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qubes-doc/project-security/verifying-signatures.rst
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====================
Verifying signatures
====================
The Qubes OS Project uses `digital signatures <https://en.wikipedia.org/wiki/Digital_signature>`__ to
guarantee the authenticity and integrity of certain important assets.
This page explains how to verify those signatures. It is extremely
important for your security to understand and apply these practices.
What digital signatures can and cannot prove
--------------------------------------------
Most people — even programmers — are confused about the basic concepts
underlying digital signatures. Therefore, most people should read this
section, even if it looks trivial at first sight.
Digital signatures can prove both **authenticity** and **integrity** to
a reasonable degree of certainty. **Authenticity** ensures that a given
file was indeed created by the person who signed it (i.e., that a third
party did not forge it). **Integrity** ensures that the contents of the
file have not been tampered with (i.e., that a third party has not
undetectably altered its contents *en route*).
Digital signatures **cannot** prove, e.g., that the signed file is not
malicious. In fact, there is nothing that could stop someone from
signing a malicious program (and it happens from time to time in
reality).
The point is that we must decide who we will trust (e.g., Linus
Torvalds, Microsoft, or the Qubes Project) and assume that if a trusted
party signed a given file, then it should not be malicious or
negligently buggy. The decision of whether to trust any given party is
beyond the scope of digital signatures. It’s more of a social and
political decision.
Once we decide to trust certain parties, digital signatures are useful,
because they make it possible for us to limit our trust only to those
few parties we choose and not to worry about all the bad things that can
happen between them and us, e.g., server compromises (qubes-os.org will
surely be compromised one day, so :ref:`don’t blindly trust the live version of this site <introduction/faq:should i trust this website?>`), dishonest IT staff
at the hosting company, dishonest staff at the ISPs, Wi-Fi attacks, etc.
We call this philosophy :ref:`distrusting the infrastructure <introduction/faq:what does it mean to "distrust the infrastructure"?>`.
By verifying all the files we download that purport to be authored by a
party we’ve chosen to trust, we eliminate concerns about the bad things
discussed above, since we can easily detect whether any files have been
tampered with (and subsequently choose to refrain from executing,
installing, or opening them).
However, for digital signatures to make sense, we must ensure that the
public keys we use for signature verification are the original ones.
Anybody can generate a cryptographic key that purports to belong to “The
Qubes OS Project,” but of course only the keys that we (the real Qubes
developers) generate are the genuine ones. The rest of this page
explains how to verify the authenticity of the various keys used in the
project and how to use those keys to verify certain important assets.
OpenPGP software
----------------
We use `PGP <https://en.wikipedia.org/wiki/Pretty_Good_Privacy>`__
(specifically, the
`OpenPGP <https://en.wikipedia.org/wiki/Pretty_Good_Privacy#OpenPGP>`__
standard). Before we begin, you’ll need software that can manage PGP
keys and verify PGP signatures. Any program that complies with the
OpenPGP standard will do, but here are some examples for popular
operating systems:
**Linux:** `GnuPG <https://gnupg.org/download/index.html>`__
(`documentation <https://www.gnupg.org/documentation/>`__). Open a
terminal and use the ``gpg2`` command. If you don’t already have GnuPG
installed, install it via your distro’s package manager or from the
GnuPG website.
**Mac:** `GPG Suite <https://gpgtools.org/>`__
(`documentation <https://gpgtools.tenderapp.com/kb>`__). Open a terminal
to enter commands.
**Windows:** `Gpg4win <https://gpg4win.org/download.html>`__
(`documentation <https://www.gpg4win.org/documentation.html>`__). Use
the Windows command line (``cmd.exe``) to enter commands.
Throughout this page, we’ll use GnuPG via the ``gpg2`` command. If that
doesn’t work for you, try ``gpg`` instead. If that still doesn’t work,
please consult the documentation for your specific program (see links
above) and the `troubleshooting FAQ <#troubleshooting-faq>`__ below.
How to import and authenticate the Qubes Master Signing Key
-----------------------------------------------------------
Many important Qubes OS Project assets (e.g., ISOs, RPMs, TGZs, and Git
objects) are digitally signed by an official team member’s key or by a
release signing key (RSK). Each such key is, in turn, signed by the
`Qubes Master Signing Key (QMSK) <https://keys.qubes-os.org/keys/qubes-master-signing-key.asc>`__
(``0x427F11FD0FAA4B080123F01CDDFA1A3E36879494``). In this way, the QMSK
is the ultimate root of trust for the Qubes OS Project.
The developer signing keys are set to expire after one year, while the
QMSK and RSKs have no expiration date. The QMSK was generated on and is
kept only on a dedicated, air-gapped “vault” machine, and the private
portion will (hopefully) never leave this isolated machine.
Before we proceed, you must first complete the prerequisite step of
`installing OpenPGP software <#openpgp-software>`__.
Once you have appropriate OpenPGP software installed, there are several
ways to get the QMSK.
