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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>

<p>Digital signature is a process ensuring that a certain package was
    generated by its developers and has not been tampered with. Below we explain
    why it is important and how to verify that the Tor program you download is
    the one we have created and has not been modified by some attacker.</p>

<p>Digital signature is a cryptographic mechanism. If you want to learn more
    about how it works see <a href="https://www.gnupg.org/documentation/">
    https://www.gnupg.org/documentation/</a>.</p>

<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?
    Digital signatures ensure that the package you are downloading was created by
    our developers. It uses a cryptographic mechanism which outputs a sequence of
    characters that is always the same unless the software has not been tampered
    with.</p>

<p>For many Tor users it is important to verify that the Tor software is authentic
    as they have very real adversaries who might try to give them a fake version
    of Tor.</p>

<p>If the Tor package has been modified by some attacker it is not safe to use.
    It doesn't matter how secure and anonymous Tor is if you're not running the real Tor.</p>

<p>There are a variety of attacks that can be used to make you download a fake
    version of Tor. For example, an attacker could trick you into thinking some other
    website is a great place to download Tor. That's why you should
    always download Tor from <a href="https://www.torproject.org"><b>https</b>://www.torproject.org/</a>.</p>

<p><a href="https://www.torproject.org">https://www.torproject.org/</a> uses https.
    Https is the secure version of the http protocol which uses encryption and authentication between your
    browser and the website. This makes 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 to download Tor <a href="<page
    docs/faq>#GetTor">somewhere else</a>.</p>

<p>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="https://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,
    torbrowser-install-<version-torbrowserbundle>_en-US.exe is accompanied by
    torbrowser-install-<version-torbrowserbundle>_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>

<p>We now show how you can verify the downloaded file's digital signature on
    different operating systems. Please notice that a signature is dated the moment
    the package has been signed. Therefore every time a new file is uploaded a new
    signature is generated with a different date. As long as you have verified the
    signature you should not worry that the reported date may vary.
    </p>

<h3>Windows</h3>
    <hr>
    <p>You need to have GnuPG installed before
    you can verify signatures. Download it from <a
    href="https://gpg4win.org/download.html">https://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>The Tor Browser team signs Tor Browser releases. Import its key
    (0x4E2C6E8793298290) by starting <i>cmd.exe</i> and typing:</p>
    <pre>"C:\Program Files\Gnu\GnuPg\gpg.exe" --keyserver pool.sks-keyservers.net --recv-keys 0x4E2C6E8793298290</pre>
    <p>After importing the key, you can verify that the fingerprint
    is correct:</p>
    <pre>"C:\Program Files\Gnu\GnuPg\gpg.exe" --fingerprint 0x4E2C6E8793298290</pre>
    <p>You should see:</p>
    <pre>
    pub   4096R/93298290 2014-12-15
          Key fingerprint = EF6E 286D DA85 EA2A 4BA7  DE68 4E2C 6E87 9329 8290
    uid                  Tor Browser Developers (signing key) <torbrowser@torproject.org>
    sub   4096R/F65C2036 2014-12-15
    sub   4096R/D40814E0 2014-12-15
    sub   4096R/C3C07136 2016-08-24
</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\torbrowser-install-<version-torbrowserbundle>_en-US.exe.asc \
    C:\Users\Alice\Desktop\torbrowser-install-<version-torbrowserbundle>_en-US.exe</pre>
    <p>The output should say "Good signature": </p>
    <pre>
    gpg: Signature made Tue 24 Jan 2015 09:29:09 AM CET using RSA key ID D40814E0
    gpg: Good signature from "Tor Browser Developers (signing key) <torbrowser@torproject.org>"
    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: EF6E 286D DA85 EA2A 4BA7  DE68 4E2C 6E87 9329 8290</pre>
    <p>Currently valid subkey fingerprints are:
    <pre>
    5242 013F 02AF C851 B1C7  36B8 7017 ADCE F65C 2036
    BA1E E421 BBB4 5263 180E  1FC7 2E1A C68E D408 14E0
    A430 0A6B C93C 0877 A445  1486 D148 3FA6 C3C0 7136</pre></p>
    <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 and Linux</h3>
    <hr>

<p>You need to have GnuPG installed before you can verify
    signatures. If you are using Mac OS X, you can install it from <a
    href="https://www.gpgtools.org/">https://www.gpgtools.org/</a>. If you
    are using Linux, then it's probably you already have GnuPG in your
    system, as most Linux distributions come with it preinstalled.
    </p>

