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verifying-signatures.wml

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    <a href="<page index>">Home &raquo; </a>
    <a href="<page docs/verifying-signatures>">Verifying Signatures</a>
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    <h1>How to verify signatures for packages</h1>
    <hr>

<h3>What is a signature and why should I check it?</h3>
    <hr>

<p>How do you know that the Tor program you have is really the
    one we made? Many Tor users have very real adversaries who might
    try to give them a fake version of Tor &mdash; and it doesn't matter
    how secure and anonymous Tor is if you're not running the real Tor.</p>

<p>An attacker could try a variety of attacks to get you to download
    a fake Tor. For example, he could trick you into thinking some other
    website is a great place to download Tor. That's why you should
    always download Tor from <b>https</b>://www.torproject.org/. The
    https part means there's encryption and authentication between your
    browser and the website, making it much harder for the attacker
    to modify your download. But it's not perfect. Some places in the
    world block the Tor website, making users try <a href="<page
    docs/faq>#GetTor">somewhere else</a>. Large
    companies sometimes force employees to use a modified browser,
    so the company can listen in on all their browsing. We've even <a
    href="https://blog.torproject.org/blog/diginotar-debacle-and-what-you-should-do-about-it">seen</a>
    attackers who have the ability to trick your browser into thinking
    you're talking to the Tor website with https when you're not.</p>

<p>Some software sites list <a
    href="http://en.wikipedia.org/wiki/Cryptographic_hash_function">sha1
    hashes</a> alongside the software on their website, so users can
    verify that they downloaded the file without any errors. These
    "checksums" help you answer the question "Did I download this file
    correctly from whoever sent it to me?" They do a good job at making
    sure you didn't have any random errors in your download, but they
    don't help you figure out whether you were downloading it from the
    attacker. The better question to answer is: "Is this file that I
    just downloaded the file that Tor intended me to get?"</p>

<h3>Where do I get the signatures and the keys that made them?</h3>
    <hr>
    <p>Each file on <a href="<page download/download>">our download
    page</a> is accompanied by a file with the same name as the
    package and the extension ".asc". These .asc files are GPG
    signatures. They allow you to verify the file you've downloaded
    is exactly the one that we intended you to get. For example,
    tor-browser-2.3.25-13_en-US.exe is accompanied by
    tor-browser-2.3.25-13_en-US.exe.asc. For a list
    of which developer signs which package, see our <a href="<page docs/signing-keys>">signing keys</a> page.</p>
    <h3>Windows</h3>
    <hr>
    <p>You need to have GnuPG installed before
    you can verify signatures. Download it from <a
    href="http://gpg4win.org/download.html">http://gpg4win.org/download.html</a>.</p>
    <p>Once it's installed, use GnuPG to import the key that signed your
    package. Since GnuPG for Windows is a command-line tool, you will need
    to use <i>cmd.exe</i>. Unless you edit your PATH environment variable,
    you will need to tell Windows the full path to the GnuPG program. If
    you installed GnuPG with the default values, the path should be
    something like this: <i>C:\Program Files\Gnu\GnuPg\gpg.exe</i>.</p>
    <p>Erinn Clark signs the Tor Browsers. Import her key
    (0x416F061063FEE659) by starting <i>cmd.exe</i> and typing:</p>
    <pre>"C:\Program Files\Gnu\GnuPg\gpg.exe" --keyserver x-hkp://pool.sks-keyservers.net --recv-keys 0x416F061063FEE659</pre>
    <p>After importing the key, you can verify that the fingerprint
    is correct:</p>
    <pre>"C:\Program Files\Gnu\GnuPg\gpg.exe" --fingerprint 0x416F061063FEE659</pre>
    <p>You should see:</p>
    <pre>
    pub   2048R/63FEE659 2003-10-16
          Key fingerprint = 8738 A680 B84B 3031 A630  F2DB 416F 0610 63FE E659
    uid                  Erinn Clark &lt;erinn@torproject.org&gt;
    uid                  Erinn Clark &lt;erinn@debian.org&gt;
    uid                  Erinn Clark &lt;erinn@double-helix.org&gt;
    sub   2048R/EB399FD7 2003-10-16
</pre>
    <p>To verify the signature of the package you downloaded, you will need
    to download the ".asc" file as well. Assuming you downloaded the
    package and its signature to your Desktop, run:</p>
    <pre>"C:\Program Files\Gnu\GnuPg\gpg.exe" --verify C:\Users\Alice\Desktop\tor-browser-2.3.25-13_en-US.exe.asc C:\Users\Alice\Desktop\tor-browser-2.3.25-13_en-US.exe</pre>
    <p>The output should say "Good signature": </p>
    <pre>
    gpg: Signature made Wed 31 Aug 2011 06:37:01 PM EDT using RSA key ID 63FEE659
    gpg: Good signature from "Erinn Clark &lt;erinn@torproject.org&gt;"
    gpg:                 aka "Erinn Clark &lt;erinn@debian.org&gt;"
    gpg:                 aka "Erinn Clark &lt;erinn@double-helix.org&gt;"
    gpg: WARNING: This key is not certified with a trusted signature!
    gpg:          There is no indication that the signature belongs to the owner.
    Primary key fingerprint: 8738 A680 B84B 3031 A630  F2DB 416F 0610 63FE E659
    </pre>
    <p>
    Notice that there is a warning because you haven't assigned a trust
    index to this person. This means that GnuPG verified that the key made
    that signature, but it's up to you to decide if that key really belongs
    to the developer. The best method is to meet the developer in person and
    exchange key fingerprints.
    </p>
    <h3>Mac OS X</h3>
    <hr>