- If you’re on Qubes OS, it’s available in every qube (`except dom0 <https://github.com/QubesOS/qubes-issues/issues/2544>`__):
.. code:: bash
$ gpg2 --import /usr/share/qubes/qubes-master-key.asc
- If you’re on Fedora, you can get it in the
`distribution-gpg-keys <https://github.com/xsuchy/distribution-gpg-keys>`__
package:
.. code:: bash
$ dnf install distribution-gpg-keys
$ gpg2 --import /usr/share/distribution-gpg-keys/qubes/*
- If you’re on Debian, it may already be included in your keyring.
- Fetch it with GPG:
.. code:: bash
$ gpg2 --fetch-keys https://keys.qubes-os.org/keys/qubes-master-signing-key.asc
- Get it from a public
`keyserver <https://en.wikipedia.org/wiki/Key_server_%28cryptographic%29#Keyserver_examples>`__
(specified on first use with ``--keyserver <URI>`` along with
keyserver options to include key signatures), e.g.:
.. code:: bash
$ gpg2 --keyserver-options no-self-sigs-only,no-import-clean --keyserver hkp://keyserver.ubuntu.com --recv-keys 0x427F11FD0FAA4B080123F01CDDFA1A3E36879494
- Download it as a file, then import the file.
Here are some example download locations:
- :doc:`Qubes security pack </project-security/security-pack>`
- `Qubes keyserver <https://keys.qubes-os.org/keys/qubes-master-signing-key.asc>`__
- `Email to qubes-devel <https://groups.google.com/d/msg/qubes-devel/RqR9WPxICwg/kaQwknZPDHkJ>`__
- `Email to qubes-users <https://groups.google.com/d/msg/qubes-users/CLnB5uFu_YQ/ZjObBpz0S9UJ>`__
Once you have the key as a file, import it:
.. code:: bash
$ gpg2 --import /<PATH_TO_FILE>/qubes-master-signing-key.asc
Once you’ve obtained the QMSK, you must verify that it’s authentic
rather than a forgery. Anyone can create a PGP key with the name “Qubes
Master Signing Key” and the short key ID ``0x36879494``, so you cannot
rely on these alone. You also should not rely on any single website, not
even over HTTPS.
So, what *should* you do? One option is to use the PGP `Web of Trust <https://en.wikipedia.org/wiki/Web_of_trust>`__. In addition, some
operating systems include the means to acquire the QMSK securely. For
example, on Fedora, ``dnf install distribution-gpg-keys`` will get you
the QMSK along with several other Qubes keys. On Debian, your keyring
may already contain the necessary keys.
Perhaps the most common route is to rely on the key’s fingerprint, which
is a string of 40 alphanumeric characters, like this:
.. code:: bash
427F 11FD 0FAA 4B08 0123 F01C DDFA 1A3E 3687 9494
Every PGP key has one of these fingerprints, which uniquely identifies
it among all PGP keys. (On the command line, you can view a key’s
fingerprint with the ``gpg2 --fingerprint <KEY_ID>`` command.)
Therefore, if you know the genuine QMSK fingerprint, then you always
have an easy way to confirm whether any purported copy of it is
authentic, simply by comparing the fingerprints.
But how do you know which fingerprint is the real one? After all, :ref:`this website could be compromised <introduction/faq:should i trust this website?>`, so
the fingerprint you see here may not be genuine. That’s why we strongly
suggest obtaining the fingerprint from *multiple independent sources in several different ways*, then comparing the strings of letters and
numbers to make sure they match.
For the purpose of convincing yourself that you know the authentic QMSK
fingerprint, spaces and capitalization don’t matter. In other words, all
of these fingerprints are considered the same:
.. code:: bash
427F 11FD 0FAA 4B08 0123 F01C DDFA 1A3E 3687 9494
427f 11fd 0faa 4b08 0123 f01c ddfa 1a3e 3687 9494
427F11FD0FAA4B080123F01CDDFA1A3E36879494
427f11fd0faa4b080123f01cddfa1a3e36879494
Instead, what matters is that *all* the characters are present in
*exactly* the same order. If even one character is different, the
fingerprints should not be considered the same. Even if two fingerprints
have all the same characters, if any of those characters are in a
different order, sequence, or position, then the fingerprints should not
be considered the same.
However, for the purpose of *searching for*, *looking up*, or *entering*
keys, spaces and capitalization can matter, depending on the software or
tool you’re using. You may need to try different variations (e.g., with
and without spaces). You may also sometimes see (or need to enter) the
entire fingerprint prefixed with ``0x``, as in:
.. code:: bash
0x427F11FD0FAA4B080123F01CDDFA1A3E36879494
0x427f11fd0faa4b080123f01cddfa1a3e36879494
The ``0x`` prefix is sometimes used to indicate that the string
following it is a hexadecimal value, and some PGP-related tools may
require this prefix. Again, for the purpose of convincing yourself that
you know the authentic QMSK fingerprint, you may safely ignore the
``0x`` prefix, as it is not part of the fingerprint. As long as the
40-character string after the ``0x`` matches exactly, the fingerprint is
considered the same. The ``0x`` prefix only matters if the software or
tool you’re using cares about it.
