ssl auto upgrade

[totaam]

Expanding the scope of #3299, the server can tell the client that it supports ssl and then the client may decide to upgrade the tcp or ws socket to ssl / wss.

See also #3305

[totaam]

Done in the commit above.
Caveats:

• upgrading ws websockets to wss does not work (not sure why) so it is not enabled
• still some occasional timeouts stealing the connection - replacing the timer with a thread join would be more reliable and often faster
• we could save the certificate that the server gives us and then enable ssl verification - meh

---

Example output:

$ xpra start :100 --bind-tcp=:10000 --ssl-cert=./cert.pem --ssl-key=./key.pem 

$ xpra id tcp://localhost:10000/ 
upgrading tcp socket: 127.0.0.1:56714 <- 127.0.0.1:10000 to ssl
SSL handshake complete, TLSv1.2
 ECDHE-RSA-AES256-GCM-SHA384, 256 bits
display=:100
machine-id=a9980b37933d4f86817014d8c2439a5a
pid=415233
platform=linux
session-name=xterm
session-type=seamless
uuid=fdc66c6ffa1c4706b35ee9ef5df0d4ce

[totaam]

No longer enabled by default because this causes TCP connections to servers with self-signed certificates to upgrade to SSL, then fail because the certificate is not "valid".
We could send the signature as part of the upgrade, I guess.

[totaam]

We could detect self-signed certs and refuse to auto-upgrade in that case: SO: Determine if SSL certificate is self signed using Python

[themoonisacheese]

We could detect self-signed certs and refuse to auto-upgrade in that case: SO: Determine if SSL certificate is self signed using Python

why not instead do it the ssh way and keep a list of known-good servers that may authenticate themselves using a ssc?

in SSH, connecting to a previously-unknown server displays its hostname, its host key fingerprint and if approved, adds it to .ssh/known-hosts. if the host key ever changes, ssh will throw big red warnings at the user.

i propose doing the same for ssl certs, let the user accept that this is lower security than a CA-signed cert, but still have their back wrt to changing certs. Overall, blindly trusting a self-signed cert is better for security than simply not upgrading, because having a self-signed cert still means the traffic is encrypted in-flight.

[totaam]

why not instead do it the ssh way and keep a list of known-good servers that may authenticate themselves using a ssc?
(..)

We already do that for SSL connections, it just isn't wired up for upgraded connections.
Details can be found in #3305