What kind of encryption is used?

Authentication uses a 4096 bits RSA key. (3072 bits on oldest servers)
The current recommended key size considered safe until 2030 is 2048 bits.

VPN traffic encryption is performed with the best cipher available to OpenVPN (with a recent version AES 256 GCM) using a random 256 bits key re-generated regularly, and unique to a VPN connection.

Key Exchange uses 3072 bits Diffie-Hellman parameters.
A 2048 bits key is considered safe until 2030.