mirror of
https://github.com/QubesOS/qubes-doc.git
synced 2024-10-01 01:25:40 -04:00
390 lines
17 KiB
Markdown
390 lines
17 KiB
Markdown
---
|
||
layout: doc
|
||
title: Split GPG
|
||
permalink: /doc/split-gpg/
|
||
redirect_from:
|
||
- /en/doc/split-gpg/
|
||
- /doc/SplitGpg/
|
||
- /doc/UserDoc/SplitGpg/
|
||
- /wiki/UserDoc/SplitGpg/
|
||
- /doc/open-pgp/
|
||
- /en/doc/open-pgp/
|
||
- /doc/OpenPGP/
|
||
- /doc/UserDoc/OpenPGP/
|
||
- /wiki/UserDoc/OpenPGP/
|
||
---
|
||
|
||
Qubes Split GPG
|
||
===============
|
||
|
||
What is Split GPG and why should I use it instead of the standard GPG?
|
||
----------------------------------------------------------------------
|
||
Split GPG implements a concept similar to having a smart card with your
|
||
private GPG keys, except that the role of the "smart card" plays another Qubes
|
||
AppVM. This way one, not-so-trusted domain, e.g. the one where Thunderbird is
|
||
running, can delegate all crypto operations, such as encryption/decryption
|
||
and signing to another, more trusted, network-isolated, domain. This way
|
||
the compromise of your domain where Thunderbird or another client app is
|
||
running -- arguably a not-so-unthinkable scenario -- does not allow the
|
||
attacker to automatically also steal all your keys. (We should make a rather
|
||
obvious comment here that the so-often-used passphrases on private keys are
|
||
pretty meaningless because the attacker can easily set up a simple backdoor
|
||
which would wait until the user enters the passphrase and steal the key then.)
|
||
|
||
The diagram below presents the big picture of Split GPG architecture.
|
||
|
||
|
||
|
||
### Advantages of Split GPG vs. traditional GPG with a smart card ###
|
||
|
||
It is often thought that the use of smart cards for private key storage
|
||
guarantees ultimate safety. While this might be true (unless the attacker
|
||
can find a usually-very-expensive-and-requiring-physical-presence way to
|
||
extract the key from the smart card) but only with regards to the safety of
|
||
the private key itself. However, there is usually nothing that could stop
|
||
the attacker from requesting the smart card to perform decryption of all the
|
||
user documents the attacker has found or need to decrypt. In other words,
|
||
while protecting the user's private key is an important task, we should not
|
||
forget that ultimately it is the user data that are to be protected and that
|
||
the smart card chip has no way of knowing the requests to decrypt documents
|
||
are now coming from the attacker's script and not from the user sitting in
|
||
front of the monitor. (Similarly the smart card doesn't make the process
|
||
of digitally signing a document or a transaction in any way more secure --
|
||
the user cannot know what the chip is really signing. Unfortunately this
|
||
problem of signing reliability is not solvable by Split GPG)
|
||
|
||
With Qubes Split GPG this problem is drastically minimized, because each time
|
||
the key is to be used the user is asked for consent (with a definable time
|
||
out, 5 minutes by default), plus is always notified each time the key is used
|
||
via a tray notification from the domain where GPG backend is running. This
|
||
way it would be easy to spot unexpected requests to decrypt documents.
|
||
|
||
|
||
|
||
|
||
### Current limitations ###
|
||
|
||
- Current implementation requires importing of public keys to the vault
|
||
domain. This opens up an avenue to attack the gpg running in the backend domain
|
||
via a hypothetical bug in public key importing code. See ticket [#474] for more
|
||
details and plans how to get around this problem, as well as the section on
|
||
[using split GPG with subkeys] below.
|
||
|
||
- It doesn't solve the problem of allowing the user to know what is to be
|
||
signed before the operation gets approved. Perhaps the GPG backend domain
|
||
could start a Disposable VM and have the to-be-signed document displayed
|
||
there? To Be Determined.
