Re: [bitcoin-dev] An alternative: OP_CAT & OP_CHECKSIGFROMSTACK

["Russell O'Connor" <roconnor@blockstream.io>]

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Hi Jimmy,

The message could really be anything.  For example, in discreet log
contracts, AFAIU, you might have a specific public key from a trusted third
party (the Oracle) that is signs the closing price of corn in BTC on
2019-05-23 with a particular nonce dedicated to that product-date pair, in
which case the message would be the price expressed in binary.  In the case
of amortized secure multiparty computations, the message is protocol
specific binary data that consists of a counter (or counters), concatenated
with shares of secret data that is used to construct the result of the
multiparty computation.  In the case of transaction reflection, the message
would be a duplicate copy of the tapscript signed transaction data (about
244 bytes of data plus a 64 byte prefix).

As you note, the message is likely to constructed from a value computed
from a mix of witness and committed data, though the message might be pure
witness data, as in the discreet log contract example.  In that the
discreet log contract example, you'd probably duplicate the integer value
and do further processing (e.g. compare it to some other committed value).

On Thu, May 23, 2019 at 1:36 PM Jimmy Song <jaejoon@gmail.com> wrote:

> Hi Russell,
>
> This is probably a dumb question, but I'd like to get some clarity on your
> proposal.
>
> OP_CHECKSIGFROMSTACKVERIFY would pop off a signature, message and pubkey.
> Presumably, the message would then have to get constructed as part of the
> Script execution. What would such a message look like? What, in other
> words, would you be signing and would that be similar to what signatures
> sign now? Would it be a single blob that incorporates all the input/output
> information in some hashed manner (like BIP143)? Or would you need separate
> signatures for different parts of the transaction? Or is it something more
> complicated like aggregating multiple signatures over different parts of
> the transaction?
>
> Best,
>
> Jimmy
>
> On Thu, May 23, 2019 at 8:35 AM Russell O'Connor via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> 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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>

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