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

"Ephemeral anchor" names two separate policies shipped in two different releases; see package-relay/SKILL.md.

• Pay-to-Anchor (P2A) — keyless standard output OP_1 <0x4e73>, a 2-byte witness v1 program (not Taproot, whose programs are 32 bytes). Standard to spend since Bitcoin Core 28.0 (October 2024); specified by BIP433, Status Draft as of September 2026. Default dust limit 240 sat.
• Ephemeral dust — Bitcoin Core 29.0 (April 2025) permits one dust output, any script type and down to value=0, provided the creating tx pays zero fee.
• The dust must be spent by anything that spends the parent's unconfirmed outputs, i.e. in the same package (mempool rule).
• A bare OP_TRUE scriptPubKey is not a standard output template and will not relay; BIP433 presents P2A as the compact replacement for sh(OP_TRUE).
• Combined with TRUC v3, replaces BOLT-3's two 330-sat keyed anchor outputs with a single keyless shared_anchor P2A output under zero_fee_commitments: 240 sat in the normal case, dropping below 240 sat (down to 0) only when the commitment has less than that left over, in which case the commitment pays zero fee. See BOLT-3 03-transactions.md, "shared_anchor Output (zero_fee_commitments)".

See also

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