P2WSH Script - Full Witness Execution

6. P2WSH Script — Full Witness Execution

Overview

Pay-to-Witness-Script-Hash (P2WSH) is the SegWit equivalent of P2SH, introduced alongside P2WPKH in BIP 141. Instead of a 20-byte hash, the witness program is a 32-byte SHA256 hash of a witness script, providing stronger pre-image resistance. P2WSH enables complex scripts (multisig, timelocks, custom logic) to benefit from SegWit's weight discount and malleability fix.

Script Structure

Locking script (scriptPubKey) — 34 bytes:

OP_0 <32-byte witness program>

The 32-byte witness program is SHA256(witnessScript) — note: only SHA256, not HASH160.

scriptSig: Empty.

Witness:

<n items satisfying witnessScript>
<serialized witnessScript>

Byte-Level Anatomy

scriptPubKey (34 bytes):
00            <- OP_0 (witness version 0)
20            <- push 32 bytes
<32-byte SHA256(witnessScript)>

Witness for 2-of-3 multisig:
05            <- 5 witness items
00            <- empty dummy (OP_CHECKMULTISIG bug)
47            <- sig1 length
<sig1>
47            <- sig2 length
<sig2>
XX            <- witnessScript length
<witnessScript bytes>

Execution Rules

For a 32-byte version 0 witness program:

1. The last witness item is deserialized as the witnessScript.

2. SHA256(witnessScript) is computed and compared to the 32-byte witness program.

3. If they match, the witnessScript is executed with the remaining witness items as the initial stack.

Witness stack (before script execution):
[<dummy>, <sig1>, <sig2>]

witnessScript (2-of-3 multisig):
OP_2 <pk1> <pk2> <pk3> OP_3 OP_CHECKMULTISIG

Execution:
→ OP_CHECKMULTISIG verifies 2 of 3 signatures
→ Stack: [1]
→ VALID

Security Upgrade: 32-byte Hash

P2WSH uses a 32-byte SHA256 hash instead of the 20-byte HASH160 used in P2SH. This matters for complex scripts:

P2SH collision resistance: 2^80 (birthday attack on 160-bit hash)
P2WSH collision resistance: 2^128 (birthday attack on 256-bit hash)

For advanced multisig and contract scripts where the script itself is large, the stronger hash prevents theoretical script-substitution attacks.