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.
It's 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@packages.debian.org.
MANPAGE AUTHORS
This man-page is based on the work of Javier Fernndez-Sanguino Pea,
Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
AUTHOR
Jason Gunthorpe
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