The answer is definitely the first two, maybe the third.
In the first case, TrustyKeys generated Key AB.
That's the key used to protect these messages,
so any message over this channel that TrustyKeys can intercept it can read.
TrustyKeys can impersonate any customer because it has a key shared with them.
It can generate a fake channel, make it seem like Alice is talking to Colleen
but pretend to be Colleen in that communication.
And finally, if there's a malicious party C,
well, maybe they could tamper with messages along these channels.
I didn't provide enough details on what we're using for encryption here
and what mode of operation is used and other details
to know whether or not that's possible.
Let's assume we could solve this problem.
These other two are really big problems, and there's no way to solve these
if we're using a third party to generate our keys.
We also haven't solved the key distribution problem
because we still needed to set up all these shared secret keys
between TrustyKeys and every other party.
We haven't really solved our problem.
This would only work if everyone completely trusted TrustyKeys,
which seems unlikely, even with the green box,
and if there was a secure way for everyone to establish
a shared secret key with TrustyKeys.
