[Taler] reduce attack surface (Case 2)

[Fabian Kirsch]

Dear all,

in the refreshing process the link creation relies on the customer to 
provide the correct E_\gamma.
This is because only E_i (i \neq \gamma) gets checked.

So the mint could earn some black market money by providing hints on 
gamma or even predictable gamma selection.
The customer can therefore use a foreign Cp(gamma) for which he does not 
know Cs(gamma)
Then the customer can provide correct E_i for all i \neq \gamma.
The mint has clean records for the audits.
The customer successfully broke the link and performed a hidden 
transaction to the owner of Cs(gamma)

We have to find a source for the selection of gamma which is not in the 
hands of a possible tax evader.

Greetings
  Fabian