2015-12-29 05:24:02 -08:00
|
|
|
NOTE: Not implemented in code, only included as part of the wire protocol for
|
|
|
|
future implementation!
|
|
|
|
|
|
|
|
There are multiple R-value hashes supported in HTLCs in the wire protocol. This
|
|
|
|
is to support conditional multiparty payments, e.g. 2-of-3 "escrow", which is
|
|
|
|
one of the biggest use cases of bitcoin scripting today. An example use case is
|
2015-12-29 17:10:00 -08:00
|
|
|
a 3rd party escrow verifies whether a seller should be paid. This design is
|
|
|
|
such that the escrow is not a traditional custodial escrow, but instead
|
2015-12-29 05:24:02 -08:00
|
|
|
determines who should get the money in the event of non-cooperation.
|
|
|
|
|
2015-12-29 17:10:00 -08:00
|
|
|
In this implementation, we are including *wire protocol support* but not
|
2015-12-29 05:24:02 -08:00
|
|
|
writing code yet for 2-of-3, it is to be implemented later. Arbitrary N-of-M
|
2015-12-29 17:10:00 -08:00
|
|
|
can be supported with M values higher than 3 and lower than max script size,
|
|
|
|
but let's keep this simple for now!
|
2015-12-29 05:24:02 -08:00
|
|
|
|
|
|
|
How it works: Require 2-of-3 R-value preimages (from 3 hashes) in order for the
|
|
|
|
HTLC to be fulfilled. For each hop in the payment, it requires this 2-of-3
|
|
|
|
condition. The timeout minimum for each hop in the path is at least the minimum
|
|
|
|
agreed contractual escrow timeout. This means each hop consumes a higher amount
|
|
|
|
of time-value (due to much longer timeouts along all channels in the path),
|
|
|
|
which does have greater pressure towards lower hop-distances compared to
|
|
|
|
straight payments.
|
|
|
|
|
|
|
|
This is a slightly different way of thinking about things. It's not signatures
|
|
|
|
that the escrow produces (or for that matters any of the 3-parties in the
|
|
|
|
2-of-3). It's some secret which is revealed to authorize payment. So if the
|
|
|
|
Escrow wants the payment to go through, they disclose the secret (R-value) to
|
|
|
|
the recipient. If the recipient is unable to produce 2-of-3, after the agreed
|
2015-12-29 17:10:00 -08:00
|
|
|
timeout, the sender will be refunded. Sender and receiver can agree to
|
|
|
|
authorize payment in most cases where there is cooperation, escrow is only
|
|
|
|
contacted if there is non-cooperation.
|
2015-12-29 05:24:02 -08:00
|
|
|
|
2015-12-29 17:10:00 -08:00
|
|
|
Supported in the wire protocol for the unit8 (two 4-bit N-of-M):
|
|
|
|
17 (00010001): 1-of-1
|
|
|
|
34 (00100010): 2-of-2
|
|
|
|
35 (00100011): 2-of-3 [with Recipient being 1 of the two N parties]
|
|
|
|
51 (00110011): 3-of-3
|
2015-12-29 05:24:02 -08:00
|
|
|
|
2015-12-29 17:10:00 -08:00
|
|
|
I think the only ones that really matter are 1-of-1, 2-of-3, and 2-of-2. 1-of-2
|
|
|
|
and 1-of-3 doesn't make sense if the recipient must consent to receiving funds
|
|
|
|
anyway (pushing funds w/o consent is tricky due to pay-to-contract-hash) so
|
|
|
|
that's basically a 1-of-1.
|
2015-12-29 05:24:02 -08:00
|
|
|
|
2015-12-29 17:10:00 -08:00
|
|
|
Assume the order in the stack is Sender, Escrow, Recipient.
|
2015-12-29 05:24:02 -08:00
|
|
|
|
2015-12-29 17:10:00 -08:00
|
|
|
For PAID 2-of-3 Escrow+Recipient, the HTLC stack is:
|
|
|
|
<BobSig> <0> <EscrowPreimageR> <RecipientPreimageR> <0>
|
2015-12-29 05:24:02 -08:00
|
|
|
|
|
|
|
If it's REFUND because 2-of-3 has not been redeemed in time:
|
|
|
|
<AliceSig> <1>
|
|
|
|
|
|
|
|
Script (we use OP_1/OP_0 to distinctly show computed true/false. 0/1 is for
|
|
|
|
supplied data as part of the sigScript/redeemScript stack):
|
|
|
|
-------------------------------------------------------------------------------
|
|
|
|
//Paid
|
|
|
|
OP_IF
|
2015-12-29 17:10:00 -08:00
|
|
|
<CSVDelay> OP_DROP OP_CSV
|
2015-12-29 05:24:02 -08:00
|
|
|
|
2015-12-29 17:10:00 -08:00
|
|
|
//Stack: <BobSig> <0> <EscrowPreimageR> <RecipientPreimageR>
|
|
|
|
//Recipient must agree to receive funds.