|
||
|
||
|
||
Configuring Split GPG
|
||
---------------------
|
||
|
||
In dom0, make sure the `qubes-gpg-split-dom0` package is installed.
|
||
|
||
sudo qubes-dom0-update qubes-gpg-split-dom0
|
||
|
||
If using templates based on Debian, make sure you have the `qubes-gpg-split`
|
||
package installed.
|
||
|
||
sudo apt-get install qubes-gpg-split
|
||
|
||
For Fedora.
|
||
|
||
sudo yum install qubes-gpg-split
|
||
|
||
Start with creating a dedicated AppVM for storing your keys (the GPG backend
|
||
domain). It is recommended that this domain be network disconnected (set its
|
||
netvm to `none`) and only used for this one purpose. In later examples this
|
||
AppVM is named `work-gpg`, but of course it might have any other name.
|
||
|
||
### Setting up the GPG backend domain ###
|
||
|
||
Make sure the gpg is installed there and there are some private keys in the
|
||
keyring, e.g.:
|
||
|
||
[user@work-gpg ~]$ gpg -K
|
||
/home/user/.gnupg/secring.gpg
|
||
-----------------------------
|
||
sec 4096R/3F48CB21 2012-11-15
|
||
uid Qubes OS Security Team <security@qubes-os.org>
|
||
ssb 4096R/30498E2A 2012-11-15
|
||
(...)
|
||
|
||
This is pretty much all that is required. However one might also want to modify
|
||
the default timeout which tells the backend for how long the user's approval
|
||
for key access should be valid (default 5 minutes). This is adjustable via
|
||
`QUBES_GPG_AUTOACCEPT` variable. One can override it e.g. in `~/.bash_profile`:
|
||
|
||
[user@work-gpg ~]$ echo "export QUBES_GPG_AUTOACCEPT=86400" >> ~/.bash_profile
|
||
|
||
### Configuring the client apps to use Split GPG backend ###
|
||
|
||
Normally it should be enough to set the `QUBES_GPG_DOMAIN` to the GPG backend
|
||
domain name and use `qubes-gpg-client` in place of `gpg`, e.g.:
|
||
|
||
[user@work ~]$ export QUBES_GPG_DOMAIN=work-gpg
|
||
[user@work ~]$ gpg -K
|
||
[user@work ~]$ qubes-gpg-client -K
|
||
/home/user/.gnupg/secring.gpg
|
||
-----------------------------
|
||
sec 4096R/3F48CB21 2012-11-15
|
||
uid Qubes OS Security Team <security@qubes-os.org>
|
||
ssb 4096R/30498E2A 2012-11-15
|
||
(...)
|
||
|
||
[user@work ~]$ qubes-gpg-client secret_message.txt.asc
|
||
(...)
|
||
|
||
Note that running normal `gpg -K` in the demo above shows no private keys
|
||
stored in this AppVM.
|
||
|
||
A note on `gpg` and `gpg2`:
|
||
|
||
Throughout this guide, we refer to `gpg`, but note that Split-GPG uses `gpg2`
|
||
under the hood for compatibility with programs like Enigmail (which now supports
|
||
only `gpg2`). If you encounter trouble while trying to set up Split-GPG, make
|
||
sure you're using `gpg2` for your configuration and testing, since keyring data
|
||
may differ between the two installations.
|
||
|
||
## Using Thunderbird + Enigmail with Split GPG ##
|
||
|
||
However, when using Thunderbird with Enigmail extension it is
|
||
not enough, because Thunderbird doesn't preserve the environment
|
||
variables. Instead it is recommended to use a simple script provided by
|
||
`/usr/bin/qubes-gpg-client-wrapper` file by pointing Enigmail to use this
|
||
script instead of the standard GnuPG binary:
|
||
|
||
|
||
|
||
The script also sets the `QUBES_GPG_DOMAIN` variable automatically based on
|
||
the content of the file `/rw/config/gpg-split-domain`, which should be set to
|
||
the name of the GPG backend VM. This file survives the AppVM reboot, of course.