The general idea of “comparing fingerprints” is to go out into the world
(whether digitally, physically, or both) and find other 40-character
strings purporting to be the QMSK fingerprint, then compare them to your
own purported QMSK fingerprint to ensure that the sequence of
alphanumeric characters is exactly the same (again, regardless of spaces
or capitalization). If any of the characters do not match or are not in
the same order, then at least one of the fingerprints is a forgery. Here
are some ideas to get you started:
- Check the fingerprint on various websites (e.g., `mailing lists <https://groups.google.com/g/qubes-devel/c/RqR9WPxICwg/m/kaQwknZPDHkJ>`__,
`discussion forums <https://forum.qubes-os.org/t/1441/9>`__,
`social <https://twitter.com/rootkovska/status/496976187491876864>`__
`media <https://www.reddit.com/r/Qubes/comments/5bme9n/fingerprint_verification/>`__,
`personal websites <https://andrewdavidwong.com/fingerprints.txt>`__).
- Check against PDFs, photographs, and videos in which the fingerprint
appears (e.g., `slides from a talk <https://hyperelliptic.org/PSC/slides/psc2015_qubesos.pdf>`__,
on a
`T-shirt <https://twitter.com/legind/status/813847907858337793/photo/2>`__,
or in the `recording of a presentation <https://youtu.be/S0TVw7U3MkE?t=2563>`__).
- Ask people to post the fingerprint on various mailing lists, forums,
and chat rooms.
- Download old Qubes ISOs from different sources and check the included
Qubes Master Signing Key.
- Repeat the above over Tor.
- Repeat the above over various VPNs and proxy servers.
- Repeat the above on different networks (work, school, internet cafe,
etc.).
- Text, email, call, video chat, snail mail, or meet up with people you
know to confirm the fingerprint.
- Repeat the above from different computers and devices.
Once you’ve observed enough matching fingerprints from enough
independent sources in enough different ways that you feel confident
that you have the genuine fingerprint, keep it in a safe place. Every
time you need to check whether a key claiming to be the QMSK is
authentic, compare that key’s fingerprint to your trusted copy and
confirm they match.
Now that you’ve imported the authentic QMSK, set its trust level to
“ultimate” so that it can be used to automatically verify all the keys
signed by the QMSK (in particular, RSKs).
.. code:: bash
$ gpg2 --edit-key 0x427F11FD0FAA4B080123F01CDDFA1A3E36879494
gpg (GnuPG) 1.4.18; Copyright (C) 2014 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
pub 4096R/36879494 created: 2010-04-01 expires: never usage: SC
trust: unknown validity: unknown
[ unknown] (1). Qubes Master Signing Key
gpg> fpr
pub 4096R/36879494 2010-04-01 Qubes Master Signing Key
Primary key fingerprint: 427F 11FD 0FAA 4B08 0123 F01C DDFA 1A3E 3687 9494
gpg> trust
pub 4096R/36879494 created: 2010-04-01 expires: never usage: SC
trust: unknown validity: unknown
[ unknown] (1). Qubes Master Signing Key
Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)
1 = I don't know or won't say
2 = I do NOT trust
3 = I trust marginally
4 = I trust fully
5 = I trust ultimately
m = back to the main menu
Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y
pub 4096R/36879494 created: 2010-04-01 expires: never usage: SC
trust: ultimate validity: unknown
[ unknown] (1). Qubes Master Signing Key
Please note that the shown key validity is not necessarily correct
unless you restart the program.
gpg> q
Now, when you import any of the release signing keys and many Qubes team
member keys, they will already be trusted in virtue of being signed by
the QMSK.
As a final sanity check, make sure the QMSK is in your keyring with the
correct trust level.
.. code:: bash
$ gpg2 -k "Qubes Master Signing Key"
pub rsa4096 2010-04-01 [SC]
427F11FD0FAA4B080123F01CDDFA1A3E36879494
uid [ultimate] Qubes Master Signing Key
If you don’t see the QMSK here with a trust level of “ultimate,” go back
and follow the instructions in this section carefully and consult the
`troubleshooting FAQ <#troubleshooting-faq>`__ below.
How to import and authenticate release signing keys
---------------------------------------------------
Every Qubes OS release is signed by a **release signing key (RSK)**,
which is, in turn, signed by the Qubes Master Signing Key (QMSK).
Before we proceed, you must first complete the following prerequisite
steps:
1. `Install OpenPGP software. <#openpgp-software>`__
2. `Import and authenticate the QMSK. <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__
After you have completed these two prerequisite steps, the next step is
to obtain the correct RSK. The filename pattern for RSKs is
``qubes-release-X-signing-key.asc``, where ``X`` is either a major or
minor Qubes release number, such as ``4`` or ``4.2``. There are several
ways to get the RSK for your Qubes release.
