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APT-SECURE(8) 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 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.
TRUSTED ARCHIVES
The chain of trust from an apt archive to the end user is made up of different steps.
apt-secure is the last step in this chain, trusting an archive does not mean that the
packages that you trust it do not contain malicious code but means that you trust the
archive maintainer. Its the archive maintainer responsibility to ensure that the archive
integrity is correct.
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. This upload in order to become effective needs
to be signed by a key of a maintainer within the Debian maintainer's keyring (available in
the debian-keyring package). Maintainer's 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, an MD5 sum of the package is computed and put in the Packages
file. The MD5 sum 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 (which is created once a year and
distributed through the FTP server. This key is also on the Debian keyring.
Any end user can check the signature of the Release file, extract the MD5 sum of a package
from it and compare it with the MD5 sum of the package he downloaded. Prior to version 0.6
only the MD5 sum of the downloaded Debian package was checked. Now both the MD5 sum and
the signature of the Release file are checked.
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, a malicious agent can
introduce himself in 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 Debian 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.
USER CONFIGURATION
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 provide 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 Release.gpg files from the
archives you have configured.
ARCHIVE 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 -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 archive.
Whenever the contents of the archive changes (new packages are added or removed) the
archive maintainer has to follow the first two steps previously outlined.
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 (available also 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 AT packages.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.
AUTHOR
Jason Gunthorpe
Author.
COPYRIGHT
Copyright (C) 1998-2001 Jason Gunthorpe
NOTES
1. Debian Security Infrastructure
http://www.debian.org/doc/manuals/securing-debian-howto/ch7.en.html
2. Strong Distribution HOWTO
http://www.cryptnet.net/fdp/crypto/strong_distro.html
3. APT bug page
http://bugs.debian.org/src:apt
Linux 28 October 2008 APT-SECURE(8)
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On Apache
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