1APT-SECURE(8) APT APT-SECURE(8)
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6 apt-secure - Archive authentication support for APT
7
9 Starting with version 0.6, APT contains code that does signature
10 checking of the Release file for all repositories. This ensures that
11 data like packages in the archive can't be modified by people who have
12 no access to the Release file signing key. Starting with version 1.1
13 APT requires repositories to provide recent authentication information
14 for unimpeded usage of the repository. Since version 1.5 changes in the
15 information contained in the Release file about the repository need to
16 be confirmed before APT continues to apply updates from this
17 repository.
18 Note: All APT-based package management front-ends like apt-get(8),
20 aptitude(8) and synaptic(8) support this authentication feature, so
21 this manpage uses APT to refer to them all for simplicity only.
22
24 If an archive has an unsigned Release file or no Release file at all
25 current APT versions will refuse to download data from them by default
26 in update operations and even if forced to download front-ends like
27 apt-get(8) will require explicit confirmation if an installation
28 request includes a package from such an unauthenticated archive.
29 You can force all APT clients to raise only warnings by setting the
31 configuration option Acquire::AllowInsecureRepositories to true.
32 Individual repositories can also be allowed to be insecure via the
33 sources.list(5) option allow-insecure=yes. Note that insecure
34 repositories are strongly discouraged and all options to force apt to
35 continue supporting them will eventually be removed. Users also have
36 the Trusted option available to disable even the warnings, but be sure
37 to understand the implications as detailed in sources.list(5).
38 A repository which previously was authenticated but would loose this
40 state in an update operation raises an error in all APT clients
41 irrespective of the option to allow or forbid usage of insecure
42 repositories. The error can be overcome by additionally setting
43 Acquire::AllowDowngradeToInsecureRepositories to true or for Individual
44 repositories with the sources.list(5) option
45 allow-downgrade-to-insecure=yes.
46
48 The chain of trust from an APT archive to the end user is made up of
49 several steps. apt-secure is the last step in this chain; trusting an
50 archive does not mean that you trust its packages not to contain
51 malicious code, but means that you trust the archive maintainer. It's
52 the archive maintainer's responsibility to ensure that the archive's
53 integrity is preserved.
54 apt-secure does not review signatures at a package level. If you
56 require tools to do this you should look at debsig-verify and debsign
57 (provided in the debsig-verify and devscripts packages respectively).
58 The chain of trust in Debian starts (e.g.) when a maintainer uploads a
60 new package or a new version of a package to the Debian archive. In
61 order to become effective, this upload needs to be signed by a key
62 contained in one of the Debian package maintainer keyrings (available
63 in the debian-keyring package). Maintainers' keys are signed by other
64 maintainers following pre-established procedures to ensure the identity
65 of the key holder. Similar procedures exist in all Debian-based
66 distributions.
67 Once the uploaded package is verified and included in the archive, the
69 maintainer signature is stripped off, and checksums of the package are
70 computed and put in the Packages file. The checksums of all of the
71 Packages files are then computed and put into the Release file. The
72 Release file is then signed by the archive key for this Debian release,
73 and distributed alongside the packages and the Packages files on Debian
74 mirrors. The keys are in the Debian archive keyring available in the
75 debian-archive-keyring package.
76 End users can check the signature of the Release file, extract a
78 checksum of a package from it and compare it with the checksum of the
79 package they downloaded by hand - or rely on APT doing this
80 automatically.
81 Notice that this is distinct from checking signatures on a per package
83 basis. It is designed to prevent two possible attacks:
84 • Network "man in the middle" attacks. Without signature checking,
86 malicious agents can introduce themselves into the package download
87 process and provide malicious software either by controlling a
88 network element (router, switch, etc.) or by redirecting traffic to
89 a rogue server (through ARP or DNS spoofing attacks).
90 • Mirror network compromise. Without signature checking, a malicious
92 agent can compromise a mirror host and modify the files in it to
93 propagate malicious software to all users downloading packages from
94 that host.
95 However, it does not defend against a compromise of the master server
97 itself (which signs the packages) or against a compromise of the key
98 used to sign the Release files. In any case, this mechanism can
99 complement a per-package signature.
100
102 A Release file contains beside the checksums for the files in the
103 repository also general information about the repository like the
104 origin, codename or version number of the release.
105 This information is shown in various places so a repository owner
107 should always ensure correctness. Further more user configuration like
108 apt_preferences(5) can depend and make use of this information. Since
109 version 1.5 the user must therefore explicitly confirm changes to
110 signal that the user is sufficiently prepared e.g. for the new major
111 release of the distribution shipped in the repository (as e.g.
112 indicated by the codename).
113
115 apt-key is the program that manages the list of keys used by APT to
116 trust repositories. It can be used to add or remove keys as well as
117 list the trusted keys. Limiting which key(s) are able to sign which
118 archive is possible via the Signed-By in sources.list(5).
119 Note that a default installation already contains all keys to securely
121 acquire packages from the default repositories, so fiddling with
122 apt-key is only needed if third-party repositories are added.
123 In order to add a new key you need to first download it (you should
125 make sure you are using a trusted communication channel when retrieving
126 it), add it with apt-key and then run apt-get update so that apt can
127 download and verify the InRelease or Release.gpg files from the
128 archives you have configured.
129
131 If you want to provide archive signatures in an archive under your
132 maintenance you have to:
133 • Create a toplevel Release file, if it does not exist already. You
135 can do this by running apt-ftparchive release (provided in
136 apt-utils).
137 • Sign it. You can do this by running gpg --clearsign -o InRelease
139 Release and gpg -abs -o Release.gpg Release.
140 • Publish the key fingerprint, so that your users will know what key
142 they need to import in order to authenticate the files in the
143 archive. It is best to ship your key in its own keyring package
144 like Debian does with debian-archive-keyring to be able to
145 distribute updates and key transitions automatically later.
146 • Provide instructions on how to add your archive and key. If your
148 users can't acquire your key securely the chain of trust described
149 above is broken. How you can help users add your key depends on
150 your archive and target audience ranging from having your keyring
151 package included in another archive users already have configured
152 (like the default repositories of their distribution) to leveraging
153 the web of trust.
154 Whenever the contents of the archive change (new packages are added or
156 removed) the archive maintainer has to follow the first two steps
157 outlined above.
158
160 apt.conf(5), apt-get(8), sources.list(5), apt-key(8), apt-
161 ftparchive(1), debsign(1), debsig-verify(1), gpg(1)
162 For more background information you might want to review the Debian
164 Security Infrastructure[1] chapter of the Securing Debian Manual (also
165 available in the harden-doc package) and the Strong Distribution
166 HOWTO[2] by V. Alex Brennen.
167
169 APT bug page[3]. If you wish to report a bug in APT, please see
170 /usr/share/doc/debian/bug-reporting.txt or the reportbug(1) command.
171
173 APT was written by the APT team <apt@packages.debian.org>.
174
176 This man-page is based on the work of Javier Fernández-Sanguino Peña,
177 Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
178
180 Jason Gunthorpe
181 APT team
183
185 1. Debian Security Infrastructure
186 https://www.debian.org/doc/manuals/securing-debian-howto/ch7
187 2. Strong Distribution HOWTO
189 http://www.cryptnet.net/fdp/crypto/strong_distro.html
190 3. APT bug page
192 http://bugs.debian.org/src:apt
193APT 2.5.5 06 August 2016 APT-SECURE(8)