6.6. Pakket Authenticiteit Controleren

Beveiliging is heel belangrijk voor de beheerders van Falcot Corp. Overeenkomstig moeten ze er zeker van zijn dat ze enkel pakketten installeren die gegarandeerd van Debian komen zonder dat er onderweg mee geknoeid is. Een computer kraker zou kunnen proberen om kwaadaardige code toe te voegen aan een anderzijds legitiem pakket. Zo'n pakket, indien geïnstalleerd, kan alles doen waar de kraker het voor ontworpen heeft, inclusief bijvoorbeeld het openbaar maken van wachtwoorden of vertrouwelijke informatie. Om dit risico tegen te gaan, biedt Debian een manipuleer-veilig zegel om te garanderen — tijdens de installatie — dat een pakket echt van de officiële onderhouder komt en niet is aangepast door derden.

The seal works with a chain of cryptographic hashes and a signature and is explained in detail in apt-secure(8). Starting with Debian 10 Buster the signed file is the InRelease file, provided by the Debian mirrors. There is also a legacy file called Release. Both contain a list of the Packages files (including their compressed forms, Packages.gz and Packages.xz, and the incremental versions), along with their SHA256 hashes, which ensures that the files haven't been tampered with. These Packages files contain a list of the Debian packages available on the mirror, along with their hashes, which ensures in turn that the contents of the packages themselves haven't been altered either. The difference between InRelease and Release is that the former is cryptographically signed in-line, whereas the latter provides a detached signature in the form of the file Release.gpg.

APT needs a set of trusted GnuPG public keys to verify signatures in the InRelease and Release.gpg files available on the mirrors. It gets them from files in /etc/apt/trusted.gpg.d/ and from the /etc/apt/trusted.gpg keyring (managed by the apt-key command). The official Debian keys are provided and kept up-to-date by the debian-archive-keyring package which puts them in /etc/apt/trusted.gpg.d/:

# ls /etc/apt/trusted.gpg.d/
debian-archive-bullseye-automatic.gpg
debian-archive-bullseye-security-automatic.gpg
debian-archive-bullseye-stable.gpg
debian-archive-buster-automatic.gpg
debian-archive-buster-security-automatic.gpg
debian-archive-buster-stable.gpg
debian-archive-stretch-automatic.gpg
debian-archive-stretch-security-automatic.gpg
debian-archive-stretch-stable.gpg

IN DE PRAKTIJK Vertrouwde sleutels toevoegen

Wanneer een pakket bron van derden wordt toegevoegd aan het sources.list bestand, moet APT verteld worden om de corresponderende GPG authenticatie sleutel te vertrouwen (anders blijft het klagen dat het de authenticiteit van de pakketten komende van deze bron niet kan verzekeren). De eerste stap is natuurlijk het verkrijgen van de publieke sleutel. Meestal wordt de sleutel aangeboden als een klein tekst bestand, welke we key.asc zullen noemen in het volgende voorbeeld.

To add the key to APT's trusted keyring, the administrator can just put it in a *.gpg file in /etc/apt/trusted.gpg.d/ (*.asc files are discouraged and can cause problems). This is supported since Debian Stretch. With older releases, you had to run apt-key add < key.asc, which is considered obsolete already.

While this is very common behavior, it also creates a possible issue. APT does not know which key belongs to which repository. If one of the keys in APT's keyring matches the signature of an archive, APT considers the archive as authenticated. Once a key has been added to the trusted keyring, a malicious attacker could actually provide its own "version" of a Debian mirror signed by the same key. It might therefor actually be better to put the key as *.gpg file into /usr/share/keyrings/ and use the Signed-By field in sources.list for third party repositories as shown in the example below.

Imagine, that the Falcot Corp. runs their own public repository and you want to use it. They have signed it and provide a publicly available key(ring) as well to download. To restrict the key to the repository, you can simply do this:

$ sudo wget -O falcot-keyring.gpg -P /usr/share/keyrings https://packages.falcot.com/debian/repository.gpg

deb [ signed-by=/usr/share/keyrings/falcot-keyring.gpg ] https://packages.falcot.com/debian bullseye main

Beware that adding and using third party repositories always poses a security and stability risk!

Once the appropriate keys are in the keyring, APT will check the signatures before any risky operation, so that frontends will display a warning if asked to install a package whose authenticity can't be ascertained.

Note, that binary packages are usually not signed. The integrity of a package can only be confirmed by checking its hashsums against a trusted (and possibly signed) hashsum source.

SPECIFIC CASE Unsigned repositories

While in general unsigned repositories shouldn't be used, some users might still require them, for example to provide some locally built packages. APT can be forced to accept such a repository by prepending the repository URL using allow-insecure=yes or trusted=yes. We will demonstrate it using the example from Paragraaf 15.3, “Creating a Package Repository for APT”:

deb [ trusted=yes ] http://packages.falcot.com/ updates/
deb [ trusted=yes ] http://packages.falcot.com/ internal/