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 any other property, 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, of course, that people must choose who they will trust (e.g., Linus Torvalds, Microsoft, the Qubes Project, etc.) and assume that if a given file was signed by a trusted party, then it should not be malicious or buggy in some horrible way. But the decision of whether to trust any given party is beyond the scope of digital signatures. It's more of a sociological 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 In The Middle" between us and them, e.g., server compromises (qubes-os.org will surely be compromised one day), dishonest IT staff at the hosting company, dishonest staff at the ISPs, Wi-Fi attacks, etc.
By verifying all the files we download which 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 GPG key pair that purports to belong to "The Qubes Project," but of course only the key pair that we (i.e., the Qubes developers) generated is the legitimate one. The next section explains how to verify the validity of the Qubes signing keys.
Every file published by the Qubes Project (ISO, RPM, TGZ files and git repositories) is digitally signed by one of the developer or release signing keys. Each such key is signed by the Qubes Master Signing Key ([`0x36879494`](https://keys.qubes-os.org/keys/qubes-master-signing-key.asc)).
The public portion of the Qubes Master Signing Key can be imported directly from a [ keyserver](https://en.wikipedia.org/wiki/Key_server_%28cryptographic%29#Keyserver_examples) (specified on first use with --keyserver URI, keyserver saved in `~/.gnupg/gpg.conf`), e.g.,
For additional security we also publish the fingerprint of the Qubes Master Signing Key ([`0x36879494`](https://keys.qubes-os.org/keys/qubes-master-signing-key.asc)) here in this document:
There should also be a copy of this key at the project's main website, in the [Qubes Security Pack](/doc/security-pack/), and in the archives of the project's [developer](https://groups.google.com/forum/#!msg/qubes-devel/RqR9WPxICwg/kaQwknZPDHkJ) and [user](https://groups.google.com/d/msg/qubes-users/CLnB5uFu_YQ/ZjObBpz0S9UJ) mailing lists.
Once you have obtained the Qubes Master Signing Key ([`0x36879494`](https://keys.qubes-os.org/keys/qubes-master-signing-key.asc)), you should verify the fingerprint of this key very carefully by obtaining copies of the fingerprint from trustworthy independent sources and comparing them to the downloaded key's fingerprint to ensure they match. Then set its trust level to "ultimate" (oh, well), so that it can be used to automatically verify all the keys signed by the Qubes Master Signing Key:
$ gpg --edit-key 0x36879494
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
For example: Qubes OS Release 2 Signing Key ([`0x0A40E458`](https://keys.qubes-os.org/keys/qubes-release-2-signing-key.asc)) is used for all Release 2 ISO images.
You can also download all the currently used developers' signing keys and current and older release signing keys (and also a copy of the Qubes Master Signing Key) from the [keys directory on our server](https://keys.qubes-os.org/keys/) and from the [Qubes Security Pack](/doc/security-pack/).
The developer signing keys are set to be valid for 1 year only, while the Qubes Master Signing Key ([`0x36879494`](https://keys.qubes-os.org/keys/qubes-master-signing-key.asc)) has no expiration date. This latter key was generated and is kept only within a dedicated, air-gapped "vault" machine, and the private portion will (hopefully) never leave this isolated machine.
The Release 2 Signing Key ([`0x0A40E458`](https://keys.qubes-os.org/keys/qubes-release-2-signing-key.asc)) used to sign this ISO image should be signed by the Qubes Master Signing Key ([`0x36879494`](https://keys.qubes-os.org/keys/qubes-master-signing-key.asc)):
Each ISO is accompanied by a plain text file ending in `.DIGESTS`. This file contains the output of running several different crytographic hash functions on the ISO in order to obtain alphanumeric outputs known as "digests." For example, `Qubes-R2-x86_64-DVD.iso` is accompanied by `Qubes-R2-x86_64-DVD.iso.DIGESTS` which has the following content:
Four digests have been computed for this ISO. The hash functions used, in order from top to bottom, are MD5, SHA1, SHA256, and SHA512. One way to verify that the ISO you downloaded matches any of these is by using `openssl` from the command line:
(Notice that the outputs match the values from the `.DIGESTS` file.)
However, it is possible that an attacker replaced `Qubes-R2-x86_64-DVD.iso` with a malicious ISO, computed the hash values for that ISO, and replaced the values in `Qubes-R2-x86_64-DVD.iso.DIGESTS` with his own set of values. Therefore, ideally, we should also verify the authenticity of the listed hash values. Since `Qubes-R2-x86_64-DVD.iso.DIGESTS` is a clearsigned PGP file, we can use `gpg` to verify it from the command line:
$ gpg -v --verify Qubes-R2-x86_64-DVD.iso.DIGESTS
gpg: armor header: Hash: SHA256
gpg: armor header: Version: GnuPG v1
gpg: original file name=''
gpg: Signature made 2015-08-01T22:27:18 UTC using RSA key ID 0A40E458
gpg: using PGP trust model
gpg: Good signature from "Qubes OS Release 2 Signing Key"
gpg: textmode signature, digest algorithm SHA256
The signature is good. Assuming our copy of the `Qubes OS Release 2 Signing Key` is also authentic (see above), we can be confident that these hash values came from the Qubes devs.
Developers who fetch code from our Git server should always verify tags on the latest commit. Any commits that are not followed by a signed tag should not be trusted!
