| Since 2.4.0, OpenVPN has official support for elliptic curve crypto. Elliptic |
| curves are an alternative to RSA for asymmetric encryption. |
| |
| Elliptic curve crypto ('ECC') can be used for the ('TLS') control channel only |
| in OpenVPN; the data channel (encrypting the actual network traffic) uses |
| symmetric encryption. ECC can be used in TLS for authentication (ECDSA) and key |
| exchange (ECDH). |
| |
| Key exchange (ECDH) |
| ------------------- |
| OpenVPN 2.4.0 and newer automatically initialize ECDH parameters. When ECDSA is |
| used for authentication, the curve used for the server certificate will be used |
| for ECDH too. When autodetection fails (e.g. when using RSA certificates) |
| OpenVPN lets the crypto library decide if possible, or falls back to the |
| secp384r1 curve. |
| |
| An administrator can force an OpenVPN/OpenSSL server to use a specific curve |
| using the --ecdh-curve <curvename> option with one of the curves listed as |
| available by the --show-curves option. Clients will use the same curve as |
| selected by the server. |
| |
| Note that not all curves listed by --show-curves are available for use with TLS; |
| in that case connecting will fail with a 'no shared cipher' TLS error. |
| |
| Authentication (ECDSA) |
| ---------------------- |
| Since OpenVPN 2.4.0, using ECDSA certificates works 'out of the box'. Which |
| specific curves and cipher suites are available depends on your version and |
| configuration of the crypto library. The crypto library will automatically |
| select a cipher suite for the TLS control channel. |
| |
| Support for generating an ECDSA certificate chain is available in EasyRSA (in |
| spite of it's name) since EasyRSA 3.0. The parameters you're looking for are |
| '--use-algo=ec' and '--curve=<curve_name>'. See the EasyRSA documentation for |
| more details on generating ECDSA certificates. |
