qubes-doc/security/split-gpg.md

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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 DisposableVM and have the to-be-signed document displayed there? To Be Determined.

• The Split GPG client will fail to sign or encrypt if the private key in the GnuPG backend is protected by a passphrase. It will give an Inappropriate ioctl for device error. Do not set passphrases for the private keys in the GPG backend domain. Doing so won't provide any extra security anyway, as explained above and below. If you are generating a new key pair, or if you have a private key that already has a passphrase, you can use gpg2 --edit-key <key_id> then passwd to set an empty passphrase. Note that pinentry might show an error when you try to set an empty passphrase, but it will still make the change. (See this StackExchange answer for more information.)

Configuring Split GPG

In dom0, make sure the qubes-gpg-split-dom0 package is installed.

[user@dom0 ~]$ sudo qubes-dom0-update qubes-gpg-split-dom0

If using templates based on Debian or Whonix, make sure you have the qubes-gpg-split package installed.

[user@debian-8 ~]$ sudo apt install qubes-gpg-split

For Fedora.

[user@fedora-25 ~]$ sudo dnf 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

Please be aware of the caveat regarding passphrase-protected keys in the Current limitations section.

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
[root@work ~]$ echo "work-gpg" > /rw/config/gpg-split-domain

Qubes 4.0 Specifics

New qrexec policies in Qubes R4.0 by default require the user to enter the name of the domain containing GPG keys each time it is accessed. To improve usability for Thunderbird+Enigmail, in dom0 place the following line at the top of the file /etc/qubes-rpc/policy/qubes.Gpg:

work-email  work-gpg  allow

where work-email is the Thunderbird+Enigmail AppVM and work-gpg contains your GPG keys.

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

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

• 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 does not. An adversary who does possess the passphrase can simply use it to legally extend the expiration date of the key (or remove it entirely). An adversary who does not possess the passphrase 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.^* 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.)

^*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 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
• ​"Creating the Perfect GPG Keypair" by Alex Cabal
• ​"GPG Offline Master Key w/ smartcard" maintained by Abel Luck
• ​"Using GnuPG with QubesOS" by Alex