Re: Packet number encryption

[Jana Iyengar <[e-mail]>]

I'm going to try a different attempt at moving towards
convergence. Thinking about this some more, and picking up something that
Mirja said way earlier in this thread, I think the following decomposition
is useful.

1. In the transport, all packet numbers start at 0, increasing
monotonically. ACK frames carry these packet numbers, and therefore do not
have to span large gaps and do not have to deal with increases in the size
of the largest acked. Basically, no gaps are visible within the transport
processing engine.
2. Before sending a packet on the wire, and before processing the packet,
QUIC transforms the packet number with some function, replacing the visible
packet number with the transformed value.

I think we can all agree that (1) is goodness. A sender is also free to use
packet numbers from non-contiguous spaces here FWIW, but that's entirely
independent of what's visible on the wire. This helps me separate transport
processing complexity from the rest of it, which I think is helpful.

I think we are disagreeing on the precise properties we want from the
transformation in (2). If we can agree on these properties, we can then
figure out whether it's possible and how to construct such a transform.
Here's a union of what folks are concerned about:
1. Packet number must be unlinkable across connection ID change (for
migration.)
2. Packet number must start at an arbitrary value, to avoid ossification of
the first packet number.
3. Some sequencing information -- a few bits of the packet number perhaps
-- should be revealed (for monitoring. Number of bits TBD.)
4. Any packet number transformation should not be compute intensive.

I'm not looking for a construction, but I'd like to agree on the problem
first. Does this sound like the set of properties we want? Or is there a
contradiction among these properties that I'm not seeing?

On Tue, Feb 6, 2018 at 9:56 AM, Mike Bishop <[e-mail]> wrote:

> Packet number encryption *does* improve linkability equivalently to random
> jumps, but without the fragmentation of the ACK packet which those jumps
> otherwise cause.  So I'm with Stephen, we don't yet have agreement on that
> assertion.
>
> -----Original Message-----
> From: QUIC [mailto:[e-mail]] On Behalf Of Stephen Farrell
> Sent: Tuesday, February 6, 2018 7:35 AM
> To: Mirja Kühlewind <[e-mail]>; Mikkel Fahnøe
> Jørgensen <[e-mail]>; Brian Trammell (IETF) <[e-mail]>;
> Jana Iyengar <[e-mail]>
> Cc: Praveen Balasubramanian <[e-mail]>; QUIC WG <[e-mail]>;
> Roberto Peon <[e-mail]>; Lubashev, Igor <[e-mail]>; Salz,
> Rich <[e-mail]>
> Subject: Re: Packet number encryption
>
>
>
> On 06/02/18 15:23, Mirja Kühlewind wrote:
> >
> > It's also my understanding that we do agree that packet number
> > encryption does not help likability (and thereby privacy) a lot, or at
> > least that the benefits we might get (or not) do not justify the
> > additional complexity.
>
> FWIW, which isn't much, I don't (yet) agree to the above.
> I reckon it'll be a while before we know how linkability pans out. I do
> agree that packet number encryption is not a panacea for unlinkability, but
> it may help, or may even end up being required, to do a good job wrt
> linkability.
>
> I also heard people say it was less complex for them so "additional
> complexity" may also not be quite right. (I guess "differently complex" is
> correct though.)
>
> S.
>
> --
> PGP key change time for me.
> New-ID 7B172BEA; old-ID 805F8DA2 expires Jan 24 2018.
> NewWithOld sigs in keyservers.
> Sorry if that mucks something up;-)
>