In this tutorial series, we'll break down the process of signing a Bitcoin transaction into 7 detailed posts. We'll work with a real example from BIP143's test vectors to explain each step of the signing process.
You can find the complete tutorial series on our website at bitcoindevs.xyz/decoding/transaction-signing.
The transaction we'll be working with has two inputs, but we'll focus specifically on signing the second input which is a P2WPKH (native SegWit) input.
The first input is a legacy P2PK input which uses a different signing method that we won't cover now.
Our goal is to create and sign this transaction:
Unsigned Raw Transaction: ---------------------- 0100000002fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f 0000000000eeffffffef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57 b90ec68a0100000000ffffffff02202cb206000000001976a9148280b37df378db99f66f85 c95a783a76ac7a6d5988ac9093510d000000001976a9143bde42dbee7e4dbe6a21b2d50ce2 f0167faa815988ac11000000 Transaction Breakdown: ---------------------- nVersion: 01000000 txin: 02 fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f 00000000 00 eeffffff ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a 01000000 00 ffffffff txout: 02 202cb20600000000 1976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac 9093510d00000000 1976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac nLockTime: 11000000`}
Below is the complete transaction data. You can refer back to this whenever you need to check any details during the following steps:
Inputs
| Input | Type | Previous Tx ID | Output Index | Amount |
|---|---|---|---|---|
| #0 | P2PK | 9f96ade4b41d5433f4eda31e1738ec2b36f6e7d1420d94a6af99801a88f7f7ff | 0 | 6.25 BTC |
| #1 | P2WPKH | 8ac60eb9575db5b2d987e29f301b5b819ea83a5c6579d282d189cc04b8e151ef | 1 | 6 BTC |
Outputs
| Output | Amount | ScriptPubKey |
|---|---|---|
| #0 | 1.1234 BTC (112,340,000 satoshis) | 1976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac |
| #1 | 2.2345 BTC (223,450,000 satoshis) | 1976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac |
Steps We'll Follow
-
Create Basic Transaction Structure
-
Add Inputs & Outputs
-
Generate ScriptCode
-
Calculate Transaction Digest
-
Calculate Preimage & Sighash
-
Generate Signature
-
Build Witness Data
-
Assemble Final Transaction
Each step will be explained in detail in the following sections, with code examples and tests using this exact transaction data.