François' Blog

OpenVPN and Modern Crypto (Part II)

Published on 2020-09-11

This blog post is a copy of a blog post I wrote for the eduVPN blog...

Last year we decided to investigate the OpenVPN client support of TLSv1.3 and EdDSA (Ed25519). One reason for doing this is, to stay current with algorithm recommendations by experts and move away from RSA. As EdDSA is easier to implement securely and has built-in protections against attacks that other curves, most notably, the NIST curves, do not have, there are fewer things that can go wrong. This can make the VPN more secure.

The other reason is performance. Generating RSA keys is slow, very slow. As we currently generate the keys on the server, this potentially results in high CPU load when many clients want to obtain a (new) certificate at around the same time, for example at the start of the work day. For a service with hundreds or thousands of users, this can create problems. Also, on a Raspberry Pi, yes eduVPN / Let's Connect! supports the Raspberry Pi, it is slow to generate RSA keys, which can take many seconds. No fun!

A simple benchmark running on a laptop from 2012, and Raspberry Pi 3 Model B+ shows the clear difference. The benchmark generates a self signed CA then generates 50 keys and signs each of them using the CA. The time varies per execution, but they show a clear, very big difference. The time between brackets is key generation and signing per certificate, on average.

Key Type Laptop Raspberry Pi
RSA 63s (1.26s) 368s (7.36s)
ECDSA 1s 4s
EdDSA 0s 1s

We decided to check the status of the clients again to investigate whether it is possible to upgrade to TLSv1.3 and EdDSA in the next version of eduVPN / Let's Connect!. Luckily, much has changed since last year and support for EdDSA and TLSv1.3 looks a lot better now!

The eduVPN / Let's Connect! 2.x server meanwhile supports EdDSA (and ECDSA) out of the box, but will keep RSA as the default. However the server can easily be configured to use ECDSA or EdDSA.

We'll again go over the list of clients that were tested last year. The updated results can be found in the table.

Application Works? Version
OpenVPN Community (Windows) Yes 2.4.9 on Windows 10
Passepartout Yes 1.12.0 (2390) on iOS
Viscosity (Windows, macOS) Yes 1.8.6 (Windows, macOS)
Tunnelblick (macOS) Yes 3.8.3a (build 5521)
OpenVPN for Android Yes 0.7.16
OpenVPN Connect (iOS) Yes 3.2.0 (3253)
OpenVPN Connect (Android) Yes 3.2.2 (5027)
Linux Yes Modern Distributions

This looks good! With modern Linux distributions we mean Fedora, Debian 10, Ubuntu 20.04, and any other distribution or OS with OpenSSL >= 1.1.1.

It seems we should be able to update to TLSv1.3 and EdDSA in the next major version of eduVPN / Let's Connect!. The eduVPN / Let's Connect! apps are based on OpenVPN (Community) and the TunnelKit library used by Passepartout. We'll even keep supporting the standard OpenVPN clients!