|
||
|
||
[user@work ~]$ sudo bash
|
||
[user@work ~]$ echo "work-gpg" > /rw/config/gpg-split-domain
|
||
|
||
A note on passphrases:
|
||
|
||
You may experience trouble when attempting to use a PGP key *with a passphrase*
|
||
along with Split-GPG and Enigmail. If you do, you may need to remove the
|
||
passphrase from your (sub)key(s) in order to get Split-GPG working correctly.
|
||
As mentioned above, we do not believe PGP key passphrases to be significant
|
||
from a security perspective.
|
||
|
||
## Using Git with Split GPG ##
|
||
|
||
Git can be configured to used with Split-GPG, something useful if you would
|
||
like to contribute to the Qubes OS Project as every commit is required to be
|
||
signed. The most basic `~/.gitconfig` file to with working Split-GPG looks
|
||
something like this.
|
||
|
||
[user]
|
||
name = YOUR NAME
|
||
email = YOUR EMAIL ADDRESS
|
||
signingkey = YOUR KEY ID
|
||
|
||
[gpg]
|
||
program = qubes-gpg-client-wrapper
|
||
|
||
Your key id is the public id of your signing key, which can be found by running
|
||
`qubes-gpg-client -k`. In this instance, the key id is DD160C74.
|
||
|
||
[user@work ~]$ qubes-gpg-client -k
|
||
/home/user/.gnupg/pubring.kbx
|
||
-----------------------------
|
||
pub rsa4096/DD160C74 2016-04-26
|
||
uid Qubes User
|
||
|
||
To sign commits, you now add the "-S" flag to your commit command, which should
|
||
prompt for Split-GPG usage. If you would like automatically sign all commits,
|
||
you can add the following snippet to `~/.gitconfig`.
|
||
|
||
[commit]
|
||
gpgsign = true
|
||
|
||
Lastly, if you would like to add aliases to sign and verify tags using the
|
||
conventions the Qubes OS Project recommends, you can add the following snippet
|
||
to `~/.gitconfig`.
|
||
|
||
[alias]
|
||
stag = "!id=`git rev-parse --verify HEAD`; git tag -s user_${id:0:8} -m \"Tag for commit $id\""
|
||
vtag = !git tag -v `git describe`
|
||
|
||
Replace `user` with your short, unique nickname. Now you can use `git stag` to
|
||
add a signed tag to a commit and `git vtag` to verify the most recent tag that
|
||
is reachable from a commit.
|
||
|
||
## Importing public keys ###
|
||
|
||
Use `qubes-gpg-import-key` in the client AppVM to import the key into the
|
||
GPG backend VM. Of course a (safe, unspoofable) user consent dialog box is
|
||
displayed to accept this.
|
||
|
||
[user@work ~]$ export QUBES_GPG_DOMAIN=work-gpg
|
||
[user@work ~]$ qubes-gpg-import-key ~/Downloads/marmarek.asc
|
||
|
||
|
||
|
||
<br />
|
||
|
||
Advanced: Using Split GPG with Subkeys
|
||
--------------------------------------
|
||
Users with particularly high security requirements may wish to use Split
|
||
GPG with [subkeys]. However, this setup
|
||
comes at a significant cost: It will be impossible to sign other people's keys
|
||
with the master secret key without breaking this security model. Nonetheless,
|
||
if signing others' keys is not required, then Split GPG with subkeys offers
|
||
unparalleled security for one's master secret key.
