Re: Packet number encryption

[Mikkel Fahnøe Jørgensen <[e-mail]>]

Sounds reasonable to me. You could add that the transformation should be space efficient.

________________________________
From: Jana Iyengar <[e-mail]>
Sent: Wednesday, February 7, 2018 12:00:32 AM
To: Mike Bishop
Cc: Stephen Farrell; Mirja Kühlewind; Mikkel Fahnøe Jørgensen; Brian Trammell (IETF); Praveen Balasubramanian; QUIC WG; Roberto Peon; Lubashev, Igor; Salz, Rich
Subject: Re: Packet number encryption

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]<mailto:[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]<mailto:[e-mail]>] On Behalf Of Stephen Farrell
Sent: Tuesday, February 6, 2018 7:35 AM
To: Mirja Kühlewind <[e-mail]<mailto:[e-mail]>>; Mikkel Fahnøe Jørgensen <[e-mail]<mailto:[e-mail]>>; Brian Trammell (IETF) <[e-mail]<mailto:[e-mail]>>; Jana Iyengar <[e-mail]<mailto:[e-mail]>>
Cc: Praveen Balasubramanian <[e-mail]<mailto:[e-mail]>>; QUIC WG <[e-mail]<mailto:[e-mail]>>; Roberto Peon <[e-mail]<mailto:[e-mail]>>; Lubashev, Igor <[e-mail]<mailto:[e-mail]>>; Salz, Rich <[e-mail]<mailto:[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;-)