Clients will now always add 87 bytes of padding to the clientRequest,
and Servers will always send the PRNG seed frame unpadded, and bundled
with the serverResponse.
Why 87 bytes? The amount of data that the server sends is 87.
This fixes#5.
Instead of including the previous secretbox in the input when
calculating the SipHash-2-4 digest used to generate the obfuscation
mask, use only the nonce. This is significantly faster, and if someone
breaks obfs4 by exploiting the low amount of input entropy between each
invocation (a counter incrementing by 1), I hope they publish the
attack on the PRF.
This breaks wire protocol compatibility.
This paves the way for having servers use the same seed for all
incoming connections, across multiple startup/shutdown cycles. As
opposed to the current situation where each Obfs4Listener will
randomly generate it's seed at creation time.
Additionally, use 256 bit seeds (128 bit SipHash-2-4 key + 16 bytes of
initial material).
In theory this is easier on the garbage collector. Probably could
reuse more of the intermediary buffers by stashing them in the
connection state, but that makes the code kind of messy. This should
be an improvement.
The same algorithm as ScrambleSuit is used, except:
* SipHash-2-4 in OFB mode is used to create the distribution.
* The system CSPRNG is used when sampling the distribution.
This fixes most of #3, all that remains is generating and sending a
persistent distribution on the server side to the client.
On second thought instead of using log.Panicf(), panic() and do the
logging with recover(). This somewhat centralizes logging in
obfs4proxy, which will be easier to change when I invariably decide to
do logging differently in the future.
This adds preliminary support for data padding by adding another layer
of encapsulation inside each AEAD frame containing a type and length.
For now, data is still sent unpadded, but the infrastructure for
supporting it is mostly there.
Additionally, use log.Panic[f]() instead of panic through out the code
so that some panics are logged.
Write timeouts are obnoxious to handle as the frame encoder state
already is updated to cover the entire payload for the Write() call
that timed out. In theory it is possible to buffer the pending data,
but that causes Write() to voilate the semantics of the interface.
Like ScrambleSuit, a random interval between 1x and 5x of additional
data from the peer is read and immediately discarded before closing.
Additionally, obfs4 will close off invalid connections anywhere between
0 and 60 seconds after it determines that the incoming connection will
never complete the handshake successfully.
* The old and the busted: obfs4-[client,server].
* The new hotness: obfs4client.
* Add obfs4.ServerHandshake() that servers need to call after a
successful return from Accept(). This allows implementations to
move the handshake into a goroutine or whatever.
