Starting with version 0.6, apt contains code that does signature
checking of the Release file for all archives. This ensures that
packages in the archive can't be modified by people who have no access
to the Release file signing key.
If a package comes from a archive without a signature, or with a
signature that apt does not have a key for, that package is considered
untrusted, and installing it will result in a big warning. apt-get
will currently only warn for unsigned archives; future releases might
force all sources to be verified before downloading packages from them.
The package frontends apt-get(8), aptitude(8) and synaptic(8) support
this new authentication feature.
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 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 the Debian Maintainers keyring (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.
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
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
complement a per-package signature.
apt-key is the program that manages the list of keys used by apt. It
can be used to add or remove keys, although an installation of this
release will automatically contain the default Debian archive signing
keys used in the Debian package repositories.
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, that way your users will know what key
they need to import in order to authenticate the files in the
Whenever the contents of the archive change (new packages are added or
removed) the archive maintainer has to follow the first two steps
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 chapter of the Securing Debian Manual
(available also in the harden-doc package) and the Strong Distribution
HOWTO by V. Alex Brennen.
APT bug page. If you wish to report a bug in APT, please see
/usr/share/doc/debian/bug-reporting.txt or the reportbug(1) command.
APT was written by the APT team <firstname.lastname@example.org>.
This man-page is based on the work of Javier Fernandez-Sanguino Pena,
Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
APT 1.0.1ubuntu2 09 June 2012 APT-SECURE(8)
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