Re: [saag] On PKI vs. Pinning (SAAG 108 preview)

[Stephen Farrell <[e-mail]>]

Hiya,

Good topic.

On 28/07/2020 20:13, Benjamin Kaduk wrote:
> Can we make a classification of what types of protocols are naturally
> suited for PKI scenarios and what types of protocols are naturally suited
> for pinning?

FYI, in our survey of covid-tracing Apps most but not all
of them do pin. When those apps are uploading keys, it'd
not be good for that to be logged in an enterprise security
appliance. That seems to generalise to any applications
where the relevant entities would not like their data to
end up logged like that (by accident or design), and where
the application not working when behind a MITM is ok, or
where one can expect the application to be whitelisted by
such MITMs. ISTM that may be a fairly broad class of
applications, certainly enough to be of interest. I'd
guess some of the operators of MITMs might prefer to not
be able to see some of that application data too.

> 
> If you're going to pin, how does it matter if you pin a (raw) public key, a
> self-signed non-CA cert, an arbitrary cert, an intermediate CA, a root CA,
> etc?

Raw public key seems too much a foot-gun. Even with
pinning, you still need to depend on a CA for TLS, so
it seems to make sense to pin to your currently
preferred CA(s) plus one other independent CA. We've
seen that in at least one of the above apps.

I'd guess the choice of root vs. intermediate might
depend, not sure there's an obviously correct choice
for that, for all cases.

> 
> Is there benefit in arranging for a description of the system where the
> ability to pin is just a degenerate case of a PKI, with a single trust
> anchor and no real hierarchy?

I don't get the question sorry.

> Do we want all protocols that specify one to be usable with the other (and
> document it that way)?
> 
> Is there utility in writing a document to cover any of the above topics?

I think such a document would be useful. If it doesn't
already exist then doing that as an informational RFC or
BCP would be useful. If one exists already that's good
enough then pointing at that may be fine.

Cheers,
S.