Stanza encryption

So, lets finally do it and solve Stanza Encryption!

We probably agree that FULL stanza encryption is not really a good idea, since there is always information that needs to be accessible to the server (processing hints, delay tags, recipient and sender addresses (duh). So a better approach would be partial stanza encryption, which only encrypts parts of the stanza. You can imagine this as kind of an envelope element in which sensible extension elements are added and which is afterwards encrypted and appended to the message.

If we want to create a specification that follows this idea, we may want to take inspiration from XEP-0373: OpenPGP for XMPP, which uses exactly that mechanism. So a first step to get started would be to find a way to move OX's OpenPGP Content Elements out of OX into a new XEP which generalizes this idea for arbitrary E2EE encryption methods.

Also we need to specify a white-/blacklist which dictates which elements (don't) belong into the content element / message to prevent implementation mistakes.