Custom Python ScriptSig Decoder

In this final simulation, we will write a script that parses a raw ScriptSig from a Legacy transaction. We will identify the signature and the public key, and deconstruct the lengths of these data pushes.

The Python ScriptSig Decoder

def decode_scriptsig(scriptsig_hex):
    # Convert hex string to bytes
    data = bytes.fromhex(scriptsig_hex)
    cursor = 0

    print(f"--- Decoding Raw ScriptSig ---")

    # 1. Read Signature Length (Usually 0x47 or 0x48)
    sig_len = data[cursor]
    cursor += 1

    # 2. Extract Signature (DER Encoded)
    signature = data[cursor:cursor+sig_len]
    cursor += sig_len

    # 3. Read Public Key Length (Usually 0x21 for compressed)
    pubkey_len = data[cursor]
    cursor += 1

    # 4. Extract Public Key
    pubkey = data[cursor:cursor+pubkey_len]

    # Analysis
    print(f"\n[Signature Data]")
    print(f"[*] Length: {sig_len} bytes")
    print(f"[*] SIGHASH: 0x{signature[-1]:02x}")
    print(f"[*] Raw:    {signature.hex()[:30]}...")

    print(f"\n[Public Key Data]")
    print(f"[*] Length: {pubkey_len} bytes")
    print(f"[*] Type:   {'Compressed' if pubkey_len == 33 else 'Uncompressed'}")
    print(f"[*] Hex:    {pubkey.hex()}")

# --- Simulation ---
# A real P2PKH ScriptSig hex:
# [47] -> Sig Len (71)
# [3044...01] -> DER Signature + SIGHASH_ALL
# [21] -> Pubkey Len (33)
# [02...8f] -> Compressed Pubkey
sample_scriptsig = (
    "47" + 
    "3044022026859367ca7e2c943806297491730419266993a46611f71a0673c683c316226a022003c274191730419266993a46611f71a0673c683c316226a01" + 
    "21" + 
    "0281cd6495d3550e553c3066d40026a090ef666993a46611f71a0673c683c31622"
)

decode_scriptsig(sample_scriptsig)

How to Run the Decoder

Ensure you have Python 3 installed.
Copy the code into a file named scriptsig_decoder.py.
Run it using python3 scriptsig_decoder.py.

Technical Takeaways

Prefix-Based Parsing: Note how we use the first byte to tell us how many bytes to read next. This is how Bitcoin's streaming parser works.
SIGHASH Discovery: The last byte of the signature (0x01 in our example) is the SIGHASH flag. This is what makes the signature "valid" for the whole transaction.
Transparency: Even though the signature looks like random gibberish, it has a strict internal structure (DER) that allows nodes to verify it.

Congratulations! You have completed the ScriptSig module. You now understand the "Key" that unlocks the world's most valuable ledger.

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