APT-SECURE(8)

📦
version 3.3.5 · 15 versions
, in arch rolling, nixpkgs unstable

✨ apt-secure ✨

apt-secure — archive authentication support for APT (yes, that's the manual page 8 one, bestie 💅)

🌸 NAME 🌸

apt-secure — archive authentication support for APT. That's it, that's the whole vibe: it's the thing that makes sure the stuff you're downloading is actually the stuff you think you're downloading 🫶

🎀 DESCRIPTION 🎀

okay so starting with version 0.6, APT has had code that does signature checking on the Release file for all repositories. Translation: nobody who lacks access to the Release file signing key can go in and modify data like the packages in the archive. Iconic behavior, honestly.

Starting with version 1.1, APT requires repositories to hand over recent authentication information — like, if you want unimpeded usage of the repository, you gotta keep it fresh ✨

And since version 1.5, changes in the info contained in the Release file about the repository need to be confirmed before APT keeps applying updates from that repository. No sneaky little changes on the down-low, periodt.

💡 Note: all APT-based package management front-ends like apt-get(8), aptitude(8) and synaptic(8) support this authentication feature — so this manpage just says "APT" to refer to all of them, purely for simplicity, no favorites being played 💖

🔑 USER CONFIGURATION 🔑

Keys should usually be included inside their corresponding .sources by embedding the ASCII-armored key in the Signed-By option. To do that: replace the empty line with a dot, and then indent all lines by two spaces. See sources.list(5) for more info 🎀

Alternatively — and this is the lowkey classic move — keys may be placed in /etc/apt/keyrings for local keys, or /usr/share/keyrings for keys managed by packages, and then referenced by:

Signed-By: /etc/apt/keyrings/example-archive-keyring.asc

...as an option in a .sources file, or using:

deb [signed-by=/etc/apt/keyrings/example-archive-keyring.asc] ...

...in the legacy .list format. This is useful for APT versions prior to 2.4, which do not support embedded keys. And pls remember: ASCII-armored keys must use an extension of .asc, and unarmored keys an extension of .gpg — do not mix those up bestie 😭

To generate keys suitable for use in APT using GnuPG, you'll need to use:

gpg --export-options export-minimal [--armor] --export

Earlier solutions involving --keyring file --import no longer work with recent GnuPG versions — they use a new internal format ("GPG keybox database") and honestly it ate the old approach alive 💀

✨ Note: a default installation already contains all the keys to securely acquire packages from the default repositories, so managing keys is only needed if third-party repositories get added. The extrepo package can manage several external repositories with ease — chef's kiss 👑

💀 UNSIGNED REPOSITORIES 💀

If an archive has an unsigned Release file or no Release file at all, current APT versions will refuse to download data from them by default in update operations. And even if forced to download, front-ends like apt-get(8) will require explicit confirmation if an installation request includes a package from such an unauthenticated archive. Not me being dramatic, that's just the rules 🙈

You can force all APT clients to raise only warnings by setting the configuration option Acquire::AllowInsecureRepositories to true. Individual repositories can also be allowed to be insecure via the sources.list(5) option allow-insecure=yes.

⚠️ fr fr: insecure repositories are strongly discouraged, and all options to force apt to keep supporting them will eventually be removed. Like, gone-gone. Users also have the Trusted option available to disable even the warnings — but be sure you understand the implications as detailed in sources.list(5) before you go turning that on 🚨

A repository which previously was authenticated but would lose this state in an update operation raises an error in all APT clients, irrespective of the option to allow or forbid usage of insecure repositories. That error can be overcome by additionally setting Acquire::AllowDowngradeToInsecureRepositories to true, or for individual repositories with the sources.list(5) option allow-downgrade-to-insecure=yes.

👑 SIGNED REPOSITORIES 👑

The chain of trust from an APT archive to the end user is made up of several steps. apt-secure is the last step in this chain. And bestie, listen: trusting an archive does not mean you trust its packages not to contain malicious code — it means you trust the archive maintainer. It's the archive maintainer's responsibility to ensure the archive's integrity is preserved. Main character energy, but make it accountability 💅

Also apt-secure does not review signatures at a package level. If you require tools to do that, you should look at debsig-verify and debsign (provided in the debsig-verify and devscripts packages respectively) 🦋

🧚 The chain of trust, explained 🧚

The chain of trust in Debian starts (e.g.) when a maintainer uploads a new package or a new version of a package to the Debian archive. In order to become effective, this upload needs to be signed by a key contained in one of the Debian package maintainer keyrings (available in the debian-keyring package). Maintainers' keys are signed by other maintainers following pre-established procedures to ensure the identity of the key holder. Similar procedures exist in all Debian-based distributions ✨

