Manual: apt-secure
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 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 ubuntu-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 Ubuntu does with
ubuntu-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 .
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 3.2.0 23 November 2024 APT-SECURE(8)