On Tuesday, 15 August 2017 18:27:27 CEST Colm MacCárthaigh wrote: > On Tue, Aug 15, 2017 at 1:55 PM, Hubert Kario <hka...@redhat.com> wrote: > > On Tuesday, 15 August 2017 00:55:50 CEST Colm MacCárthaigh wrote: > >> On Mon, Aug 14, 2017 at 8:16 PM, Hubert Kario <hka...@redhat.com> wrote: > >> ... and even today with very low > >> latency networks and I/O schedulers it remains very difficult to > >> measure that kind of timing difference remotely. > > > > simply not true[1], you can measure the times to arbitrary precision with > > any real world network connection, it will just take more tries, not > > infinite tries > > Surely the Nyquist limits apply? The fundamental resolution of > networks is finite. Clock cycles are measured in partial billionths of > a second, but even 10Gbit/sec networks use framing (85 byte minimum) > in a way that gives you a resolution of around 70 billionths of a > second. Nyquist says that to measure a signal you need a sampling > resolution twice that of the signal itself ... that's about 2 orders > of magnitude of distance to cover in this case.
Nyquist applies to a single sample, not to multiple sampling of the same signal. In other words, if I sample a signal 4 times, and once I get 0 and 3 times I get 1, then assuming uniform distribution[1] I can deduce that the signal is closer to 0.75 than it is to 1, 0 or 0.5. 1 - yes, it's a "spherical cow in a vacuum" example > >> But per the post, the > >> larger point is that it is prudent to be cautious. > > > > exactly, unless you can show that the difference is not measurable, under > > all conditions, you have to assume that it is. > > > >> > When you are careful on the application level (which is fairly simple > >> > when > >> > you just are sending acknowledgement message), the timing will still be > >> > leaked. > >> > >> There are application-level and tls-implementation-level approaches > >> that can prevent the network timing leak. The easiest is to only write > >> TLS records during fixed period slots. > > > > sure it is, it also limits available bandwidth and it will always use that > > amount of bandwidth, something which is not always needed > > Constant-time schemes work by taking the maximum amount of time in > every case. This fundamentally reduces the throughput; because small > payloads don't get a speed benefit. My point is that if I don't care about the side channel of presence or absence of the communication, then I am limited by the size of the maximal record, not amount of records I can send in a second. So my bandwidth is limited in "Transaction Per Second" sense, not actual bandwidth (measured in bytes per second) > > we are not concerned if the issue can be workarouded, we want to be sure > > that the TLS stack does not undermine application stack work towards > > constant time behaviour > > The TLS stack can take a constant amount of time to encrypt/decrypt a > record, regardless of padding length, but it's very difficult to see > how it can pass data to/from the application in constant time; besides > the approach I outlined, which you don't like. As I said in a different email, in C you pass a pointer and length, that can be returned in constant time quite easily (even in read(3)-like API). encryption is indeed much harder > Note that these problems get harder with larger amounts of padding. > Today the lack of padding makes passive traffic analysis attacks very > easy. It's extremely feasible for an attacker to categorize request > and content lengths (e.g. every page on Wikipedia) and figure out what > page is user is browsing. That's a practical attack, that definitely > works, today, and it's probably the most practical and most serious > attack that we do know works. The fix for that attack is padding, and > quite large amounts are needed to defeat traffic analysis. But that > will make the timing challenges harder. In that context: it's > important to remember; so far those timing attacks have not been > practical. We don't want to optimize for the wrong problem. True, that being said, I'd prefer if we did release protocol in which we can't poke holes before the official release... -- Regards, Hubert Kario Senior Quality Engineer, QE BaseOS Security team Web: www.cz.redhat.com Red Hat Czech s.r.o., Purkyňova 115, 612 00 Brno, Czech Republic
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