6.6. Vérification d'authenticité des paquets

Étant donné l'importance qu'accordent les administrateurs de Falcot SA à la sécurité, ils veulent s'assurer de n'installer que des paquets garantis provenant de Debian et non altérés en cours de route. En effet, un pirate pourrait tenter d'agir indirectement sur des machines en modifiant un paquet Debian diffusé afin d'y ajouter les instructions de son choix. Si un paquet ainsi modifié est installé, ces instructions agiront, par exemple afin de dérober les mots de passe. C'est pourquoi Debian offre un moyen de s'assurer que le paquet installé provient bien de son mainteneur et qu'il n'a subi aucune modification par un tiers : il existe un mécanisme de scellement des paquets.

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

EN PRATIQUE Ajouter des clés de confiance

Lorsqu'une source de paquets tierce est ajoutée au fichier sources.list, il faut désormais porter à la connaissance de APT la clé de confiance correspondante (sans quoi il se plaindra constamment qu'il ne peut pas vérifier l'authenticité des paquets contenus dans le dépôt concerné). Pour cela, il faut avant tout récupérer la clé publique en question : la plupart du temps, elle sera fournie sous la forme d'un petit fichier texte (qui sera nommé cle.asc dans les exemples ci-dessous).

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 Section 15.3, « Créer une archive de paquets pour APT »:

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