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.
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:
o 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).
o 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).
USER CONFIGURATION
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.
REPOSITORY CONFIGURATION
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
apt-utils).
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 Debian does with debian-archive-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
outlined above.
SEE ALSO
apt.conf(5), apt-get(8), sources.list(5), apt-key(8), 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 Fernandez-Sanguino Pena,
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-howto/ch7
2. Strong Distribution HOWTO
http://www.cryptnet.net/fdp/crypto/strong_distro.html
3. APT bug page
http://bugs.debian.org/src:apt
APT 2.1.7 06 August 2016 APT-SECURE(8)