Something wrong with updates and appcenter

Question

I would like to update all of my programs, but something wrong.

$ sudo apt update && sudo apt full-upgrade
[sudo] password for racztamaska:           
Hit:1 http://gb.archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://ppa.launchpad.net/elementary-os/stable/ubuntu xenial InRelease
Ign:3 http://dl.google.com/linux/chrome/deb stable InRelease                        
Hit:4 http://ppa.launchpad.net/elementary-os/os-patches/ubuntu xenial InRelease     
Get:5 http://gb.archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]         
Hit:6 http://dl.google.com/linux/chrome/deb stable Release                          
Hit:7 http://packages.elementary.io/appcenter xenial InRelease                      
Get:8 http://gb.archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB]       
Get:9 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]        
Get:10 https://repo.skype.com/deb stable InRelease [4,487 B]  
Err:10 https://repo.skype.com/deb stable InRelease             
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3
Reading package lists... Done 
W: GPG error: https://repo.skype.com/deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3
E: The repository 'https://repo.skype.com/deb stable InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

What is the problem? What is public_key?

Please, help me.

Thank you

Tamas

Answer 1

Per the Debian Wiki:

Public key cryptography is based on pairs of keys, a public key and a private key. The public key is given out to the world; the private key must be kept a secret. Anyone possessing the public key can encrypt a message so that it can only be read by someone possessing the private key. It's also possible to use a private key to sign a file, not encrypt it. If a private key is used to sign a file, then anyone who has the public key can check that the file was signed by that key. Anyone who doesn't have the private key can't forge such a signature.

To solve the error messages shown by the Terminal and install Skype, you need to add Skype's public key which can be done by running:

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