docs: add PSBT funding tutorial
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Oliver Gugger
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docs/psbt.md
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docs/psbt.md
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# PSBT
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This document describes various use cases around the topic of Partially Signed
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Bitcoin Transactions (PSBTs). Currently only channel funding is possible with
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PSBTs but more features are planned.
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See [BIP174](https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki) for
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a full description of the PSBT format and the different _roles_ that a
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participant in a PSBT can have.
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## Opening a channel by using a PSBT
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This is a step-by-step guide on how to open a channel with `lnd` by using a PSBT
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as the funding transaction.
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We will use `bitcoind` to create and sign the transaction just to keep the
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example simple. Of course any other PSBT compatible wallet could be used and the
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process would likely be spread out over multiple signing steps. The goal of this
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example is not to cover each and every possible edge case but to help users of
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`lnd` understand what inputs the `lncli` utility expects.
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The goal is to open a channel of 1'234'567 satoshis to the node
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`03db1e56e5f76bc4018cf6f03d1bb98a7ae96e3f18535e929034f85e7f1ca2b8ac` by using
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a PSBT. That means, `lnd` can have a wallet balance of `0` and is still able to
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open a channel. We'll jump into an example right away.
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The new funding flow has a small caveat: _Time matters_.
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When opening a channel using the PSBT flow, we start the negotiation
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with the remote peer immediately so we can obtain their multisig key they are
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going to use for the channel. Then we pause the whole process until we get a
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fully signed transaction back from the user. Unfortunately there is no reliable
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way to know after how much time the remote node starts to clean up and "forgets"
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about the pending channel. If the remote node is an `lnd` node, we know it's
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after 10 minutes. **So as long as the whole process takes less than 10 minutes,
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everything should work fine.**
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### 1. Use the new `--psbt` flag in `lncli openchannel`
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The new `--psbt` flag in the `openchannel` command starts an interactive dialog
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between `lncli` and the user. Below the command you see an example output from
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a regtest setup. Of course all values will be different.
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```bash
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$ lncli openchannel --node_key 03db1e56e5f76bc4018cf6f03d1bb98a7ae96e3f18535e929034f85e7f1ca2b8ac --local_amt 1234567 --psbt
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Starting PSBT funding flow with pending channel ID fc7853889a04d33b8115bd79ebc99c5eea80d894a0bead40fae5a06bcbdccd3d.
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PSBT funding initiated with peer 03db1e56e5f76bc4018cf6f03d1bb98a7ae96e3f18535e929034f85e7f1ca2b8ac.
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Please create a PSBT that sends 0.01234567 BTC (1234567 satoshi) to the funding address bcrt1qh33ghvgjj3ef625nl9jxz6nnrz2z9e65vsdey7w5msrklgr6rc0sv0s08q.
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Example with bitcoind:
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bitcoin-cli walletcreatefundedpsbt [] '[{"bcrt1qh33ghvgjj3ef625nl9jxz6nnrz2z9e65vsdey7w5msrklgr6rc0sv0s08q":0.01234567}]'
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Or if you are using a wallet that can fund a PSBT directly (currently not
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possible with bitcoind), you can use this PSBT that contains the same address
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and amount: cHNidP8BADUCAAAAAAGH1hIAAAAAACIAILxii7ESlHKdKpP5ZGFqcxiUIudUZBuSedTcB2+geh4fAAAAAAAA
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Paste the funded PSBT here to continue the funding flow.
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Base64 encoded PSBT:
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```
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The command line now waits until a PSBT is entered. We'll create one in the next
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step. Make sure to use a new shell window/tab for the next commands and leave
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the prompt from the `openchannel` running as is.
