Re: [tor-dev] RFC: obfs4 (Name not final)

[Michael Rogers <michael@xxxxxxxxxxxxxxxx>]

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On 23/05/14 13:16, Philipp Winter wrote:
> - ScrambleSuit's framing mechanism is vulnerable to this attack: 
> <http://www.isg.rhul.ac.uk/~kp/SandPfinal.pdf> In a nutshell, the
> receiver needs to decrypt the ScrambleSuit header before it is able
> to verify the HMAC which makes it possible for an attacker to 
> tamper with the length fields.  While there are probably simpler
> attacks, it would be nice to have a fix for this problem.

In the next version of the Briar transport protocol we're addressing
that problem by dividing each frame into two parts. The first part is
a fixed-length header, the second is a variable-length body. Each part
is separately encrypted and MACed. The header contains the length of
the body.

This requires two MACs per frame, but I prefer that to the
alternatives: using fixed-length frames, or using the decrypted length
field before checking whether it's been tampered with.

Cheers,
Michael
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