- If you have access to an existing Qubes installation, the release
keys are available in dom0 in
``/etc/pki/rpm-gpg/RPM-GPG-KEY-qubes-*``. These can be
:ref:`copied <user/how-to-guides/how-to-copy-from-dom0:copying **from** dom0>` into other
qubes for further use. In addition, every other qube contains the
release key corresponding to that installation’s release in
``/etc/pki/rpm-gpg/RPM-GPG-KEY-qubes-*``. If you wish to use one of
these keys, make sure to import it into your keyring, e.g.:
.. code:: bash
$ gpg2 --import /etc/pki/rpm-gpg/RPM-GPG-KEY-qubes-*
- Fetch it with GPG:
.. code:: bash
$ gpg2 --keyserver-options no-self-sigs-only,no-import-clean --fetch-keys https://keys.qubes-os.org/keys/qubes-release-X-signing-key.asc
- Download it as a file. You can find the RSK for your Qubes release on
the :doc:`downloads </user/downloading-installing-upgrading/downloads>` page. You can also download all the
currently used developers’ signing keys, RSKs, and the Qubes Master
Signing Key from the :doc:`Qubes security pack </project-security/security-pack>` and
the `Qubes keyserver <https://keys.qubes-os.org/keys/>`__. Once
you’ve downloaded your RSK, import it with GPG:
.. code:: bash
$ gpg2 --keyserver-options no-self-sigs-only,no-import-clean --import ./qubes-release-X-signing-key.asc
Now that you have the correct RSK, you simply need to verify that it is
signed by the QMSK:
.. code:: bash
$ gpg2 --check-signatures "Qubes OS Release X Signing Key"
pub rsa4096 2017-03-06 [SC]
5817A43B283DE5A9181A522E1848792F9E2795E9
uid [ full ] Qubes OS Release X Signing Key
sig!3 1848792F9E2795E9 2017-03-06 Qubes OS Release X Signing Key
sig! DDFA1A3E36879494 2017-03-08 Qubes Master Signing Key
gpg: 2 good signatures
This is just an example, so the output you receive may not look exactly
the same. What matters is the line with a ``sig!`` prefix showing that
the QMSK has signed this key. This verifies the authenticity of the RSK.
Note that the ``!`` flag after the ``sig`` tag is important because it
means that the key signature is valid. A ``sig-`` prefix would indicate
a bad signature, and ``sig%`` would mean that gpg encountered an error
while verifying the signature. It is not necessary to independently
verify the authenticity of the RSK, since you already verified the
authenticity of the QMSK.
As a final sanity check, make sure the RSK is in your keyring with the
correct trust level:
.. code:: bash
$ gpg2 -k "Qubes OS Release"
pub rsa4096 2017-03-06 [SC]
5817A43B283DE5A9181A522E1848792F9E2795E9
uid [ full ] Qubes OS Release X Signing Key
If you don’t see the correct RSK here with a trust level of “full” or
higher, go back and follow the instructions in this section carefully,
and consult the `troubleshooting FAQ <#troubleshooting-faq>`__ below.
How to obtain and authenticate other signing keys
-------------------------------------------------
Please see the :doc:`Qubes security pack </project-security/security-pack>` documentation.
How to verify the cryptographic hash values of Qubes ISOs
---------------------------------------------------------
There are two ways to verify Qubes ISOs: cryptographic hash values and
detached PGP signatures. Both methods are equally secure. Using just one
method is sufficient to verify your Qubes ISO. Using both methods is not
necessary, but you can do so if you like. One method might be more
convenient than another in certain circumstances, so we provide both.
This section covers cryptographic hash values. For the other method, see
`how to verify detached PGP signatures on Qubes ISOs <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__.
Before we proceed, you must first complete the following prerequisite
steps:
1. `Install OpenPGP software. <#openpgp-software>`__
2. `Import and authenticate the Qubes Master Signing Key. <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__
3. `Import and authenticate your release signing key. <#how-to-import-and-authenticate-release-signing-keys>`__
Each Qubes ISO is accompanied by a set of **cryptographic hash values**
contained in a plain text file ending in ``.DIGESTS``, which can find on
the :doc:`downloads </user/downloading-installing-upgrading/downloads>` page alongside the ISO. This file
contains the output of running several different cryptographic hash
functions on the ISO (a process known as “hashing”) to obtain
alphanumeric outputs known as “hash values” or “digests.”
One convenient property of hash values is that they can be generated on
any computer. This means, for example, that you can download a Qubes ISO
on one computer, hash it, then visually compare that hash value to the
one you generated or have saved on a different computer.
In addition to the ``.DIGESTS`` files on the :doc:`downloads </user/downloading-installing-upgrading/downloads>`
page alongside each ISO, and you can always find all the digest files
for every Qubes ISO in the :doc:`Qubes security pack </project-security/security-pack>`.
