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406 lines
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406 lines
18 KiB
Markdown
---
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layout: doc
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title: Split GPG
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permalink: /doc/split-gpg/
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redirect_from:
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- /en/doc/split-gpg/
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- /doc/SplitGpg/
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- /doc/UserDoc/SplitGpg/
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- /wiki/UserDoc/SplitGpg/
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- /doc/open-pgp/
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- /en/doc/open-pgp/
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- /doc/OpenPGP/
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- /doc/UserDoc/OpenPGP/
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- /wiki/UserDoc/OpenPGP/
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---
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Qubes Split GPG
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===============
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What is Split GPG and why should I use it instead of the standard GPG?
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----------------------------------------------------------------------
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Split GPG implements a concept similar to having a smart card with your
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private GPG keys, except that the role of the "smart card" plays another Qubes
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AppVM. This way one, not-so-trusted domain, e.g. the one where Thunderbird is
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running, can delegate all crypto operations, such as encryption/decryption
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and signing to another, more trusted, network-isolated, domain. This way
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the compromise of your domain where Thunderbird or another client app is
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running -- arguably a not-so-unthinkable scenario -- does not allow the
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attacker to automatically also steal all your keys. (We should make a rather
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obvious comment here that the so-often-used passphrases on private keys are
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pretty meaningless because the attacker can easily set up a simple backdoor
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which would wait until the user enters the passphrase and steal the key then.)
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The diagram below presents the big picture of Split GPG architecture.
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### Advantages of Split GPG vs. traditional GPG with a smart card ###
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It is often thought that the use of smart cards for private key storage
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guarantees ultimate safety. While this might be true (unless the attacker
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can find a usually-very-expensive-and-requiring-physical-presence way to
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extract the key from the smart card) but only with regards to the safety of
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the private key itself. However, there is usually nothing that could stop
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the attacker from requesting the smart card to perform decryption of all the
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user documents the attacker has found or need to decrypt. In other words,
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while protecting the user's private key is an important task, we should not
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forget that ultimately it is the user data that are to be protected and that
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the smart card chip has no way of knowing the requests to decrypt documents
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are now coming from the attacker's script and not from the user sitting in
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front of the monitor. (Similarly the smart card doesn't make the process
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of digitally signing a document or a transaction in any way more secure --
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the user cannot know what the chip is really signing. Unfortunately this
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problem of signing reliability is not solvable by Split GPG)
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With Qubes Split GPG this problem is drastically minimized, because each time
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the key is to be used the user is asked for consent (with a definable time
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out, 5 minutes by default), plus is always notified each time the key is used
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via a tray notification from the domain where GPG backend is running. This
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way it would be easy to spot unexpected requests to decrypt documents.
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### Current limitations ###
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- Current implementation requires importing of public keys to the vault
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domain. This opens up an avenue to attack the gpg running in the backend domain
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via a hypothetical bug in public key importing code. See ticket [#474] for more
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details and plans how to get around this problem, as well as the section on
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[using split GPG with subkeys] below.
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- It doesn't solve the problem of allowing the user to know what is to be
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signed before the operation gets approved. Perhaps the GPG backend domain
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could start a Disposable VM and have the to-be-signed document displayed
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there? To Be Determined.
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Configuring Split GPG
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---------------------
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In dom0, make sure the `qubes-gpg-split-dom0` package is installed.
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[user@dom0 ~]$ sudo qubes-dom0-update qubes-gpg-split-dom0
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If using templates based on Debian or Whonix, make sure you have the `qubes-gpg-split`
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package installed.
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[user@debian-8 ~]$ sudo apt install qubes-gpg-split
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For Fedora.
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[user@fedora-25 ~]$ sudo dnf install qubes-gpg-split
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Start with creating a dedicated AppVM for storing your keys (the GPG backend
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domain). It is recommended that this domain be network disconnected (set its
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netvm to `none`) and only used for this one purpose. In later examples this
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AppVM is named `work-gpg`, but of course it might have any other name.
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### Setting up the GPG backend domain ###
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Make sure the gpg is installed there and there are some private keys in the
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keyring, e.g.:
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[user@work-gpg ~]$ gpg -K
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/home/user/.gnupg/secring.gpg
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-----------------------------
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sec 4096R/3F48CB21 2012-11-15
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uid Qubes OS Security Team <security@qubes-os.org>
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ssb 4096R/30498E2A 2012-11-15
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(...)
