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Merge pull request #64 from jrakibi/01-02-siganture-topics
Add 11 topics on tx signature
2 parents 97fbb96 + 8045533 commit 343a199

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‎decoding/transaction-signing-00.mdx

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---
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title: "Step 0: Create Base Transaction"
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date: 2024-01-25
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lastmod: "2024-01-25"
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draft: false
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category: Transactions
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layout: TopicBanner
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order: 0
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icon: "FaClipboardList"
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images:
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[
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"/bitcoin-topics/static/images/topics/thumbnails/transaction-module/signature/tx-thumbnail-signature-0.jpg"
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]
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---
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In this step, we define the base structure of the transaction, which includes:
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- Version (4 bytes)
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- Marker/Flag (required for SegWit transactions)
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- Locktime (4 bytes)
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Inputs, outputs, and witness data will be added in the next steps.
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<div className="dark:hidden w-full rounded-xl overflow-hidden full-width">
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<SvgDisplay
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src="/bitcoin-topics/static/images/topics/transactions/signature/signature6.svg"
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width="100%"
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height="auto"
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/>
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</div>
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<div className="hidden dark:block w-full rounded-xl overflow-hidden full-width">
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<SvgDisplay
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src="/bitcoin-topics/static/images/topics/transactions/signature/signature6.svg"
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width="100%"
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height="auto"
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/>
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</div>
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<ExpandableAlert
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title="SegWit vs Legacy Transactions"
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type="important"
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expandable={true}
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initialLines={3}
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>
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If a transaction has at least one SegWit input (native or wrapped), it must
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include:
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- Marker byte (0x00)
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- Flag byte (0x01)
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</ExpandableAlert>
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The transaction structure at this stage is:
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<CodeSnippet
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language="text"
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highlightLines={[4, 5, 6, 12]}
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showLineNumbers={true}
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code={`Transaction Breakdown:
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═══════════════════════════════════════════════════════════════════════════════════
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version: 01000000
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marker: 00
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flag: 01
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in: # We'll add inputs in the next step
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out: # We'll add outputs later
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locktime: 11000000`}
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/>
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## Code Implementation
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<CodeSnippet
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code={`def create_basic_tx(
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version: int,
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inputs: list,
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outputs: list,
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locktime: int,
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segwit: bool = True
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) -> bytes:
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# 4-byte version in little-endian
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tx_version = int_to_little_endian(version, 4)
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# Marker + Flag for segwit (only if segwit=True)
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marker_flag = b'\\x00\\x01' if segwit else b''
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# Number of inputs/outputs (varint)
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in_count = varint(len(inputs))
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out_count = varint(len(outputs))
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# Serialize inputs and outputs
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serialized_inputs = b''.join(inputs)
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serialized_outputs = b''.join(outputs)
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# Locktime (4 bytes)
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tx_locktime = int_to_little_endian(locktime, 4)
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return (
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tx_version +
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marker_flag +
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in_count +
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serialized_inputs +
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out_count +
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serialized_outputs +
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tx_locktime
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)`}
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language="python"
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/>
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Let's now add inputs and outputs in the next step!

‎decoding/transaction-signing-01.mdx

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---
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title: "Step 1: Create Transaction Inputs"
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date: 2024-01-25
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lastmod: "2024-01-25"
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draft: false
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category: Transactions
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layout: TopicBanner
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order: 1
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icon: "FaClipboardList"
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images:
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[
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"/bitcoin-topics/static/images/topics/thumbnails/transaction-module/signature/tx-thumbnail-signature-1.jpg"
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]
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---
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Each input in a Bitcoin transaction must specify:
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- Transaction ID (32 bytes): Points to the UTXO being spent
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- Output Index (4 bytes): Which output from that transaction
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- ScriptSig: Placeholder for the unlocking script
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- Sequence (4 bytes): Usually 0xFFFFFFFF
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<div className="w-full rounded-xl overflow-hidden full-width">
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<SvgDisplay
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src="/bitcoin-topics/static/images/topics/transactions/signature/signature7.svg"
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width="100%"
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height="auto"
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/>
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</div>
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The following table summarizes our UTXOs (Transaction IDs shown in big-endian format):
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| Input | Transaction ID (big-endian) | Output Index | Sequence |
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| ----- | ---------------------------------------------------------------- | ------------ | -------- |
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| #1 | 9f96add4e4db413543df3eea1781c3be62637f1e2dd44069fa99801a88f7f7ff | 0 | eeffffff |
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| #2 | 8ac60eb9575db5b2d987e29f301b5b819ea83a5c6579d282d189cc04b8e151ef | 1 | ffffffff |
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_Note: In the transaction serialization below, these Transaction IDs are converted to little-endian format as required by the Bitcoin protocol._
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<CodeSnippet
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language="text"
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highlightLines={[6, 8, 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, 20]}
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showLineNumbers={true}
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code={`Transaction Breakdown:
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═══════════════════════════════════════════════════════════════════════════════════
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Version: 01000000
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Input Count: 02
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input[0]:
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Previous TXID: fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f
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Output Index: 00000000
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ScriptSig Size: 00
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ScriptSig:
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Sequence: eeffffff
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input[1]:
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Previous TXID: ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a
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Output Index: 01000000
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ScriptSig Size: 00
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ScriptSig:
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Sequence: ffffffff
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Locktime: 11000000`}
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/>
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<ExpandableAlert
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title="Why are scriptSig fields empty?"
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type="info"
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expandable={true}
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>
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Transaction signing follows a specific order:
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1. First, we create the
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transaction structure with empty scriptSig fields
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2. Then, we calculate the
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signature hash (sighash) for each input
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3. Finally, we add the signatures to
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create the complete transaction
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</ExpandableAlert>
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## Code Implementation
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<CodeSnippet
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code={`def create_input(
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txid: str,
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vout: int,
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script_sig: bytes = b'',
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sequence: bytes = b'\\xff\\xff\\xff\\xff'
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) -> bytes:
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# Convert txid from hex string and reverse (to little-endian)
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txid_bytes = bytes.fromhex(txid)[::-1]
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# Convert vout to 4 bytes, little-endian
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vout_bytes = int_to_little_endian(vout, 4)
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# Script length and script
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script_sig_length = varint(len(script_sig))
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return (
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txid_bytes + # 32 bytes
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vout_bytes + # 4 bytes
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script_sig_length + # 1 byte
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script_sig + # variable
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sequence # 4 bytes
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)`}
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language="python"
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/>
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<ExpandableAlert title="Helper Functions" type="info" expandable={true}>
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Two essential encoding functions:
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1. int_to_little_endian: Converts integers to little-endian byte format
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2. varint: Encodes variable-length integers used for counts and lengths
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<CodeSnippet
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code={`def int_to_little_endian(value: int, length: int) -> bytes:
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"""Convert an integer to little-endian bytes"""
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return value.to_bytes(length, 'little')
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def varint(n: int) -> bytes:
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"""Encode an integer as a variable length integer"""
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if n < 0xfd:
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return bytes([n])
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elif n <= 0xffff:
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return b'\\xfd' + n.to_bytes(2, 'little')
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elif n <= 0xffffffff:
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return b'\\xfe' + n.to_bytes(4, 'little')
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else:
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return b'\\xff' + n.to_bytes(8, 'little')`}
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language="python"
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/>
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</ExpandableAlert>

