Backport "Safeguard from potential attacks against OCB2" #4251
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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.
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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.
Backported from #4227