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qubes-doc/project-security/verifying-signatures.md
Andrew David Wong 0cca6ec53d
Explain "major version number" 
https://forum.qubes-os.org/t/7027/
2021-10-20 08:42:22 -07:00

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---
lang: en
layout: doc
permalink: /security/verifying-signatures/
redirect_from:
- /doc/verifying-signatures/
- /en/doc/verifying-signatures/
- /doc/VerifyingSignatures/
- /wiki/VerifyingSignatures/
ref: 211
title: 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 it was not forged by a
third party). **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 given file was signed by
a trusted party, 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 make the decision 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 us and them, e.g., server compromises (qubes-os.org will surely be
compromised one day, so [don't blindly trust the live version of this
site](/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 [distrusting the
infrastructure](/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 any sense, we must ensure that the
public keys we use for signature verification are indeed 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).
Now, 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)):
```shell_session
$ 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:
```shell_session
$ 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:
```shell_session
$ 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.:
```shell_session
$ 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:
- [Qubes security pack](/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:
```shell_session
$ 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 in a secure way. 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. Every PGP
key has a fingerprint that uniquely identifies it among all PGP keys (viewable
with `gpg2 --fingerprint <KEY_ID>`). 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.
For example, here is the QMSK fingerprint:
```
pub 4096R/36879494 2010-04-01
Key fingerprint = 427F 11FD 0FAA 4B08 0123 F01C DDFA 1A3E 3687 9494
uid Qubes Master Signing Key
```
But how do you know that this is the real fingerprint? After all, [this website
could be compromised](/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*.
Here are some ideas for how to do that:
- 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/there-is-no-way-to-validate-qubes-master-signing-key/1441/9?u=adw),
[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)).
- Download old Qubes ISOs from different sources and check the included Qubes
Master Signing Key.
- Ask people to post the fingerprint on various mailing lists, forums, and chat
rooms.
- 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 obtained the fingerprint from enough independent sources in enough
different ways that you feel confident that you know 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).
```
$ 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.
```
$ 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)
The first step is to obtain the correct RSK. The filename of the RSK for your
Qubes OS release is usually `qubes-release-X-signing-key.asc`, where `X` is the
[major version number](https://semver.org/) of your Qubes release. For example,
if you were installing release `1.2.3`, you would replace `X` with `1`,
resulting in `qubes-release-1-signing-key.asc`. 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
[copied](/doc/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.:
```
$ gpg2 --import /etc/pki/rpm-gpg/RPM-GPG-KEY-qubes-*
```
- Fetch it with GPG:
```shell_session
$ 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 [downloads](/downloads/) page. You can also download all the
currently used developers' signing keys, RSKs, and the Qubes
Master Signing Key from the [Qubes security pack](/security/pack/) and the
[Qubes keyserver](https://keys.qubes-os.org/keys/). Once you've downloaded
your RSK, import it with GPG:
```shell_session
$ 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:
```shell_session
$ 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 will not look exactly the
same. What matters is the line that shows that this key is signed by the QMSK
with a `sig!` prefix. 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:
```
$ 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 [Qubes security pack](/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
[downloads](/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") in order 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 one you generated
or have saved on a different computer.
In addition to the `.DIGESTS` files on the [downloads](/downloads/) page
alongside each ISO, and you can always find all the digest files for every
Qubes ISO in the [Qubes security pack](/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:
```
-----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:
```shell_session
$ 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 to be 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:
```shell_session
$ 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:
```shell_session
$ 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 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 [downloads](/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:
```shell_session
$ 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 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. [Import and authenticate keys from the Qubes security pack
(qubes-secpack).](/security/pack/) Please see our [PGP key
policies](/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,
the user's key has not been replaced by an admin, 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: [distrusting the
infrastructure](/faq/#what-does-it-mean-to-distrust-the-infrastructure).)
### How to verify a signature on a Git tag
```shell_session
$ git tag -v <tag name>
```
or
```shell_session
$ git verify-tag <tag name>
```
### How to verify a signature on a Git commit
```shell_session
$ git log --show-signature <commit ID>
```
or
```shell_session
$ 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). Read this page again 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 basic 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 read this page again 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 [help, support, mailing lists, and forum](/support/) for
places where you can ask!
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