Re: Exposing TLS & Certificate Information in Javascript

On Mar 3, 2014 3:21 PM, "Lyor Goldstein" <lgoldstein@vmware.com> wrote:
>
> I was wondering about the feasibility of the following feature that could
> benefit from exposing the TLS certification chain:
>
> - Server generates a random challenge in the web page
> - Client signs a hash of the challenge + the full TLS certification chain
> - as obtained from the javascript DOM model (the proposed enhancement)
> - Server makes sure that the signature matches - using the client's
> pre-registered public key and knowledge of its own certificate
>
> In effect, we are doing a variation on certificate pinning - even if the
> MITM has obtained a CA certificate and is able to fool the client into
> believing that a trusted signed certificate is presented, it cannot
> convince the server since (a) it does not have the clients private key and
> (b) the server "knows" which certificate it provided. Therefore, even if
> the MITM can create a certificate with its own key, it cannot forge the
> server's certificate (but with its own key), thus the client's signature
> will not match.
>

What prevents the attacker from directing the client JS to sign the
original server's certificate chain in script (eg: using RSASSA)?

It would need to be an entirely new signature algorithm in order to be
secured.

If we do go down that route, it is functionally identical to what FIDO
would like to use WebCrypto for (signing a combination of 'untrusted' data
- the hash - and 'trusted' data only the UA supplies - the origin, channel
ID, etc)

Of course, to evaluate why to expose this, it requires understanding why
you want to, and whether any proposal can meet those security requirements.
For reinventing pinning, I think any proposal will be fundamentally flawed,
but perhaps you were just making the analogy for brevity.

To emphasize: I do not believe WebCrypto can or should try to prevent MITM.

> P.S. Even leaving this aside - I was wondering why is it such an effort to
> expose currently loaded page certificate chain (for read-only) ? After
> all, the browser has this information anyway
>
>

TL;DR - don't reinvent pinning in JS. Use real pinning.

Because there is no such concept as "a connection" in a web page. Its an
artifact of loading, but there is no way to reliably and consistently know
what the certificate(s) were.

There's the connection used to load the HTML. The connection used to load
the JS (since surely you're not doing inline JS). And any number of other
connections. And maybe it was cached. Or used ServiceWorker/AppCache. And
maybe the connection followed a series of redirects that involved different
servers. Or renegotiations.

The Web is connectionless.

Certificate information is fundamentally connection oriented.

And before anyone says "Use what you show in the UI for the lock", this
fundamentally ignores the use case of why you want to bind to the cert -
and how you can fundamentally subvert it.