How to verify signatures for packages


What is a signature and why should I check it?


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 — and it doesn't matter how secure and anonymous Tor is if you're not running the real Tor.

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 https://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 somewhere else. Large companies sometimes force employees to use a modified browser, so the company can listen in on all their browsing. We've even seen attackers who have the ability to trick your browser into thinking you're talking to the Tor website with https when you're not.

Some software sites list sha1 hashes 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?"

Where do I get the signatures and the keys that made them?


Each file on our download page 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 signing keys page.

Windows


You need to have GnuPG installed before you can verify signatures. Download it from http://gpg4win.org/download.html.

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 cmd.exe. 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: C:\Program Files\Gnu\GnuPg\gpg.exe.

Erinn Clark signs the Tor Browser Bundles. Import her key (0x416F061063FEE659) by starting cmd.exe and typing:

"C:\Program Files\Gnu\GnuPg\gpg.exe" --keyserver x-hkp://pool.sks-keyservers.net --recv-keys 0x416F061063FEE659

After importing the key, you can verify that the fingerprint is correct:

"C:\Program Files\Gnu\GnuPg\gpg.exe" --fingerprint 0x416F061063FEE659

You should see:

    pub   2048R/63FEE659 2003-10-16
          Key fingerprint = 8738 A680 B84B 3031 A630  F2DB 416F 0610 63FE E659
    uid                  Erinn Clark <erinn@torproject.org>
    uid                  Erinn Clark <erinn@debian.org>
    uid                  Erinn Clark <erinn@double-helix.org>
    sub   2048R/EB399FD7 2003-10-16

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:

"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

The output should say "Good signature":

    gpg: Signature made Wed 31 Aug 2011 06:37:01 PM EDT using RSA key ID 63FEE659
    gpg: Good signature from "Erinn Clark <erinn@torproject.org>"
    gpg:                 aka "Erinn Clark <erinn@debian.org>"
    gpg:                 aka "Erinn Clark <erinn@double-helix.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: 8738 A680 B84B 3031 A630  F2DB 416F 0610 63FE E659
    

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.

Mac OS X


You need to have GnuPG installed before you can verify signatures. You can install it from http://www.gpgtools.org/.

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

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

After importing the key, you can verify that the fingerprint is correct:

gpg --fingerprint 0x416F061063FEE659

You should see:

    pub   2048R/63FEE659 2003-10-16
          Key fingerprint = 8738 A680 B84B 3031 A630  F2DB 416F 0610 63FE E659
    uid                  Erinn Clark <erinn@torproject.org>
    uid                  Erinn Clark <erinn@debian.org>
    uid                  Erinn Clark <erinn@double-helix.org>
    sub   2048R/EB399FD7 2003-10-16
    

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:

gpg --verify /Users/Alice/TorBrowser-4.0-osx-i386-en-US.dmg{.asc*,}

The output should say "Good signature":

    gpg: Signature made Wed 31 Aug 2011 06:37:01 PM EDT using RSA key ID 63FEE659
    gpg: Good signature from "Erinn Clark <erinn@torproject.org>"
    gpg:                 aka "Erinn Clark <erinn@debian.org>"
    gpg:                 aka "Erinn Clark <erinn@double-helix.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: 8738 A680 B84B 3031 A630  F2DB 416F 0610 63FE E659
    

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.

Linux


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".

If you're using the Debian Tor (not Tor Browser) packages, you should read the instructions on importing these keys to apt.

If you're using the RPMs (for Tor, not Tor Browser), you can manually verify the signatures on the RPM packages by

rpm -K filename.rpm

See http://www.gnupg.org/documentation/ to learn more about GPG.


Verifying sha256sums (advanced)


Build reproducibility is a security property of Tor Browser Bundle 3.0 and later. Anyone can build the Tor Browser Bundle 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 hashes will confirm that multiple people have built Tor Browser Bundles identical to the download.

The steps below walk through this process:

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

Scripts to automate these steps have been written, but to use them you will need to modify them yourself with the latest Tor Browser Bundle filename.