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This is pretty much all that is required. However one might also want to modify
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the default timeout which tells the backend for how long the user's approval
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for key access should be valid (default 5 minutes). This is adjustable via
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`QUBES_GPG_AUTOACCEPT` variable. One can override it e.g. in `~/.bash_profile`:
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[user@work-gpg ~]$ echo "export QUBES_GPG_AUTOACCEPT=86400" >> ~/.bash_profile
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### Configuring the client apps to use Split GPG backend ###
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Normally it should be enough to set the `QUBES_GPG_DOMAIN` to the GPG backend
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domain name and use `qubes-gpg-client` in place of `gpg`, e.g.:
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[user@work ~]$ export QUBES_GPG_DOMAIN=work-gpg
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[user@work ~]$ gpg -K
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[user@work ~]$ qubes-gpg-client -K
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/home/user/.gnupg/secring.gpg
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-----------------------------
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sec 4096R/3F48CB21 2012-11-15
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uid Qubes OS Security Team <security@qubes-os.org>
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ssb 4096R/30498E2A 2012-11-15
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(...)
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[user@work ~]$ qubes-gpg-client secret_message.txt.asc
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(...)
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Note that running normal `gpg -K` in the demo above shows no private keys
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stored in this AppVM.
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A note on `gpg` and `gpg2`:
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Throughout this guide, we refer to `gpg`, but note that Split-GPG uses `gpg2`
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under the hood for compatibility with programs like Enigmail (which now supports
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only `gpg2`). If you encounter trouble while trying to set up Split-GPG, make
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sure you're using `gpg2` for your configuration and testing, since keyring data
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may differ between the two installations.
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## Using Thunderbird + Enigmail with Split GPG ##
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However, when using Thunderbird with Enigmail extension it is
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not enough, because Thunderbird doesn't preserve the environment
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variables. Instead it is recommended to use a simple script provided by
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`/usr/bin/qubes-gpg-client-wrapper` file by pointing Enigmail to use this
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script instead of the standard GnuPG binary:
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The script also sets the `QUBES_GPG_DOMAIN` variable automatically based on
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the content of the file `/rw/config/gpg-split-domain`, which should be set to
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the name of the GPG backend VM. This file survives the AppVM reboot, of course.
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[user@work ~]$ sudo bash
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[root@work ~]$ echo "work-gpg" > /rw/config/gpg-split-domain
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A note on passphrases:
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You may experience trouble when attempting to use a PGP key *with a passphrase*
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along with Split-GPG and Enigmail. If you do, you may need to remove the
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passphrase from your (sub)key(s) in order to get Split-GPG working correctly.
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As mentioned above, we do not believe PGP key passphrases to be significant
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from a security perspective.
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## Using Git with Split GPG ##
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Git can be configured to used with Split-GPG, something useful if you would
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like to contribute to the Qubes OS Project as every commit is required to be
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signed. The most basic `~/.gitconfig` file to with working Split-GPG looks
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something like this.
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[user]
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name = YOUR NAME
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email = YOUR EMAIL ADDRESS
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signingkey = YOUR KEY ID
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[gpg]
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program = qubes-gpg-client-wrapper
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Your key id is the public id of your signing key, which can be found by running
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`qubes-gpg-client -k`. In this instance, the key id is DD160C74.
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[user@work ~]$ qubes-gpg-client -k
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/home/user/.gnupg/pubring.kbx
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-----------------------------
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pub rsa4096/DD160C74 2016-04-26
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uid Qubes User
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To sign commits, you now add the "-S" flag to your commit command, which should
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prompt for Split-GPG usage. If you would like automatically sign all commits,
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you can add the following snippet to `~/.gitconfig`.
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[commit]
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gpgsign = true
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Lastly, if you would like to add aliases to sign and verify tags using the
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conventions the Qubes OS Project recommends, you can add the following snippet
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to `~/.gitconfig`.
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[alias]
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stag = "!id=`git rev-parse --verify HEAD`; git tag -s user_${id:0:8} -m \"Tag for commit $id\""
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vtag = !git tag -v `git describe`
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Replace `user` with your short, unique nickname. Now you can use `git stag` to
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add a signed tag to a commit and `git vtag` to verify the most recent tag that
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is reachable from a commit.
