Subject: Re: [Bitcoin-development] Bitcoin at POS using BIP70, NFC and offline payments - implementer feedback

From: Andy Schroder <[email-masked]>
Date: Tue, 24 Feb 2015 17:50:46 -0500

We can change "resource" to "Session ID" if you want.

I think the URL scheme should be:

bitcoin:[address]?r=bt:<mac>&s=<PublicKey>

But when connecting to the mac, the client indicates the SessionID in 
the header, and as you say, SessionID is derived in some way from PublicKey.

This is a slightly different format than both of your suggestions below, 
but seems to make more sense based on what you said in your entire 
message. The other thing is it can be used with more protocols without 
taking up more space in the URL.

However, by loosing the h= parameter, I think we are now loosing some 
benefit it brought to https based connections if the customer doesn't 
want to use bluetooth. Right?

Also, you talk about a new public key (and session ID) for each tap. I 
guess I'm wondering about this though. If the public key is compromised 
on the first tap, isn't their payment request already compromised?

Since we are securing everything, can we change the message header 
format from what Schildbach's bitcoin wallet implements to something 
more consistent? Maybe we can create a new UUID for this secure service 
so Schildbach's bitcoin wallet can still maintain backwards compatibility.

Andy Schroder

On 02/24/2015 05:14 PM, Eric Voskuil wrote:
>>    * Add a "s=" parameter that uses a unique public key for each session.
>>      This public key identifies the payee to the payer and payer to the
>>      payee.
> This would be the simple model, which just tacks on another parameter to
> the bitcoin URL:
>
> bitcoin:[address]?bt=<mac>&s=<key>
>
> But we should also look at the more flexible "r#" approach from your
> existing TBIPs, which would yield:
>
> bitcoin:[address]?r=bt:<mac>/<key>
>
> and incorporate the "payment_url" list.
>
>>    * Use a base58 encoding to save space and reduce the character set
>>      slightly.
> :)
>
>>    * Get rid of the resource? If a terminal is accepting payment from
>>      multiple customers simultaneously, it should be smart enough to
>>      distinguish between customers based on the public key they are
>>      encrypting the data with. Is this approach feasible?
> Yes, it is not necessary on the URL. But an id is useful in helping the
> BT terminal identify the session without having to try all of its
> outstanding keys until it finds one that works.
>
> I proposed that the resource name ("session id" may be a better name) be
> deterministically derived from the session key. Given the design change
> to pass an EC public key it would need to be derived from that key (not
> from the session key because the receiver would not have a copy before
> decrypting the first BT message). So any function on the public key that
> reduces it to a smaller length, fixed width should be fine. Hashing it
> first may be better as is prevents disclosure of any bits of the public
> key, which should be treated as a secret during the session.
>
>>    * When you said a new public key for each tap, do you see that as
>>      every single tap, or do you consider multiple taps from the same
>>      customer the same tap?
> Yes, since there would be no other way to distinguish between customers
> in some scenarios and this is the safest approach. We certainly won't
> run out of numbers, and unused sessions can be discarded based on any
> number of criteria, including discarding all but the most recent. That
> may may be sufficient for your vending machines given there's little if
> any call for parallelism.
>
> e