BIP 18 hashScriptCheck explained: review its dataSig and scriptCheck model, sigop accounting, compatibility plan and relationship to deployed P2SH.
This guide explains BIP 18 hashScriptCheck in plain English. It covers the problem behind the BIP, why it matters and whether the proposal is part of Bitcoin today.
BIP 18 hashScriptCheck: dataSig replaced scriptSig with push-only data, its final element was the scriptCheck, and hashScriptCheck replaced scriptPubKey with a commitment to that script.
A node operator is checking BIP 18 hashScriptCheck. A near match or a block before activation would use legacy script processing. The model attempted to describe data and executable policy more explicitly while remaining interpretable by legacy software through the familiar HASH160 template.
dataSig replaced scriptSig with push-only data, its final element was the scriptCheck, and hashScriptCheck replaced scriptPubKey with a commitment to that script. The model attempted to describe data and executable policy more explicitly while remaining interpretable by legacy software through the familiar HASH160 template.
The committed output was required to match OP_HASH160, a 20-byte hash push and OP_EQUAL exactly. A near match or a block before activation would use legacy script processing. Exact template recognition is consensus-sensitive: accepting extra opcodes or non-minimal lengths would change which validation branch runs.
The dataSig had to contain pushes only, the final scriptCheck had to hash to the output commitment, and execution with the earlier data elements preloaded had to finish successfully with true on top. Each step creates a distinct rejection reason useful for test vectors and incident diagnosis.
CHECKSIG operations counted individually, small preceded CHECKMULTISIG constructions counted by their explicit key number, and other CHECKMULTISIG cases counted as twenty. Static counting bounded block verification cost even when branches were not executed. Template and pool software needed the same conservative interpretation.
The proposal described non-standard behaviour for old relay nodes, permissive old-node block interpretation, coinbase signalling and a coordinated activation threshold. That history shows why a hash-equality outer script did not by itself make the inner spending rule active for all validators.
BIP 18 stated that it was protocol-identical to BIP 16 while framing and deprecating the legacy fields differently. Production Bitcoin documentation normally uses BIP 16 terminology: scriptSig, redeem script and P2SH scriptPubKey. Mixing the alternate names into current runbooks can obscure what deployed software actually exposes.
Developers test the proposal with made-up data on an isolated test network. They check normal cases and deliberately invalid cases. Different implementations should reach the same result before anyone relies on the proposal.
BIP 18 hashScriptCheck: dataSig replaced scriptSig with push-only data, its final element was the scriptCheck, and hashScriptCheck replaced scriptPubKey with a commitment to that script.
dataSig replaced scriptSig with push-only data, its final element was the scriptCheck, and hashScriptCheck replaced scriptPubKey with a commitment to that script.
The committed output was required to match OP_HASH160, a 20-byte hash push and OP_EQUAL exactly.
The dataSig had to contain pushes only, the final scriptCheck had to hash to the output commitment, and execution with the earlier data elements preloaded had to finish successfully with true on top.
BIP 18 offers a precise historical vocabulary for the P2SH transition, including resource accounting and compatibility. Current operations should validate the deployed BIP 16 behaviour and use BIP 18 only to understand the design record.
Check the current specification status and the documentation for the exact implementation you operate before moving production funds or changing a mining node.
BIP 441 guide to the Draft tapleaf 0xC2 script-restoration proposal, varops, stack limits, unsigned arithmetic, restored opcodes and safety testing.
BIP 440 guide to the Draft transaction-wide variable-operation budget, weight scaling, opcode cost classes, benchmarks, validation and soft-fork status.
BIP 361 guide to the Draft informational post-quantum migration and legacy-signature sunset proposal, phases, prerequisites, risks and readiness planning.
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