On Wed, Aug 21, 2013 at 10:44:40PM +0200, Sebastian Wiesinger wrote:
> I just did, here is the PCAP:
>
> http://www.karotte.org/smtp-gmx.pcap
The client sends an "internal error" alert. It is not clear what
problem it is encountering. The server elects:
Cipher Suite: TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA (0xc009)
and the client purports to support the curve in the server certificate.
I don't have the expertise to try to debug the server's key exchange
message, but it it is typically secp256r1 aka prime256v1, which the
client purports to support.
> > There are no known practical attacks on 256-bit EC keys and 384-bit
> > EC is slower. AES-128 with EC-256 is sufficiently secure for SMTP
> > TLS. Though I expect that if the sender has trouble with 384-bit
> > EC, they'll have trouble with EC in general.
>
> I found no real guidance in regards to EC so I chose a higher one.
It may be overkill, but it should work. I am afraid the best path
forward is for GMX to debug this with their client software.
--
Viktor.
