Commits on Jun 5, 2020

1. Safeguard from potential attacks against OCB2 …

   OCB2 is known to be broken under certain conditions:
   https://eprint.iacr.org/2019/311
   
   To execute the universal attacks described in the paper, an attacker needs
   access to an encryption oracle that allows it to perform encryption queries with
   attacker-chosen nonce. Luckily in Mumble the encryption nonce is a fixed counter
   which is far too restrictive for the universal attacks to be feasible against
   Mumble.
   
   The basic attacks do not require an attacker-chosen nonce and as such are more
   applicable to Mumble. They are however of limited use and do require an en- and
   a decryption oracle which Mumble seemingly does not provide at the same time.
   
   To be on the safe side, this commit implements the counter-cryptanalysis
   measure described in the paper in section 9 for the sender and receiver side.
   This way if either server of client are patched, their communication is almost
   certainly (merely lacking formal proof) not susceptible to the attacks described
   in the paper.

   

   Johni0702 authored and Krzmbrzl committed Jun 5, 2020
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