Hand-Coding a Bitcoin Transaction
Bitcoin is an open-source, decentralized cryptocurrency that allows users to create and transmit transactions without the need for intermediaries like banks. One of the key components of a Bitcoin transaction is the script Sig, which is used to verify the sender’s identity and ensure the integrity of the transaction.
In this article, we will walk through the process of hand-coding a Bitcoin transaction using the standard format as defined by the Bitcoin protocol.
The Transaction Format
A Bitcoin transaction consists of the following components:
prevhash: The previous block hash
index: The index of the previous transaction (0-indexed)
scriptSig: A script signature, which is used to verify the sender’s identity and ensure the integrity of the transaction
vsize: The size of the transaction data
vpri: The public key of the sender
data: The transaction data
Script Signatures
A script signature consists of a series of numbers that are prefixed with “OP_”. Each number represents an operation that is performed on the sender’s balance. The most common operations are:
OP_1: Set the sender’s balance to 0
OP_2: Increase the sender’s balance by the value specified in the following transaction data
Here is an example of a script signature:
4a76a51c7f9b6a6a0000000000000000000000000000000000000000000000001e80fe2004a5ef6d6
This script signature means that the sender’s balance should be set to 0. In this example, the transaction data OP_2 specifies a value of 100.
Hand-Coding a Bitcoin Transaction
To hand-code a Bitcoin transaction, we need to construct a new block and add the required components to it.
Here is an example of how to hand-code a Bitcoin transaction:
Input:
Previous tx: f5d8ee39a430901c91a5917b9f2dc19d6d1a0e9cea205b009ca73dd04470b9a6
Index: 0
ScriptSig:
4a76a51c7f9b6a6a000000000000000000000000000000000000000000000000100
vsize: 20 bytes (transaction data only)
vpri: 3047023278d0782e2bfc35d41ed29cdab4ea5ef6dc
data:
OP_1: 0
OP_3: f5d8ee39a430901c91a5917b9f2dc19d6d1a0e9cea205b009ca73dd04470b9a6
This block consists of:
prevhash: The previous block hash, which is set to 0 (since we are starting from scratch)
index: The index of the previous transaction, which is 0
scriptSig: A script signature that sets the sender’s balance to 0 and increases their balance by 100 using OP_2
vsize: The size of the transaction data, which is set to 20 bytes (transaction data only)
vpri: The public key of the sender, which is set to a hardcoded value for this example
data: A script signature that increases the sender’s balance by 100 using OP_2
Signing and Verifying the Transaction
To sign the transaction, we need to create a new Bitcoin wallet and load it with our public key. Then, we can use the wallet’s signing API to generate a digital signature for the transaction.
Here is an example of how to sign the transaction:
import bitcoin
Load the wallet
wallet = bitcoin.new_wallet()
Create a new block
block = bitcoin.NewBlock()
Set the previous hash and index
previous_hash = '0'
index = 0
Add the script signature and data to the block
block.set_script Sig(4a76a51c7f9b6a6a000000000000000000000000000000000000000000000100)
block.add_data OP_1, OP_3, index, 'f5d8ee39a430901c91a5917b9f2dc19d6d1a0e9cea205b009ca73dd04470b9a6', 100)
Sign the block
signature = wallet.sign_block(block)
Print the signed block
print(bitcoin.PrintBlock(block, signature))
This code creates a new Bitcoin wallet and loads it with our public key. Then, it sets up a new block and adds the required script signatures and data to it. Finally, it signs the block using the wallet’s signing API and prints the signed block.