Step 1: Create Transaction Inputs

Each input in a Bitcoin transaction must specify:

Transaction ID (32 bytes): Points to the UTXO being spent
Output Index (4 bytes): Which output from that transaction
ScriptSig: Placeholder for the unlocking script
Sequence (4 bytes): Usually 0xFFFFFFFF

Input	Transaction ID (big-endian)	Output Index	Sequence
#1	9f96add4e4db413543df3eea1781c3be62637f1e2dd44069fa99801a88f7f7ff	0	eeffffff
#2	8ac60eb9575db5b2d987e29f301b5b819ea83a5c6579d282d189cc04b8e151ef	1	ffffffff

Note: In the transaction serialization below, these Transaction IDs are converted to little-endian format as required by the Bitcoin protocol.

text

1Transaction Breakdown:
2═══════════════════════════════════════════════════════════════════════════════════
3
4Version: 01000000
5
6Input Count: 02
7
8input[0]:
9Previous TXID: fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f
10Output Index: 00000000
11ScriptSig Size: 00
12ScriptSig:
13Sequence: eeffffff
14
15input[1]:
16Previous TXID: ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a
17Output Index: 01000000
18ScriptSig Size: 00
19ScriptSig:
20Sequence: ffffffff
21
22Locktime: 11000000

Why are scriptSig fields empty?

Transaction signing follows a specific order:

First, we create the transaction structure with empty scriptSig fields
Then, we calculate the signature hash (sighash) for each input
Finally, we add the signatures to create the complete transaction

Code Implementation

python

def create_input(
    txid: str,
    vout: int,
    script_sig: bytes = b'',
    sequence: bytes = b'\xff\xff\xff\xff'
) -> bytes:
    # Convert txid from hex string and reverse (to little-endian)
    txid_bytes = bytes.fromhex(txid)[::-1]
    
    # Convert vout to 4 bytes, little-endian
    vout_bytes = int_to_little_endian(vout, 4)
    
    # Script length and script
    script_sig_length = varint(len(script_sig))
    
    return (
        txid_bytes +           # 32 bytes
        vout_bytes +          # 4 bytes
        script_sig_length +   # 1 byte
        script_sig +          # variable
        sequence              # 4 bytes
    )

Helper Functions

Two essential encoding functions:

int_to_little_endian: Converts integers to little-endian byte format
varint: Encodes variable-length integers used for counts and lengths

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Next: Step 2: Create Transaction Outputs