[Openvpn-devel] Proposal: tls-crypt-v2 (client-specific tls-crypt keys)

[Steffan K. <ste...@fo...>]

Hi,

As mentioned previously on hackathons, IRC and the mailing list, we've
been working on tls-crypt (which is available in 2.4) and a variant that
requires a bit more provisioning, but is better suited for large VPN
setups and also useful for VPN providers: tls-crypt-v2.  This is a
public request for comments on the design of this feature.

The existing --tls-auth and tls-crypt options use a pre-shared group
key, which is shared amongst all clients and servers in an OpenVPN
deployment.  If any client or server is compromised, the attacker will
have access to this shared key, and it will no longer provide any
security.  To reduce the risk of loosing pre-shared keys, tls-crypt-v2
adds the ability to supply each client with a unique tls-crypt key.

Also, even if all peers succeed in keeping the key secret, the
--tls-crypt key lifetime is limited to roughly 8000 years, divided by
the number of clients (see the --tls-crypt section of the man page).
Using client-specific keys, we lift this lifetime requirement to roughly
8000 years for each client key (which "Should Be Enough For Everybody
(tm)").

The full specification is attached.  Antonio and I have been working on
an implementation, a preview is available on:
https://github.com/syzzer/openvpn/tree/tls-crypt-v2-preview
(NOT READY FOR PRODUCTION USE!  Also, this is not a stable branch and
might be force-pushed or disappear at any moment.)

Any comments (both on the design as well as the text), bug reports or
suggestions are welcome!

-Steffan