[tor-dev] Moving key material out of the main tor process

[Linus Nordberg]

Hi,

tl;dr How to move key material out of tor?

## The idea of a vault component

ahf and others in the network team have been discussing the
possibility of a "vault" component in tor, for moving private keys out
of the tor process. Separating secret key material from the code
handling data from the network seems valuable and providing a
component making different implementations "pluggable" would allow for
anyone to use their favourite technology without touching the tor
code base. Examples are local trusted execution environments like Intel
SGX and Arm TrustZone and various HSM's and security keys/tokens.

One way of implementing this would be to define a protocol, for a
vault component to talk to a daemon running on the same host as tor,
over some IPC mechanism. This protocol would allow tor to request a
signature over a hash, or a document, in a certain key. Whether the
daemon has access to the key material or has to forward the request to
a separate device or hardware component is irrelevant to the protocol
and the vault component.

Even if the design focuses on signatures it should probably take
encryption and decryption into account, to be added later.


## The vault as a possible match for TorHSM

Such a vault component would fit well with a project where Peter Stuge
and myself are building an HSM for Tor directory authority signing
keys [0] based on the CrypTech design [1].

[0] https://trac.cryptech.is/wiki/ExternalProjectsTorHSM
[1] https://cryptech.is/

One of the options for tor to delegate the signing of votes and
consensuses to such a device would be to use a vault component as
described above. We would then have a signing daemon responsible for
the USB communication with the TorHSM device. Another option would be
to build support directly into tor for communicating with the device,
through a library capable of a USB vendor specific protocol defined by
TorHSM.

There are pros and cons with both options. I'd like to hear what the
Tor developers community think would be best.


## Asynchronous signing API

Vault or not, tor needs to change the way signing is done to be able
to move key material out of the main process. First, external devices
may need more time to perform a signature operation than what tor can
accept to spend blocking in its main thread. Second, an external
device might disappear or malfunction, requiring the possibility for
an operation to time out.

This can be implemented either with callbacks or with a state machine
with more well defined state transitions. Maybe there are more
options that I didn't think of.


## Protocol choice

If we decide to go for the vault component with a separate daemon we
should consider using the ssh-agent protocol for tor to talk to the
daemon. Apart from being already defined there are multiple well
tested implementations of this protocol. It also has a couple of
features we might want, such as forgetting a certain key and
locking/unlocking of the vault. Finally, it's extensible and should
accommodate potential additions we might want to make later.