[Bitcoin-development] ECDH in the payment protocol
I wrote an article about an ECDH extension for BIP 70:
https://medium.com/p/cb2f81962c1b
The article is meant for people who don't follow bitcoin-development so
I'll summarise it here:
- The notion of being able to publish a piece of data once and use it to
receive lots of payments without any coordination is really useful. This is
the idea behind the stealth address proposal.
- Stealth addresses don't fit with the payment protocol, because they're
a new kind of address (obviously).
- Stealth addresses are not backwards compatible. If you give someone a
stealth address and their wallet doesn't support it, they can't pay you,
not even with worse privacy. Sometimes people may optionally want that
behaviour but stealth addresses have it all the time.
- The proposed stealth address design makes huge sacrifices to try and
keep everything within the block chain. It bloats the chain with OP_RETURN
stuff that isn't a part of the validation. But more seriously, the only way
to make it efficient enough for lightweight clients is to reduce the
stealthyness. The more efficient you make your address the less private
it becomes. This is somewhat similar to the dilemma we have with Bloom
filtering, except Bloom filters are transient and can only be used to link
addresses by someone who observes them on the wire. Stealth addresses
record the relationship in the block chain forever.
- The design makes these sacrifices to avoid moving data around outside
the block chain. But with BIP 70 that's the direction we're heading in
anyway. So by adding ECDH to the payment protocol and putting our effort
into making BIP 70 work really well for everyone, we end up killing
multiple birds with one stone. The same work that resolves the privacy
problems inherent in the stealth address design also allows us to attach
messages to payments and other commonly requested features.
There's a straw man in the article that I recreate here:
message Output {
optional uint64 amount = 1 [default = 0];
optional bytes script = 2;
*optional boolean accept_ecdh = 3; // Requires script to be a
pay-to-pubkey output.*
}
message Payment {
optional bytes merchant_data = 1;
repeated bytes transactions = 2;
repeated Output refund_to = 3;
optional string memo = 4;
*repeated bytes ecdh_nonces = 5;*
}
The way the nonces are combined to arrive at the address could be the same
as in the current stealth address spec. A wallet that doesn't understand
ECDH but does understand raw BIP 70 would deliver the money to the base
address, which receiving wallets would look for too - so it's backwards
compatible. The nonces stay out of the block chain. The transactions are
delivered directly to the recipient so there's no problems with trying to
make it fit with Bloom/prefix filtering.
To make this work there obviously has to be a backchannel from payer to
payee. BIP 70 is mostly used by web shops today so that back channel is
just HTTPS to the website itself, but shops benefit less from ECDH than
others do. So we need a simple email-like store and forward network where
HTTP POSTs to a server get queued up and delivered later (or possibly
forwarded to another store-and-forward network like the Android push
network). Most of the article discusses how best to build such a thing.
The justification for the original stealth address design can be summed up
as it's easier to [ab]use the Bitcoin network for delivery of short
messages than use a different system. But there are just so many features
we may want to add into the Payment message in future it seems better to
crack the SaF problem earlier rather than continue trying to jam a square
peg into a round hole. There are lots of very reliable SAF networks around
(email, instant messaging, etc) so it doesn't seem infeasible.
Thoughts welcome!