If the filename of your ISO is ``Qubes-RX-x86_64.iso``, then the name of
the digest file for that ISO is ``Qubes-RX-x86_64.iso.DIGESTS``, where
``X`` is a specific release of Qubes. The digest filename is always the
same as the ISO filename followed by ``.DIGESTS``. Since the digest file
is a plain text file, you can open it with any text editor. Inside, you
should find text that looks similar to this:
.. code:: bash
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256
3c951138b8b9867d8657f173c1b58b82 *Qubes-RX-x86_64.iso
1fc9508160d7c4cba6cacc3025165b0f996c843f *Qubes-RX-x86_64.iso
6b998045a513dcdd45c1c6e61ace4f1b4e7eff799f381dccb9eb0170c80f678a *Qubes-RX-x86_64.iso
de1eb2e76bdb48559906f6fe344027ece20658d4a7f04ba00d4e40c63723171c62bdcc869375e7a4a4499d7bff484d7a621c3acfe9c2b221baee497d13cd02fe *Qubes-RX-x86_64.iso
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2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=e9oD
-----END PGP SIGNATURE-----
Four digests have been computed for this ISO. The hash functions used,
in order from top to bottom, are MD5, SHA-1, SHA-256, and SHA-512. One
way to verify that the ISO you downloaded matches any of these hash
values is by using the respective ``*sum`` command:
.. code:: bash
$ md5sum -c Qubes-RX-x86_64.iso.DIGESTS
Qubes-RX-x86_64.iso: OK
md5sum: WARNING: 23 lines are improperly formatted
$ sha1sum -c Qubes-RX-x86_64.iso.DIGESTS
Qubes-RX-x86_64.iso: OK
sha1sum: WARNING: 23 lines are improperly formatted
$ sha256sum -c Qubes-RX-x86_64.iso.DIGESTS
Qubes-RX-x86_64.iso: OK
sha256sum: WARNING: 23 lines are improperly formatted
$ sha512sum -c Qubes-RX-x86_64.iso.DIGESTS
Qubes-RX-x86_64.iso: OK
sha512sum: WARNING: 23 lines are improperly formatted
The ``OK`` response tells us that the hash value for that particular
hash function matches. The program also warns us that there are 23
improperly formatted lines, but this is expected. This is because each
file contains lines for several different hash values (as mentioned
above), but each ``*sum`` program verifies only the line for its own
hash function. In addition, there are lines for the PGP signature that
the ``*sum`` programs do not know how to read. Therefore, it is safe to
ignore these warning lines.
Another way is to use ``openssl`` to compute each hash value, then
compare them to the contents of the digest file:
.. code:: bash
$ openssl dgst -md5 Qubes-RX-x86_64.iso
MD5(Qubes-RX-x86_64.iso)= 3c951138b8b9867d8657f173c1b58b82
$ openssl dgst -sha1 Qubes-RX-x86_64.iso
SHA1(Qubes-RX-x86_64.iso)= 1fc9508160d7c4cba6cacc3025165b0f996c843f
$ openssl dgst -sha256 Qubes-RX-x86_64.iso
SHA256(Qubes-RX-x86_64.iso)= 6b998045a513dcdd45c1c6e61ace4f1b4e7eff799f381dccb9eb0170c80f678a
$ openssl dgst -sha512 Qubes-RX-x86_64.iso
SHA512(Qubes-RX-x86_64.iso)= de1eb2e76bdb48559906f6fe344027ece20658d4a7f04ba00d4e40c63723171c62bdcc869375e7a4a4499d7bff484d7a621c3acfe9c2b221baee497d13cd02fe
(Notice that the outputs match the values from the digest file.)
However, it is possible that an attacker replaced
``Qubes-RX-x86_64.iso`` with a malicious ISO, computed the hash values
for that malicious ISO, and replaced the values in
``Qubes-RX-x86_64.iso.DIGESTS`` with his own set of values. Therefore,
we should also verify the authenticity of the listed hash values. Since
``Qubes-RX-x86_64.iso.DIGESTS`` is a clearsigned PGP file, we can use
GPG to verify the signature in the digest file:
.. code:: bash
$ gpg2 -v --verify Qubes-RX-x86_64.iso.DIGESTS
gpg: armor header: Hash: SHA256
gpg: armor header: Version: GnuPG v2
gpg: original file name=''
gpg: Signature made Tue 20 Sep 2016 10:37:03 AM PDT using RSA key ID 03FA5082
gpg: using PGP trust model
gpg: Good signature from "Qubes OS Release X Signing Key"
gpg: textmode signature, digest algorithm SHA256
This is just an example, so the output you receive will not look exactly
the same. What matters is the line that says
``Good signature from "Qubes OS Release X Signing Key"``. This confirms
that the signature on the digest file is good.
If you don’t see a good signature here, go back and follow the
instructions in this section carefully, and consult the `troubleshooting FAQ <#troubleshooting-faq>`__ below.
How to verify detached PGP signatures on Qubes ISOs
---------------------------------------------------
There are two ways to verify Qubes ISOs: cryptographic hash values and
detached PGP signatures. Both methods are equally secure. Using just one
method is sufficient to verify your Qubes ISO. Using both methods is not
necessary, but you can do so if you like. One method might be more
convenient than another in certain circumstances, so we provide both.
This section covers detached PGP signatures. For the other method, see
`how to verify the cryptographic hash values of Qubes ISOs <#how-to-verify-the-cryptographic-hash-values-of-qubes-isos>`__.
