A GPG key is used to sign the repository to ensure authenticity, integrity and trust while updating the packages. Every 2-3 years, the Kali team either extends the lifetime of the GPG key used to sign the APT repository or replaces it with a new key. This can cause errors for users who haven’t updated their kali-archive-keyring package in a long time. The error will look like this:

kali@kali:~$ sudo apt update
[sudo] password for kali:
Get:1 https://http.kali.org/kali kali-rolling InRelease [41.5 kB]
Err:1 https://http.kali.org/kali kali-rolling InRelease
  Sub-process /usr/bin/sqv returned an error code (1), error message is: Missing key 827C8569F2518CC677FECA1AED65462EC8D5E4C5, which is needed to verify signature.
Fetched 41.5 kB in 3s (16.5 kB/s)
82 packages can be upgraded. Run 'apt list --upgradable' to see them.
Warning: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. OpenPGP signature verification failed: https://http.kali.org/kali kali-rolling InRelease: Sub-process /usr/bin/sqv returned an error code (1), error message is: Missing key 827C8569F2518CC677FECA1AED65462EC8D5E4C5, which is needed to verify signature.
Warning: Failed to fetch https://http.kali.org/kali/dists/kali-rolling/InRelease  Sub-process /usr/bin/sqv returned an error code (1), error message is: Missing key 827C8569F2518CC677FECA1AED65462EC8D5E4C5, which is needed to verify signature.
Warning: Some index files failed to download. They have been ignored, or old ones used instead.

To avoid this problem in the future:

Keep your system updated regularly, especially the kali-archive-keyring package.

If your Kali installation is more than 2 years old, it may no longer be supported. Consider updating to the latest release to continue receiving updates.

Another way to resolve this issue is to retrieve the latest key and store it in a place where apt will find it.

kali@kali:~$ sudo wget https://archive.kali.org/archive-keyring.gpg -O /usr/share/keyrings/kali-archive-keyring.gpg