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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-R3.1-x86_64.iso) matches its
signature (Qubes-R3.1-x86_64.iso.asc):
$ gpg -v --verify Qubes-R3.1-x86_64.iso.asc Qubes-R3.1-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
Having problems verifying the ISO images? Make sure you have the corresponding release signing key and see this thread:
https://groups.google.com/group/qubes-devel/browse_thread/thread/4bdec1cd19509b38/9f8e219c41e1b232
Verifying Digests
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-R3.1-x86_64.iso is accompanied by
Qubes-R3.1-x86_64.iso.DIGESTS which has the following content:
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256
f99634b05d15f6bb2ac02ee03e4338a0 *Qubes-R3.1-x86_64.iso
990b7765ee209b42b3cad78673463daae769c729 *Qubes-R3.1-x86_64.iso
2d82a684d507ad5789ed83272af4311fd04375e782364d5dd9df4b3d7118cc28 *Qubes-R3.1-x86_64.iso
083d6cfc3fb5dc97fd91d8f9f70301c154e3674114ff1727b0415c2c663b233c22e0830d0bfc1f7a532549d7e39c6ef5cfde6a90a650343b47ba57d3e8e92ca7 *Qubes-R3.1-x86_64.iso
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1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=FiJ5
-----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-R3.1-x86_64.iso.DIGESTS
Qubes-R3.1-x86_64.iso: OK
md5sum: WARNING: 23 lines are improperly formatted
$ sha1sum -c Qubes-R3.1-x86_64.iso.DIGESTS
Qubes-R3.1-x86_64.iso: OK
sha1sum: WARNING: 23 lines are improperly formatted
$ sha256sum -c Qubes-R3.1-x86_64.iso.DIGESTS
Qubes-R3.1-x86_64.iso: OK
sha256sum: WARNING: 23 lines are improperly formatted
$ sha512sum -c Qubes-R3.1-x86_64.iso.DIGESTS
Qubes-R3.1-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-R3.1-x86_64.iso
MD5(Qubes-R3.1-x86_64.iso)= f99634b05d15f6bb2ac02ee03e4338a0
$ openssl dgst -sha1 Qubes-R3.1-x86_64.iso
SHA1(Qubes-R3.1-x86_64.iso)= 990b7765ee209b42b3cad78673463daae769c729
$ openssl dgst -sha256 Qubes-R3.1-x86_64.iso
SHA256(Qubes-R3.1-x86_64.iso)= 2d82a684d507ad5789ed83272af4311fd04375e782364d5dd9df4b3d7118cc28
$ openssl dgst -sha512 Qubes-R3.1-x86_64.iso
SHA512(Qubes-R3.1-x86_64.iso)=
083d6cfc3fb5dc97fd91d8f9f70301c154e3674114ff1727b0415c2c663b233c22e0830d0bfc1f7a532549d7e39c6ef5cfde6a90a650343b47ba57d3e8e92ca7
(Notice that the outputs match the values from the .DIGESTS file.)
However, it is possible that an attacker replaced Qubes-R3.1-x86_64.iso with
a malicious ISO, computed the hash values for that ISO, and replaced the values
in Qubes-R3.1-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-R3.1-x86_64.iso.DIGESTS is a clearsigned PGP file, we can use
gpg to verify it from the command line:
$ gpg -v --verify Qubes-R3.1-x86_64.iso.DIGESTS
gpg: armor header: Hash: SHA256
gpg: armor header: Version: GnuPG v1
gpg: original file name=''
gpg: Signature made Wed 09 Mar 2016 03:35:48 AM PST 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 tags on the latest commit. Any commits that are not followed by a signed tag should not be trusted!
To verify a signature on a git tag, you can use:
$ git tag -v <tag name>