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

[Eric Voskuil <eric@voskuil.org>]

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On 02/23/2015 09:53 PM, Andy Schroder wrote:
> 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?

Because the presumption was that there was not an additional secret in
the URI. If the public key is reused then anyone can spoof a payer and
obtain payment requests.

Adding a secret to the URI can resolve this, as long as it is encrypted
with the public key before being transmitted back to BT. Otherwise the
secret can be intercepted and replayed to the terminal, encrypted with
the well-known public key.

So if you want to treat the "resource" as a secret this would work.
However the resource was designed as a public session identifier,
leading the byte stream. This changes it to private session identifier,
which loses some utility.

Also, reuse of the public key introduces a forward secrecy problem and
the potential for persistent seller impersonation in the case of
undiscovered key compromise.

But there's really no benefit to reusing the key. An ephemeral key
resolves these issues and can also seed the public resource name.

> 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)

Don't you have someone stop by the pump once a week and empty out the
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.

With a secure channel that identifies the parties by proximity, the
reason for the payment request signature is for the payer to obtain a
non-repudiation guarantee. But it also serves as a defense-in-depth
solution to a compromise of the channel (though does not offer a benefit
in the case of seller terminal/cert compromise).

> 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
> wallet-to-wallet.

In that case the cert provides no benefit. A self-signed cert can be
repudiated and if the channel is compromised anyone can sign the payment
request.

> It sounds like you are saying I'm proposing the customer is going to
> need a certificate signed by CA? If so, why? 

This was not a serious proposal, it was to point out what would become
necessary if the payer could not be identified by proximity.

In the case where a public key is reused, any payer can contact the BT
terminal and obtain the payment request. If the merchant can't rely on
proximity (i.e. can't trust the integrity of the NFC connection) then he
would have to fall back on some other means of identifying the payer. A
mutual verbal/visual confirmation could work, but the point of of NFC+BT
is elimination of that hassle.

Yes, it sounds a bit wild, but I have seen on this list a serious
proposal to have people broadcast their photo, having the merchant
select them and push to them the payment request. Of course anyone can
spoof another's image, so at some point your image would need to be
certified, and hence a CA.

I wouldn't go there, but was just making the point.

> I don't need this for any https website I visit.

When you go to a web site you first establish a private communication.
The site doesn't know who you are (hopefully). Then you log on with your
secret, or proof of it, establishing who you are. Customer identity
problem solved.

Or you create an account, providing your relevant identity information
which effectively becomes who you are to the site.

Or you shop anonymously and when you go to check out they know that if
you pay, you get permission to direct the product shipment. And only you
can see the bill. This because your session binds your shopping to your
bill and payment.

However when you go to the local adult shop to pick up some love toys,
the person at the counter has no idea who's asking their terminal for a
payment request. You having the shop's public cert doesn't help them
with that problem (nor does some anonymous signal sending them a photo
of you). Protecting your privacy ironically requires that they know who
you are - electronically. That means some sort of crazy consumer cert
(not sure that would fly in the love shop)... or trust in
(electronically anonymous) proximity.

> 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.

The payment request is private. It's a (potentially signed) proposal to
contract. It can contain interesting information.

> Those bitcoin addresses are never used. It's just
> like a shopping cart on a website where someone aborts payment and
> cancels the order.

Very much so, but in that case your neighbors can't read your potential
transactions because your session is secured.

> 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).

It can work, but you just end up putting an additional value on the URI
(for watchers), requiring legacy addresses (for non-watchers), adding
P2SH scripts to the BT broadcast of the public key, and adding another
BT round trip to obtain a public key before establishing the session.

A few bytes on the NFC tap is a non-issue, especially in comparison to
the additional complexity and BT traffic. Those choices are really all
based on providing private offline transaction support originating from
generally not private QR code scanning. But QR+BT is not the same as NFC+BT.

Honestly I think it would be reasonable to use the technique with QR+BT,
accepting the limitations for the legacy system while not unduly
burdening NFC+BT just for an unachievable cross-consistency goal. Always
passing the key on the URL for NFC but giving a non-NFC wallet the
option to ask a BT terminal for a public key seems not just reasonable
but optimal if we want to support the QR+BT scenario.

Note also that the BT-only scenario is different as well (see recent
discussion on Airbitz BLE wallet, resulting in the RedPhone-based
proposal). And finally, QR-only and NFC-only are also different. The
URIs can be consistent, but the communication protocol will vary.

> 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.

Yes, but even with a manual button you could have these problems. The
data transfer needs to be proximate as well.

> 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?

I think this question is covered above.

> 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. 

It doesn't actually add another step to the protocol, just some
different but simple code on each end. The only downside is that it
extends the NFC URL about 23 characters.

> If we do this, can we then safely get rid of the h= parameter?

Absolutely, and I believe Mike ack'd this on a previous post today.

> That should make Mike Hearn happy, and also may alleviate the base64url
> debate?

Others may not be aware of the encoding squabble (not sure if it
qualifies as debate). In the proposed URL, it affects the mac address
and the key:

bitcoin:[address]?bt:<mac>/<key>

base58:
bitcoin:?r=bt:12rAs9mM/234KF8hPkXq5pa6pT1wnJC3hVH7W6yB2Wtw24ztzGtBc4

base64url:
bitcoin:?r=bt:ABBgss5s/A3xWlhB1GI_t2NMR9Zq9E47hZOzmZ6eZTS8sbq-liugh

I prefer base58 because it's available to all bitcoin libraries, nearly
as compact as base64 (+1 byte in our example) and better standardized.
Some embedded device people might care about having to incorporate
base64 as well as base58.

It's also better looking (no - or _ characters) and more consistent in
the proposed URL (all three values would be base58, as opposed to one
base58 and two base64url). There may be some idea that base58 is just
for bitcoin addresses (not true) or designed for humans... that's sort
of the point, but it's also good for URLs.

e

> 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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