Section: APT (8)
Updated: 09 June 2012
Starting with version 0.6, apt contains code that does signature checking of the Release file for all archives. This ensures that packages in the archive can't be modified by people who have no access to the Release file signing key.
If a package comes from a archive without a signature, or with a signature that apt does not have a key for, that package is considered untrusted, and installing it will result in a big warning. apt-get will currently only warn for unsigned archives; future releases might force all sources to be verified before downloading packages from them.
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; trusting an archive does not mean that you trust its packages not to contain malicious code, but means that you trust the archive maintainer. It's the archive maintainer's responsibility to ensure that the archive's integrity is preserved.
apt-secure does not review signatures at a package level. If you require tools to do this you should look at debsig-verify and debsign (provided in the debsig-verify and devscripts packages respectively).
The chain of trust in Debian starts 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 the Debian Maintainers keyring (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.
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 rely on APT doing this automatically.
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).
- • 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.
However, it does not defend against a compromise of the Debian 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.
apt-key is the program that manages the list of keys used by apt. It can be used to add or remove keys, although an installation of this release will automatically contain the default Debian archive signing keys used in the Debian package repositories.
In order to add a new key you need to first download it (you should make sure you are using a trusted communication channel when retrieving it), add it with apt-key and then run apt-get update so that apt can download and verify the InRelease or Release.gpg files from the archives you have configured.
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, that way your users will know what key they need to import in order to authenticate the files in the archive.
For more background information you might want to review the m[blue]Debian Security Infrastructurem chapter of the Securing Debian Manual (available also in the harden-doc package) and the m[blue]Strong Distribution HOWTOm by V. Alex Brennen.
APT was written by the APT team <[email protected]>.
- Debian Security Infrastructure
- Strong Distribution HOWTO
- APT bug page