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## Importing public keys ###
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Use `qubes-gpg-import-key` in the client AppVM to import the key into the
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GPG backend VM. Of course a (safe, unspoofable) user consent dialog box is
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displayed to accept this.
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[user@work ~]$ export QUBES_GPG_DOMAIN=work-gpg
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[user@work ~]$ qubes-gpg-import-key ~/Downloads/marmarek.asc
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<br />
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Qubes 4.0
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---------
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New qrexec policies in Qubes R4.0 by default require the user to enter the name
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of the domain containing GPG keys each time it is accessed. To improve usability
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for Thunderbird+Enigmail, in `dom0` place the following line at the top of the file
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`/etc/qubes-rpc/policy/qubes.Gpg`:
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```
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work-email work-gpg allow
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```
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where `work-email` is the Thunderbird+Enigmail AppVM and `work-gpg` contains
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your GPG keys.
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<br />
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Advanced: Using Split GPG with Subkeys
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--------------------------------------
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Users with particularly high security requirements may wish to use Split
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GPG with [subkeys]. However, this setup
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comes at a significant cost: It will be impossible to sign other people's keys
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with the master secret key without breaking this security model. Nonetheless,
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if signing others' keys is not required, then Split GPG with subkeys offers
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unparalleled security for one's master secret key.
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### Setup Description ###
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In this example, the following keys are stored in the following locations
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(see below for defintions of these terms):
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| PGP Key(s) | VM Name |
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| ---------- | ------------ |
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| `sec` | `vault` |
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| `ssb` | `work-gpg` |
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| `pub` | `work-email` |
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<br />
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* `sec` (master secret key)
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Depending on your needs, you may wish to create this as a **certify-only
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(C)** key, i.e., a key which is capable only of signing (a.k.a.,
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"certifying") other keys. This key may be created *without* an expiration
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date. This is for two reasons. First, the master secret key is never to
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leave the `vault` VM, so it is extremely unlikely ever to be obtained by
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an adversary (see below). Second, an adversary who *does* manage to obtain
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the master secret key either possesses the passphrase to unlock the key
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(if one is used) or does not. An adversary who *does* possess the passphrase
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can simply use it to legally extend the expiration date of the key
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(or remove it entirely). An adversary who does *not* possess the passphrase
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cannot use the key at all. In either case, an expiration date provides no
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additional benefit.
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By the same token, however, having a passphrase on the key is of little
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value. An adversary who is capable of stealing the key from your `vault`
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would almost certainly also be capable of stealing the passphrase as
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you enter it. An adversary who obtains the passphrase can then use it
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in order to change or remove the passphrase from the key. Therefore,
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using a passphrase at all should be considered optional. It is, however,
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recommended that a **revocation certificate** be created and safely stored
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in multiple locations so that the master keypair can be revoked in the
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(exceedingly unlikely) event that it is ever compromised.
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* `ssb` (secret subkey)
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Depending on your needs, you may wish to create two different subkeys: one
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for **signing (S)** and one for **encryption (E)**. You may also wish to
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give these subkeys reasonable expiration dates (e.g., one year). Once these
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keys expire, it is up to you whether to *renew* these keys by extending the
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expiration dates or to create *new* subkeys when the existing set expires.
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On the one hand, an adversary who obtains any existing encryption subkey
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(for example) will be able to use it in order to decrypt all emails (for
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example) which were encrypted to that subkey. If the same subkey were to
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continue to be used--and its expiration date continually extended--only
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that one key would need to be stolen (e.g., as a result of the `work-gpg`
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VM being compromised; see below) in order to decrypt *all* of the user's
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emails. If, on the other hand, each encryption subkey is used for at most
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approximately one year, then an adversary who obtains the secret subkey will
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be capable of decrypting at most approximately one year's worth of emails.
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On the other hand, creating a new signing subkey each year without
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renewing (i.e., extending the expiration dates of) existing signing
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subkeys would mean that all of your old signatures would eventually
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read as "EXPIRED" whenever someone attempts to verify them. This can be
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problematic, since there is no consensus on how expired signatures should
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be handled. Generally, digital signatures are intended to last forever,
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so this is a strong reason against regularly retiring one's signing subkeys.
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* `pub` (public key)
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This is the complement of the master secret key. It can be uploaded to
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keyservers (or otherwise publicly distributed) and may be signed by others.