<p>You need to have GnuPG installed before you can verify
    signatures. You can install it from <a
    href="http://www.gpgtools.org/">http://www.gpgtools.org/</a>.
    </p>

<p>Once it's installed, use GnuPG to import the key that signed
    your package. Erinn Clark signs the Tor Browsers. Import her
    key (0x416F061063FEE659) by starting the terminal (under "Applications")
    and typing:</p>

<pre>gpg --keyserver x-hkp://pool.sks-keyservers.net --recv-keys 0x416F061063FEE659</pre>

<p>After importing the key, you can verify that the fingerprint
    is correct:</p>

<pre>gpg --fingerprint 0x416F061063FEE659</pre>

<p>You should see:</p>
    <pre>
    pub   2048R/63FEE659 2003-10-16
          Key fingerprint = 8738 A680 B84B 3031 A630  F2DB 416F 0610 63FE E659
    uid                  Erinn Clark &lt;erinn@torproject.org&gt;
    uid                  Erinn Clark &lt;erinn@debian.org&gt;
    uid                  Erinn Clark &lt;erinn@double-helix.org&gt;
    sub   2048R/EB399FD7 2003-10-16
    </pre>

<p>To verify the signature of the package you downloaded, you will need
    to download the ".asc" file as well. Assuming you downloaded the
    package and its signature to your Desktop, run:</p>

<pre>gpg --verify /Users/Alice/TorBrowser-<version-torbrowserbundleosx32>-osx32_en-US.dmg{.asc*,}</pre>

<p>The output should say "Good signature": </p>

<pre>
    gpg: Signature made Wed 31 Aug 2011 06:37:01 PM EDT using RSA key ID 63FEE659
    gpg: Good signature from "Erinn Clark &lt;erinn@torproject.org&gt;"
    gpg:                 aka "Erinn Clark &lt;erinn@debian.org&gt;"
    gpg:                 aka "Erinn Clark &lt;erinn@double-helix.org&gt;"
    gpg: WARNING: This key is not certified with a trusted signature!
    gpg:          There is no indication that the signature belongs to the owner.
    Primary key fingerprint: 8738 A680 B84B 3031 A630  F2DB 416F 0610 63FE E659
    </pre>

<p>
    Notice that there is a warning because you haven't assigned a trust
    index to this person. This means that GnuPG verified that the key made
    that signature, but it's up to you to decide if that key really belongs
    to the developer. The best method is to meet the developer in person and
    exchange key fingerprints.
    </p>

<h3>Linux</h3>
    <hr>

<p>Most Linux distributions come with gpg preinstalled, so users
    who want to verify the Tor Browser for Linux (or the source
    tarball) can just follow along with the instructions above for
    "Mac OS X". </p>

<p>If you're using the <b>Debian</b> Tor (not Tor Browser) packages, you 
    should read the
    instructions on <a href="<page docs/debian>#packages">importing
    these keys to apt</a>.</p>

<p>If you're using the <b>RPMs</b> (for Tor, not Tor Browser), you can 
    manually verify the
    signatures on the RPM packages by <pre>rpm -K filename.rpm</pre></p>

<p>See <a
    href="http://www.gnupg.org/documentation/">http://www.gnupg.org/documentation/</a>
    to learn more about GPG.</p>

