I think were talking about a lot of the same things. There is one key piece of information that I was not thinking about until you made it clear. Why the payee needs to identify the payer. In my fuel pump application, they really don't, so I wasn't thinking closely about these other situations. With my fuel pump, it won't even let you do anything until you sign a transaction and submit it. So, the payment request contains no personal information, it's just a request for payment, and not for anything specific. It doesn't know or care what you are going to buy until you make a prepayment, because there is no use in trying to start doing business without a signed transaction. This approach minimizes risk because once you dispense a fuel, or anything else, it's not like you can easily give it back if you happened to not have any funds. It also makes it a higher chance for a confirmation before the customer leaves. Other transactions have similar post payment traditions, like a restaurant (not fast food), where the seller doesn't know if you actually have enough money until you've already consumed the food, but this work flow seems to be a culturally driven one rather than risk driven.

In the discussion about an https website, there are many websites where no login or authentication by the client required to have a private connection. With a shopping website though, the customer can identify themselves without logging in by saying (in band) what they are intending to buy after the private connection has been established. At a cash register in person the items being purchased have no tie to the customer. The items being purchased were communicated to the seller out of band (in real life) and insecurely to establish that link. You are trying to make a tie between that list of items and the buyer separately, and that is why some unique identifier needs to be transmitted via NFC.

Stepping a bit back: I guess I'm wondering if it would be useful to encourage an opposite work flow where a micro payment channel is setup for most purchases. For example, if I go to the grocery store, it usually takes a minute or so to check out. If I immediately tap and open up a payment channel with the store when I start checkout, rather than finish, there can be more network propagation of that transaction while they are scanning all the items. They'll see the channel is open and start adding all the items I want to buy to that micro payment channel. I'm identified because I made a payment, not because I've transmitted a unique resource or used a unique public key to encrypt communication. A payment terminal would not allow for new payment channels to be open until the currently active one is closed. If I don't have enough funds left in the payment channel, they just stop scanning items. There may be some additional privacy implications of setting up micro payment channels all the time for everyday purchases. It also may not work for every sales case, so we may still need some way to authenticate the payer with a unique identifier. So, maybe we don't want to do this, but it is just a thought to consider.

So, unless someone thinks what I am proposing in my previous paragraph has any potential (as a complete solution, not a complement to solutions), the plan is the following:

 * Get rid of the "h=" parameter
 * 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
 * Use a base58 encoding to save space and reduce the character set
 * 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?
 * 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?

Andy Schroder

On 02/24/2015 06:28 AM, Eric Voskuil wrote:
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
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

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
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
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:




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.


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
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:


In comparison to a symmetric key:


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.


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.

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