[Openvpn-users] NCP ciphers negotiation question

[Dmitry Melekhov]

Hello!


Just wrote on 2.4.1 server

ncp-ciphers AES-256-CBC:AES-256-GCM:AES-128-GCM

by mistake and then can not connect using 2.4.1 client,

because server pushed AES-256-CBC to client and it is not in ncp-ciphers
default client list.


Yes, this is documented:

"For servers, the first cipher from cipher_list will be pushed to
clients that support cipher negotiation." , but is it really negotiation,

if server pushes only one cipher to client and there are two ciphers in
list on client and server they can use and no connection because

server chooses only first from list ? :-(

I guess negotiation is choosing cipher from both sides list...


Thank you!

[Gert Doering]

Hi,

Post by Dmitry Melekhov
I guess negotiation is choosing cipher from both sides list...

Not today.

It is what is documented today: choose the first cipher in the cipher
list on the server.

In the future, "real" negotiations are planned (client tells the server
what it can do, server picks one of those ciphers if in its list of permitted
chiphers), but this is not totally easy to do within the limits of what
the openvpn handshake mechanism allows us to do. It will happen, but we're
happy that we got 2.4 with "what we have so far" out, so 2.3 users can
slowly migrate to AEAD.

gert

[Dmitry Melekhov]

Post by Gert Doering
Hi,

Post by Dmitry Melekhov
I guess negotiation is choosing cipher from both sides list...

Not today.
It is what is documented today: choose the first cipher in the cipher
list on the server.
In the future, "real" negotiations are planned (client tells the server
what it can do, server picks one of those ciphers if in its list of permitted
chiphers), but this is not totally easy to do within the limits of what
the openvpn handshake mechanism allows us to do. It will happen, but we're
happy that we got 2.4 with "what we have so far" out, so 2.3 users can
slowly migrate to AEAD.

Thank you very much for clarification!