apt-secure - Archive authentication support for APT
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. Starting with version 1.1 APT
to provide recent authentication information for unimpeded usage of the
repository. Since version 1.5 changes in the information contained in the
Release file about the repository need to be confirmed before APT continues to
apply updates from this repository.
Note: All APT-based package management front-ends like apt-get
(8) and synaptic
(8) support this authentication feature,
so this manpage uses APT to refer to them all for simplicity only.
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
will require explicit confirmation if an installation request includes a
package from such an unauthenticated archive.
You can force all APT clients to raise only warnings by setting the
configuration option Acquire::AllowInsecureRepositories
Individual repositories can also be allowed to be insecure via the
(5) option allow-insecure=yes. Note that insecure
repositories are strongly discouraged and all options to force apt to continue
supporting them will eventually be removed. Users also have the Trusted
option available to disable even the warnings, but be sure to understand the
implications as detailed in sources.list
A repository which previously was authenticated but would loose this state in an
operation raises an error in all APT clients irrespective of the
option to allow or forbid usage of insecure repositories. The error can be
overcome by additionally setting
to true or for Individual
repositories with the sources.list
The chain of trust from an APT archive to the end user is made up of several
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
(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 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 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 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
A Release file contains beside 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. Further more user configuration like
(5) can depend and make use of this information. Since
version 1.5 the user must therefore explicitly confirm changes to signal that
the user is sufficiently prepared e.g. for the new major release of the
distribution shipped in the repository (as e.g. indicated by the codename).
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
Note that a default installation already contains all keys to securely acquire
packages from the default repositories, so fiddling with apt-key
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
and then run apt-get update
so that apt can download and
verify the InRelease or Release.gpg files from the archives you have
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
•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 to be 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. 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
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.
For more background information you might want to review the Debian Security
 chapter of the Securing Debian Manual (also available in
the harden-doc package) and the Strong Distribution HOWTO
 by V. Alex
APT bug page
. If you wish to report a bug in APT, please see
/usr/share/doc/debian/bug-reporting.txt or the reportbug
APT was written by the APT team <email@example.com>.
This man-page is based on the work of Javier Fernández-Sanguino
Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
- Debian Security Infrastructure
- Strong Distribution HOWTO
- APT bug page