|
||
|
||
### Setup Description ###
|
||
|
||
In this example, the following keys are stored in the following locations
|
||
(see below for defintions of these terms):
|
||
|
||
| PGP Key(s) | VM Name |
|
||
| ---------- | ------------ |
|
||
| `sec` | `vault` |
|
||
| `ssb` | `work-gpg` |
|
||
| `pub` | `work-email` |
|
||
|
||
<br />
|
||
|
||
* `sec` (master secret key)
|
||
|
||
Depending on your needs, you may wish to create this as a **certify-only
|
||
(C)** key, i.e., a key which is capable only of signing (a.k.a.,
|
||
"certifying") other keys. This key may be created *without* an expiration
|
||
date. This is for two reasons. First, the master secret key is never to
|
||
leave the `vault` VM, so it is extremely unlikely ever to be obtained by
|
||
an adversary (see below). Second, an adversary who *does* manage to obtain
|
||
the master secret key either possesses the passphrase to unlock the key
|
||
(if one is used), or he does not. If he does, then he can simply use
|
||
the passphrase in order to legally extend the expiration date of the key
|
||
(or remove it entirely). If he does not, then he cannot use the key at
|
||
all. In either case, an expiration date provides no additional benefit.
|
||
|
||
By the same token, however, having a passphrase on the key is of little
|
||
value. An adversary who is capable of stealing the key from your `vault`
|
||
would almost certainly also be capable of stealing the passphrase as
|
||
you enter it. An adversary who obtains the passphrase can then use it
|
||
in order to change or remove the passphrase from the key. Therefore,
|
||
using a passphrase at all should be considered optional. It is, however,
|
||
recommended that a **revocation certificate** be created and safely stored
|
||
in multiple locations so that the master keypair can be revoked in the
|
||
(exceedingly unlikely) event that it is ever compromised.
|
||
|
||
* `ssb` (secret subkey)
|
||
|
||
Depending on your needs, you may wish to create two different subkeys: one
|
||
for **signing (S)** and one for **encryption (E)**. You may also wish to
|
||
give these subkeys reasonable expiration dates (e.g., one year). Once these
|
||
keys expire, it is up to you whether to *renew* these keys by extending the
|
||
expiration dates or to create *new* subkeys when the existing set expires.
|
||
|
||
On the one hand, an adversary who obtains any existing encryption subkey
|
||
(for example) will be able to use it in order to decrypt all emails (for
|
||
example) which were encrypted to that subkey. If the same subkey were to
|
||
continue to be used--and its expiration date continually extended--only
|
||
that one key would need to be stolen (e.g., as a result of the `work-gpg`
|
||
VM being compromised; see below) in order to decrypt *all* of the user's
|
||
emails. If, on the other hand, each encryption subkey is used for at most
|
||
approximately one year, then an adversary who obtains the secret subkey will
|
||
be capable of decrypting at most approximately one year's worth of emails.
|
||
|
||
On the other hand, creating a new signing subkey each year without
|
||
renewing (i.e., extending the expiration dates of) existing signing
|
||
subkeys would mean that all of your old signatures would eventually
|
||
read as "EXPIRED" whenever someone attempts to verify them. This can be
|
||
problematic, since there is no consensus on how expired signatures should
|
||
be handled. Generally, digital signatures are intended to last forever,
|
||
so this is a strong reason against regularly retiring one's signing subkeys.
|
||
|
||
* `pub` (public key)
|
||
|
||
This is the complement of the master secret key. It can be uploaded to
|
||
keyservers (or otherwise publicly distributed) and may be signed by others.
|
||
|
||
* `vault`
|
||
|
||
This is a network-isolated VM. The initial master keypair and subkeys are
|
||
generated in this VM. The master secret key *never* leaves this VM under
|
||
*any* circumstances. No files or text is *ever* [copied] or [pasted] into
|
||
this VM under *any* circumstances.
|
||
|
||
* `work-gpg`
|
||
|
||
This is a network-isolated VM. This VM is used *only* as the GPG backend for
|
||
`work-email`. The secret subkeys (but *not* the master secret key) are
|
||
[copied] from the `vault` VM to this VM. Files from less trusted VMs are
|
||
*never* [copied] into this VM under *any* circumstances.
|
||
|
||
* `work-email`
|
||
|
||
This VM has access to the mail server. It accesses the `work-gpg` VM via
|
||
the Split GPG protocol. The public key may be stored in this VM so that
|
||
it can be attached to emails and for other such purposes.
