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[tor-dev] ScrambleSuit's replay protection incomplete

In short: The implementation of ScrambleSuit's replay protection is
incomplete which means that an active adversary can circumvent it.  All
the credit for this discovery goes to Lasse Øverlier.

ScrambleSuit uses Uniform Diffie-Hellman as one of its authentication
mechanisms.  To defend against replay attacks, a sever caches the HMAC
of a client's authentication message.

The attack works as follows.  In the first step, an active adversary
(e.g., a censor trying to detect ScrambleSuit) observes a client
authenticate successfully towards a ScrambleSuit server and captures the
server's Uniform Diffie-Hellman response.  In the second step, the
adversary replays the captured response to the very same server.  Since
the server did not cache the HMAC of its own response, it will interpret
the replayed data as legitimate authentication message of a new client
and respond with an authentication response.  The adversary now
successfully tricked the server into responding despite not knowing the
shared secret.  This creates a noteworthy distinguisher which can help
identifying ScrambleSuit.

Luckily, it's easy to fix this problem.  Introducing message types would
be one option but it would break backwards compatibility.  The easiest
fix which retains backwards compatibility is to make the server also
cache its own HMACs which are part of the response to a client's
authentication message.  The downside is that it doubles the size of the
replay table but that's tolerable.

Note that obfs4 is not affected by this problem because a client's and a
server's authentication message are different.

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