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qubes-doc/project-security/verifying-signatures.md
Andrew David Wong 78b0742362
Replace RSK attributes in examples with variables 
Users keep getting confused because their output doesn't match the
examples, even though the text already says, "This is just an example,
so the output you receive may not look exactly the same."

Recent examples of this:
- https://www.reddit.com/r/Qubes/comments/1f2b221/
- https://forum.qubes-os.org/t/29384

Specifically, users seem to get confused because their RSK has a
different date and key ID than the one in the example (which makes
sense, because RSKs change with each release), so replacing these
specific values with variables may avert some confusion.
2024-10-06 04:06:46 -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 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 [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 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)):
```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 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:
```
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, [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*, 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:
```
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:
```
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).
```shell_session
$ 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)
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
[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.:
```shell_session
$ 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 YYYY-MM-DD [SC]
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
uid [ full ] Qubes OS Release X Signing Key
sig!3 XXXXXXXXXXXXXXXX YYYY-MM-DD Qubes OS Release X Signing Key
sig! DDFA1A3E36879494 YYYY-MM-DD 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:
```shell_session
$ gpg2 -k "Qubes OS Release X Signing Key"
pub rsa4096 YYYY-MM-DD [SC]
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
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") 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 [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:
```shell_session
-----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 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 <TIME> 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 [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 <TIME> 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 [installation security
considerations](/doc/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 [written your desired Qubes
ISO onto the USB
drive](/doc/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:
```shell_session
$ 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:
```shell_session
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 [downloads
page](/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:
```shell_session
$ 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).)
```shell_session
$ dd if=/dev/sdX bs=1M count=<ISO_SIZE> iflag=count_bytes | gpg -v --verify Qubes-RX-x86_64.iso.asc -
gpg: Signature made <TIME>
gpg: using RSA key XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
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. [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,
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: [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). 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 [help, support, mailing lists, and forum](/support/) for
places where you can ask!
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