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323 lines
15 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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---
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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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a compromise of your domain where the Thunderbird or other 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](#advanced-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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- Verifying detached signatures does not work (see \#900). You have to have
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public keys in AppVM and some means to use different command to verify
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them. Both git and Enigmail does not allow that and you have to choose
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between Split GPG and PGP/MIME.
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Configuring and using Split GPG
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-------------------------------
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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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### Configuring Thunderbird/Enigmail for use 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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[user@work ~]$ echo "work-gpg" > /rw/config/gpg-split-domain
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*NOTE*: A recent engimail update, version `thunderbird-enigmail-1.7-1`,
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introduced changes in how Enigmail expects to execute GPG binary
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and so requires an updated split-gpg package with version \>=
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`qubes-gpg-split-2.0.7-1`. Please make sure you have all the latest qubes
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packages installed in your template.
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### How to use `gpg2` instead of `gpg` ###
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In your GPG backend domain's TemplateVM:
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1. `sudo vim /etc/qubes-rpc/qubes.Gpg`
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2. Change `/usr/bin/gpg` to `/usr/bin/gpg2`.
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3. Ensure that your key has a **blank passphrase**. If not, you will encounter
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an error.
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4. Shut down the TemplateVM and restart the GPG backend domain.
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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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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](https://wiki.debian.org/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 he does not. If he does, then he can simply use
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the passphrase in order to legally extend the expiration date of the key
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(or remove it entirely). If he does not, then he cannot use the key at
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all. In either case, an expiration date provides no 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
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subkeys are generated in this VM. The master secret key *never*
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leaves this VM under *any* circumstances. No files or text is *ever*
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[copied](/doc/CopyingFiles#on-inter-domain-file-copy-security) or
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[pasted](/doc/CopyPaste#on-copypaste-security) into this VM under *any*
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circumstances.
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* `work-gpg`
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This is a network-isolated VM. This VM is used *only* as the
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GPG backend for `work-email`. The secret subkeys (but *not*
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the master secret key) are [copied](/doc/CopyingFiles) from the
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`vault` VM to this VM. Files from less trusted VMs are *never*
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[copied](/doc/CopyingFiles#on-inter-domain-file-copy-security) into this
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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](https://en.wikipedia.org/wiki/Mail_user_agent) 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](/doc/DataLeaks). 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](/doc/SoftwareUpdateVM#notes-on-trusting-your-template-vms)
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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](https://groups.google.com/d/topic/qubes-users/Kwfuern-R2U/discussion)
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- ["Creating the Perfect GPG Keypair" by Alex Cabal](https://alexcabal.com/creating-the-perfect-gpg-keypair/)
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- ["GPG Offline Master Key w/ smartcard" maintained by Abel Luck](https://gist.github.com/abeluck/3383449)
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- ["Using GnuPG with QubesOS" by Alex](https://apapadop.wordpress.com/2013/08/21/using-gnupg-with-qubesos/)
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