We decided to investigate what it would take to modernize the cryptography used to establish the TLS control channel with OpenVPN. See our current configuration documentation as used by eduVPN. The reason for looking into this is to get improved security and increased performance, when switching to TLSv1.3 and possibly switching key type. This is relevant for the connection setup when establishing a OpenVPN connection.
What we tried to accomplish these two tasks specifically:
- Use TLSv1.3 instead of TLSv1.2;
- Use Ed25519 instead of RSA for the OpenVPN client and server keys.
In order to test with TLSv1.3 and Ed25519, we used our own CA for issuing certificates, vpn-ca that also recently added support for Ed25519 keys and certificates and will in the near future replace easy-rsa in eduVPN.
In order to modify the configuration, we can specify that the minimum version of TLS that can be used is 1.3.
tls-version-min 1.3
We no longer need the tls-cipher configuration option, as all ciphers supported by TLSv1.3 are secure. The client can decide.
It turns out, not many (existing) OpenVPN clients are capable of supporting TLSv1.3 and/or Ed25519 keys out of the box.
| Client | Works? | Notes |
|---|---|---|
| OpenVPN Community (Windows) | No | OpenSSL 1.1.0j |
| TunnelKit | No | OpenSSL-Apple (1.1.0l.4) (1) |
| Viscosity (Windows, macOS) | No | OpenSSL 1.0.2t (2) |
| Tunnelblick (macOS) | Yes | Requires Config Change (3) |
| OpenVPN for Android | Yes | has OpenSSL 1.1.1a |
| OpenVPN Connect (iOS) | No | OpenSSL 1.0.2s, mbed TLS 2.7.12 |
| OpenVPN Connect (Android) | No | OpenSSL 1.0.2s, mbed TLS 2.7.12 |
| Linux | Maybe | when distribution has OpenSSL >= 1.1.1 |
The eduVPN clients are based on some of the above, so whatever the above clients support is important for eduVPN as well!
Clients depending on OpenSSL needs to use OpenSSL >= 1.1.1, otherwise there is no support for TLSv1.3 and Ed25519. When a client uses mbed TLS, there is no support for TLSv1.3 or Ed25519 at all.
Interestingly, some clients use OpenSSL versions that are no longer supported upstream. Only 1.1.1 and 1.0.2 are still supported. See the OpenSSL Release Strategy.
For eduVPN specifically, there may be some additional hurdles to take: the platform may not support Ed25519 keys in its platform native certificate store. More research is needed here, but having “Yes” in the “Works?” column above is a good first step, even when we are required to use RSA keys for a little while longer.
Notes
(1) We opened an issue upstream
(2) We sent a mail on 2019-10-11 to support@sparklabs.com to ask for updating OpenSSL as used by Viscosity. Their response: “…we’ll likely be adopting 1.1.1 with an upcoming OpenVPN 2.4.x release…”
(3) In Tunnelblick, the “OpenVPN version” needs to be changed to 2.4.7 - OpenSSL v1.1.1d under “Configurations”. We tested with Tunnelblick 3.8.2beta01 (build 5401)
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