Final answer:
To ensure the integrity of a downloaded package, use GNU Privacy Guard (GPG) keys to verify that the package's signature matches the one provided by the maintainer, ensuring it has not been altered.
Step-by-step explanation:
To ensure the integrity of a downloaded package, you can use GNU Privacy Guard (GPG) keys. GPG keys are a tool that allows you to verify that the package you've downloaded has not been tampered with and is indeed the authentic package released by the maintainers. When a package is released, the maintainer generates a signature using their private GPG key. As a user, you can verify this signature using the maintainer's public GPG key. The apt-get and yum commands are package managers for Linux that can also check signatures if configured to do so, but by themselves, they are not tools specifically for verifying integrity. Knoppix is a Linux distribution that is not related to verifying package integrity.