Re: [TLS] Should exporter keys be updated with post-handshake authentication and/or KeyUpdate?

[Hugo Krawczyk <[e-mail]>]

On Mon, Jul 11, 2016 at 9:13 PM, Martin Thomson <[e-mail]>
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.

Hugo


>
> On 12 July 2016 at 05:39, Douglas Stebila <[e-mail]> 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
> > [e-mail]
> > https://www.ietf.org/mailman/listinfo/tls
>
> _______________________________________________
> TLS mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/tls
>