Server certificate hash checking

[David Woodhouse]

> So, I finally got around to integrating the new library on the Android
> side, and I was wondering how you envisioned the UI working.

Well, I give you the DER and you can do whatever the hell you like with it :)

The openconnect_get_peer_cert_details() function is just a helper in case
you want it.

> With the old API, I was able to request a SHA1 of the whole
> certificate DER and present that to the user.  They could compare the
> SHA1 to the output from e.g. "openssl x509 -fingerprint -sha1 -in
> foo.pem" or "View Certificate" in Firefox.  The known-good SHA1 could
> also be provided by a sysadmin, posted on a sticky note, or read off
> over the phone.  This fingerprinting convention is reasonably standard
> across different clients and OSes.
>
> On Android I intentionally did not add a "View Certificate" button as
> it is trivial to make a spoofed cert that has authentic data in all of
> the human-readable fields.  Allowing the user to assess risk based on
> that content creates a false sense of security.  So they are forced to
> use the hash to make a decision.
>
> But now we're getting a different hash from
> openconnect_get_peer_cert_hash(), and I'm not sure what the user
> should be comparing it against, other than asking "is it the same as
> last time?"  Unless the new fingerprinting scheme is widely used, it
> doesn't help authenticate the server when they're connecting for the
> first time.
>
> So maybe we need a new library function to return the old
> (standardized) hash, and once that is accepted through the UI, cache
> the pubkey hash and use that for future comparisons?

That seems to make sense. In fact openconnect_sha1() is already exported,
albeit under OPENCONNECT_PRIVATE, and you could just pass the DER to it. I
have no real objection to exporting it properly, or adding an
openconnect_get_peer_cert_sha1() again for this purpose. We should
probably also ensure the sha1 is included in the
openconnect_get_peer_cert_hash() output.

Nikos, what do you think?


-- 
dwmw2