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I tried updating with apt-get update && apt-get upgrade
as well as with apt-get update --fix-missing and
apt apt-get clean && apt-get update && apt-get dist-upgrade but it always fails.
Whenever I type in apt-get update, I get the following error:

apt-get update
Get:1 http://ftp.halifax.rwth-aachen.de/kali kali-rolling InRelease [30.5 kB]
Err:1 http://ftp.halifax.rwth-aachen.de/kali kali-rolling InRelease
The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux Repository <devel@kali.org>
Hit:2 https://packagecloud.io/AtomEditor/atom/any any InRelease  
Reading package lists... Done   
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://ftp.halifax.rwth-aachen.de/kali kali-rolling InRelease: The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux Repository <devel@kali.org>
W: Failed to fetch http://http.kali.org/kali/dists/kali-rolling/InRelease  The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux Repository <devel@kali.org>
W: Some index files failed to download. They have been ignored, or old ones used instead.

What does the invalid signature EXPKEYSIG ED444FF07D8D0BF6 mean and how could I make it valid?

Is there any way to fix this so that I can update the repositories properly? Your help would be appreciated a lot!
Thanks in advance.

marked as duplicate by Kiwy, Stephen Kitt, Jeff Schaller, Rui F Ribeiro, DarkHeart Mar 13 '18 at 11:27

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1

Try the following command:

 wget -q -O - https://archive.kali.org/archive-key.asc  | apt-key add

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