Before we proceed, you must first complete the following prerequisite
steps:
1. `Install OpenPGP software. <#openpgp-software>`__
2. `Import and authenticate the Qubes Master Signing Key. <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__
3. `Import and authenticate your release signing key. <#how-to-import-and-authenticate-release-signing-keys>`__
Every Qubes ISO is released with a **detached PGP signature** file,
which you can find on the :doc:`downloads </user/downloading-installing-upgrading/downloads>` page alongside the
ISO. If the filename of your ISO is ``Qubes-RX-x86_64.iso``, then the
name of the signature file for that ISO is ``Qubes-RX-x86_64.iso.asc``,
where ``X`` is a specific release of Qubes. The signature filename is
always the same as the ISO filename followed by ``.asc``.
Download both the ISO and its signature file. Put both of them in the
same directory, then navigate to that directory. Now, you can verify the
ISO by executing this GPG command in the directory that contains both
files:
.. code:: bash
$ gpg2 -v --verify Qubes-RX-x86_64.iso.asc Qubes-RX-x86_64.iso
gpg: armor header: Version: GnuPG v1
gpg: Signature made Tue 08 Mar 2016 07:40:56 PM PST using RSA key ID 03FA5082
gpg: using PGP trust model
gpg: Good signature from "Qubes OS Release X Signing Key"
gpg: binary signature, digest algorithm SHA256
This is just an example, so the output you receive will not look exactly
the same. What matters is the line that says
``Good signature from "Qubes OS Release X Signing Key"``. This confirms
that the signature on the ISO is good.
If you don’t see a good signature here, go back and follow the
instructions in this section carefully, and consult the `troubleshooting FAQ <#troubleshooting-faq>`__ below.
How to re-verify installation media after writing
-------------------------------------------------
*This is an optional section intended for advanced users.*
After you have authenticated your Qubes ISO and written it onto your
desired medium (such as a USB drive or optical disc), you can re-verify
the data that has been written to your medium. Why would you want to do
this when you’ve already verified the original ISO? Well, it’s
conceivable that a sufficiently sophisticated adversary might allow your
initial ISO verification to succeed (so as not to alert you that your
machine has been compromised, for example), then surreptitiously modify
the data as it is being written onto your installation medium, resulting
in a compromised Qubes installer. This might increase the odds that the
attack goes undetected. One way to mitigate this risk is to re-verify
the installer after writing it onto an installation medium that cannot
be altered, such as a USB drive with a properly-implemented physical
write-protect switch and firmware that is either unflashable or
cryptographically-signed (or both), as discussed in our :doc:`installation security considerations </user/downloading-installing-upgrading/install-security>`.
This section will walk through an example of re-verifying the installer
on such a device. We begin by assuming that you have just :ref:`written your desired Qubes ISO onto the USB drive <user/downloading-installing-upgrading/installation-guide:copying the iso onto the installation medium>`.
First, unplug your USB drive and flip the write protect switch so that
the data on the drive can no longer be altered. If you have a different
computer from the one you used to create the installation medium,
consider using that computer. If not, try to at least use a fresh VM
(e.g., if it’s a Qubes system). The idea is that the original machine
may have been compromised, and using a different one for re-verification
forces your hypothetical adversary to compromise an additional machine
in order to succeed.
Now, our goal is to perform the same verification steps as we did with
the original ISO, except, this time, we’ll be reading the installer data
directly from the write-protected USB drive instead of from the original
ISO file. First, let’s compute the SHA-256 hash value of the data on the
drive. (This assumes you’re already familiar with `how to verify the cryptographic hash values of Qubes ISOs <#how-to-verify-the-cryptographic-hash-values-of-qubes-isos>`__.)
In order to do this, we have to know the exact size, in bytes, of the
original ISO. There are two ways to get this information: from the ISO
itself and from the Qubes website. Here’s an example of the first way:
.. code:: bash
$ dd if=/dev/sdX bs=1M count=$(stat -c %s /path/to/iso) iflag=count_bytes | sha256sum
(Where ``/dev/sdX`` is your USB drive and ``/path/to/iso`` is the path
to your Qubes ISO.)
This command reads exactly the number of bytes of your Qubes ISO
(obtained with ``stat -c %s /path/to/iso``) from the USB drive and pipes
them into ``sha256sum``. The output should look something like this:
.. code:: bash
0e68dd3347b68618d9e5f3ddb580bf7ecdd2166747630859b3582803f1ca8801 -
5523+0 records in
5523+0 records out
5791285248 bytes (5.8 GB, 5.4 GiB) copied, 76.3369 s, 75.9 MB/s
Note that your actual SHA-256 hash value and byte number will depend on
which Qubes ISO you’re using. This is just an example. Your SHA-256 hash
value should match the hash value of your genuine original Qubes ISO.
Now, reading the number of bytes directly from the ISO is fine, but you
may be concerned that a sufficiently sophisticated adversary may have
compromised the machine on which you’re performing this re-verification
and may therefore be capable of feeding you a false success result.
