Subject: Re: [Bitcoin-development] Positive and negative feedback on certificate validation errors

From: Wladimir <[email-masked]>
Date: Sat, 1 Mar 2014 08:26:57 +0100

On Sat, Mar 1, 2014 at 7:26 AM, Jeremy Spilman <[email-masked]> wrote:

> There's a open bug (#3628) and pull request (#3684) to provide negative
> feedback (yellow background) for a missing or invalid signature, but it
> seems like there's some debate on whether bitcoind should do that...
>

The consensus there is to treat invalid and unsigned payment requests the
same (apart from debug error logging). After all, the cost to the attacker
to remove the signature or corrupt it is exactly the same.

I do recommend testing that pull request (#3684) to see if it improves
payment request reporting, and provide testing reports or suggestions in
the github comments.

I've been very busy the last few weeks with integrating and testing other
pre-0.9 changes so I have been unable to look at the visual side of payment
request stuff much. We could use some help there.

If an attacker can avoid the negative feedback by just stripping the
> signature and setting pki_type to none, then arguably there's no security
> benefit by singling out badly signed payment requests from unsigned
> payment requests.
>

Exactly.

> So perhaps the root problem is that the positive feedback (green
> background) is not strong enough to make its absence highly conspicuous to
> the end user.
>

Well, ideas to make the difference more conspicuous are welcome. The green
background is just to make a basic distinction.

If it involves any imagery or graphics we do need contributions (with the
appropriate MIT license), no one of us is an artist.

> As an aside, how could we go about implementing the equivalent of HTTP
> Strict Transport Security for payment protocol to prevent this trivial
> signature stripping attack? Is this a possible extension field merchants
> are interested in?
>

Such a thing would be interesting for a future BIP standard. I see one
problem here: for an unsigned payment request there isn't really an
"origin". Browser URI handlers don't send the referrer either.

This rules out adding a field to the Bitcoin URI 'requests from us must be
signed from now on' (there's no us).

The server that serves the payment requests *could* serve an HSTS-like
header 'only accept signed payment requests from us from now on'. The
client needs to remember this for this server. Then if someone has
compromised that server (or hijacked DNS) to serve fake and unsigned
payment requests, the client can block these.

Neither scenario will help in the case in which the server serving the
Bitcoin URIs is compromised.

Wladimir