Re: [from ekr] More on key isolation/netflix use case

On Tue, May 15, 2012 at 10:07 AM, Seetharama Rao Durbha <
S.Durbha@cablelabs.com> wrote:

I have a similar observation on the discussion of key isolation or hiding
> the key from JS in yesterday's call. In my opinion, any key generated by
> the JS (as a result of invoking the API) should be accessible to the JS.


This seems overly permissive to me. Maybe geolocation-style access control
UI is out of scope for this API, but it seems conceivable to me that an
application could, for example, want to have JS generate a signing key that
requires some user interaction to use. It would be useful to be able to
guarantee that an xss or other malicious JS could only use the key as a
signing oracle (which requires user interaction), and not be able to
actually steal the key itself.

Emily


-----Original Message-----
> From: Wendy Seltzer [mailto:wseltzer@w3.org]
> Sent: Monday, May 14, 2012 11:53 AM
> To: public-webcrypto@w3.org
> Subject: Fwd: [from ekr] More on key isolation/netflix use case
>
> (I'm not sure why this didn't go through directly, since Eric is
> subscribed as an Invited Expert -- perhaps with a different email
> address?)
>
> -------- Original Message --------
> Date: Mon, 14 May 2012 16:07:28 +0000
> From: Eric Rescorla <ekr@rtfm.com>
>
>
> The Netflix use case document posted by Mitch shows an example of a DH key
> exchange designed to create a secure key between Alice and Bob without the
> JS getting it.
>
>    To support Diffie-Hellman key exchange using WebCrypto, we might do
> something like this:
>
>    // In this example, we use the following webcrypto APIs:
>    // DiffieHellman object ctor
>    //   DiffieHellman(p, g)
>    //
>    // (member function) generate() internally creates 'a' & returns 'A'
>    // 'a' is never visible in Javascript
>    //   generate()
>    //
>    // (member function) computeSS() takes 'B' & calculates 'ss'
>    //   computeSS(B)
>
>    // example usage of above APIs to create 'ss'
>    var dh = new DiffieHellman(p, g);
>    var A = dh.generate();
>    // we now send 'p', 'g', and 'A' to the server which responds with 'B'
>    // after receiving 'B' we generate 'ss' which stays inside our dh object
>    dh.computeSS(B);
>
>    At this point, we have created a shared secret which is inaccessible
>    to Javascript, but we can't yet do anything useful with it. In order
>    to transform the shared secret into something usable we need to use a
>    key derivation algorithm (RFC 2631? or something simpler?) to compress
>    or expand the keying material 'ss' to keying data which is the
>    appropriate size for some other algorithm."
>
> I agree that this creates a shared secret not known to the JS, but what
> stops the JS from mounting a MITM attack. I.e., it generates it's own DH
> key pair (c, C) and provides C to boththe local browser and the remote end.
> At this point, it shares K_ac with the browser and K_bc with Bob. Absent
> some method for verifying that a DH share came out of a compliant browser,
> it's not clear to me what security benefit has been achieved here.
>
> -Ekr
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