Date: Thu, 29 Dec 2011 20:08:39 +0100
From: Pieter Wuille <pieter.wuille@gmail.com>
To: bitcoin-development@lists.sourceforge.net
Message-ID: <20111229190838.GA29609@ulyssis.org>
References: <alpine.LRH.2.00.1112290111310.22327@theorem.ca>
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Subject: Re: [Bitcoin-development] Alternative to OP_EVAL
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On Thu, Dec 29, 2011 at 01:55:03AM -0500, roconnor@theorem.ca wrote:
> Gavin asked me to come up with an alternative to OP_EVAL, so here is my 
> proposal.
> 
> OP_CODEHASH Initial Proposal

If we're again brainstorming about alternatives for OP_EVAL, I'll do my own.

It is called OP_CHECKEDEVAL, and is specified as follows:
* It looks at the two elements most recently (in code position) pushed by a literal,
  and not yet consumed by another OP_CHECKEDEVAL. These are S (the serialized script),
  and H (its hash). This implies it defines its own literal-only stack, where all
  literals push to, and only OP_CHECKEDEVAL pops from. This special stack has the
  advantage of allowing static analysis - one does not need to execute any operations
  to find out which data will end up on it. Note that "skipped" code (inside the
  ignored part of an IF-THEN-ELSE) can still push to the literal stack.
* For the "outer script", it does not have any effect at all, except for:
  * 2 elements popped from the literal-only stack
  * potentially causing failure
  It does not touch the main stack, alt stack or any other part of the execution state
  not listed above.
* Failure is caused when either of these conditions hold:
  * No two elements remain on the literal-only stack
  * The Hash(S) != H
  * The inner script execution caused failure
* For the execution of the inner script:
  * It is executed in a completely new and independent execution environnement
  * It executes the deserialized S
  * It inherits the main stack and alt stack (without the serialized script and the hash
    themselves) from the outer execution.

This requires OP_CHECKEDEVAL to immediately follow the push of script and hash,
so the code in the pair < <script> OP_CHECKEDEVAL > can be parsed and interpreted as code, 
allowing static analysis.

As OP_CHECKEDEVAL has absolutely no effects except for potentially causing failure, it
is very similar to the OP_NOPx it would replace, and guarantees that interpreting
OP_CHECKEDEVAL as OP_NOPx can never cause the script to become invalid if it wasn't
already.

A basic pay-to-script-hash scriptPubKey is very short:
  
  <scriptHash> OP_CHECKEDEVAL

And it is redeemed using:

  <script inputs> <script>

Furthermore, the implementation is very similar to what was already done for
OP_EVAL. Modifications:
* EvalScriptInner needs less by-ref arguments, as it cannot modify the parent's state.
* A literal-only stack needs to be maintained.


I believe this combines all advantages:
* Easy spend-to-script-hash (shorter than OP_EVAL)
* Backward compatible (guaranteed by construction, instead of separately enforced like with OP_EVAL)
* Statically analyzable (though it requires deserializing the script data).
* Possibility to introduce a new language inside (not done in this proposal)

Only disadvantages:
* Slightly less flexible than OP_EVAL, as it disallows dynamic interation with serialized scripts.
* Static code analyzers need to deserialize script data.

Credits: gmaxwell for the idea of a literal-only stack

-- 
Pieter