|
||
|
||
### Security Benefits ###
|
||
|
||
In the standard Split GPG setup, there are at least two ways in
|
||
which the `work-gpg` VM might be compromised. First, an attacker
|
||
who is capable of exploiting a hypothetical bug in `work-email`'s
|
||
[MUA] could gain control of
|
||
the `work-email` VM and send a malformed request which exploits a hypothetical
|
||
bug in the GPG backend (running in the `work-gpg` VM), giving the attacker
|
||
control of the `work-gpg` VM. Second, a malicious public key file which is
|
||
imported into the `work-gpg` VM might exploit a hypothetical bug in the GPG
|
||
backend which is running there, again giving the attacker control of the
|
||
`work-gpg` VM. In either case, such an attacker might then be able to leak
|
||
both the master secret key and its passphrase (if any is used, it would
|
||
regularly be input in the work-gpg VM and therefore easily obtained by an
|
||
attacker who controls this VM) back to the `work-email` VM or to another VM
|
||
(e.g., the `netvm`, which is always untrusted by default) via the Split GPG
|
||
protocol or other [covert channels]. Once the master secret
|
||
key is in the `work-email` VM, the attacker could simply email it to himself
|
||
(or to the world).
|
||
|
||
In the alternative setup described in this section (i.e., the subkey
|
||
setup), even an attacker who manages to gain access to the `work-gpg` VM
|
||
will not be able to obtain the user's master secret key since it is simply
|
||
not there. Rather, the master secret key remains in the `vault` VM, which
|
||
is extremely unlikely to be compromised, since nothing is ever copied or
|
||
transferred into it.<sup>\*</sup> The attacker might nonetheless be able to
|
||
leak the secret subkeys from the `work-gpg` VM in the manner described above,
|
||
but even if this is successful, the secure master secret key can simply be
|
||
used to revoke the compromised subkeys and to issue new subkeys in their
|
||
place. (This is significantly less devastating than having to create a new
|
||
*master* keypair.)
|
||
|
||
<sup>\*</sup>In order to gain access to the `vault` VM, the attacker
|
||
would require the use of, e.g., a general Xen VM escape exploit
|
||
or a [signed, compromised package which is already installed in the
|
||
TemplateVM][trusting-templates]
|
||
upon which the `vault` VM is based.
|
||
|
||
### Subkey Tutorials and Discussions ###
|
||
|
||
(Note: Although the tutorials below were not written with Qubes Split GPG
|
||
in mind, they can be adapted with a few commonsense adjustments. As always,
|
||
exercise caution and use your good judgment.)
|
||
|
||
- ["OpenPGP in Qubes OS" on the qubes-users mailing list][openpgp-in-qubes-os]
|
||
- ["Creating the Perfect GPG Keypair" by Alex Cabal][cabal]
|
||
- ["GPG Offline Master Key w/ smartcard" maintained by Abel Luck][luck]
|
||
- ["Using GnuPG with QubesOS" by Alex][apapadop]
|
||
|
||
|
||
[#474]: https://github.com/QubesOS/qubes-issues/issues/474
|
||
[using split GPG with subkeys]: #advanced-using-split-gpg-with-subkeys
|
||
[subkeys]: https://wiki.debian.org/Subkeys
|
||
[copied]: /doc/copying-files#on-inter-domain-file-copy-security
|
||
[pasted]: /doc/copy-paste#on-copypaste-security
|
||
[MUA]: https://en.wikipedia.org/wiki/Mail_user_agent
|
||
[covert channels]: /doc/data-leaks
|
||
[trusting-templates]: /doc/SoftwareUpdateVM#notes-on-trusting-your-template-vms
|
||
[openpgp-in-qubes-os]: https://groups.google.com/d/topic/qubes-users/Kwfuern-R2U/discussion
|
||
[cabal]: https://alexcabal.com/creating-the-perfect-gpg-keypair/
|
||
[luck]: https://gist.github.com/abeluck/3383449
|
||
[apapadop]: https://apapadop.wordpress.com/2013/08/21/using-gnupg-with-qubesos/
|
||
|