Bitcoin Scripts

Bitcoin Script

Quick refs: opcodes.md, output-types.md, tapscript.md

Bitcoin Script is a stack-based, non-Turing-complete (no loops), forth-like language. Each opcode pushes/pops bytes from a stack. Validity = stack contains a single non-zero element after execution.

Execution model

1. Initialize empty stack.
2. Concatenate scriptSig + scriptPubKey (legacy) — execute serialized. (Disabled post-BIP16 P2SH for the segwit/taproot path.)
3. (For SegWit/Taproot) Witness stack pushes are pre-loaded onto the stack, then scriptPubKey (or witness script) executes.
4. After execution: top of stack must be non-zero / true.

Output type catalogue

P2PK, P2PKH = pre-2012 patterns, still consensus-valid. Mainnet wallets should use SegWit v0 (P2WPKH/P2WSH) as default and Taproot (P2TR) where feasible.

Sig encoding

• ECDSA (legacy/segwit v0): DER-encoded signature + 1-byte sighash flag suffix (<r,s> low-s only since BIP146).
• Schnorr (Taproot): 64-byte raw signature. Optional 1-byte sighash if not SIGHASH_DEFAULT (0x00) → 65 bytes.

Witness construction (SegWit v0)

For P2WPKH spending:

witness:
  <signature DER + sighash byte>
  <pubkey 33 bytes>

For P2WSH spending:

witness:
  <stack items consumed by witnessScript>
  ...
  <witnessScript bytes>           ← last element, hashed for verification

Witness construction (Taproot)

Two paths:

• Key path (default, cheapest, indistinguishable from single-sig):

  witness: [<schnorr sig>]
  

• Script path:

  witness: [<args for leaf script> ... <leaf script> <control block>]
  

  Control block: 0xc0 | parity + internal pubkey (32) + merkle path.

Standardness

Mempool policy enforces:

• No OP_RETURN outputs > 80 bytes data (configurable, default).
• No multi-sig with > 3 keys via "bare" multisig (post-P2SH only).
• No non-canonical signatures (low-s, DER strict).
• Tapscript opcodes: OP_CHECKSIGADD is consensus-active; new opcodes added per leaf-version policy.

See also

• taproot/SKILL.md
• miniscript/SKILL.md — script policy compiler
• descriptors/SKILL.md