On Mon, Jul 11, 2016 at 7:56 PM, Hugo Krawczyk <h...@ee.technion.ac.il> wrote:
> > > On Mon, Jul 11, 2016 at 9:13 PM, Martin Thomson <martin.thom...@gmail.com> > wrote: > >> Not taking any position on the question, which I think is a fine thing >> to ask, but... >> >> I'd just like to point out that the example is flawed in the sense >> that the system permits both users to share state. When Alice logs >> out, that needs to include any state that might have been accumulated >> to Alice. This necessarily includes any sessions, including that TLS >> connection. >> >> If we imagine that this is a browser, then you also need to flush >> caches and remove cookies before the system is usable by another user. >> There might be operating system level things as well. Machines in >> internet cafes often create temporary accounts, or even rebuild the >> entire machine between users for this reason. >> >> Back to the question... >> One challenge with this is that exporters are often used to compare >> things. For instance, one side signs an exported value, the other >> validates the signature by independently exporting the same value. >> Getting different values for a particular exporter will cause some >> classes of things to fail in subtle ways. >> > > This is unrelated > to the issue > s > raised by Douglas > , but if the exporter *key* is intended for use as a unique session > identifier > > (or a sort of "channel binding") then calling it a "key" is misleading. > For example, while a key of 128 bits is perfectly fine (e.g. for AES-128), > such length is insufficient as a channel binding string (where resistance > to birthday attacks seems necessary). I do not see a note on this in the > TLS document or RFC 5705. > I filed the following issue so we would remember this: https://github.com/tlswg/tls13-spec/issues/540 -Ekr > Hugo > > >> >> On 12 July 2016 at 05:39, Douglas Stebila <dsteb...@gmail.com> wrote: >> > Some of the discussions I've had with people about post-handshake >> client authentication have raised the question of whether application >> traffic secrets should be updated automatically upon post-handshake client >> authentication: the thinking being that every change in context should be >> accompanied by a change in keying material. I used to think that was a >> good idea for TLS 1.3, although it was recently argued to me that if we >> view the application traffic secrets as being "internal" to the TLS >> protocol, then the change in client authentication status doesn't change >> the confidential or integrity properties of the record layer, it just >> serves as a "marker" to the application that certain portions of the >> application data were associated with certain authentication contexts. I >> was convinced that this can be safely accomplished without a change in >> application traffic secret key material. >> > >> > But I'm not sure that the same applies to *exporter* keys. Should >> exported keying material change as the authentication context changes? >> > >> > Consider a long-lived TLS connection, where different users come and >> go. For example, a web browser on a public terminal may have established a >> long-lived TLS connection to a particular website, and send subsequent >> requests to the same website over the same TLS connection. Now imagine two >> users use the terminal one after another: >> > >> > 1: initial handshake on a public terminal >> > 2: [time passes] >> > 3: Alice starts browsing >> > 4: Alice does post-handshake client authentication >> > 5: Alice purchases something >> > 6: Alice hits "logout" at the application layer >> > 7: [time passes] >> > 8: Bob starts using the terminal >> > 9: Bob does post-handshake client authentication >> > 10: Bob purchases something >> > 11: Bob hits "logout" at the application layer >> > >> > TLS 1.3 will tell the application about events 4 and 9. Events 6 and >> 11 happen at the application layer rather than the TLS layer (since I don't >> think TLS 1.3 has a client-de-authentication option). But putting this all >> together, the application will learn all the correct authentication >> contexts: 1-3 is anonymous, 4-5 is Alice, 6-8 is anonymous, 9-10 is Bob, >> 11-onwards is anonymous. >> > >> > Now imagine that we use keying material exporters in on lines 5 and 10: >> > >> > 1: initial handshake on a public terminal >> > 2: [time passes] >> > 3: Alice starts browsing >> > 4: Alice does post-handshake client authentication >> > *5: Alice presses the "export keying material" button >> > 6: Alice hits "logout" at the application layer >> > 7: [time passes] >> > 8: Bob starts using the terminal >> > 9: Bob does post-handshake client authentication >> > *10: Bob presses the "export keying material" button >> > 11: Bob hits "logout" at the application layer >> > >> > Since the exporter master secret is not updated when client >> authentication changes, Alice and Bob will export the same keying material >> at steps 5 and 10. If the intended goal of this exported key is for Alice >> to obtain confidentiality in some other use, this will not be achieved, >> since Bob will obtain the same exported key. >> > >> > Now, a proviso is that RFC 5705 allows for the application to mix a >> "context value" into the export, which could mitigate this, but that is >> optional. >> > >> > So it seems to me like in at least some scenarios, exported keying >> material should be associated with authentication context. It is less >> clear to me if the same holds true for KeyUpdates. >> > >> > Douglas >> > _______________________________________________ >> > TLS mailing list >> > TLS@ietf.org >> > https://www.ietf.org/mailman/listinfo/tls >> >> _______________________________________________ >> TLS mailing list >> TLS@ietf.org >> https://www.ietf.org/mailman/listinfo/tls >> > > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls > >
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