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    <refmiscinfo class="manual">APT</refmiscinfo>
  17  </refmeta>
  18
  19 <!-- NOTE: This manpage has been written based on the
  20      Securing Debian Manual ("Debian Security
  21      Infrastructure" chapter) and on documentation
  22      available at the following sites:
  23      http://wiki.debian.net/?apt06
  24      http://www.syntaxpolice.org/apt-secure/
  25      http://www.enyo.de/fw/software/apt-secure/
  26 -->
  27 <!-- TODO: write a more verbose example of how it works with
  28      a sample similar to
  29      http://www.debian-administration.org/articles/174
  30      ?
  31 -->
  32
  33
  34  <!-- Man page title -->
  35  <refnamediv>
  36     <refname>apt-secure</refname>
  37     <refpurpose>Archive authentication support for APT</refpurpose>
  38  </refnamediv>
  39
  40  <refsect1><title>Description</title>
  41    <para>
  42    Starting with version 0.6, <command>apt</command> contains code
  43    that does signature checking of the Release file for all
  44    archives. This ensures that packages in the archive can't be
  45    modified by people who have no access to the Release file signing
  46    key.
  47    </para>
  48
  49    <para>
  50    If a package comes from a archive without a signature or with a
  51    signature that apt does not have a key for that package is
  52    considered untrusted and installing it will result in a big
  53    warning. <command>apt-get</command> will currently only warn
  54    for unsigned archives, future releases might force all sources
  55    to be verified before downloading packages from them.
  56    </para>
  57
  58    <para>
  59    The package frontends &apt-get;, &aptitude; and &synaptic; support this new
  60    authentication feature.
  61    </para>
  62 </refsect1>
  63
  64  <refsect1><title>Trusted archives</title>
  65
  66    <para>
  67    The chain of trust from an apt archive to the end user is made up of
  68    different steps. <command>apt-secure</command> is the last step in
  69    this chain, trusting an archive does not mean that the packages
  70    that you trust it do not contain malicious code but means that you
  71    trust the archive maintainer. It's the archive maintainer
  72    responsibility to ensure that the archive integrity is correct.
  73    </para>
  74
  75    <para>apt-secure does not review signatures at a
  76    package level. If you require tools to do this you should look at
  77    <command>debsig-verify</command> and
  78    <command>debsign</command> (provided in the debsig-verify and
  79    devscripts packages respectively).</para>
  80
  81    <para>
  82    The chain of trust in Debian starts when a maintainer uploads a new
  83    package or a new version of a package to the Debian archive. This
  84    upload in order to become effective needs to be signed by a key of
  85    a maintainer within the Debian maintainer's keyring (available in
  86    the debian-keyring package). Maintainer's keys are signed by
  87    other maintainers following pre-established procedures to
  88    ensure the identity of the key holder.
  89    </para>
  90
  91    <para>
  92    Once the uploaded package is verified and included in the archive,
  93    the maintainer signature is stripped off, an MD5 sum of the package
  94    is computed and put in the Packages file. The MD5 sum of all of the
  95    packages files are then computed and put into the Release file. The
  96    Release file is then signed by the archive key (which is created
  97    once a year) and distributed through the FTP server. This key is
  98    also on the Debian keyring.
  99    </para>
 100
 101    <para>
 102    Any end user can check the signature of the Release file, extract the MD5
 103    sum of a package from it and compare it with the MD5 sum of the
 104    package he downloaded. Prior to version 0.6 only the MD5 sum of the
 105    downloaded Debian package was checked. Now both the MD5 sum and the
 106    signature of the Release file are checked.
 107    </para>
 108
 109    <para>Notice that this is distinct from checking signatures on a
 110    per package basis. It is designed to prevent two possible attacks:
 111    </para>
 112
 113     <itemizedlist>
 114        <listitem><para><literal>Network "man in the middle"
 115        attacks</literal>. Without signature checking, a malicious
 116        agent can introduce himself in the package download process and
 117        provide malicious software either by controlling a network
 118        element (router, switch, etc.) or by redirecting traffic to a
 119        rogue server (through arp or DNS spoofing
 120        attacks).</para></listitem>
 121
 122        <listitem><para><literal>Mirror network compromise</literal>.
 123         Without signature checking, a malicious agent can compromise a
 124         mirror host and modify the files in it to propagate malicious
 125         software to all users downloading packages from that
 126         host.</para></listitem>
 127     </itemizedlist>
 128
 129    <para>However, it does not defend against a compromise of the
 130    Debian master server itself (which signs the packages) or against a
 131    compromise of the key used to sign the Release files. In any case,
 132    this mechanism can complement a per-package signature.</para>
 133 </refsect1>
 134
 135  <refsect1><title>User configuration</title>
 136    <para>
 137    <command>apt-key</command> is the program that manages the list
 138    of keys used by apt. It can be used to add or remove keys although
 139    an installation of this release will automatically provide the
 140    default Debian archive signing keys used in the Debian package
 141    repositories.
 142    </para>
 143    <para>
 144    In order to add a new key you need to first download it
 145    (you should make sure you are using a trusted communication channel
 146    when retrieving it), add it with <command>apt-key</command> and
 147    then run <command>apt-get update</command> so that apt can download
 148    and verify the <filename>Release.gpg</filename> files from the archives you
 149    have configured.
 150    </para>
 151 </refsect1>
 152
 153 <refsect1><title>Archive configuration</title>
 154    <para>
 155    If you want to provide archive signatures in an archive under your
 156    maintenance you have to:
 157    </para>
 158
 159      <itemizedlist>
 160        <listitem><para><emphasis>Create a toplevel Release
 161        file</emphasis>,  if it does not exist already. You can do this
 162        by running <command>apt-ftparchive release</command>
 163        (provided in apt-utils).</para></listitem>
 164
 165       <listitem><para><emphasis>Sign it</emphasis>. You can do this by running
 166       <command>gpg -abs -o Release.gpg Release</command>.</para></listitem>
 167
 168       <listitem><para><emphasis>Publish the key fingerprint</emphasis>,
 169       that way your users will know what key they need to import in
 170       order to authenticate the files in the
 171       archive.</para></listitem>
 172
 173     </itemizedlist>
 174
 175     <para>Whenever the contents of the archive changes (new packages
 176     are added or removed) the archive maintainer has to follow the
 177     first two steps previously outlined.</para>
 178
 179 </refsect1>
 180
 181 <refsect1><title>See Also</title>
 182 <para>
 183 &apt-conf;, &apt-get;, &sources-list;, &apt-key;, &apt-ftparchive;,
 184 &debsign; &debsig-verify;, &gpg;
 185 </para>
 186
 187 <para>For more background information you might want to review the
 188 <ulink
 189 url="http://www.debian.org/doc/manuals/securing-debian-howto/ch7.en.html">Debian
 190 Security Infrastructure</ulink> chapter of the Securing Debian Manual
 191 (available also in the harden-doc package) and the
 192 <ulink url="http://www.cryptnet.net/fdp/crypto/strong_distro.html"
 193 >Strong Distribution HOWTO</ulink> by V. Alex Brennen.  </para>
 194
 195 </refsect1>
 196
 197  &manbugs;
 198  &manauthor;
 199
 200 <refsect1><title>Manpage Authors</title>
 201
 202 <para>This man-page is based on the work of Javier Fernández-Sanguino
 203 Peña, Isaac Jones, Colin Walters, Florian Weimer and Michael Vogt.
 204 </para>
 205
 206 </refsect1>
 207
 208
 209 </refentry>
 210