21 IncTyler Julian
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In this tutorial we'll show you how to create a bitcoin-payable notary public service. Any user can pay bitcoin to a server endpoint that writes a custom message to the blockchain to be stored forever.
After reading, you should be able to write data to the blockchain and read it back whenever you'd like. You'll also gain experience using the various bitcoin classes to build your own transactions.
You will need the following items:
- A 21 Bitcoin Computer, with built-in mining and software
- The latest version of the 21 software, obtained by running
21 updateas described in the [setup][18] process. - Do the [Introduction to the 21 Bitcoin Computer][19] tutorial
- Do the [Working with Multiple 21 Users][20] tutorial
- Do the [Bitcoin-Payable API][21] tutorial
If you've done all that, you are ready to go. Let's get started!
Follow the instructions in the
[Working With Multiple 21 Users][20] tutorial to set
up the twenty-client, twenty-server, and twenty-server-2 users
and install Flask.
IMPORTANT: You will need to have a nonzero blockchain balance in
twenty-client to do this example. While logged in as
twenty-client, run 21 mine followed by 21 flush and wait until
21 status shows that you have a nonzero blockchain balance. This may
take anywhere from ~20 minutes to more than an hour, depending on the
speed of mining.
Log into the Bitcoin Computer as the twenty-server. Create a project
folder to house our app and an empty file as follows:
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cd ~/
mkdir notary-server && cd notary-server
touch notary-server.py
The server is going to need a blockchain balance for this example, so mine some bitcoin and flush it to your bitcoin wallet:
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21 mine
21 flush
The transaction will be broadcast to the blockchain within a 20 minute window following the flush. You can check to see when this transaction is confirmed by running:
21 status
Now open notary-server.py in your text editor and type in the following
code:
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#!/usr/bin/env python3
# import the flask web microframework
from flask import Flask, request
# import from the 21 Bitcoin Developer Library
from two1.lib.wallet.two1_wallet import Two1Wallet
from two1.lib.blockchain.twentyone_provider import TwentyOneProvider
from two1.lib.bitcoin import utils
from two1.lib.bitcoin import PublicKey, Script
from two1.lib.bitcoin import Transaction, TransactionInput, TransactionOutput
from two1.lib.bitserv.flask import Payment
# Create application objects
app = Flask(__name__)
wallet = Two1Wallet(Two1Wallet.DEFAULT_WALLET_PATH, TwentyOneProvider())
payment = Payment(app, wallet)
@app.route('/write-message')
@payment.required(1000)
def write_message():
"""Write a message to the blockchain."""
msg = request.args.get('message')
# Create a bitcoin script object with our message
max_length = 40
if (len(msg) > max_length):
return ('Message contains more than %s characters and may not be accepted.' % str(max_length), 404)
message_script = Script('OP_RETURN 0x{}'.format(utils.bytes_to_str(msg.encode())))
# Define the fee we're willing to pay for the tx
tx_fee = 3000
# Get the first UTXO from our set that can cover the fee
utxo = None
for utxo_addr, utxos in wallet.get_utxos().items():
for u in utxos:
if u.value > tx_fee:
utxo = u
break
if utxo:
break
if not utxo:
return ('No confirmed UTXOs available to pay for the transaction.', 404)
# Build the transaction inputs (there is only one, but Transaction expects a list)
inputs = [TransactionInput(outpoint=utxo.transaction_hash,
outpoint_index=utxo.outpoint_index,
script=utxo.script,
sequence_num=0xffffffff)]
outputs = []
# Build one output with our custom message script
outputs.append(TransactionOutput(value=0, script=message_script))
# Build another output to pay the UTXO money back to one of our addresses
_, change_key_hash = utils.address_to_key_hash(wallet._accounts[0].get_next_address(True))
outputs.append(TransactionOutput(value=utxo.value - tx_fee,
script=Script.build_p2pkh(change_key_hash)))
# Build an unsigned transaction object
txn = Transaction(version=Transaction.DEFAULT_TRANSACTION_VERSION,
inputs=inputs,
outputs=outputs,
lock_time=0)
# Sign the transaction with the correct private key
private_key = wallet.get_private_key(utxo_addr)
signed = txn.sign_input(input_index=0,
hash_type=Transaction.SIG_HASH_ALL,
private_key=private_key,
sub_script=utxo.script)
# Broadcast the transaction
tx = wallet.broadcast_transaction(txn.to_hex())
return tx
if __name__ == '__main__':
app.run(host='0.0.0.0')
As soon as 21 status shows a nonzero "spendable balance
on the Blockchain", start the micropayments server:
python3 notary-server.py
Your bitcoin-enabled notary web-service is up and running! Let's create a client to purchase your notary service for a bitcoin micropayment.