After all, if your adversary knows the answer you’re looking for —
namely, a match to the genuine ISO — and has access to that very ISO in
the same re-verification environment, then there is little to prevent
him from simply hashing the original ISO and feeding you that result
(perhaps while also reading from the USB drive and piping it into
``/dev/null`` so that you see the light on the USB drive blinking to
support the illusion that the data is being read from the USB drive).
Therefore, in order to make things a bit more difficult for your
hypothetical adversary, you may instead wish to perform the
re-verification in an environment that has never seen the original ISO,
e.g., a separate offline computer or a fresh VM the storage space of
which is too small to hold the ISO. (Note: If you’re doing this in
Qubes, you can attach the block device from sys-usb to a separate new
qube. You don’t have to perform the re-verification directly in
sys-usb.) In that case, you’ll have to obtain the size of the ISO in
bytes and enter it into the above command manually. You can, of course,
obtain the size by simply using the ``stat -c %s /path/to/iso`` command
from above on the machine that has the ISO. You can also obtain it from
the Qubes website by hovering over any ISO download button on the
:doc:`downloads page </user/downloading-installing-upgrading/downloads>`. (You can also view these values
directly in the downloads page’s `source data <https://github.com/QubesOS/qubesos.github.io/blob/master/_data/downloads.yml>`__.)
Once you have the exact size of the ISO in bytes, simply insert it into
the same command, for example:
.. code:: bash
$ dd if=/dev/sdX bs=1M count=5791285248 iflag=count_bytes | sha256sum
If you wish to compute the values of other hash functions, you can
replace ``sha256sum``, e.g., with ``md5sum``, ``sha1sum``, or
``sha512sum``.
In addition to checking hash values, you can also use GnuPG to verify
the detached PGP signature directly against the data on the USB drive.
(This assumes you’re already familiar with `how to verify detached PGP signatures on Qubes ISOs <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__.)
.. code:: bash
$ dd if=/dev/sdX bs=1M count=<ISO_SIZE> iflag=count_bytes | gpg -v --verify Qubes-RX-x86_64.iso.asc -
gpg: Signature made Thu 14 Jul 2022 08:49:38 PM PDT
gpg: using RSA key 5817A43B283DE5A9181A522E1848792F9E2795E9
gpg: using pgp trust model
gpg: Good signature from "Qubes OS Release X Signing Key" [full]
gpg: binary signature, digest algorithm SHA256, key algorithm rsa4096
5523+0 records in
5523+0 records out
5791285248 bytes (5.8 GB, 5.4 GiB) copied, 76.6013 s, 75.6 MB/s
(Where ``/dev/sdX`` is your USB drive, ``<ISO_SIZE>`` is the size of the
original ISO in bytes, and ``Qubes-RX-x86_64.iso.asc`` is the detached
signature file of the original ISO.)
This command reads the exact number of bytes from your USB drive as the
size of the original ISO and pipes them into ``gpg``. The usual form of
a ``gpg`` verification command is
``gpg --verify <SIGNATURE> <SIGNED_DATA>``. Our command is using shell
redirection in order to use data from your USB drive as the
``<SIGNED_DATA>``, which is why the ``-`` at the end of the command is
required. Remember that you still must have properly imported and
trusted the
`QMSK <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__
and appropriate
`RSK <#how-to-import-and-authenticate-release-signing-keys>`__ in order
for this to work. You should receive a ``Good signature`` message for
the appropriate RSK, which should be signed by a copy of the QMSK that
you previously confirmed to be genuine.
How to verify signatures on Git repository tags and commits
-----------------------------------------------------------
Before we proceed, you must first complete the following prerequisite
steps:
1. `Install OpenPGP software. <#openpgp-software>`__
2. `Import and authenticate the Qubes Master Signing Key. <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__
3. :doc:`Import and authenticate keys from the Qubes security pack (qubes-secpack). </project-security/security-pack>` Please see our :ref:`PGP key policies <project-security/security-pack:pgp key policies>` for important
information about these keys.
Whenever you use one of the `Qubes repositories <https://github.com/QubesOS>`__, you should use Git to
verify the PGP signature in a tag on the latest commit or on the latest
commit itself. (One or both may be present, but only one is required.)
If there is no trusted signed tag or commit on top, any commits after
the latest trusted signed tag or commit should **not** be trusted. If
you come across a repo with any unsigned commits, you should not add any
of your own signed tags or commits on top of them unless you personally
vouch for the trustworthiness of the unsigned commits. Instead, ask the
person who pushed the unsigned commits to sign them.
You should always perform this verification on a trusted local machine
with properly authenticated keys rather than relying on a third party,
such as GitHub. While the GitHub interface may claim that a commit has a
verified signature from a member of the Qubes team, this is only
trustworthy if GitHub has performed the signature check correctly, the
account identity is authentic, an admin has not replaced the user’s key,
GitHub’s servers have not been compromised, and so on. Since there’s no
way for you to be certain that all such conditions hold, you’re much
better off verifying signatures yourself. (Also see: :ref:`distrusting the infrastructure <introduction/faq:what does it mean to "distrust the infrastructure"?>`.)
