Re: [PATCH RFC] GnuTLS: Support TOFU certificate checking.

[Ted Zlatanov <tzz@lifelogs.com>]

On Wed, 08 Oct 2014 17:19:12 +0200 Toke Høiland-Jørgensen <toke@toke.dk> wrote: 

TH> Ted Zlatanov <tzz@lifelogs.com> writes:

>> I like the simplicity of it. It would be nice to do this inside Emacs
>> itself (it's OK if it requires some kind of `emacs --batch' call,
>> doesn't have to be immediate). Either way, the errors should tell us
>> specifically what to run from the command line in these two cases:

TH> I have an updated version where I split out the parameters into
TH> :tofu-strict and :tofu-auto, where the latter will automatically add a
TH> certificate that hasn't been seen before (and fail on mismatch only).
TH> This might be more suitable to have turned out by default.

That's very good.

TH> Removing things from the key store has to be done manually, though; but
TH> that goes for gnutls-cli as well (and even for ssh).

The certificate management UI will take care of that.

>> I think so. But Emacs creates a Emacs-specific homedir for the GnuPG
>> keychain, for verification of the package archives, in
>> `~/.emacs.d/gnupg' which is an equally valid approach. So I don't have
>> a strong opinion.
>> 
>> What's the drawback of having a dedicated Emacs store? Do any other
>> programs besides `gnutls-cli` use the global GnuTLS store?

TH> Well, any programs that use gnutls and pass NULL as the trust store will
TH> share the site default. Using this also has the nice side effect of not
TH> having to come up with a portable way to find a suitable file name (I'm
TH> sure this is solved elsewhere in the emacs code but it saved me from
TH> going looking ;)).

Emacs has this function already, e.g. `(locate-user-emacs-file "certs")'

I think it's better to make the store private than shared by default, so
I'd just give the user the choice to use nil (translated to NULL in C).

>> I think that's pretty tricky with GnuTLS because it expects all the
>> validations to be C callbacks and just hands off the connection at the
>> end. You're not supposed to interact with the session during the
>> validation, IIUC. So it will probably require two attempts.

TH> Noted. I checked what gnutls-cli does (by way of packet dumps), and it
TH> seems to keep the connection open, presumably in the middle of the
TH> handshake, while waiting for the user to decide whether to trust it. So
TH> presumably something similar could be done by Emacs, and I think it's
TH> more a matter of whether or not it's possible to call back up into lisp
TH> from this part of the code.

That would be great, please see how far you get with the exploration.
Your contribution is already very useful so I am excited to see it evolve.

>> Yes, that would be nice and clean. They can simply be attached as
>> symbol properties to the error. Maybe you can adjust
>> `gnutls_make_error'?

TH> I will look into it. I'm out of time to hack on this for a while, so for
TH> now I'll just resubmit the patch with the changes noted above, and then
TH> return to this at a later date (heh, famous last words).

OK; Lars and I will probably work on it as well as time allows in order
to get something into trunk.

Ted