacement in your system is not automatic and you need to delete the old key and add the new one.

There are two ways two solve the issue: one, using the traditional apt-key add command, the other one adding manually the new key to the keyring.

# Solution valid until Ubuntu 21.04 (including it)

You can delete the no longer valid key:

sudo apt-key del 1F3045A5DF7587C3

and then re-add the new valid key:

curl https://repo.skype.com/data/SKYPE-GPG-KEY | sudo apt-key add -

Running sudo apt update && sudo apt upgrade you should see no errors.

# Solution valid in Ubuntu 21.04 and future releases

When you try to add an APT repository key using apt-key in Ubuntu, you may see the following message:

Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).

The apt-key man page mentions that:

use of apt-key is deprecated, except for the use of apt-key del in maintainer scripts to remove existing keys from the main keyring.

So, if you use a version of Ubuntu until 21.04, you are ok using apt-key del and apt-key add, but for the following versions you must manually add the key in the keyring (in Ubuntu 21.04 both solutions work perfectly: I tested both of them).

You can delete the no longer valid key:

sudo apt-key del 1F3045A5DF7587C3

Download the key and add it to the keyring:

curl https://repo.skype.com/data/SKYPE-GPG-KEY | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/skype-stable-archive-keyring.gpg

Open the skype-stable.list file...

sudo nano /etc/apt/sources.list.d/skype-stable.list

...and modify the first line in this way:

deb [arch=amd64 signed-by=/etc/apt/trusted.gpg.d/skype-stable-archive-keyring.gpg] https://repo.skype.com/deb stable main

Running sudo apt update && sudo apt upgrade you should see no errors.