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### 2. Use `bitcoind` to create a funding transaction
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The output of the last command already gave us an example command to use with
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`bitcoind`. We'll go ahead and execute it now. The meaning of this command is
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something like "bitcoind, give me a PSBT that sends the given amount to the
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given address, choose any input you see fit":
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```bash
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$ bitcoin-cli walletcreatefundedpsbt [] '[{"bcrt1qh33ghvgjj3ef625nl9jxz6nnrz2z9e65vsdey7w5msrklgr6rc0sv0s08q":0.01234567}]'
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{
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"psbt": "cHNidP8BAH0CAAAAAbxLLf9+AYfqfF69QAQuETnL6cas7GDiWBZF+3xxc/Y/AAAAAAD+////AofWEgAAAAAAIgAgvGKLsRKUcp0qk/lkYWpzGJQi51RkG5J51NwHb6B6Hh+1If0jAQAAABYAFL+6THEGhybJnOkFGSRFbtCcPOG8AAAAAAABAR8wBBAkAQAAABYAFHemJ11XF7CU7WXBIJLD/qZF+6jrAAAA",
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"fee": 0.00003060,
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"changepos": 1
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}
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```
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We see that `bitcoind` has given us a transaction that would pay `3060` satoshi
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in fees. Fee estimation/calculation can be changed with parameters of the
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`walletcreatefundedpsbt` command. To see all options, use
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`bitcoin-cli help walletcreatefundedpsbt`.
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If we want to know what exactly is in this PSBT, we can look at it with the
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`decodepsbt` command:
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```bash
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$ bitcoin-cli decodepsbt cHNidP8BAH0CAAAAAbxLLf9+AYfqfF69QAQuETnL6cas7GDiWBZF+3xxc/Y/AAAAAAD+////AofWEgAAAAAAIgAgvGKLsRKUcp0qk/lkYWpzGJQi51RkG5J51NwHb6B6Hh+1If0jAQAAABYAFL+6THEGhybJnOkFGSRFbtCcPOG8AAAAAAABAR8wBBAkAQAAABYAFHemJ11XF7CU7WXBIJLD/qZF+6jrAAAA
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{
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"tx": {
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"txid": "374504e4246a93a45b4a2c2bc31d8adc8525aa101c7b9065db6dc01c4bdfce0a",
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"hash": "374504e4246a93a45b4a2c2bc31d8adc8525aa101c7b9065db6dc01c4bdfce0a",
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"version": 2,
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"size": 125,
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"vsize": 125,
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"weight": 500,
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"locktime": 0,
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"vin": [
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{
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"txid": "3ff673717cfb451658e260ecacc6e9cb39112e0440bd5e7cea87017eff2d4bbc",
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"vout": 0,
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"scriptSig": {
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"asm": "",
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"hex": ""
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},
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"sequence": 4294967294
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}
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],
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"vout": [
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{
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"value": 0.01234567,
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"n": 0,
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"scriptPubKey": {
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"asm": "0 bc628bb11294729d2a93f964616a73189422e754641b9279d4dc076fa07a1e1f",
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"hex": "0020bc628bb11294729d2a93f964616a73189422e754641b9279d4dc076fa07a1e1f",
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"reqSigs": 1,
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"type": "witness_v0_scripthash",
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"addresses": [
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"bcrt1qh33ghvgjj3ef625nl9jxz6nnrz2z9e65vsdey7w5msrklgr6rc0sv0s08q"
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]
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}
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},
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{
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"value": 48.98759093,
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"n": 1,
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"scriptPubKey": {
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"asm": "0 bfba4c71068726c99ce9051924456ed09c3ce1bc",
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"hex": "0014bfba4c71068726c99ce9051924456ed09c3ce1bc",
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"reqSigs": 1,
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"type": "witness_v0_keyhash",
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"addresses": [
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"bcrt1qh7aycugxsunvn88fq5vjg3tw6zwrecduvvgre5"
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]
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}
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}
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]
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},
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"unknown": {
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},
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"inputs": [
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{
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"witness_utxo": {
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"amount": 48.99996720,
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"scriptPubKey": {
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"asm": "0 77a6275d5717b094ed65c12092c3fea645fba8eb",
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"hex": "001477a6275d5717b094ed65c12092c3fea645fba8eb",
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"type": "witness_v0_keyhash",
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"address": "bcrt1qw7nzwh2hz7cffmt9cysf9sl75ezlh28tzl4n4e"
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}
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}
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}
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],
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"outputs": [
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{
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},
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{
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}
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],
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"fee": 0.00003060
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}
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```
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This tells us that we got a PSBT with a big input, the channel output and a
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change output for the rest. Everything is there but the signatures/witness data,
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which is exactly what we need.
