On Mon, Jan 13, 2014 at 2:37 PM, Roy Badami <roy@gnomon.org.uk> wrote:

> That does require trusting the third party not to later tamper with
> the payment request, though.


You have to trust the billboard owner too. If you're relying on a third
party to relay a payment instruction, that will always be an issue, hence
the signing.

Signing a payment request for an individual is easy, anyway, depending on
the kind of ID you want. If you want to sign with an email address, just go
here with a browser like Chrome/Safari/IE that uses the system keystore:

   http://www.comodo.com/home/email-security/free-email-certificate.php

They'll send you an email, you click the link to verify, and a cert will be
generated and installed by your web browser. It's actually easier than
signing up for a website. There are lots of other places that do it for
free too, I just picked the first one from a google search for [free email
certificate].

Once you've got that in your keystore, a wallet app can quite easily be
told to sign payment requests with your email address.

For a billboard I guess you'd probably be an organisation or company
instead, though an email address can work there too as long as you have a
well known domain name.