Discussion:
BIP114 Merklized Script update and 5 BIPs for new script functions
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Johnson Lau via bitcoin-dev
2017-09-08 09:49:46 UTC
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I have rewritten and simplified BIP114, and renamed it to “Merklized Script”, as a more accurate description after consulting the original proposers of MAST. It could be considered as a special case of MAST, but has basically the same functions and scaling properties of MAST.

Compared with Friedenbach’s latest tail-call execution semantics proposal, I think the most notable difference is BIP114 focuses on maintaining the static analysability, which was a reason of OP_EVAL (BIP12) being rejected. Currently we could count the number of sigOp without executing the script, and this remains true with BIP114. Since sigOp is a block-level limit, any OP_EVAL-like operation means block validity will depend on the precise outcome of script execution (instead of just pass or fail), which is a layer violation.

Link to the revised BIP114: https://github.com/jl2012/bips/blob/vault/bip-0114.mediawiki

On top of BIP114, new script functions are defined with 5 BIPs:

VVV: Pay-to-witness-public-key: https://github.com/jl2012/bips/blob/vault/bip-0VVV.mediawiki
WWW: String and Bitwise Operations in Merklized Script Version 0: https://github.com/jl2012/bips/blob/vault/bip-0WWW.mediawiki
XXX: Numeric Operations in Merklized Script Version 0: https://github.com/jl2012/bips/blob/vault/bip-0XXX.mediawiki
YYY: ECDSA signature operations in Merklized Script Version 0: https://github.com/jl2012/bips/blob/vault/bip-0YYY.mediawiki
ZZZ: OP_PUSHTXDATA: https://github.com/jl2012/bips/blob/vault/bip-0ZZZ.mediawiki

As a summary, these BIPs have the following major features:

1. Merklized Script: a special case of MAST, allows users to hide unexecuted branches in their scripts (BIP114)
2. Delegation: key holder(s) may delegate the right of spending to other keys (scripts), with or without additional conditions such as locktime. (BIP114, VVV)
3. Enabling all OP codes disabled by Satoshi (based on Elements project with modification. BIPWWW and XXX)
4. New SIGHASH definition with very high flexibility (BIPYYY)
5. Covenant (BIPZZZ)
6. OP_CHECKSIGFROMSTACK, modified from Elements project (BIPYYY)
7. Replace ~72 byte DER sig with fixed size 64 byte compact sig. (BIPYYY)

All of these features are modular and no need to be deployed at once. The very basic BIP114 (merklized script only, no delegation) could be done quite easily. BIP114 has its own versioning system which makes introducing new functions very easy.

Things I’d like to have:

1. BIP114 now uses SHA256, but I’m open to other hash design
2. Using Schnorr or similar signature scheme, instead of ECDSA, in BIPYYY.

Reference implementation: https://github.com/jl2012/bitcoin/commits/vault
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