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, torbrowser-install-4.5.2_en-US.exe is accompanied by torbrowser-install-4.5.2_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.

The Tor Browser team signs the Tor Browsers. Import its key (0x4E2C6E8793298290) by starting cmd.exe and typing:

"C:\Program Files\Gnu\GnuPg\gpg.exe" --keyserver x-hkp://pool.sks-keyservers.net --recv-keys 0x4E2C6E8793298290

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

"C:\Program Files\Gnu\GnuPg\gpg.exe" --fingerprint 0x4E2C6E8793298290

You should see:

    pub   4096R/93298290 2014-12-15
          Key fingerprint = EF6E 286D DA85 EA2A 4BA7  DE68 4E2C 6E87 9329 8290
    uid                  Tor Browser Developers (signing key) 
    sub   4096R/F65C2036 2014-12-15
    sub   4096R/D40814E0 2014-12-15
    sub   4096R/589839A3 2014-12-15

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\torbrowser-install-4.5.2_en-US.exe.asc C:\Users\Alice\Desktop\torbrowser-install-4.5.2_en-US.exe

The output should say "Good signature":

    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) "
    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

Currently valid subkey fingerprints are:

    5242 013F 02AF C851 B1C7  36B8 7017 ADCE F65C 2036
    BA1E E421 BBB4 5263 180E  1FC7 2E1A C68E D408 14E0
    05FA 4425 3F6C 19A8 B7F5  18D4 2D00 0988 5898 39A3

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 and Linux


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

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

gpg --keyserver x-hkp://pool.sks-keyservers.net --recv-keys 0x4E2C6E8793298290

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

gpg --fingerprint 0x4E2C6E8793298290

You should see:

    pub   4096R/93298290 2014-12-15
          Key fingerprint = EF6E 286D DA85 EA2A 4BA7  DE68 4E2C 6E87 9329 8290
    uid                  Tor Browser Developers (signing key) 
    sub   4096R/F65C2036 2014-12-15
    sub   4096R/D40814E0 2014-12-15
    sub   4096R/589839A3 2014-12-15
    

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:

For Mac OS X users:
gpg --verify ~/Desktop/TorBrowser-4.5.2-osx64_en-US.dmg{.asc*,}
For Linux users (change 32 to 64 if you have the 64-bit package):
gpg --verify ~/Desktop/tor-browser-linux32-4.5.2_en-US.tar.xz{.asc*,}

The output should say "Good signature":

    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) "
    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

Currently valid subkey fingerprints are:

    5242 013F 02AF C851 B1C7  36B8 7017 ADCE F65C 2036
    BA1E E421 BBB4 5263 180E  1FC7 2E1A C68E D408 14E0
    05FA 4425 3F6C 19A8 B7F5  18D4 2D00 0988 5898 39A3

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.

If you're a Linux user and 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 GnuPG.


Verifying sha256sums (advanced)


Build reproducibility is a security property 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 hashes will confirm that multiple people have built Tor Browsers identical to the download.

The steps below walk through this process:

  • Download the Tor Browser package, the sha256sums-unsigned-build.txt file, and the sha256sums-unsigned-build.txt.asc signature file. They can all be found in the same directory under https://www.torproject.org/dist/torbrowser/, for example in '4.5.1' for Tor Browser 4.5.1.
  • Retrieve the signers' GPG keys. This can be done from the command line by entering something like
    gpg --keyserver keys.mozilla.org --recv-keys 0x4E2C6E8793298290
    (This will bring you the public part of the Tor Browser developers' signing key. Other developers' key IDs can be found on this page.)
  • Verify the sha256sums-unsigned-build.txt file by executing this command:
    gpg --verify sha256sums-unsigned-build.txt.asc sha256sums-unsigned-build.txt
  • You should see a message like "Good signature from <DEVELOPER NAME>". If you don't, there is a problem. Try these steps again.
  • 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 osslsigncode and delcert.exe. Assuming you have built e.g. osslsigncode on a Linux computer you can enter
    /path/to/your/osslsigncode remove-signature \
            /path/to/your/<TOR BROWSER FILE NAME>.exe <TOR BROWSER FILE NAME>.exe
  • 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>.dmg
    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-unsigned-build.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 filename.


Verifying MAR files we ship (advanced)


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.

Assuming the verification happens on a Linux computer one first needs the mar-tools-linux*.zip out of the gitian-builder/inputs directory to remove the embedded signature(s). The steps to get the unsigned MAR file on a 64 bit Linux are

    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

Now you can compare the SHA256 sum of your-unsigned-mar-file.mar with the one provided in the sha265sums-unsigned-build.txt or sha256sums-unsigned-build.incremental.txt as outlined in Verifying sha256sums (advancded) above.