The security@parity.io GPG key, which is used to sign releases and Linux packages of polkadot, expired on the the 16th of July 2021 and has subsequently been renewed.
If you make use of our Debian/Ubuntu repositories to install Polkadot, you may need to perform the following steps to fetch the new public GPG key:
# Import the security@parity.io GPG key
gpg --recv-keys --keyserver hkps://keys.mailvelope.com 9D4B2B6EB8F97156D19669A9FF0812D491B96798
gpg --export 9D4B2B6EB8F97156D19669A9FF0812D491B96798 > /usr/share/keyrings/parity.gpg
apt update
# Install the `parity-keyring` package