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 such as banks. One of the key components of a Bitcoin transaction is the Sig script, which is used to verify the identity of the sender and ensure the integrity of the transaction.

In this article, we will walk you through hand-coding a Bitcoin transaction using the standard format defined by the Bitcoin protocol.

Transaction Format

A Bitcoin transaction consists of the following parts:

• prevhash: The hash of the previous block

• “index”: The index of the previous transaction (0-indexed)

• scriptSig: A script signature used to verify the identity of the sender and ensure the integrity of the transaction

• vsize: The size of the transaction data

• vpri: The sender’s public key

• data: The transaction data

Script Signatures

A script signature consists of a sequence of numbers prefixed with “OP_”. Each number represents an operation 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:

۴a76a51c7f9b6a6a00000000000000000000000000000000000000000000000000000000000000000000000000000000001e80fe2004a5ef6d6

This script signature means that the sender’s balance should be set to 0. In this example, the transaction data “OP_2” specifies the value 100.

Hand coding a Bitcoin transaction

To hand-code a Bitcoin transaction, we need to build a new block and add the necessary components to it.

Here is an example of hand-coding a Bitcoin transaction:

Input:
Previous tx: f5d8ee39a430901c91a5917b9f2dc19d6d1a0e9cea205b009ca73dd04470b9a6
Index: 0
ScriptSig:
۴a76a51c7f9b6a6a0000000000000000000000000000000000000000000000000000000000000000000100
vsize: 20 bytes (transaction data only)
vpri: 3047023278d0782e2bfc35d41ed29cdab4ea5ef6dc
dates:
OP_1: 0
OP_3: f5d8ee39a430901c91a5917b9f2dc19d6d1a0e9cea205b009ca73dd04470b9a6

This block consists of:

• prevhash: Previous block hash, set to 0 (since we are starting from scratch)

• index: Index of the previous transaction, which is 0

• scriptSig: Script signature that sets the sender’s balance to zero and increases their balance by 100 using OP_2

• “vsize”: Size of the transaction data, set to 20 bytes (transaction data only)

• vpri: Sender’s public key, set to a hardcoded value in this example

• data: Script signature that increases the sender’s balance by 100 using OP_2

Signing and Verifying a Transaction

To sign a transaction, we need to create a new Bitcoin wallet and load it with our public key. We can then use the wallet’s signing API to create a digital signature for the transaction.

Here is an example of signing a transaction:

import bitcoins





Load wallet
wallet = bitcoin.new_wallet()

Create a new block
block = bitcoin.NewBlock()

Set previous hash and index
previous_hash = '0'
index=0

Add script signature and data to block
block.set_script Sig(4a76a51c7f9b6a6a0000000000000000000000000000000000000000000000000000000000000000000000000)
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. It then establishes a new block and adds the necessary script signatures and information to it. Finally, it signs the block using the wallet’s signature API and prints the signed block.