Step 4: Calculate Transaction Digest Components
After creating the basic transaction and scriptCode, we need to calculate three important hashes:
hashPrevouts: Double SHA256 of all input outpointshashSequence: Double SHA256 of all input sequence numbershashOutputs: Double SHA256 of all outputs
Implementation
def dsha256(data: bytes) -> bytes:
"""Double SHA256 hash"""
return hashlib.sha256(hashlib.sha256(data).digest()).digest()
def get_transaction_digest(inputs: List[bytes], outputs: List[bytes]) -> tuple[bytes, bytes, bytes]:
"""Calculate BIP143 transaction digest components"""
# For hashPrevouts: concatenate all outpoints (txid + vout)
outpoints = b''
sequences = b''
for tx_input in inputs:
outpoints += tx_input[:36] # first 36 bytes are outpoint (txid + vout)
sequences += tx_input[-4:] # last 4 bytes are sequence
# Calculate all three hashes
hash_prevouts = dsha256(outpoints)
hash_sequence = dsha256(sequences)
hash_outputs = dsha256(b''.join(outputs))
return hash_prevouts, hash_sequence, hash_outputsTest Vector from BIP143
Let's verify our implementation against the official BIP143 test vectors.
You can check the official BIP143 specification to confirm that our generated hashes match exactly with the test vectors provided there.
# Expected hashes from BIP143 test vector
hashPrevouts = "96b827c8483d4e9b96712b6713a7b68d6e8003a781feba36c31143470b4efd37"
hashSequence = "52b0a642eea2fb7ae638c36f6252b6750293dbe574a806984b8e4d8548339a3b"
hashOutputs = "863ef3e1a92afbfdb97f31ad0fc7683ee943e9abcf2501590ff8f6551f47e5e5"Next Step
With these digest components ready, we'll move on to creating the complete signature hash preimage.