You can't run a trusted client on an untrusted machine.
A hostile user has complete access to the machine state, including
memory, stack, and register values. He/she can log all communication
between the client and the server and create a fake client that
duplicates the "authentification" procedure of the real client, but
without displaying ads.
It's a lost cause, but luckily for the people running the free
Internet access programs, most users won't do this.
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