Re: [TLS] Fully encrypted and authenticated headers (was Re: Encrypting record headers: practical for TLS 1.3 after all?)

[Watson Ladd <[e-mail]>]

On Wed, Dec 2, 2015 at 10:34 AM, Jacob Appelbaum <[e-mail]> wrote:
> On 12/2/15, Eric Rescorla <[e-mail]> wrote:
>> On Wed, Dec 2, 2015 at 1:07 AM, Bryan Ford <[e-mail]> wrote:
>>
>>> On 02 Dec 2015, at 06:02, Martin Thomson <[e-mail]>
>>> wrote:
>>> > On 1 December 2015 at 08:22, Bryan A Ford <[e-mail]>
>>> > wrote:
>>> >> The 2-byte length field in each record's header no longer indicates
>>> >> the length of the *current* record but instead indicates the length of
>>> >> the *next* record.
>>> >
>>> > Ensuring that you know the length of the *next* record is difficult
>>> > and could dramatically degrade latency, or adding extra bogus padding
>>> > or extra bogus records.  For instance, I can always send in bursts of
>>> > two packets, a one octet packet that promises the remainder of the
>>> > burst and one that promises a single octet packet.  At that point, I
>>> > get to do what I've always done and you have gained little other than
>>> > an increase in packet size of around 19 octets (best case).
>>>
>>> That type of inefficiency is extremely easy to avoid; please read the
>>> rest
>>> of my proposal where I discussed exactly that at length.  Yes, a
>>> particularly stupid implementation could send everything in bursts of two
>>> packets, but it’s ridiculously easy for a slightly smarter implementation
>>> to avoid doing that.  And what you’ve gained is complete encryption and
>>> integrity-checking of the whole TLS record before any part is
>>> interpreted,
>>> which seems like a nontrivial security improvement.
>>
>>
>> It's not really clear to me what the anti-traffic-analysis benefit of your
>> proposal
>> is over and above just padding everything to a fixed size. That's certainly
>> far
>> easier for the implementation to do, especially in typical stacks where the
>> application just calls SSL_Write (or whatever) and so there's no obvious
>> API point to provide the "next length", so as a practical matter the stack
>> will very often if not always be in "last block" mode.
>>
>
> I think that it eliminates all static distinguisher in the protocol
> for all data covered by the encryption. That is a fantastically
> wonderful benefit.

What's a "static distinguisher"? Padding solves this problem as well,
but it also solves problems resulting from TCP segmentation down the
stack, which header encryption doesn't. What does header encryption
offer that padding does not?

Sincerely,
Watson

-- 
"Man is born free, but everywhere he is in chains".
--Rousseau.