README.ec

   1 Since 2.4.0, OpenVPN has official support for elliptic curve crypto. Elliptic
   2 curves are an alternative to RSA for asymmetric encryption.
   3
   4 Elliptic curve crypto ('ECC') can be used for the ('TLS') control channel only
   5 in OpenVPN; the data channel (encrypting the actual network traffic) uses
   6 symmetric encryption. ECC can be used in TLS for authentication (ECDSA) and key
   7 exchange (ECDH).
   8
   9 Key exchange (ECDH)
  10 -------------------
  11 OpenVPN 2.4.0 and newer automatically initialize ECDH parameters. When ECDSA is
  12 used for authentication, the curve used for the server certificate will be used
  13 for ECDH too. When autodetection fails (e.g. when using RSA certificates)
  14 OpenVPN lets the crypto library decide if possible, or falls back to the
  15 secp384r1 curve. The list of groups/curves that the crypto library will choose
  16 from can be set with the --tls-groups <grouplist> option.
  17
  18 An administrator can force an OpenVPN/OpenSSL server to use a specific curve
  19 using the --ecdh-curve <curvename> option with one of the curves listed as
  20 available by the --show-groups option. Clients will use the same curve as
  21 selected by the server.
  22
  23 Note that not all curves listed by --show-groups are available for use with TLS;
  24 in that case connecting will fail with a 'no shared cipher' TLS error.
  25
  26 Authentication (ECDSA)
  27 ----------------------
  28 Since OpenVPN 2.4.0, using ECDSA certificates works 'out of the box'. Which
  29 specific curves and cipher suites are available depends on your version and
  30 configuration of the crypto library. The crypto library will automatically
  31 select a cipher suite for the TLS control channel.
  32
  33 Support for generating an ECDSA certificate chain is available in EasyRSA (in
  34 spite of it's name) since EasyRSA 3.0. The parameters you're looking for are
  35 '--use-algo=ec' and '--curve=<curve_name>'. See the EasyRSA documentation for
  36 more details on generating ECDSA certificates.