Decrypting Raw ScriptOutput: Extracting ScriptPubKey

When receiving transactions from Bitcoin Core using zeroMQ, you are likely dealing with raw script output in JSON format. The script field contains a serialized version of a Bitcoin script, which is what we need to extract the scriptPubKey. This type represents the public key that can be used for digital signatures and other cryptographic operations.

In this article, we will walk through how to retrieve the scriptpubkey (the actual bytes representing the public key) from the raw script output.

Understanding Bitcoin ScriptFormat

Before we dive into the code, let’s quickly review the format of a Bitcoin script. The “script” field in Bitcoin Core transactions typically consists of a serialized version of the following:

• OP_DUP: duplicates the current operand (in this case, the value)

• OP_P2PKH: specifies that we are dealing with private keys

• OP_D2SH: converts the script to a human-readable format

• …and so on

The raw script output typically looks like this:

{ value: 200000,
script: 
}

Decoding ScriptOutput

To extractscriptPubKey, you need to we decode the raw script output. We can use the following Python code snippet as an example:

“python

import json

def extract_scriptpubkey(script_output):

Split the script into individual elements

elements = script_output[‘script’].split(‘\x00’)

Initialize the scriptpubkey variable

script_pubkey = None

Iterate through each element and try to decode it as a script signature

for element in elements:

if element.startswith(‘OP_DUP’):

Extract the value of the duplicate operand (the input)

value = int.from_bytes(element[7:9], byteorder=’big’)

Check if this is an OP_P2PKH instruction with a private key signature

if “P2SH” in element and “OP_01” in element:

Extract the public key from the input (assuming 1-based indexing)

public_key = int.from_bytes(element[9:16], byteorder=’big’)

Set the scriptpubkey variable with the extracted public key

if scriptpubkey is None:

scriptpubkey = bytes([public_key])

else:

scriptpubkey += bytes([public_key])

elif “P2SH” in element and “OP_01” in element:

Extract the public key from the input (assuming it is a 1-based indexing)

public_key = int.from_bytes(element[9:16], byteorder=’big’)

Set the scriptpubkey variable with an inactive value for the OP_DUP statement

if scriptpubkey is None:

scriptpubkey = bytes([public_key])

else:

scriptpubkey += bytes([public_key])

elif “OP_01” in element:

Extract the public key from the input (assuming it is a 1-based indexing)

public_key = int.from_bytes(element[8:16], byteorder=’big’)

Set the scriptpubkey variable to an inactive value for the OP_DUP statement

if script_pubkey is None:

script_pubkey = bytes([public_key])

else:

script_pubkey += bytes([public_key])

return script_pubkey

Usage example

script_output = json.loads(‘{“value”: 200000, “script”: “

script_pubkey = extract_scriptpubkey(script_output)

print(script_pubkey.