<p>The next step is to use GnuPG to import the key that signed
    your package. The Tor Browser team signs Tor Browser releases. Import its
    key (0x4E2C6E8793298290) by starting the terminal (under "Applications"
    in Mac OS X) and typing:</p>

<pre>gpg --keyserver pool.sks-keyservers.net --recv-keys 0x4E2C6E8793298290</pre>

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

<pre>gpg --fingerprint 0x4E2C6E8793298290</pre>

<p>You should see:</p>
    <pre>
    pub   4096R/93298290 2014-12-15
          Key fingerprint = EF6E 286D DA85 EA2A 4BA7  DE68 4E2C 6E87 9329 8290
    uid                  Tor Browser Developers (signing key) <torbrowser@torproject.org>
    sub   4096R/F65C2036 2014-12-15
    sub   4096R/D40814E0 2014-12-15
    sub   4096R/C3C07136 2016-08-24</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 Downloads folder, run:</p>

<strong>For Mac OS X users</strong>:<br />
    <pre>gpg --verify ~/Downloads/TorBrowser-<version-torbrowserbundleosx64>-osx64_en-US.dmg{.asc*,}</pre>

<strong>For Linux users</strong> (change 64 to 32 if you have the 32-bit package):<br />
    <pre>gpg --verify tor-browser-linux64-<version-torbrowserbundlelinux64>_en-US.tar.xz.asc \
    tor-browser-linux64-<version-torbrowserbundlelinux64>_en-US.tar.xz</pre>

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

<pre>
    gpg: Signature made Tue 24 Jan 2015 09:29:09 AM CET using RSA key ID D40814E0
    gpg: Good signature from "Tor Browser Developers (signing key) <torbrowser@torproject.org>"
    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: EF6E 286D DA85 EA2A 4BA7  DE68 4E2C 6E87 9329 8290</pre> <p> Currently valid subkey fingerprints are:
    <pre>
    5242 013F 02AF C851 B1C7  36B8 7017 ADCE F65C 2036
    BA1E E421 BBB4 5263 180E  1FC7 2E1A C68E D408 14E0
    A430 0A6B C93C 0877 A445  1486 D148 3FA6 C3C0 7136</pre></p>
    <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>

<p>
    If you're a Linux user and 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>.
    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="https://www.gnupg.org/documentation/">https://www.gnupg.org/documentation/</a>
    to learn more about GnuPG.</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 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 <tt>sha256sums-unsigned-build.txt</tt>
      file, and the <tt>sha256sums-unsigned-build.txt.asc</tt> signature file.
      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 '<version-torbrowserbundlelinux64>'
      for Tor Browser <version-torbrowserbundlelinux64>.</li>
      <li>In case your operating system is adding the .txt extension
      automatically to the SHA256 sums signature file strip it again by running
      <pre>mv sha256sums-unsigned-build.txt.asc.txt sha256sums-unsigned-build.txt.asc</pre>
      <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 0x4E2C6E8793298290</pre>
      (This will bring you the public part of the Tor Browser developers'
       signing key. Other developers' key IDs can be found on
      <a href="<page docs/signing-keys>">this
      page</a>.)</li>
      <li>Verify the sha256sums-unsigned-build.txt file by executing this
      command:
      <pre>gpg --verify sha256sums-unsigned-build.txt.asc sha256sums-unsigned-build.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>If you want to verify a Windows Tor Browser package you need to first
      strip off the authenticode signature of it. Tools that can be used for
      this purpose are <a href="http://osslsigncode.sourceforge.net">osslsigncode</a> and
      <a href="http://forum.xda-developers.com/showthread.php?t=416175">delcert.exe</a>.
      Assuming you have built e.g. <tt>osslsigncode</tt> on a Linux computer you can enter
      <pre>/path/to/your/osslsigncode remove-signature &#92;
        /path/to/your/&lt;TOR BROWSER FILE NAME&gt;.exe &lt;TOR BROWSER FILE NAME&gt;.exe</pre></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>
      <p>On Linux you can run</p>
      <pre>sha256sum &lt;TOR BROWSER FILE NAME&gt;.tar.gz</pre>
      without having to download a utility. Note: this does not work for OS X
      yet due to Apple's codesigning requirement.</li>
      <li>You will see a string of letters and numbers.</li>
      <li>Open <tt>sha256sums-unsigned-build.txt</tt> 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="https://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
    export LD_LIBRARY_PATH=/path/to/MAR/file/mar-tools
    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-unsigned-build.txt</tt> or
    <tt>sha256sums-unsigned-build.incremental.txt</tt> as outlined in
    <a href="#BuildVerification">Verifying sha256sums (advancded)</a> above.</p>

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