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Sign Introduction Information to be able to discover MITM attack

Torsten Grote requested to merge 364-introduction-responses-should-be-signed into master

The local introducee doesn't know whether each piece of information received from the introducer originates from the remote introducee or has been replaced by the introducer, i.e. whether the introducer is carrying out a man-in-the-middle attack.

The introduction protocol doesn't aim to detect or prevent man-in-the-middle attacks. We only aim to establish that if the remote identity public key is not replaced then the remote ephemeral public key, transport properties and timestamp are not replaced either. This MR adds a MAC and a signature to the introduction protocol's ACK message to fulfill that aim. See #364 (closed) for the detailed security argument.

Later, when the local introducee verifies that the remote identity public key belongs to a particular person (#513), she can also be sure that the remote ephemeral public key, transport properties and timestamp originated from that person.

Closes #364 (closed)

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