Recently there have been some tapscript proposals, SIGHASH_ANYPREVOUT and OP_CHECKOUTPUTHASHVERIFY, that aim to enable particular new features for Bitcoin via new Script operations. However, I think that these proposals miss the mark when it comes to how they approach Bitcoin Script and language features.
Bitcoin Script appears designed to be a flexible programmable system that provides generic features to be composed to achieve various purposes. Thus, when we design new language features for Script, we should be striving, as much as possible, to similarly build general purpose tools which can in turn be used for a variety of purposes. I feel the SIGHASH_ANYPREVOUT and OP_CHECKOUTPUTHASHVERIFY proposals fail to achieve these design goals. They are both are designed with very narrow applications in mind, while also going out of their way to extend the semantic domain of the interpretation of Bitcoin operations in new ways that complicate their specification. In the case of SIGHASH_ANYPREVOUT, the semantic domain is extended by adding new counters to track the use of various v0 and v2 signature types. In the case of OP_CHECKOUTPUTHASHVERIFY, it employs a new context-sensitive operation that peeks at the value of surrounding opcodes. Instead, I propose that, for the time being, we simply implement OP_CAT and OP_CHECKSIGFROMSTACKVERIFY. OP_CAT pops two byte arrays off the stack and pushes their concatenation back onto the stack. OP_CHECKSIGFROMSTACKVERIFY pops a signature, message, and pubkey off the stack and performs a bip-schnorr verification on the SHA256 hash of the message. In concert, these two operations enable: * Oracle signature verification, including discrete log contracts. * Amortized secure multiparty computations (see "Amortizing Secure Computation with Penalties" by Kumaresan and Bentov). * Transaction introspection including: + Simulated SIGHASH_ANYPREVOUT, which are necessarily chaperoned simply by the nature of the construction. + Decide if a transaction has exactly one input or not. (etc.) + Weak covenants, which can verify output scripts to see if they are among a set of predefined values or verify the output hash. and presumably more applications as well. For better or for worse, without an OP_PUBKEYTWEEK operation available, the more interesting recursive-covenants remain largely out of reach, with the exception of a recursive covenant that is only able to send back to its own address, possibly abusing its own TXO value as a state variable. All this is accomplished by two straightforward opcodes whose semantics are pure computational operations on stack values. The only semantic side-effect is that OP_CHECKSIGFROMSTACKVERIFY would count towards the existing 'sigops_passed' count. Moreover, I feel that adding these operations does not preclude adding more specialized opcodes in the future as an optimization for whatever popular constructions come up, once we know what those are. I feel that this style of generic building blocks truly embodies what is meant by "programmable money".
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