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

Andy Schroder info at AndySchroder.com
Tue Feb 24 05:53:30 UTC 2015

I was saying provide a public key via NFC (or a public key fingerprint 
and then send the full public key over bluetooth). Instead of providing 
a new public key on each tap, why can't the payee just stop accepting 
connections from new parties on that "resource" after a session key has 
been received from the first person? If the person decides to have there 
friend or family pay for them instead and cancel the payment, they could 
just hit cancel on the POS or something (on my fuel pump I have a switch 
that needs to be turned, the purpose of this is to avoid wasting too 
many addresses) and/or do another NFC tap (if you're providing QR codes 
you'd still need a button of some kind though so it knows to refresh 
it), or the POS can just provide a completely new payment request to any 
new connections on that same "resource" which use a different session key.

I feel like the authentication of the payer to the payee in any future 
connections after they receive the session key from them (which was 
encrypted with the payees public key), comes from the fact that they are 
sending responses back that are encrypted using the session key they 
gave to the payee. The way I am seeing it is that the NFC tap or QR code 
scan is acting in addition to the visual name check on the signature 
verification in the wallet. If the certificate used isn't signed by a CA 
(self signed), it may be fine as long as you heard about it via NFC or 
QR code. I don't think it will require PKI and should still work 

It sounds like you are saying I'm proposing the customer is going to 
need a certificate signed by CA? If so, why? I don't need this for any 
https website I visit. It's not like the payee is sending anything to 
the payer that is private. The payment request only becomes private if 
something is actually received to it, otherwise, it is just discarded 
and it doesn't matter. Those bitcoin addresses are never used. It's just 
like a shopping cart on a website where someone aborts payment and 
cancels the order.

At one point I was thinking we could do something similar to Mike 
Hearn's suggestion in another recent e-mail where we re-use some 
existing part of the bitcoin URI to bootstrap some trust in a public key 
that the payee next sends via bluetooth after the NFC connection. Now 
that I'm reviewing my notes though, I can't see how this will work with 
a watching only wallet or if no backwards compatible (to BIP21) bitcoin 
address is presented in the URI (as Mike said).

What I was saying above about how you can stop accepting connections on 
that "resource" after a session key has been received by the first 
person could be problematic though. An evil person could just start 
making connections to every device they can, just to be mean, which 
would not allow the POS operator to receive payments from their real 
customers. If you do the other option I proposed, which is to just keep 
giving out new payment requests, you have other problems (on top of 
wasting addresses), which are that you can still have mean people giving 
you a denial of service attach on your hardware, or you could have an 
unusual situation where two people pay (don't know why they would do 
this though), so that is why I'm suggesting a manual tap or button press 
or switch turn being required.

I guess as more of a abuse filter, a new "resource" could be given 
instead with each tap, and the POS would just ignore all requests to an 
inactive resource. You may say, why not send a new public key (as you 
suggested) instead of a new "resource" with each tap (or button press if 
using QR codes), and then you can skip the sending of a static public 
key (or public key fingerprint), and ignore any data that is not 
encrypted with that public key. Maybe that is a better idea because it 
will shorten the bitcoin URI. However, I don't think its required from a 
privacy standpoint, it primarily just aids in combining the public key 
fingerprint with the changing "resource" name used to filter abuse. Or, 
am I missing something?

So, after thinking through the abuse scenarios I mentioned above, I 
think I am agreeing with you, but the reason I'm writing all this is to 
hopefully just get some feedback on my logic to learn something from 
this discussion. I do think sending a unique public key over NFC has to 
be better than a unique session key. It adds one more step, but seems to 
help. If we do this, can we then safely get rid of the h= parameter? 
That should make Mike Hearn happy, and also may alleviate the base64url 

Andy Schroder

On 02/23/2015 09:55 PM, Eric Voskuil wrote:
> Andy, adding to my previous post below:
> On 02/23/2015 01:40 AM, Eric Voskuil wrote:
>> On 02/22/2015 11:36 PM, Andy Schroder wrote:
> ...
>>> It's possible a really sophisticated modification could be done where
>>> the attacker encrypts and decrypts the communication and then relays to
>>> each party (without them knowing or any glitches detected), but I guess
>>> I'm not sure how easy that would be on such a close proximity device?
>> If the NFC tap is sufficiently private, privacy is easy to achieve for
>> the subsequent communication. If it is not, privacy can be completely
>> compromised. The question is only how much more difficult is the attack.
>> With the public cert tap, the level of difficulty is much lower for
>> capturing selected payment requests. The interloper no longer needs to
>> invade the space of the NFC terminal and can instead impersonate the
>> payer from a safe distance. Nobody gets paid, but privacy is compromised.
> This problem in the preceding paragraph can be resolved by sending a
> unique public key on each NFC tap. In that case an attacker would need
> to monitor the NFC communication.
> The talk of wrapping the connection in SSL led me to believe you were
> talking about a static public certificate. However that's not a
> necessary assumption here and may not be what you intended.
>> The level of difficulty in the case where the interloper wants to taint
>> transactions may appear lower, but it is not:
>> With the session key tap the interloper must compromise the NFC location
>> and then monitor the BT traffic. Monitoring BT traffic without being
>> party to the connection is presumably not rocket surgery, but not
>> standard BT design either.
>> With the public cert tap the interloper must also compromise the NFC
>> location and communicate over BT. Therefore the hardware and physical
>> attack requirements are similar. The only added difficulty is that the
>> attack on the NFC terminal attack is active (modifying the MAC address
>> directing the payer to the BT service).
> I believe your central claim was that the difference in the two
> bootstrapping approaches (public key vs. session key) is that by using a
> unique public key per tap, the attack requires an active vs. passive
> attack on the NFC terminal. I just wanted to make clear here that I
> agree with that assessment.
> The symmetric key approach is based on the idea that these attacks are
> comparable in difficulty and otherwise identical in privacy loss.
> However, the difference in implementation amounts to about +23
> additional encoded characters for the BT/LE URL, assuming use of the
> secp256k1 curve for DHE. This is really not a material issue in the case
> of the NFC tap. The entire URI+URL could be as small as:
> bitcoin:?r=bt:12rAs9mM/79bq48xJaMgqR9YNxnWhqHHM1JB52nxn6VFXBHTP2zrP
> In comparison to a symmetric key:
> bitcoin:?r=bt:12rAs9mM/12drXXUifSrRnXLGbXg8E
> It also does not change the protocol design or complexity at all - it
> would just swap out an AES key for a secp256k1 public key.
> bitcoin:[address]?bt:<mac>/<key>
> If that gets us aligned I'm all for it.
>> However impersonating the payer is just a matter of software - no more
>> difficult than the session key attack. In fact it may be much easier to
>> implement, as the attack can use supported BT features because the
>> attacker has directed the payer to connect to him and is connecting to
>> the receiver as if he was a payer.
>> But it gets worse for the public cert tap, since a more sophisticated
>> attacker can set himself up in the same position without subverting the
>> NFC terminal at all. By broadcasting a more powerful BT service on the
>> same advertised MAC address, the attacker can capture traffic and relay
>> it to the intended service.
> I'm retracting the last paragraph, since the interloper, without
> invading the NFC connection (by substituting the public cert), could not
> read the relayed traffic. It was getting late :/
>> So in sum, reliance on a public cert makes the communication less
>> private under the same physical set of constraints. The difference
>> results from the receiver allowing non-proximate payers to impersonate
>> proximate payers from a distance by generating their own session keys
>> and submitting them over BT.
> e

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