I wanted to try out a real trade using lightning network. I don't know of any
pizza places near me that accept lightning bitcoin yet but a friend from London
agreed to do it and he sub contracted out the pizza delivery to a local shop.
In short, I paid bitcoin using the lightning network and he arranged for pizza
to be delivered to me. In this trade my friend is just a middle man that is
taking the risk on accepting lightning payments, but it demonstrates the basic
premise of how this works for everyday transactions. It could just as well be
the pizza shop accepting the payment directly with their own lightning node.
I wanted two pizzas and to try to do it as close to atomically as possible. I
didn't want to prepay and end up with no pizza. As far as I know we don't yet
have pizza/bitcoin atomic swap software but we improvised and decided that I
would need to provide the payment hash preimage to the delivery driver in order
to claim my pizza. If I can't produce the preimage, proving that I paid, then
the pizza would not be handed over and it would be destroyed. This works
because I can't get the preimage without paying the invoice. I agreed to open
a channel and fund it with a sufficient amount for what we estimated the cost
would end up being. After we agreed to these terms my friend was able to
verify that I funded a channel on the blockchain, which shows that I at least
have the money (bitcoin). He is taking on some entrepreneurial risk and
prepaying his sub contractor to prepare and deliver the pizza to me, but at
this point I have not risked my bitcoins, they're just committed to a channel.
I was given a bolt11 invoice which I decoded with the c-lightning cli to verify
everything was as agreed:
$ ./lightning-cli decodepay
lnbc6490u1pdfrjhcpp5jyxuuskqw53apgqvtxa7emcrz5vs0qr2sxjayxv7jj70jznnl94sdp5x9vycgzrdpjk2umeypgxj7n6vykzqvfqg3jkcatcv5s9q6t60fssxqyzx2qcqpgaue37x27yp3pn4cr6wuprvwedncz4kavqh83cp3l0vwfrprj0xj8cedkfmjdzea0xpp0jazfcyy77cq37ej6d3xvmujmgu56pe56ktcqa3vcys
{ "currency" : "bc", "timestamp" : 1519504120, "created_at" : 1519504120, "expiry" : 72000, "payee" :
"0397b318c5e0d09b16e6229ec50744c8a7a8452b2d7c6d9855c826ff14b8fa8b27", "msatoshi" : 649000000, "description" : "1XL Cheesy Pizza, 1 Deluxe Pizza", "min_final_cltv_expiry" : 8,
"payment_hash" : "910dce42c07523d0a00c59bbecef03151907806a81a5d2199e94bcf90a73f96b", "signature" :
"3045022100ef331f195e206219d703d3b811b1d96cf02adbac05cf1c063f7b1c91847279a402207c65b64ee4d167af3042f97449c109ef6011f665a6c4ccdf25b4729a0e69ab2f" }
When the pizza delivery arrived, I was asked "What is the preimage?" by the
driver. At this point I paid the invoice and instantly received the preimage in return.
$ ./lightning-cli pay
lnbc6490u1pdfrjhcpp5jyxuuskqw53apgqvtxa7emcrz5vs0qr2sxjayxv7jj70jznnl94sdp5x9vycgzrdpjk2umeypgxj7n6vykzqvfqg3jkcatcv5s9q6t60fssxqyzx2qcqpgaue37x27yp3pn4cr6wuprvwedncz4kavqh83cp3l0vwfrprj0xj8cedkfmjdzea0xpp0jazfcyy77cq37ej6d3xvmujmgu56pe56ktcqa3vcys
{ "preimage" : "7241e3f185148625894b8887ad459babd26540fc12124c3a7a96c937d89da8c1",
"tries" : 1 }
In the interest of keeping it simple we agreed that the preimage would just be
the first and last 4 characters of the hex string. So my answer was 7241-a8c1.
I wrote this on a notepad and presented it to the driver who compared it to
his own notepad, at which point I was given the pizza. It's probably not a
good practice to share the preimage. The delivery driver didn't have the full
string, only enough to verify that I had it.
How do you get the preimage for your invoice? In c-lightning you can do it
like this:
$ ./lightning-cli invoice 12345 label description
{ "payment_hash" : "e04dfbd4adc634779b560c8e7072f883d5f17a3e32a33603bfc90a8682873d44", "expiry_time" :
1519523498, "expires_at" : 1519523498, "bolt11" :
"lnbc123450p1pdfyzy6pp5upxlh49dcc680x6kpj88quhcs02lz737x23nvqaley9gdq5884zqdqjv3jhxcmjd9c8g6t0dccqpg802ys4s4z3rpm6d8zvdgq397wewh5kaz527hnglz9xsmjxfjrhe3mxq9pp7pqm0pwcwm748tav4am97gqrvnzxnlw5uxxawgw4vcywgphj26nf"
}
$ sqlite3 ~/.lightning/lightningd.sqlite3 "SELECT quote(payment_key) FROM invoices
ORDER BY id DESC LIMIT 1"
X'D3BE7E68D8B38B15A5194AEA131A21429A1987085C95A0631273273546FF5ED8'
Then you can verify that it's indeed the correct preimage by hashing it again
and comparing it to the payment_hash in the invoice above:
$ echo "D3BE7E68D8B38B15A5194AEA131A21429A1987085C95A0631273273546FF5ED8" | xxd
-r -p | sha256sum
e04dfbd4adc634779b560c8e7072f883d5f17a3e32a33603bfc90a8682873d44 -
Note that you should not share the preimage with anyone.
So is there any point to doing this instead of an on chain transaction? For
what I described here, probably not. The goal was just to play around with
c-lightning and do something more than shuffling a few satoshi back and forth.
Maybe eventually pizza shops will have their own lightning nodes and I can open
channels to them directly.
Some pics of my family enjoying the pizza here:
http://eclipse.heliacal.net/~solar/bitcoin/lightning-pizza/
-Laszlo
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