Security and Privacy Considerations

Hi all,

I've started documenting potential security and privacy attacks against
the Web Payments API here:

https://github.com/w3c/webpayments/wiki/Security-and-Privacy-Considerations

It contains a number of attacks that I could think off just off of the
top of my head this morning. I'll try to add more attacks as I have
time. Clearly we'll need multiple thorough security reviews by experts
and the hope is that this page can prime their thinking wrt. the types
of attacks that the system we're building is susceptible to.

-- manu