On Digital Signatures and Key Verification

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 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.

Importing 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 (0xDDFA1A3E36879494).

The public portion of the Qubes Master Signing Key can be imported directly from a keyserver (specified on first use with --keyserver <URI>, keyserver saved in ~/.gnupg/gpg.conf), e.g.,

gpg --keyserver pool.sks-keyservers.net --recv-keys 0x427F11FD0FAA4B080123F01CDDFA1A3E36879494

or downloaded here and imported with gpg,

$ gpg --import ./qubes-master-signing-key.asc 

or fetched directly with gpg.

$ gpg --fetch-keys https://keys.qubes-os.org/keys/qubes-master-signing-key.asc

For additional security we also publish the fingerprint of the Qubes Master Signing Key here in this document:

pub   4096R/36879494 2010-04-01
      Key fingerprint = 427F 11FD 0FAA 4B08 0123  F01C DDFA 1A3E 3687 9494
uid   Qubes Master Signing Key

There should also be a copy of this key at the project’s main website, in the Qubes Security Pack, and in the archives of the project’s developer and user mailing lists.

Once you have obtained the Qubes Master Signing Key, 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
                     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 you can easily download any of the developer or release signing keys that happen to be used to sign particular ISO, RPM, TGZ files or git tags.

For example, the Qubes OS Release 3 Signing Key (0xCB11CA1D03FA5082) is used for all Release 3 ISO images:

$ gpg --recv-keys 0xC52261BE0A823221D94CA1D1CB11CA1D03FA5082
gpg: requesting key 03FA5082 from hkp server keys.gnupg.net
gpg: key 03FA5082: public key "Qubes OS Release 3 Signing Key" imported
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   1  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: depth: 1  valid:   1  signed:   0  trust: 1-, 0q, 0n, 0m, 0f, 0u
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)

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 Qubes OS Keyserver and from the Qubes Security Pack.

The developer signing keys are set to be valid for 1 year only, while the Qubes Master Signing Key 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.

You can now verify the ISO image (Qubes-R3.2-x86_64.iso) matches its signature (Qubes-R3.2-x86_64.iso.asc):

$ gpg -v --verify Qubes-R3.2-x86_64.iso.asc Qubes-R3.2-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 3 Signing Key"
gpg: binary signature, digest algorithm SHA256

The Release 3 Signing Key used to sign this ISO image should be signed by the Qubes Master Signing Key:

$ gpg --list-sig 03FA5082
pub   4096R/03FA5082 2014-11-19
uid                  Qubes OS Release 3 Signing Key
sig 3        03FA5082 2014-11-19  Qubes OS Release 3 Signing Key
sig          36879494 2014-11-19  Qubes Master Signing Key

Verifying Digests

Each ISO is also 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” or “hash values.” These hash values are provided as an alternative verification method to PGP signatures (though the .DIGESTS file is itself also PGP-signed — see below). If you’ve already verified the signatures on the ISO directly, then verifying digests is not necessary. You can always find all the .DIGESTS files for every Qubes ISO in the Qubes Security Pack.

As an example, Qubes-R3.2-x86_64.iso is accompanied by Qubes-R3.2-x86_64.iso.DIGESTS which has the following content:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

3c951138b8b9867d8657f173c1b58b82 *Qubes-R3.2-x86_64.iso
1fc9508160d7c4cba6cacc3025165b0f996c843f *Qubes-R3.2-x86_64.iso
6b998045a513dcdd45c1c6e61ace4f1b4e7eff799f381dccb9eb0170c80f678a *Qubes-R3.2-x86_64.iso
de1eb2e76bdb48559906f6fe344027ece20658d4a7f04ba00d4e40c63723171c62bdcc869375e7a4a4499d7bff484d7a621c3acfe9c2b221baee497d13cd02fe *Qubes-R3.2-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, SHA1, SHA256, and SHA512. One way to verify that the ISO you downloaded matches any of these hash values is by using the respective *sum programs:

$ md5sum -c Qubes-R3.2-x86_64.iso.DIGESTS
Qubes-R3.2-x86_64.iso: OK
md5sum: WARNING: 23 lines are improperly formatted
$ sha1sum -c Qubes-R3.2-x86_64.iso.DIGESTS
Qubes-R3.2-x86_64.iso: OK
sha1sum: WARNING: 23 lines are improperly formatted
$ sha256sum -c Qubes-R3.2-x86_64.iso.DIGESTS
Qubes-R3.2-x86_64.iso: OK
sha256sum: WARNING: 23 lines are improperly formatted
$ sha512sum -c Qubes-R3.2-x86_64.iso.DIGESTS
Qubes-R3.2-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 which the *sum programs do not know how to read.

Another way is to use openssl to compute each hash value, then compare them to the contents of the .DIGESTS file.:

$ openssl dgst -md5 Qubes-R3.2-x86_64.iso
MD5(Qubes-R3.2-x86_64.iso)= 3c951138b8b9867d8657f173c1b58b82
$ openssl dgst -sha1 Qubes-R3.2-x86_64.iso
SHA1(Qubes-R3.2-x86_64.iso)= 1fc9508160d7c4cba6cacc3025165b0f996c843f
$ openssl dgst -sha256 Qubes-R3.2-x86_64.iso
SHA256(Qubes-R3.2-x86_64.iso)= 6b998045a513dcdd45c1c6e61ace4f1b4e7eff799f381dccb9eb0170c80f678a
$ openssl dgst -sha512 Qubes-R3.2-x86_64.iso
SHA512(Qubes-R3.2-x86_64.iso)= de1eb2e76bdb48559906f6fe344027ece20658d4a7f04ba00d4e40c63723171c62bdcc869375e7a4a4499d7bff484d7a621c3acfe9c2b221baee497d13cd02fe

(Notice that the outputs match the values from the .DIGESTS file.)

However, it is possible that an attacker replaced Qubes-R3.2-x86_64.iso with a malicious ISO, computed the hash values for that ISO, and replaced the values in Qubes-R3.2-x86_64.iso.DIGESTS with his own set of values. Therefore, ideally, we should also verify the authenticity of the listed hash values. Since Qubes-R3.2-x86_64.iso.DIGESTS is a clearsigned PGP file, we can use gpg to verify it from the command line:

$ gpg -v --verify Qubes-R3.2-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 3 Signing Key"
gpg: textmode signature, digest algorithm SHA256

The signature is good. Assuming our copy of the Qubes OS Release 3 Signing Key is also authentic (see above), we can be confident that these hash values came from the Qubes devs.

Verifying Qubes Code

Developers who fetch code from our Git server should always verify the PGP signature of the tag on the latest commit. In some cases, commits themselves may also be signed. Any unsigned commit that is not followed by a signed tag should not be trusted!

To verify a signature on a git tag:

$ git tag -v <tag name>

or

$ git verify-tag <tag name>

To verify a signature on a git commit:

$ git log --show-signature <commit ID>

or

$ git verify-commit <commit ID>