apt-getアップデートKali Linuxエラー

更新しようとしましたが、apt-get update && apt-get upgrade
常に失敗apt-get update --fix-missingしました
apt apt-get clean && apt-get update && apt-get dist-upgrade。
を入力するたびに、apt-get update次のエラーが発生します。

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 <[email protected]>
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 <[email protected]>
W: Failed to fetch http://http.kali.org/kali/dists/kali-rolling/InRelease  The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux Repository <[email protected]>
W: Some index files failed to download. They have been ignored, or old ones used instead.

無効な署名とは何を意味EXPKEYSIG ED444FF07D8D0BF6し、それを有効にするには？

リポジトリを正しく更新できるように、この問題を解決する方法はありますか？ご協力ありがとうございます！
よろしくお願いします。