APT-SECURE(8) APT APT-SECURE(8)
NAME
apt-secure - Archive authentication support for APT
DESCRIPTION
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
requires repositories 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),
aptitude(8) and synaptic(8) support this authentication feature, so this
manpage uses APT to refer to them all for simplicity only.
USER CONFIGURATION
Keys should usually be included inside their corresponding .sources by
embedding the ASCII-armored key in the Signed-By option. To do so,
replace the empty line with a dot, and then indent all lines by two
spaces. See sources.list(5) for more information.
Alternatively, 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 option in a
.sources file or using deb
[signed-by=/etc/apt/keyrings/example-archive-keyring.asc] ... in the
legacy .list format. This may be useful for APT versions prior to 2.4,
which do not support embedded keys. ASCII-armored keys must use an
extension of .asc, and unarmored keys an extension of .gpg.
To generate keys suitable for use in APT using GnuPG, you will need to
use the gpg --export-options export-minimal [--armor] --export command.
Earlier solutions involving --keyring file --import no longer work with
recent GnuPG versions as they use a new internal format ("GPG keybox
database").
Note that a default installation already contains all keys to securely
acquire packages from the default repositories, so managing keys is only
needed if third-party repositories are added. The extrepo package can be
used to manage several external repositories with ease.
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.
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. 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(5).
A repository which previously was authenticated but would loose 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. The 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; 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
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 signature.
INFORMATION CHANGES
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
apt_preferences(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).
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 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
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.
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.
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.
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
APT 3.0.3 23 November 2024 APT-SECURE(8)
Generated by dwww version 1.16 on Tue Dec 16 04:52:48 CET 2025.