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@ -15,59 +15,35 @@ 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.
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 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).
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.
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.
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).
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.
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`).
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.,
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
@ -79,22 +55,16 @@ 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:
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][devel-master-key-msg] and [user][user-master-key-msg] [mailing lists].
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][devel-master-key-msg] and [user][user-master-key-msg] [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 multiple independent sources and comparing
them to the downloaded key's fingerprint to ensure they match. Here are some
ideas:
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 multiple independent sources and comparing them to the downloaded key's fingerprint to ensure they match.
Here are some ideas:
* Use the PGP Web of Trust.
* Check the key against different keyservers.
@ -107,14 +77,10 @@ ideas:
(e.g., slides from a talk or on a T-shirt).
* Repeat all of the above from different computers and devices.
In addition, some operating systems have built-in keyrings containing keys
capable of validating the Qubes Master Signing Key. For example, if you have
a Debian system, then your debian-keyring may already contain the necessary
keys.
In addition, some operating systems have built-in keyrings containing keys capable of validating the Qubes Master Signing Key.
For example, if you have a Debian system, then your debian-keyring may already contain the necessary keys.
Once you're confident that you have the legitimate Qubes Master Signing Key,
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:
Once you're confident that you have the legitimate Qubes Master Signing Key, 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
@ -157,11 +123,9 @@ automatically verify all the keys signed by the Qubes Master Signing Key:
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.
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:
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
@ -172,17 +136,11 @@ used for all Release 3 ISO images:
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].
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.
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`):
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
@ -191,8 +149,7 @@ signature (`Qubes-R3.2-x86_64.iso.asc`):
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:
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
@ -204,17 +161,13 @@ 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].
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:
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
@ -241,10 +194,9 @@ As an example, `Qubes-R3.2-x86_64.iso` is accompanied by
=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:
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
@ -259,16 +211,12 @@ respective `*sum` programs:
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.
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.:
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
@ -281,12 +229,9 @@ to the contents of the `.DIGESTS` file.:
(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:
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
@ -297,9 +242,9 @@ Since `Qubes-R3.2-x86_64.iso.DIGESTS` is a clearsigned PGP file, we can use
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.
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
--------------------