Once the uploaded package is verified and included in the archive, the maintainer signature is stripped off, and checksums of the package are computed and put in the Packages file. The checksums of all of the Packages files are then computed and put into the Release file. The Release file is then signed by the archive key for this Debian release, and distributed alongside the packages and the Packages files on Debian mirrors. The keys are in the Debian archive keyring available in the debian-archive-keyring package 🔑

End users can check the signature of the Release file, extract a checksum of a package from it and compare it with the checksum of the package they downloaded by hand — or just rely on APT doing this automatically, which is what literally everyone does 🫶

Notice that this is distinct from checking signatures on a per package basis. It is designed to prevent two possible attacks:

  • Network "man in the middle" attacks. Without signature checking, malicious agents can introduce themselves into the package download process and provide malicious software either by controlling a network element (router, switch, etc.) or by redirecting traffic to a rogue server (through ARP or DNS spoofing attacks). Rude, honestly.
  • Mirror network compromise. Without signature checking, a malicious agent can compromise a mirror host and modify the files in it to propagate malicious software to all users downloading packages from that host. Absolutely not the vibe 🙅

However — and this is the important part — it does not defend against a compromise of the master server itself (which signs the packages) or against a compromise of the key used to sign the Release files. In any case, this mechanism can complement a per-package signature 💜

🌈 INFORMATION CHANGES 🌈

A Release file contains, besides the checksums for the files in the repository, also general information about the repository — like the origin, codename or version number of the release 🌸

This information is shown in various places, so a repository owner should always ensure correctness. Furthermore, user configuration like apt_preferences(5) can depend on and make use of this information. Since version 1.5, the user must therefore explicitly confirm changes to signal that they're sufficiently prepared — e.g. for the new major release of the distribution shipped in the repository (as e.g. indicated by the codename). It's giving "confirm you're ready for the glow-up" 💅

🛠️ REPOSITORY CONFIGURATION 🛠️

If you want to provide archive signatures in an archive under your maintenance, you have to:

  • Create a toplevel Release file, if it does not exist already. You can do this by running apt-ftparchive release (provided in apt-utils).
  • Sign it. You can do this by running:
gpg --clearsign -o InRelease Release

and:

gpg -abs -o Release.gpg Release
  • Publish the key fingerprint, so that your users will know what key they need to import in order to authenticate the files in the archive. It is best to ship your key in its own keyring package like Debian does with debian-archive-keyring, so you're able to distribute updates and key transitions automatically later 👑
  • Provide instructions on how to add your archive and key. If your users can't acquire your key securely, the chain of trust described above is broken — like, shattered. How you can help users add your key depends on your archive and target audience, ranging from having your keyring package included in another archive users already have configured (like the default repositories of their distribution) to leveraging the web of trust 🕸️

Whenever the contents of the archive change (new packages are added or removed), the archive maintainer has to follow the first two steps outlined above. Every single time. No skipping 💖

🦋 SEE ALSO 🦋

apt.conf(5), apt-get(8), sources.list(5), apt-ftparchive(1), debsign(1), debsig-verify(1), gpg(1)

For more background information you might want to review the Debian Security Infrastructure[1] chapter of the Securing Debian Manual (also available in the harden-doc package) and the Strong Distribution HOWTO[2] by V. Alex Brennen 📚

🐛 BUGS 🐛

APT bug page[3]. If you wish to report a bug in APT, please see /usr/share/doc/debian/bug-reporting.txt or the reportbug(1) command. Found a bug? Tell on it (kindly) 💜

💖 AUTHOR 💖

APT was written by the APT team <apt@packages.debian.org>.

🎀 MANPAGE AUTHORS 🎀

This man-page is based on the work of Javier Fernández-Sanguino Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt. Legends, all of them 🌟

⭐ AUTHORS ⭐

Jason Gunthorpe

APT team

📎 NOTES 📎

  1. Debian Security Infrastructure https://www.debian.org/doc/manuals/securing-debian-manual/ch07
  2. Strong Distribution HOWTO http://www.cryptnet.net/fdp/crypto/strong_distro.html
  3. APT bug page https://bugs.debian.org/src:apt

okay that's the whole page bestie, you ate that 💅 — this iconic little document is from APT 3.3.5, dated 23 November 2024, and it's page APT-SECURE(8) of the APT manual. now go forth and verify your signatures, ily 🫶✨🎀

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