Ao executar o apt-get no Kali Linux Re4son-Kali-Pi+ pela primeira vez, deparei-me com a seguinte mensagem de erro:

An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://mirror.pwnieexpress.com/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.

A correção é bem simples:

# apt-key adv --keyserver hkp://keys.gnupg.net --recv-keys 7D8D0BF6

Em seguida execute:

# apt-get update

Pronto!