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

Andy Schroder info at AndySchroder.com
Sat Feb 28 09:46:15 UTC 2015

Manually quoting a reply from Andreas that was sent privately while the 
e-mail list was 2 days delayed delivering messages ....

On 02/25/2015 02:45 AM, Andreas Schildbach wrote:
> Bear in mind that the "bt:" scheme is already in use by ~700.000
> installations. If we change the protocol except just wrapping a secure
> layer, we should change the scheme to for example "bs:" (Bluetooth secure).

This bs: is not a bad idea. Is bts: any better/clearer than bs:?

> That said, I don't like the idea to fold the resource name and the
> session key into one. Resource names can be shared by multiple
> protocols, for example a merchant may publish payment requests under
> bt:<mac>/r1*and*  https://<domain>/r1. If you want to save space and
> don't need resources, you can always just use bt:<mac> and a default
> resource (bt:<mac>/) is assumed.

I'm going to agree with Andreas on this. The other thing is we are 
making the resource name derived from the public key, so we are not even 
directly sending the resource name.

> Have we decided on the use (or non-use) of a DHKE (or similar) protocol
> like Mike suggested?

We are planning to send a unique public key of the payee via NFC. See 
other e-mails now that the e-mail list finally forwarded them through 
the other day.

Now for Eric's e-mail... More below.

On 02/24/2015 09:09 PM, Eric Voskuil wrote:
> On 02/24/2015 02:50 PM, Andy Schroder wrote:
>> We can change "resource" to "Session ID" if you want.
>> I think the URL scheme should be:
>> bitcoin:[address]?r=bt:<mac>&s=<PublicKey>
> This is a question of proper URL semantics, as applied to the "bt" scheme.
>  From rfc3986 [Uniform Resource Identifier (URI): Generic Syntax]:
> "The path component contains data, usually organized in hierarchical
> form, that, along with data in the non-hierarchical query component
> (Section 3.4), serves to identify a resource within the scope of the
> URI's scheme and naming authority (if any)."
> ...
> "The query component contains non-hierarchical data that, along with
> data in the path component (Section 3.3), serves to identify a resource
> within the scope of the URI's scheme and naming authority (if any). The
> query component is indicated by the first question mark ("?") character
> and terminated by a number sign ("#") character or by the end of the URI."
> https://tools.ietf.org/html/rfc3986#section-3.3
> The question therefore is whether <key> is (1) relative to the path
> (hierarchical) or (2) independent of the path and instead relative to
> the scheme and naming authority.
> The "bt" scheme does not include a naming authority, and as such the
> question is simply whether <key> is relative to "bt" or relative to the
> path, which is <mac>. Quite clearly <key> is valid only in the context
> of <mac>, not relevant to all <mac>s.
> As such one must conclude that the proper form is:
> bt:<mac>/<key>

See my comments above.

>> 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.
> Yes.
>> 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?
> I don't believe that the BIP-70 protocol over https has any need for the
> parameter. It was only useful because the NFC/BT session wasn't secured.
> So I don't think anything is lost.

This may be true. Andreas, do you agree? I feel like there was something 
in your app where it did not currently compare the domain name to domain 
name the digital signature in the payment request used though. Maybe 
this was only for bluetooth though? However, can we trust DNS though? 
Seems like it is not too hard to get an alternate signed certificate for 
a domain name, and if you can serve false DNS and/or change TCP/IP 
routing, then your secure link can break down?

>> 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?
> Yes, but that is not the problem that non-reuse is designed to resolve.
> Reuse of the public key creates a forward secrecy problem. If 1000
> sessions are recorded, and later the private key associated with the
> reused public key is compromized, all of the sessions are retroactively
> compromised.
> Another problem is persistent impersonation. If the one associated
> private key is compromised, and nobody knows it, the attacker can not
> only monitor all transactions but can selectively steal payments (if
> they aren't signed and verified). This is BTW also a good reason to not
> use HD generation of these session keys.
> Another problem is that any payer can use the well-known public key to
> obtain payment requests.
> Another problem is that without a unique public key there is no unique
> session id, so that would need to be added explicitly on the URI.

I get what you are saying, but I don't know that 2 taps with the same 
public key is the same as 1000 uses of the same public key?

>> Since we are securing everything, can we change the message header
>> format from what Schildbach's bitcoin wallet implements to something
>> more consistent?
> Could you spell this out, I'm not familiar with the implementation, just
> the proposals.

If you'll check the proposed specification, the headers in each message 
(before the serialized payment request data is sent), are consistent 
from message to message.


>> Maybe we can create a new UUID for this secure service
>> so Schildbach's bitcoin wallet can still maintain backwards compatibility.
> That may be necessary depending on the implementation of existing
> terminals, but I'm not familiar enough to speculate.

I think we probably also want to combine new UUID's with Schildbach's 
suggestion (above) to use a new "bs:" (which I suggested maybe "bts:") 
protocol scheme.

> e
>> 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
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