The GPG keys for Debian repositories are listed at https://ftp-master.debian.org/keys.html. Their expiration dates overlap in term of years from 1 version to the next version. These overlaps help ease the upgrade transition from 1 version to another version.
Here is how I add GPG keys in my Debian 10 Buster.
# For regular repositories. # Debian 9. wget -O - https://ftp-master.debian.org/keys/archive-key-9.asc \ | gpg --no-default-keyring --keyring /etc/apt/trusted.gpg --import # Debian 10. wget -O - https://ftp-master.debian.org/keys/archive-key-10.asc \ | gpg --no-default-keyring --keyring /etc/apt/trusted.gpg --import # For repositories with security updates. # Debian 9. wget -O - https://ftp-master.debian.org/keys/archive-key-9-security.asc \ | gpg --no-default-keyring --keyring /etc/apt/trusted.gpg --import # Debian 10. wget -O - https://ftp-master.debian.org/keys/archive-key-10-security.asc \ | gpg --no-default-keyring --keyring /etc/apt/trusted.gpg --import