The Thread spec says we need those for EC J-PAKE too.
However, we won't be using the information, so we can skip the parsing
functions in an EC J-PAKE only config; keep the writing functions in order to
comply with the spec.
Especially for resumed handshake, it's entirely possible for an epoch=0
ClientHello to be retransmitted or arrive so late that the server is already
at epoch=1. There is no good way to detect whether it's that or a reconnect.
However:
- a late ClientHello seems more likely that client going down and then up
again in the middle of a handshake
- even if that's the case, we'll time out on that handshake soon enough
- we don't want to break handshake flows that used to work
So the safest option is to not treat that as a reconnect.
Let the client retry longer, to make sure the server will time out before the
client gives up. Make it really longer to get a deterministic client exit
status (make sure it has time to reconnect after the server timeout).
Don't depend on srv.c in config.h, but add explicit checks. This is more
in line with other options that only make sense server-side, and also it
allows to test full config minus srv.c more easily.
Use a custom function that minimally parses the message an creates a reply
without the overhead of a full SSL context.
Also fix dependencies: needs DTLS_HELLO_VERIFY for the cookie types, and let's
also depend on SRV_C as is doesn't make sense on client.
- interrupt the connection abruptly (no close_notify)
- reconnect from the same port while server sill has an active connection from
this port.
Some real-world clients do that, see section 4.2.8 of RFC 6347.
I'm not sure this is necessary, because it is only multiplied by xm2 which is
already random and secret, but OTOH, xm2 is related to a public value, so
let's add blinding with a random value that's only use for blinding, just to
be extra sure.
