The ScriptSig is a variable-length field in transaction inputs that provides the "unlocking" data needed to spend a previous output. Let's examine our transaction to understand this better:

Let's break down the highlighted hex string from our transaction:

     48 3045...126c01 21 02cd...259d2

This ScriptSig follows the standard P2PKH (Pay to Public Key Hash) pattern, which we can identify by its two key components:

1. A DER-encoded signature followed by SIGHASH byte (3045...126c01)
2. A compressed public key (02cd...259d2)

    <signature> <pubkey>

The signature proves ownership of the private key, while the public key must hash to match the address in the previous output's ScriptPubKey.

1- Purpose and Structure

The ScriptSig serves as the "key" that unlocks the previous output's "lock" (ScriptPubKey). It typically contains:

• Digital signatures proving ownership
• Public keys or other data required by the previous output's script

For legacy transactions, the ScriptSig contains both the signature and any other data needed to satisfy the previous output's spending conditions. For segwit transactions, most of this data moves to the witness field, leaving the ScriptSig empty or minimal.

2- Implementation Example

Here's how you might parse a ScriptSig:

def parse_scriptsig(raw_tx: bytes, offset: int = 0) -> tuple[bytes, int]:
    """
    Parse a ScriptSig from raw transaction bytes
    
    Args:
        raw_tx: Raw transaction bytes
        offset: Starting position in bytes
        
    Returns:
        (scriptsig, new_offset)
    """
    # Read the script size (varint)
    script_size, offset = read_varint(raw_tx, offset)
    
    # Read the actual script bytes
    script = raw_tx[offset:offset + script_size]
    
    return script, offset + script_size

def decode_der_signature(sig_bytes: bytes) -> tuple[int, int]:
"""
Decode a DER-encoded ECDSA signature into r and s values
""" # Skip sequence byte
pos = 1 # Skip length byte
pos += 1 # Skip marker for r value
pos += 1 # Get r length
r_len = sig_bytes[pos]
pos += 1 # Get r value
r = int.from_bytes(sig_bytes[pos:pos + r_len], 'big')
pos += r_len # Skip marker for s value
pos += 1 # Get s length
s_len = sig_bytes[pos]
pos += 1 # Get s value
s = int.from_bytes(sig_bytes[pos:pos + s_len], 'big')

    return r, s

3- Common Script Types

P2PKH (Pay to Public Key Hash)

The most common legacy transaction type has this ScriptSig pattern:

   <signature> <pubkey>

P2SH (Pay to Script Hash)

For P2SH inputs, the ScriptSig contains:

   <signatures...> <redeem script>

Segwit Transactions

For native segwit transactions (P2WPKH/P2WSH), the ScriptSig is empty as the unlocking data moves to the witness field.

4- Size Considerations

The ScriptSig size is encoded as a varint before the actual script data. Common sizes are:

• P2PKH: ~107 bytes
• P2SH: Variable (depends on redeem script complexity)
• Native Segwit: 0 bytes
• P2SH-wrapped Segwit: ~23 bytes

5- Validation Rules

When validating a ScriptSig:

1. The script must parse successfully
2. The combined script (ScriptSig + ScriptPubKey) must execute without errors
3. The final stack must contain a true value
4. The script size must not exceed the maximum allowed size