|
|
|
|
OP_HASH160 <RecipientHash> OP_EQUALVERIFY
|
|
|
|
|
|
|
|
//Stack: <BobSig> <0> <EscrowPreimageR>
|
|
|
|
//Either the Sender or Escrow must consent for payment
|
2015-12-29 05:24:02 -08:00
|
|
|
OP_HASH160 <EscrowHash> OP_EQUAL
|
2015-12-29 17:10:00 -08:00
|
|
|
//Stack: <BobSig> <0> <OP_1>
|
2015-12-29 05:24:02 -08:00
|
|
|
OP_SWAP
|
2015-12-29 17:10:00 -08:00
|
|
|
//Stack: <BobSig> <OP_1> <0>
|
2015-12-29 05:24:02 -08:00
|
|
|
OP_HASH160 <SenderHash> OP_EQUAL
|
2015-12-29 17:10:00 -08:00
|
|
|
//Stack: <BobSig> <OP_1> <OP_0>
|
|
|
|
OP_BOOLOR
|
|
|
|
//Stack: <BobSig> <OP_1>
|
|
|
|
OP_VERIFY
|
2015-12-29 05:24:02 -08:00
|
|
|
|
|
|
|
<BobPubKey>
|
|
|
|
//Stack: <BobSig> <BobPubKey>
|
|
|
|
//Refund
|
|
|
|
OP_ELSE
|
2015-12-29 17:10:00 -08:00
|
|
|
<CSVDelay> OP_DROP OP_CSV
|
2015-12-29 05:24:02 -08:00
|
|
|
|
2015-12-29 17:10:00 -08:00
|
|
|
<HTLCTimeout> OP_DROP OP_CLTV
|
2015-12-29 05:24:02 -08:00
|
|
|
<AlicePubKey>
|
|
|
|
//Stack: <AliceSig> <AlicePubKey>
|
|
|
|
OP_ENDIF
|
|
|
|
OP_CHECKSIG
|
|
|
|
-------------------------------------------------------------------------------
|
|
|
|
|
|
|
|
Note: It is possible that Alice and Bob may not be Sender, Recipient, nor
|
|
|
|
Escrow!
|
|
|
|
|
|
|
|
The result? We can do 2-of-3 escrow payments which refund to the sender after a
|
2015-12-29 17:10:00 -08:00
|
|
|
timeout! The Sender and Recipient can agree to redeem and they only need to go
|
|
|
|
to the Escrow if there's a dispute. All nodes along the path gets paid or
|
|
|
|
refunded atomically, the same as a single-HTLC payment on Lightning.
|
|
|
|
|
|
|
|
Possible Resolution States:
|
|
|
|
* Recipient paid: Recipient and Sender provide R-values
|
|
|
|
* Recipient paid: Recipient and Escrow provide R-values
|
|
|
|
* Sender refunded via timeout: Sender is refunded if Recipient cannot convince
|
|
|
|
Escrow or Sender to disclose their R-value before HTLC timeout
|
|
|
|
* Payment immediately cancelled and Sender gets refunded: Payment sent in the
|
|
|
|
opposite direction enforced by same R-values (if there is sender & receiver
|
|
|
|
consent & cooperation to cancel payment)
|
|
|
|
|
|
|
|
Sender+Escrow isn't going to want to push funds w/o cooperation of Recipient.
|
|
|
|
However, it's possible to construct a script that way.
|
2015-12-29 05:24:02 -08:00
|
|
|
|
|
|
|
Ta-da! "Smart Contract(TM)" maymay.
|
|
|
|
|
2015-12-29 17:10:00 -08:00
|
|
|
Escrow-enforced immediately refundable payments (2-of-3 can immediately cancel
|
|
|
|
a payment) are also possible but requires another payment in the opposite
|
|
|
|
direction with the R-value hashed twice (the H becomes the R-value) and funds
|
|
|
|
encumbered in the opposite direction, but that's kind of annoying to write...
|
|
|
|
it's easier if immediate refund can only occur when both Recipient+Sender agree
|
|
|
|
to cancel the payment immediately (otherwise it will wait until the timeout).
|
|
|
|
|
|
|
|
Escrow is only contacted if the recipient needs to redeem and the sender is
|
|
|
|
uncooperative so this is still true to the "lazy escrow service" in Bitcoin
|
|
|
|
multisig.
|
|
|
|
|
|
|
|
(2-of-2 is also needed for payment cancellation.)
|