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### 3. Verify and sign the PSBT
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Now that we have a valid PSBT that has everything but the final
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signatures/witness data, we can paste it into the prompt in `lncli` that is
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still waiting for our input.
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```bash
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...
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Base64 encoded PSBT: cHNidP8BAH0CAAAAAbxLLf9+AYfqfF69QAQuETnL6cas7GDiWBZF+3xxc/Y/AAAAAAD+////AofWEgAAAAAAIgAgvGKLsRKUcp0qk/lkYWpzGJQi51RkG5J51NwHb6B6Hh+1If0jAQAAABYAFL+6THEGhybJnOkFGSRFbtCcPOG8AAAAAAABAR8wBBAkAQAAABYAFHemJ11XF7CU7WXBIJLD/qZF+6jrAAAA
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PSBT verified by lnd, please continue the funding flow by signing the PSBT by
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all required parties/devices. Once the transaction is fully signed, paste it
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again here.
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Base64 encoded PSBT:
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```
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We can now go ahead and sign the transaction. We are going to use `bitcoind` for
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this again, but in practice this would now happen on a hardware wallet and
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perhaps `bitcoind` would only know the public keys and couldn't sign for the
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transaction itself. Again, this is only an example and can't reflect all
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real-world use cases.
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```bash
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$ bitcoin-cli walletprocesspsbt cHNidP8BAH0CAAAAAbxLLf9+AYfqfF69QAQuETnL6cas7GDiWBZF+3xxc/Y/AAAAAAD+////AofWEgAAAAAAIgAgvGKLsRKUcp0qk/lkYWpzGJQi51RkG5J51NwHb6B6Hh+1If0jAQAAABYAFL+6THEGhybJnOkFGSRFbtCcPOG8AAAAAAABAR8wBBAkAQAAABYAFHemJ11XF7CU7WXBIJLD/qZF+6jrAAAA
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{
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"psbt": "cHNidP8BAH0CAAAAAbxLLf9+AYfqfF69QAQuETnL6cas7GDiWBZF+3xxc/Y/AAAAAAD+////AofWEgAAAAAAIgAgvGKLsRKUcp0qk/lkYWpzGJQi51RkG5J51NwHb6B6Hh+1If0jAQAAABYAFL+6THEGhybJnOkFGSRFbtCcPOG8AAAAAAABAR8wBBAkAQAAABYAFHemJ11XF7CU7WXBIJLD/qZF+6jrAQhrAkcwRAIgHKQbenZYvgADRd9TKGVO36NnaIgW3S12OUg8XGtSrE8CICmeaYoJ/U7Ecm+/GneY8i2hu2QCaQnuomJgzn+JAnrDASEDUBmCLcsybA5qXSRBBdZ0Uk/FQiay9NgOpv4D26yeJpAAAAA=",
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"complete": true
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}
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```
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Interpreting the output, we now have a complete, final, and signed transaction
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inside the PSBT. Let's give it to `lncli` to continue:
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```bash
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...
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Base64 encoded PSBT: cHNidP8BAH0CAAAAAbxLLf9+AYfqfF69QAQuETnL6cas7GDiWBZF+3xxc/Y/AAAAAAD+////AofWEgAAAAAAIgAgvGKLsRKUcp0qk/lkYWpzGJQi51RkG5J51NwHb6B6Hh+1If0jAQAAABYAFL+6THEGhybJnOkFGSRFbtCcPOG8AAAAAAABAR8wBBAkAQAAABYAFHemJ11XF7CU7WXBIJLD/qZF+6jrAQhrAkcwRAIgHKQbenZYvgADRd9TKGVO36NnaIgW3S12OUg8XGtSrE8CICmeaYoJ/U7Ecm+/GneY8i2hu2QCaQnuomJgzn+JAnrDASEDUBmCLcsybA5qXSRBBdZ0Uk/FQiay9NgOpv4D26yeJpAAAAA=
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{
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"funding_txid": "374504e4246a93a45b4a2c2bc31d8adc8525aa101c7b9065db6dc01c4bdfce0a"
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}
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```
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Success! We now have the final transaction ID of the published funding
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transaction. Now we only have to wait for some confirmations, then we can start
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using the freshly created channel.
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