doc/apt-secure.8.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
   4
   5 <!ENTITY % aptent SYSTEM "apt.ent">
   6 %aptent;
   7
   8 ]>
   9
  10 <refentry>
  11  &apt-docinfo;
  12
  13  <refmeta>
  14    <refentrytitle>apt-secure</refentrytitle>
  15    <manvolnum>8</manvolnum>
  16  </refmeta>
  17
  18 <!-- NOTE: This manpage has been written based on the
  19      Securing Debian Manual ("Debian Security
  20      Infrastructure" chapter) and on documentation
  21      available at the following sites:
  22      http://wiki.debian.net/?apt06
  23      http://www.syntaxpolice.org/apt-secure/
  24      http://www.enyo.de/fw/software/apt-secure/
  25 -->
  26 <!-- TODO: write a more verbose example of how it works with
  27      a sample similar to
  28      http://www.debian-administration.org/articles/174
  29      ?
  30 -->
  31
  32
  33  <!-- Man page title -->
  34  <refnamediv>
  35     <refname>apt-secure</refname>
  36     <refpurpose>Archive authentication support for APT</refpurpose>
  37  </refnamediv>
  38
  39  <refsect1><title>Description</title>
  40    <para>
  41    Starting with version 0.6, <command>apt</command> contains code
  42    that does signature checking of the Release file for all
  43    archives. This ensures that packages in the archive can't be
  44    modified by people who have no access to the Release file signing
  45    key.
  46    </para>
  47
  48    <para>
  49    If a package comes from a archive without a signature or with a
  50    signature that apt does not have a key for that package is
  51    considered untrusted and installing it will result in a big
  52    warning. <command>apt-get</command> will currently only warn
  53    for unsigned archives, future releases might force all sources
  54    to be verified before downloading packages from them.
  55    </para>
  56
  57    <para>
  58    The package frontends &apt-get;, &aptitude; and &synaptic; support this new
  59    authentication feature.
  60    </para>
  61 </refsect1>
  62
  63  <refsect1><title>Trusted archives</title>
  64
  65    <para>
  66    The chain of trust from an apt archive to the end user is made up of
  67    different steps. <command>apt-secure</command> is the last step in
  68    this chain, trusting an archive does not mean that the packages
  69    that you trust it do not contain malicious code but means that you
  70    trust the archive maintainer. Its the archive maintainer
  71    responsibility to ensure that the archive integrity is correct.
  72    </para>
  73
  74    <para>apt-secure does not review signatures at a
  75    package level. If you require tools to do this you should look at
  76    <command>debsig-verify</command> and
  77    <command>debsign</command> (provided in the debsig-verify and
  78    devscripts packages respectively).</para>
  79
  80    <para>
  81    The chain of trust in Debian starts when a maintainer uploads a new
  82    package or a new version of a package to the Debian archive. This
  83    upload in order to become effective needs to be signed by a key of
  84    a maintainer within the Debian maintainer's keyring (available in
  85    the debian-keyring package). Maintainer's keys are signed by
  86    other maintainers following pre-established procedures to
  87    ensure the identity of the key holder.
  88    </para>
  89
  90    <para>
  91    Once the uploaded package is verified and included in the archive,
  92    the maintainer signature is stripped off, an MD5 sum of the package
  93    is computed and put in the Packages file. The MD5 sum of all of the
  94    packages files are then computed and put into the Release file. The
  95    Release file is then signed by the archive key (which is created
  96    once a year and distributed through the FTP server. This key is
  97    also on the Debian keyring.
  98    </para>
  99
 100    <para>
 101    Any end user can check the signature of the Release file, extract the MD5
 102    sum of a package from it and compare it with the MD5 sum of the
 103    package he downloaded. Prior to version 0.6 only the MD5 sum of the
 104    downloaded Debian package was checked. Now both the MD5 sum and the
 105    signature of the Release file are checked.
