Bitcoin Transactions

Bitcoin Transactions

Quick refs: sighash.md, serialization.md, malleability.md, rbf-cpfp.md, v3-truc.md

Anatomy

version (4 bytes)
[marker (0x00) flag (0x01)]    ← only if SegWit, signals witness presence
vin count (varint)
vin[]:
  prevout: { txid (32), vout (4) }
  scriptSig (varint-prefixed)   ← legacy unlocking script
  sequence (4 bytes)
vout count (varint)
vout[]:
  value (8 bytes, sats)
  scriptPubKey (varint-prefixed)
[witness[]]                     ← if marker/flag present, one stack per input
locktime (4 bytes)

Version: 1, 2 (BIP68 relative locktimes require v≥2), 3 (TRUC, BIP431).

txid vs wtxid

• txid = SHA256d(tx without witness data) — what spends and merkle root reference.
• wtxid = SHA256d(tx with witness) — what coinbase witness commitment uses (BIP141 witness merkle root).
• For legacy non-segwit txs, txid == wtxid.

Inputs & outputs

• Input spends a previous output (txid:vout) and proves authorization via scriptSig + (for SegWit) witness stack. Value = whatever the prevout had — never serialized in the spending tx (must be looked up).
• Output locks N sats to a scriptPubKey. Address formats:
  • P2PKH (1...) — OP_DUP OP_HASH160 <20> OP_EQUALVERIFY OP_CHECKSIG
  • P2SH (3...) — OP_HASH160 <20> OP_EQUAL
  • P2WPKH (bc1q...) — OP_0 <20> (Bech32)
  • P2WSH (bc1q...) — OP_0 <32> (Bech32)
  • P2TR (bc1p...) — OP_1 <32> (Bech32m)

Sighash flags

See sighash.md for full combinatorics. Summary:

Taproot adds SIGHASH_DEFAULT (0x00 = ALL with new digest) and changes serialization (BIP341).

TRUC v3 (BIP431)

• Version=3 marks the tx as TRUC.
• Constraints: vsize ≤ 10000 vB, ≤1 unconfirmed ancestor, ≤1 unconfirmed descendant, ancestor must also be v3.
• Sibling eviction: a v3 child can replace a sibling without paying for the sibling's bandwidth.
• Designed for Lightning anchor / commitment fee bumping: enables 0-fee parent + child paying full package fee.

Ephemeral anchors

• Anchor with value=0 and trivially-spendable script (e.g., OP_TRUE).
• Must be spent in the same package as its parent (mempool rule).
• Combined with TRUC v3, replaces existing anchor outputs (BOLT-3) with no-cost variant.

See also

• scripts/SKILL.md — opcodes, output type construction
• taproot/SKILL.md — Taproot witness, SIGHASH_DEFAULT
• psbt/SKILL.md — partially-signed tx workflow