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* `vault`
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This is a network-isolated VM. The initial master keypair and subkeys are
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generated in this VM. The master secret key *never* leaves this VM under
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*any* circumstances. No files or text is *ever* [copied] or [pasted] into
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this VM under *any* circumstances.
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* `work-gpg`
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This is a network-isolated VM. This VM is used *only* as the GPG backend for
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`work-email`. The secret subkeys (but *not* the master secret key) are
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[copied] from the `vault` VM to this VM. Files from less trusted VMs are
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*never* [copied] into this VM under *any* circumstances.
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* `work-email`
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This VM has access to the mail server. It accesses the `work-gpg` VM via
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the Split GPG protocol. The public key may be stored in this VM so that
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it can be attached to emails and for other such purposes.
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### Security Benefits ###
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In the standard Split GPG setup, there are at least two ways in
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which the `work-gpg` VM might be compromised. First, an attacker
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who is capable of exploiting a hypothetical bug in `work-email`'s
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[MUA] could gain control of
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the `work-email` VM and send a malformed request which exploits a hypothetical
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bug in the GPG backend (running in the `work-gpg` VM), giving the attacker
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control of the `work-gpg` VM. Second, a malicious public key file which is
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imported into the `work-gpg` VM might exploit a hypothetical bug in the GPG
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backend which is running there, again giving the attacker control of the
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`work-gpg` VM. In either case, such an attacker might then be able to leak
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both the master secret key and its passphrase (if any is used, it would
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regularly be input in the work-gpg VM and therefore easily obtained by an
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attacker who controls this VM) back to the `work-email` VM or to another VM
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(e.g., the `netvm`, which is always untrusted by default) via the Split GPG
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protocol or other [covert channels]. Once the master secret
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key is in the `work-email` VM, the attacker could simply email it to himself
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(or to the world).
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In the alternative setup described in this section (i.e., the subkey
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setup), even an attacker who manages to gain access to the `work-gpg` VM
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will not be able to obtain the user's master secret key since it is simply
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not there. Rather, the master secret key remains in the `vault` VM, which
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is extremely unlikely to be compromised, since nothing is ever copied or
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transferred into it.<sup>\*</sup> The attacker might nonetheless be able to
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leak the secret subkeys from the `work-gpg` VM in the manner described above,
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but even if this is successful, the secure master secret key can simply be
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used to revoke the compromised subkeys and to issue new subkeys in their
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place. (This is significantly less devastating than having to create a new
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*master* keypair.)
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<sup>\*</sup>In order to gain access to the `vault` VM, the attacker
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would require the use of, e.g., a general Xen VM escape exploit
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or a [signed, compromised package which is already installed in the
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TemplateVM][trusting-templates]
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upon which the `vault` VM is based.
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### Subkey Tutorials and Discussions ###
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(Note: Although the tutorials below were not written with Qubes Split GPG
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in mind, they can be adapted with a few commonsense adjustments. As always,
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exercise caution and use your good judgment.)
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- ["OpenPGP in Qubes OS" on the qubes-users mailing list][openpgp-in-qubes-os]
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- ["Creating the Perfect GPG Keypair" by Alex Cabal][cabal]
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- ["GPG Offline Master Key w/ smartcard" maintained by Abel Luck][luck]
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- ["Using GnuPG with QubesOS" by Alex][apapadop]
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[#474]: https://github.com/QubesOS/qubes-issues/issues/474
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[using split GPG with subkeys]: #advanced-using-split-gpg-with-subkeys
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[subkeys]: https://wiki.debian.org/Subkeys
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[copied]: /doc/copying-files#on-inter-qube-file-copy-security
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[pasted]: /doc/copy-paste#on-copypaste-security
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[MUA]: https://en.wikipedia.org/wiki/Mail_user_agent
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[covert channels]: /doc/data-leaks
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[trusting-templates]: /doc/software-update-vm/#notes-on-trusting-your-templatevms
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[openpgp-in-qubes-os]: https://groups.google.com/d/topic/qubes-users/Kwfuern-R2U/discussion
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[cabal]: https://alexcabal.com/creating-the-perfect-gpg-keypair/
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[luck]: https://gist.github.com/abeluck/3383449
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[apapadop]: https://apapadop.wordpress.com/2013/08/21/using-gnupg-with-qubesos/
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