[openssl-users] DTLS handshake in WebRTC

[Matt Caswell]

On 01/03/17 05:55, Suman Paul wrote:
> I have been looking at WebRTC DTLS handshake and don’t understand the
> logic of how it works.
> 
> My Firefox client has support for both RSA and ECDSA ciphers while my
> DTLS server only supports DHE-RSA-AES128-SHA and has a RSA key. I see
> that Firefox sends a ECDSA key during client hello. What ends up
> happening is that DHE-RSA-AES128-SHA is selected. I would have
> expected the negotiation to fail due to there being no common
> ciphers.
> 
> I also verified this behavior using the OpenSSL s_server and s_client
> utilities. Seems to me that as long as s_server has a cert and key of
> the type of cipher I enforce with ‘-cipher’ option the negotiation
> succeeds irrespective of the type of key the s_client (provided that
> cipher is also supported by the client).

Your terminology is slightly confusing. No keys are sent in the
ClientHello at all. You should see a list of all the ciphersuites that
the client supports being sent in the ClientHello and then the server
should respond with a ServerHello which picks a ciphersuite from that list.

Matt