root@BlackRishi98:~# apt-get update
Get:1 http://old.kali.org/kali sana InRelease [20.3 kB]
Err:1 http://old.kali.org/kali sana InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
Get:2 http://ftp.yzu.edu.tw/Linux/kali kali-rolling InRelease [30.5 kB]
Err:2 http://ftp.yzu.edu.tw/Linux/kali kali-rolling InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
Reading package lists... Done
W: GPG error: http://old.kali.org/kali sana InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
E: The repository 'http://old.kali.org/kali sana 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.
W: GPG error: http://ftp.yzu.edu.tw/Linux/kali kali-rolling InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ED444FF07D8D0BF6
E: The repository 'http://http.kali.org/kali kali-rolling 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.