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Re: [Bitcoin-development] ECDH in the payment protocol
On Fri, May 09, 2014 at 02:05:24PM +0200, Mike Hearn wrote: It's always interesting to see the reinvention cycle happen in the Bitcoin space as ideas get proposed over and over again; I'm sure Amir Taaki will be pleased to read this as it is a slightly less sophisticated version of what he originally proposed to me for the design of what became stealth addresses. Of course we quickly rejected the idea of depending solely on a communications backchannel to retrieve funds. Any communications medium that isn't the blockchain makes the payment non-atomic, and thus creates opportunities for it to fail. Fortunately we already have the necessary ephemeral keys in standard Bitcoin transactions in pay-to-pubkey-hash and pay-to-script-hash spending scriptSigs so you don't need to compromise on reliability in exchange for transaction size as you're mistakingly assuming. You should re-read my original stealth address discussion with Gregory Maxwell on IRC if this is unclear. In any case it's a mistake to argue that some times of data in the blockchain are bloat by virtue of whether or not they happen to be executed in a script. Multisig addresses use an extra ~107 bytes of data per signature per txout spend to make it less likely for the user to lose their funds under certain conditions; op-return-using stealth addresses use an extra ~50 bytes of data per txout spend to make the user less likely to lose their funds and make their transactions more private under certain conditions.(1) Ultimately the resource being used is the same, making it silly to try to dictate the right trade-offs by brushing certain solutions as anti-social bloat and others not based on top-down edict; let the free market for fees do its job. 1) Note that the recent advancements in ECDH multi-party signing are limited in the cases they can cover; there still is a strong need for discrete key multisig, e.g. for Greenaddress.it -- 'peter'[:-1]@petertodd.org 3581a7e5d0e10205803803466240668215d178b216837386 signature.asc Description: Digital signature -- Is your legacy SCM system holding you back? Join Perforce May 7 to find out: #149; 3 signs your SCM is hindering your productivity #149; Requirements for releasing software faster #149; Expert tips and advice for migrating your SCM now http://p.sf.net/sfu/perforce___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] ECDH in the payment protocol
Of course we quickly rejected the idea of depending solely on a communications backchannel to retrieve funds. Any communications medium that isn't the blockchain makes the payment non-atomic Yes, I know you rejected this design, which is why I'm now proposing it instead. I think you made the wrong design call, but at any rate, it's something reasonable people can disagree on. Payment messages are sent directly to the merchant, who takes responsibility for broadcast. Once you delivered transactions to the merchant successfully, from your perspective the payment is made. A good store and forward network doesn't allow messages to go missing - email is an example of that (ignoring spam filters that explicitly want messages to go missing). It either gets delivered or it doesn't. So I'm not worried about atomicity. -- Is your legacy SCM system holding you back? Join Perforce May 7 to find out: #149; 3 signs your SCM is hindering your productivity #149; Requirements for releasing software faster #149; Expert tips and advice for migrating your SCM now http://p.sf.net/sfu/perforce___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] ECDH in the payment protocol
On Fri, May 09, 2014 at 05:15:52PM +0200, Mike Hearn wrote: Of course we quickly rejected the idea of depending solely on a communications backchannel to retrieve funds. Any communications medium that isn't the blockchain makes the payment non-atomic Yes, I know you rejected this design, which is why I'm now proposing it instead. I think you made the wrong design call, but at any rate, it's something reasonable people can disagree on. Payment messages are sent directly to the merchant, who takes responsibility for broadcast. Once you delivered transactions to the merchant successfully, from your perspective the payment is made. A good store and forward network doesn't allow messages to go missing - email is an example of that (ignoring spam filters that explicitly want messages to go missing). It either gets delivered or it doesn't. So I'm not worried about atomicity. Ah, you're still misunderstanding my point: You can get atomicity in the worst-case where the communications medium fails *and* stealth payments that use up no extra space in the blockchain. This gives you the best of both worlds. I haven't yet specified that mode of operation in the current draft stealth address standard, however I do plan on adding it. Notably the standard is designed to allow exactly that feature to be added in a backwards compatible way - senders who don't implement that feature, or choose not to use it, simply fall back to op-return. -- 'peter'[:-1]@petertodd.org 4d25218420094fda0891fe1d734e1a8df581bd6de7f2d0cd signature.asc Description: Digital signature -- Is your legacy SCM system holding you back? Join Perforce May 7 to find out: #149; 3 signs your SCM is hindering your productivity #149; Requirements for releasing software faster #149; Expert tips and advice for migrating your SCM now http://p.sf.net/sfu/perforce___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] ECDH in the payment protocol
