[Bitcoin-development] Proposal for P2P Wireless (Bluetooth LE) transfer of Payment URI

Eric Voskuil eric at voskuil.org
Tue Feb 10 16:55:58 UTC 2015


Martin,

I like your idea for the commit protocol in that it resolves the
vandalous address substitution attack. However, I don't see a way to
prevent privacy loss without adverse impact to the scenario.

Anyone could perform the handshake and thereby obtain the payment
request. Therefore to prevent inadvertent disclosure the customer must
visually confirm the "phrase" and then verbally tell the merchant to
proceed by sending the payment request.

One might argue that it's sufficient to preserve the integrity of the
transaction while suffering the privacy loss, especially given that a
hijacked handshake should never result in a completed transaction -
unless of course the hijacker pays.

But imagine someone purchasing their meds. HIPAA requires the checkout
queue to form behind a yellow line. That speaks directly to this question.

e

On 02/06/2015 01:07 AM, MⒶrtin HⒶboⓋštiak wrote:
> 2015-02-06 2:29 GMT+01:00 Eric Voskuil <eric at voskuil.org>:
>> On 02/05/2015 04:36 PM, Martin Habovštiak wrote:
>>> I believe, we are still talking about transactions of physical
>>> people in physical world. So yes, it's proximity based - people
>>> tell the words by mouth. :)
>>
>> Notice from my original comment:
>>
>>>>>> A MITM can substitute the key. If you don't have verifiable
>>>>>> identity associated with the public key (PKI/WoT), you need
>>>>>> a shared secret (such as a secret phrase).
>>
>> I said this could only be accomplished using a shared secret or a
>> trusted public key. Exchanging a value that is derived from a pair of
>> public keys is a distinction without a difference. The problem remains
>> that the parties must have a secure/out-of-band channel for
>> communicating this value.
>>
>> The fact that they are face-to-face establishes this channel, but that
>> brings us back to the original problem, as it requires manual
>> verification - as in visual/audible scanning of the two values for
>> comparison. At that point the visual comparison of the address, or some
>> value derived from it, is simpler.
> 
> I have never been against manual verification. What I'm trying to say
> is let's just make manual verification easier and more secure.
> Comparison of address is simpler for the coder but also simpler to
> attack. It has these problems:
> - Addresses broadcasted in plaintext (privacy issue)
> - Amounts broadcasted in plaintext (privacy issue)
> - Address is long - takes lot of time to verify (user experience issue)
> - Address prefix can be brute-forced, if too short or used to make
> "black hole" address if longer (vandalism issue)
> 
> Commit protocol can be used for both the encryption and the
> authentication while user experience is not bad and everything is
> still secure.
> 
>>
>>> In case of RedPhone, you read those words verbally over not-yet-
>>> verified channel relying on difficulty of spoofing your voice. Also
>>> the app remembers the public keys, so you don't need to verify
>>> second time.
>>
>> This is reasonable, but wouldn't help in the case of an ad-hoc
>> connection between parties who don't know each other well.
>>
>>> I suggest you to try RedPhone (called Signal on iPhone) yourself.
>>> It's free/open source, Internet-based and end-to-end encrypted. You
>>> may find it useful some day. Also I'm willing to help you with
>>> trying it after I wake up. (~8 hours: Send me private e-mail if
>>> you want to.)
>>
>> I appreciate the offer. I really don't trust *any* smartphone as a
>> platform for secure communication/data. But encrypting on the wire does
>> of course shrink the attack surface and increase the attacker's cost.
>>
>> e
>>
>>> Dňa 6. februára 2015 1:22:23 CET používateľ Eric Voskuil
>> <eric at voskuil.org> napísal:
>>
>>>> On 02/05/2015 04:04 PM, MⒶrtin HⒶboⓋštiak wrote:
>>>>> That's exactly what I though when seeing the RedPhone code, but after
>>>>> I studied the commit protocol I realized it's actually secure and