‎decoding/transaction-signing-02.mdx

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---
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title: "Step 2: Create Transaction Outputs"
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date: 2024-01-25
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lastmod: "2024-01-25"
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draft: false
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category: Transactions
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layout: TopicBanner
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order: 2
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icon: "FaClipboardList"
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images:
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[
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"/bitcoin-topics/static/images/topics/thumbnails/transaction-module/signature/tx-thumbnail-signature-2.jpg"
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]
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---
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Each output in a Bitcoin transaction requires:
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- Amount (8 bytes): Value in satoshis
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- ScriptPubKey: Locking script that defines spending conditions
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<div className="w-full rounded-xl overflow-hidden full-width">
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<SvgDisplay
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src="/bitcoin-topics/static/images/topics/transactions/signature/signature8.svg"
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width="100%"
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height="auto"
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/>
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</div>
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---
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## Code Implementation
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<CodeSnippet
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code={`def create_output(
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amount: int,
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script_pubkey: bytes
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) -> bytes:
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# Amount (8 bytes, little-endian)
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amount_bytes = int_to_little_endian(amount, 8)
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return (
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amount_bytes + # value in satoshis
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script_pubkey # includes length prefix
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)`}
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language="python"
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/>
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The following table summarizes our outputs from test vector:
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| Output | Amount (BTC) | ScriptPubKey |
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| ------ | ------------ | ---------------------------------------------------- |
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| #1 | 1.2 | 1976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac |
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| #2 | 2.2 | 1976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac |
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Our transaction structure at this stage is:
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<CodeSnippet
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language="text"
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highlightLines={[22, 24, 25, 26, 27, 29, 30, 31, 32]}
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showLineNumbers={true}
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code={`Transaction Breakdown:
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═══════════════════════════════════════════════════════════════════════════════════
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Version: 01000000
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Input Count: 02
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input[0]:
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Previous TXID: fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f
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Output Index: 00000000
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ScriptSig Size: 00
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ScriptSig:
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Sequence: eeffffff
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input[1]:
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Previous TXID: ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a
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Output Index: 01000000
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ScriptSig Size: 00
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ScriptSig:
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Sequence: ffffffff
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Output Count: 02
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output[0]:
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Amount: 202cb20600000000
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ScriptPubKey Size: 19
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ScriptPubKey: 76a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac
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output[1]:
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Amount: 9093510d00000000
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ScriptPubKey Size: 19
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ScriptPubKey: 76a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac
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Locktime: 11000000`}
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/>
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## Test Vector from BIP-143
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Let's test our code with the test vector from the <a href="https://github.com/bitcoin/bips/blob/58ffd93812ff25e87d53d1f202fbb389fdfb85bb/bip-0143.mediawiki?plain=1#L146" target="_blank">official BIP143 proposal</a>.
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<div className="flex justify-center items-center w-full full-width">
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<iframe
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src="https://trinket.io/embed/python3/e3098585e3c8"
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width="100%"
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height="100%"
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style={{
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border: "none",
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margin: 0
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}}
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allowFullScreen
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className="rounded-md shadow-sm h-[calc(50vh)]"
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></iframe>
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</div>

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