How to verify a signature on a Git tag
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. code:: bash
$ git tag -v <tag name>
or
.. code:: bash
$ git verify-tag <tag name>
How to verify a signature on a Git commit
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. code:: bash
$ git log --show-signature <commit ID>
or
.. code:: bash
$ git verify-commit <commit ID>
Troubleshooting FAQ
-------------------
Why am I getting "Can't check signature: public key not found"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You don’t have the correct `release signing key <#how-to-import-and-authenticate-release-signing-keys>`__.
Why am I getting "BAD signature from ‘Qubes OS Release X Signing Key'"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The problem could be one or more of the following:
- You’re trying to verify the wrong file(s). Reread this page
carefully.
- You’re using the wrong GPG command. Follow the provided examples
carefully, or try using ``gpg`` instead of ``gpg2`` (or vice versa).
- The ISO or `detached PGP signature file <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__ is bad
(e.g., incomplete or corrupt download). Try downloading the signature
file again from a different source, then try verifying again. If you
still get the same result, try downloading the ISO again from a
different source, then try verifying again.
Why am I getting "bash: gpg2: command not found"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You don’t have ``gpg2`` installed. Please install it using the method
appropriate for your environment (e.g., via your package manager), or
try using ``gpg`` instead.
Why am I getting "No such file or directory"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Your working directory does not contain the required files. Go back and
follow the instructions more carefully, making sure that you put all
required files in the same directory *and* navigate to that directory.
Why am I getting "can't open signed data ‘Qubes-RX-x86_64.iso' / can't hash datafile: file open error"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The correct ISO is not in your working directory.
Why am I getting "can't open ‘Qubes-RX-x86_64.iso.asc' / verify signatures failed: file open error"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The correct `detached PGP signature file <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__ is not in
your working directory.
Why am I getting "no valid OpenPGP data found"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Either you don’t have the correct `detached PGP signature file <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__, or you
inverted the arguments to ``gpg2``. (The signature file goes first.)
Why am I getting "WARNING: This key is not certified with a trusted signature! There is no indication that the signature belongs to the owner."?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
There are several possibilities: - You don’t have the `Qubes Master Signing Key <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__. -
You have not `set the Qubes Master Signing Key’s trust level correctly. <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__
- In the case of a key that is not directly signed by the Qubes Master
Signing Key, you have not `set that key’s trust level correctly. <#how-to-verify-signatures-on-git-repository-tags-and-commits>`__
Why am I getting "X signature not checked due to a missing key"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You don’t have the keys that created those signatures in your keyring.
For the purpose of verifying a Qubes ISO, you don’t need them as long as
you have the `Qubes Master Signing Key <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__
and the `release signing key <#how-to-import-and-authenticate-release-signing-keys>`__ for your
Qubes release.
Why am I seeing additional signatures on a key with "[User ID not found]" or from a revoked key?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This is just a fundamental part of how OpenPGP works. Anyone can sign
anyone else’s public key and upload the signed public key to keyservers.
Everyone is also free to revoke their own keys at any time (assuming
they possess or can create a revocation certificate). This has no impact
on verifying Qubes ISOs, code, or keys.
Why am I getting "verify signatures failed: unexpected data"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You’re not verifying against the correct `detached PGP signature file <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__.
Why am I getting "not a detached signature"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You’re not verifying against the correct `detached PGP signature file <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__.
Why am I getting "CRC error; […] no signature found […]"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You’re not verifying against the correct `detached PGP signature file <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__, or the
signature file has been modified. Try downloading it again or from a
different source.
Do I have to verify both the `detached PGP signature file <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__ and the `cryptographic hash values <#how-to-verify-the-cryptographic-hash-values-of-qubes-isos>`__?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
No, either method is sufficient by itself, but you can do both if you
like.
Why am I getting "no properly formatted X checksum lines found"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You’re not checking the correct `cryptographic hash values <#how-to-verify-the-cryptographic-hash-values-of-qubes-isos>`__.
Why am I getting "WARNING: X lines are improperly formatted"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Read `how to verify the cryptographic hash values of Qubes ISOs <#how-to-verify-the-cryptographic-hash-values-of-qubes-isos>`__
again.
Why am I getting "WARNING: 1 listed file could not be read"?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The correct ISO is not in your working directory.
I have another problem that isn't mentioned here.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Carefully reread this page to be certain that you didn’t skip any steps.
In particular, make sure you have the `Qubes Master Signing Key <#how-to-import-and-authenticate-the-qubes-master-signing-key>`__,
the `release signing key <#how-to-import-and-authenticate-release-signing-keys>`__ for your
Qubes release, *and* the `cryptographic hash values <#how-to-verify-the-cryptographic-hash-values-of-qubes-isos>`__
and/or `detached PGP signature file <#how-to-verify-detached-pgp-signatures-on-qubes-isos>`__, all for
the *correct* Qubes OS release. If your question is about GPG, please
see the `GnuPG documentation <https://www.gnupg.org/documentation/>`__.
Still have question? Please see :doc:`help, support, mailing lists, and forum </introduction/support>` for places where you can ask!
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