Easy proof of existence with Bitcoin.

proof_of_existence.py

#!/usr/bin/env python3

# Proof of existence \ Blockchain Graffiti

import hashlib
import bitcoin  # http://github.com/conio/pybitcointools <- this is this dependency

key = hashlib.sha256(b'Change me! I am your private key').digest() # Set your private key
address = bitcoin.privtoaddr(key)
print('\nDeposit Bitcoin here:\n%s\n' % address)  

# The amount of the incoming transaction
AMOUNT_BTC = 0.001

# The message you're going to write in blockchain (or the hash of the file you wish to prove the existence)
MESSAGE = b'Hi, I am a message in the blockchain! Script by twitter.com/khs9ne'

# Subtract the fee, the transaction would be around 200~250 bytes. If you are *VERY HURRY*
# estimate 300 bytes and we're ok.
# Use the current fee per byte, you can see it here: https://bitcoinfees.earn.com/
FEE_PER_BYTE_SATOSHI = 10
TX_SIZE = 300
FEE_SATOSHI = TX_SIZE * FEE_PER_BYTE_SATOSHI

# Edit here the money destination address (where to take back the money sent previously)
# Leave this address to tip me :-)
DESTINATION_ADDRESS = '3FVGopUDc6tyAP6t4P8f3GkYTJ5JD5tPwV'

# Once you have money on the address, put here the unspent coordinates
OUTPOINT = 'b1a132ce3b6f42263b2a8ce87014499ff6542dff1df2dc19cadd8ec9caa7ec93:2'

outpoints = [
    {
        'output': OUTPOINT,
        'value': int(AMOUNT_BTC * 10**8)
    }
]  

tx = bitcoin.mktx(
    outpoints,
    [
        {'value': (int(AMOUNT_BTC * 10**8)) - FEE_SATOSHI, 'address': DESTINATION_ADDRESS },
    ]
)
tx = bitcoin.mk_opreturn(MESSAGE, tx)
tx = bitcoin.sign(tx, 0, key)
print('Transaction to broadcast:\n%s' % tx)
# Here is printed the transaction to be broadcast. You can use this API to send it https://blockstream.info/tx/push