Starting with version 0.6, APT contains code that does signature
checking of the Release file for all repositories. This ensures that
data like packages in the archive can't be modified by people who have
no access to the Release file signing key.
If an archive has an unsigned Release file or no Release file at all
current APT versions will raise a warning in update operations and
front-ends like apt-get will require explicit confirmation if an
installation request includes a package from such an unauthenticated
In the future APT will refuse to work with unauthenticated repositories
by default until support for them is removed entirely. Users have the
option to opt-in to this behavior already by setting the configuration
option Acquire::AllowInsecureRepositories to false.
Note: All APT-based package management front-ends like apt-get(8),
aptitude(8) and synaptic(8) support this authentication feature, so
this manpage uses APT to refer to them all for simplicity only.
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 (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
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 Ubuntu release,
and distributed alongside the packages and the Packages files on Ubuntu
mirrors. The keys are in the Ubuntu archive keyring available in the
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
propagate malicious software to all users downloading packages from
However, 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.
apt-key is the program that manages the list of keys used by APT to
trust repositories. It can be used to add or remove keys as well as
list the trusted keys. Limiting which key(s) are able to sign which
archive is possible via the Signed-By in sources.list(5).
Note that a default installation already contains all keys to securely
acquire packages from the default repositories, so fiddling with
apt-key is only needed if third-party repositories are added.
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:
o Create a toplevel Release file, if it does not exist already. You
can do this by running apt-ftparchive release (provided in
o Sign it. You can do this by running gpg --clearsign -o InRelease
Release and gpg -abs -o Release.gpg Release.
o 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 Ubuntu does with ubuntu-keyring to be able to distribute
updates and key transitions automatically later.
o 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. 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
APT was written by the APT team <email@example.com>.
This man-page is based on the work of Javier Fernandez-Sanguino Pena,
Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
1. Debian Security Infrastructure
2. Strong Distribution HOWTO
3. APT bug page
APT 1.2.32 15 October 2015 APT-SECURE(8)
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