 106    </para>
 107
 108    <para>Notice that this is distinct from checking signatures on a
 109    per package basis. It is designed to prevent two possible attacks:
 110    </para>
 111
 112     <itemizedlist>
 113        <listitem><para><literal>Network "man in the middle"
 114        attacks</literal>. Without signature checking, a malicious
 115        agent can introduce himself in the package download process and
 116        provide malicious software either by controlling a network
 117        element (router, switch, etc.) or by redirecting traffic to a
 118        rogue server (through arp or DNS spoofing
 119        attacks).</para></listitem>
 120
 121        <listitem><para><literal>Mirror network compromise</literal>.
 122         Without signature checking, a malicious agent can compromise a
 123         mirror host and modify the files in it to propagate malicious
 124         software to all users downloading packages from that
 125         host.</para></listitem>
 126     </itemizedlist>
 127
 128    <para>However, it does not defend against a compromise of the
 129    Debian master server itself (which signs the packages) or against a
 130    compromise of the key used to sign the Release files. In any case,
 131    this mechanism can complement a per-package signature.</para>
 132 </refsect1>
 133
 134  <refsect1><title>User configuration</title>
 135    <para>
 136    <command>apt-key</command> is the program that manages the list
 137    of keys used by apt. It can be used to add or remove keys although
 138    an installation of this release will automatically provide the
 139    default Debian archive signing keys used in the Debian package
 140    repositories.
 141    </para>
 142    <para>
 143    In order to add a new key you need to first download it
 144    (you should make sure you are using a trusted communication channel
 145    when retrieving it), add it with <command>apt-key</command> and
 146    then run <command>apt-get update</command> so that apt can download
 147    and verify the <filename>Release.gpg</filename> files from the archives you
 148    have configured.
 149    </para>
 150 </refsect1>
 151
 152 <refsect1><title>Archive configuration</title>
 153    <para>
 154    If you want to provide archive signatures in an archive under your
 155    maintenance you have to:
 156    </para>
 157
 158      <itemizedlist>
 159        <listitem><para><literal>Create a toplevel Release
 160        file</literal>.  if it does not exist already. You can do this
 161        by running <command>apt-ftparchive release</command>
 162        (provided in apt-utils).</para></listitem>
 163
 164       <listitem><para><literal>Sign it</literal>. You can do this by running
 165       <command>gpg -abs -o Release.gpg Release</command>.</para></listitem>
 166
 167       <listitem><para><literal>Publish the key fingerprint</literal>,
 168       that way your users will know what key they need to import in
 169       order to authenticate the files in the
 170       archive.</para></listitem>
 171
 172     </itemizedlist>
 173
 174     <para>Whenever the contents of the archive changes (new packages
 175     are added or removed) the archive maintainer has to follow the
 176     first two steps previously outlined.</para>
 177
 178 </refsect1>
 179
 180 <refsect1><title>See Also</title>
 181 <para>
 182 &apt-conf;, &apt-get;, &sources-list;, &apt-key;, &apt-ftparchive;,
 183 &debsign; &debsig-verify;, &gpg;
 184 </para>
 185
 186 <para>For more background information you might want to review the
 187 <ulink
 188 url="http://www.debian.org/doc/manuals/securing-debian-howto/ch7.en.html">Debian
 189 Security Infrastructure</ulink> chapter of the Securing Debian Manual
 190 (available also in the harden-doc package) and the
 191 <ulink url="http://www.cryptnet.net/fdp/crypto/strong_distro.html"
 192 >Strong Distribution HOWTO</ulink> by V. Alex Brennen.  </para>
 193
 194 </refsect1>
 195
 196  &manbugs;
 197  &manauthor;
 198
 199 <refsect1><title>Manpage Authors</title>
 200
 201 <para>This man-page is based on the work of Javier Fernández-Sanguino
 202 Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
 203 </para>
 204
 205 </refsect1>
 206
 207
 208 </refentry>
 209