<hr>

<a id="BuildVerification"></a>
    <h3><a class="anchor" href="#BuildVerification">
    Verifying sha256sums (advanced)</a></h3>
    <hr>
    <p>Build reproducibility is a <a
       href="https://blog.torproject.org/blog/deterministic-builds-part-one-cyberwar-and-global-compromise">security
       property</a> of Tor Browser 3.0 and later. Anyone can build
       Tor Browser on their own machine and produce a binary that is
       bit-for-bit identical to the binary we offer on the download page.
       Fortunately, it is not necessary for everyone to build the Tor Browser
       locally to get this security. Verifying and comparing the signed list
       of <a href="https://en.wikipedia.org/wiki/Cryptographic_hash">hashes</a>
       will confirm that multiple people have built Tor Browsers
       identical to the download.</p>

<p>The steps below walk through this process:</p>

<ul>
      <li>Download the Tor Browser package, the sha256sums.txt file, and the
      sha256sums signature files. They can all be found in the same directory
      under <a href="https://www.torproject.org/dist/torbrowser/">
      https://www.torproject.org/dist/torbrowser/</a>, for example in '3.6.1'
      for TBB 3.6.1.</li>
      <li>Retrieve the signers' GPG keys. This can be done from the command
      line by entering something like
      <pre>gpg --keyserver keys.mozilla.org --recv-keys 0x29846B3C683686CC</pre>
      (This will bring you developer Mike Perry's public key. Other
      developers' key IDs can be found on
      <a href="<page docs/signing-keys>">this
      page</a>.)</li>
      <li>Verify the sha256sums.txt file by executing this command:
      <pre>gpg --verify &lt;NAME OF THE SIGNATURE FILE&gt;.asc sha256sums.txt</pre></li>
      <li>You should see a message like "Good signature from &lt;DEVELOPER
      NAME&gt;". If you don't, there is a problem. Try these steps again.</li>
      <li>Now you can take the sha256sum of the Tor Browser package. On
      Windows you can use the <a href="http://md5deep.sourceforge.net/">
      hashdeep utility</a> and run
      <pre>C:\location\where\you\saved\hashdeep -c sha256sum &lt;TOR BROWSER FILE NAME&gt;.exe</pre>
      On Mac or Linux you can run <pre>sha256sum &lt;TOR BROWSER FILE NAME&gt;.zip</pre> or <pre>sha256sum &lt;TOR BROWSER FILE NAME&gt;.tar.gz</pre> without having to download a utility.</li>
      <li>You will see a string of letters and numbers.</li>
      <li>Open sha256sums.txt in a text editor.</li>
      <li>Locate the name of the Tor Browser file you downloaded.</li>
      <li>Compare the string of letters and numbers to the left of your
      filename with the string of letters and numbers that appeared
      on your command line. If they match, you've successfully verified the
      build.</li>
    </ul>

<p><a href="https://github.com/isislovecruft/scripts/blob/master/verify-gitian-builder-signatures">Scripts</a>
    to <a href="http://tor.stackexchange.com/questions/648/how-to-verify-tor-browser-bundle-tbb-3-x">automate</a>
    these steps have been written, but to use them you will need to modify
    them yourself with the latest Tor Browser filename.</p>

<hr>

<a id="MARVerification"></a>
    <h3><a class="anchor" href="#MARVerification">
    Verifying MAR files we ship (advanced)</a></h3>
    <hr>
    <p>Starting with Tor Browser 4.5a4 we sign our MAR files which helps
    securing our update process. The downside of this is the need for additional
    instructions to verify that the MAR files we ship are indeed the ones we
    produced with our Gitian setup.</p>

<p>Assuming the verification happens on a Linux computer one first needs the
    <tt>mar-tools-linux*.zip</tt> out of the <tt>gitian-builder/inputs</tt>
    directory to remove the embedded signature(s). The steps to get the unsigned
    MAR file on a 64 bit Linux are</p>
    <pre>
    cd /path/to/MAR/file
    unzip /path/to/gitian-builder/inputs/mar-tools-linux64.zip
    mar-tools/signmar -r your-signed-mar-file.mar your-unsigned-mar-file.mar</pre>
    <p>Now you can compare the SHA256 sum of <tt>your-unsigned-mar-file.mar</tt>
    with the one provided in the <tt>sha265sums.txt</tt> or
    <tt>sha256sums.incremental.txt</tt> as outlined in <a href="#BuildVerification">Verifying
    sha256sums (advancded)</a> above.</p>

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tor-webwml.git

Fixes osx arch tag in TorBrowser's name.