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.)
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.
- 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][intro] and [below][using split GPG with subkeys].
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][se-pinentry] for more information.)
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.
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.
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.
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`.
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.
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.
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.
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.