Ah, you're still misunderstanding my point: You can get atomicity in the worst-case where the communications medium fails *and* stealth payments that use up no extra space in the blockchain. This gives you the best of both worlds. Sounds great! How does a lightweight client identify such transactions without any markers? Regardless, there are lots of other useful features that require BIP70 to work well person to person, like messages, refund addresses, etc. So extending it with ECDH makes sense in the end anyway no matter what. -- Is your legacy SCM system holding you back? Join Perforce May 7 to find out: #149; 3 signs your SCM is hindering your productivity #149; Requirements for releasing software faster #149; Expert tips and advice for migrating your SCM now http://p.sf.net/sfu/perforce___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] ECDH in the payment protocol
I believe stealth addresses and the payment protocol both have their use cases, and that they don't overlap. If you do not want to communicate with the receiver, you typically do not want them to know who is paying or for what (otherwise you're already talking to them in some way, right?). That's perfect for things like anonymous donations. In pretty much every other case, communicating directly with the receiver has benefits. Negotiation of the transaction details, messages associated with the transaction, refund information, no need to scan the blockchain for incoming transaction... and the ability to cancel if either party doesn't agree. Instead of adding stealth functionality to the payment protocol as a last resort, I'd rather see the payment protocol improve its atomicity. Either you want the data channel sender-receiver, or you don't. If it isn't available and you want it, things should fail. If you don't want it, you shouldn't try to use it in the first place. On Fri, May 9, 2014 at 5:34 PM, Mike Hearn m...@plan99.net wrote: Ah, you're still misunderstanding my point: You can get atomicity in the worst-case where the communications medium fails *and* stealth payments that use up no extra space in the blockchain. This gives you the best of both worlds. Sounds great! How does a lightweight client identify such transactions without any markers? Regardless, there are lots of other useful features that require BIP70 to work well person to person, like messages, refund addresses, etc. So extending it with ECDH makes sense in the end anyway no matter what. -- Is your legacy SCM system holding you back? Join Perforce May 7 to find out: #149; 3 signs your SCM is hindering your productivity #149; Requirements for releasing software faster #149; Expert tips and advice for migrating your SCM now http://p.sf.net/sfu/perforce ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development -- Is your legacy SCM system holding you back? Join Perforce May 7 to find out: #149; 3 signs your SCM is hindering your productivity #149; Requirements for releasing software faster #149; Expert tips and advice for migrating your SCM now http://p.sf.net/sfu/perforce ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] ECDH in the payment protocol
On Fri, May 9, 2014 at 8:13 PM, Peter Todd p...@petertodd.org wrote: I don't think we're going to find that's practical unfortunately due to change. Every payment I make ties up txouts, so if we try to base the atomicity of payments on whether or not the payee decides to broadcast the transaction the payor is stuck with txouts that they can't use until the payee makes up their mind. That leads to lots and lots of nasty edge cases. I haven't talked much about it except for on IRC, but my idea was this: * PaymentACK messages become signed (with the same key as the payment request, or using some delegation mechanism, so that the same key doesn't need to remain online). * Instead of a full Bitcoin transaction, a Payment message contains a scriptSig-less Bitcoin transaction + a limit on its byte size (and perhaps a limit on its sigop count). * The sender sends such a Payment to the receiver before actually signing the transaction (or at least, before revealing the signed form). * The receiver only ACKs if the transaction satisfies the request, is within time limits, isn't unlikely to confirm. * If the sender likes the ACK (the refund and memo fields are intact, the transaction isn't changed, the signature key is valid, ...), he either sends the full transaction (with receiver taking responsibility for broadcasting) or broadcasts it himself. Together, this means that a paymentACK + a confirmed matching Bitcoin transaction can count as proof of payment. Both parties have a chance to disagree with the transaction, and are certain all communicated data (apart from transaction signatures) in both directions happened correctly before committing. It would completely remove the chance that the Bitcoin transaction gets broadcast without the receiver liking it (for legitimate or illegitimate reasons), or without the backchannel functioning correctly. It's also compatible with doing multiple payments in one Bitcoin transaction - you can ask for ACKs from multiple parties before signing the transaction. Of course, the sender can still withhold the signed transaction (in which case nothing happened, but we probably need a second timeout), or the receiver can always claim to have never received the transaction. The sender can broadcast the transaction himself in order to prevent that, after obtaining an ACK. -- Pieter -- Is your legacy SCM system holding you back? Join Perforce May 7 to find out: #149; 3 signs your SCM is hindering your productivity #149; Requirements for releasing software faster #149; Expert tips and advice for migrating your SCM now http://p.sf.net/sfu/perforce ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