Leave your twenty-server session open, but open a new
terminal on your regular computer/laptop (or PuTTY session on Windows)
and SSH into your 21 Bitcoin Computer, this time as the
twenty-client user.
As the twenty-client user, create a new directory and a file called
notary-client.py:
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cd $HOME
mkdir notary-client
cd notary-client
touch notary-client.py
Open the notary-client.py file in your text editor and type in the
following code:
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import sys
import urllib.parse
# import from the 21 Developer Library
from two1.commands.config import Config
from two1.lib.wallet import Wallet
from two1.lib.bitrequests import BitTransferRequests
wallet = Wallet()
def write_message(raw_msg):
msg = urllib.parse.quote_plus(raw_msg)
requests = BitTransferRequests(wallet, Config().username)
# purchase the bitcoin payable endpoint
response = requests.get('http://localhost:5000/write-message?message={}'.format(msg))
# return error if there's any
if response.status_code != 200:
print("The server returned an error")
print("Status code: {}".format(response.status_code))
print("Message: {}".format(response.text))
exit(1)
# print out the transaction
print("Transaction: {}".format(response.text))
print("View it live at https://live.blockcypher.com/btc/tx/{}".format(response.text))
if __name__=='__main__':
write_message(sys.argv[1])
Close the file and run the client script:
python3 notary-client.py "Carving my name into the blockchain"
Congratulations, you just wrote a message to the blockchain that will be stored there permanently! Here's an an example transaction visible on [blockcypher][140].
It's worth tracing through exactly how this worked:
- First, we wrote a message. Our message was encoded in the script of
an output after an
OP_RETURNinstruction, which tells bitcoin nodes that this output cannot be used as an input for another transaction. Note that many nodes will reject a transaction if there is more than 40 bytes of data after anOP_RETURN. This will be extended to 80 bytes as Bitcoin Core 0.12.0 becomes standard.
Then, we created inputs. To build an input, we needed an unspent transaction output (UTXO), which is essentially just spendable bitcoin that we have in our wallet. We had to include a fee for miners, so we made sure our UTXO was large enough to cover it. * Next, we created outputs. One of our transaction outputs will be our message, and another output will send our own money (the change) back to our wallet. * Next, we built and signed the transaction. We combined the inputs and outputs into a transaction and signed it. * Broadcasting the transaction. Last but not least, we got the word out by broadcasting the transaction to the bitcoin network.
This example mixes concepts from the [Bitcoin Payable API][21] tutorial and the [Bitcoin Library][141] tutorials, showing how to build bitcoin-payable APIs that manipulate bitcoin data structures and write to the blockchain - therefore actually "costing bitcoin" in an intrinsic sense. As a next step, you might investigate the [Library and Wallet][141] tutorials.
Just as a reminder, you can send bitcoin mined or earned in your
21.co balance to the blockchain at any time by running 21 flush . A transaction will be created within 10 minutes,
and you can view the transaction id with 21 log.
Once the transaction has been confirmed, you can check the
balance in your bitcoin wallet from the command line with
wallet balance, and you can send bitcoin from your
wallet to another address with wallet sendto $BITCOIN_ADDRESS $BTC_AMOUNT --use-unconfirmed. Make sure
to use the amount in BTC, not in Satoshi; the
--use-unconfirmed flag ensures that you can send even if you
have unconfirmed transactions in your wallet.
Ready to try out your bitcoin-payable server in the wild? Or simply want to browse and purchase from other bitcoin-enabled servers? Head over to the [21 Developer Community][142] at slack.21.co to join the bitcoin machine-payable marketplace hosted on the 21 peer-to-peer network.
Check out all the Bitcoin Apps you can build with the 21 Bitcoin Computer.
[][15][][16][__][143]
Authors
Tyler Julian
If you have any questions or issues, please drop us a line at [[email protected] ][144] or join our Slack community.
This content is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License][145].
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(c) 2016, 21.co
[15]: https://twitter.com/intent/tweet?text=Hmm. 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//twitter.com/21 [147]: //medium.com/@21