>>>>> convenient way to do it. You should do that too. :)
>>>
>>>> I was analyzing the model as you described it to me. A formal analysis
>>>> of the security model of a particular implementation, based on
>>>> inference
>>> >from source code, is a bit beyond what I signed up for. But I'm
>>>> perfectly willing to comment on your description of the model if you
>>>> are
>>>> willing to indulge me.
>>>
>>>>> Shortly, how it works:
>>>>> The initiator of the connection sends commit message containing the
>>>>> hash of his temporary public ECDH part, second party sends back their
>>>>> public ECDH part and then initiator sends his public ECDH part in
>>>>> open. All three messages are hashed together and the first two bytes
>>>>> are used to select two words from a shared dictionary which are
>>>>> displayed on the screen of both the initiator and the second party.
>>>
>>>>> The parties communicate those two words and verify they match.
>>>
>>>> How do they compare words if they haven't yet established a secure
>>>> channel?
>>>
>>>>> If an attacker wants to do MITM, he has a chance of choosing right
>>>>> public parts 1:65536. There is no way to brute-force it, since that
>>>>> would be noticed immediately. If instead of two words based on the
>>>>> first two bytes, four words from BIP39 wordlist were chosen, it would
>>>>> provide entropy of 44 bits which I believe should be enough even for
>>>>> paranoid people.
>>>>>
>>>>> How this would work in Bitcoin payment scenario: user's phone
>>>>> broadcasts his name, merchant inputs amount and selects the name from
>>>>> the list, commit message is sent (and then the remaining two
>>>>> messages), merchant spells four words he sees on the screen and buyer
>>>>> confirms transaction after verifying that words match.
>>>
>>>> So the assumption is that there exists a secure (as in proximity-based)
>>>> communication channel?
>>>
>>>> e
>>>
>>>>> 2015-02-06 0:46 GMT+01:00 Eric Voskuil <eric at voskuil.org>:
>>>>>> On 02/05/2015 03:36 PM, MⒶrtin HⒶboⓋštiak wrote:
>>>>>>>> A BIP-70 signed payment request in the initial broadcast can
>>>> resolve the
>>>>>>>> integrity issues, but because of the public nature of the
>>>> broadcast
>>>>>>>> coupled with strong public identity, the privacy compromise is
>>>> much
>>>>>>>> worse. Now transactions are cryptographically tainted.
>>>>>>>>
>>>>>>>> This is also the problem with BIP-70 over the web. TLS and other
>>>>>>>> security precautions aside, an interloper on the communication,
>>>> desktop,
>>>>>>>> datacenter, etc., can capture payment requests and strongly
>>>> correlate
>>>>>>>> transactions to identities in an automated manner. The payment
>>>> request
>>>>>>>> must be kept private between the parties, and that's hard to do.
>>>>>>>
>>>>>>> What about using encryption with forward secrecy? Merchant would
>>>>>>> generate signed request containing public ECDH part, buyer would
>>>> send
>>>>>>> back transaction encrypted with ECDH and his public ECDH part. If
>>>>>>> receiving address/amount is meant to be private, use commit
>>>> protocol
>>>>>>> (see ZRTP/RedPhone) and short authentication phrase (which is hard
>>>> to
>>>>>>> spoof thanks to commit protocol - see RedPhone)?
>>>>>>
>>>>>> Hi Martin,
>>>>>>
>>>>>> The problem is that you need to verify the ownership of the public
>>>> key.
>>>>>> A MITM can substitute the key. If you don't have verifiable identity
>>>>>> associated with the public key (PKI/WoT), you need a shared secret
>>>> (such
>>>>>> as a secret phrase). But the problem is then establishing that
>>>> secret
>>>>>> over a public channel.
>>>>>>
>>>>>> You can bootstrap a private session over the untrusted network using
>>>> a
>>>>>> trusted public key (PKI/WoT). But the presumption is that you are
>>>>>> already doing this over the web (using TLS). That process is subject
>>>> to
>>>>>> attack at the CA. WoT is not subject to a CA attack, because it's
>>>>>> decentralized. But it's also not sufficiently deployed for some
>>>> scenarios.
>>>>>>
>>>>>> e
>>>>>>
>>>
>>>
>>

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