MIME-Version: 1.0
From: "Russell O'Connor" <roconnor@blockstream.io>
Date: Wed, 22 May 2019 17:01:21 -0400
Message-ID: <CAMZUoK=0YhqxguphmaKsMGZCy_NYVE_i53qGBfPVES=GAYTy1g@mail.gmail.com>
To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Content-Type: multipart/alternative; boundary="00000000000029a5ce05898045db"
Subject: [bitcoin-dev] An alternative: OP_CAT & OP_CHECKSIGFROMSTACK
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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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<div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><div di=
r=3D"ltr"><div>Recently there have been some tapscript proposals, SIGHASH_A=
NYPREVOUT and OP_CHECKOUTPUTHASHVERIFY, that aim to enable particular new f=
eatures for Bitcoin via new Script operations.=C2=A0 However, I think that =
these proposals miss the mark when it comes to how they approach Bitcoin Sc=
ript and language features.</div><div><br></div><div>Bitcoin Script appears=
 designed to be a flexible programmable system that provides generic featur=
es to be composed to achieve various purposes.=C2=A0 Thus, when we design n=
ew 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.</div><div><br></div><div>I feel the SIGHASH_ANYPREVOU=
T and OP_CHECKOUTPUTHASHVERIFY proposals fail to achieve these design goals=
.=C2=A0 They are both are designed with very narrow applications in mind, w=
hile also going out of their way to extend the semantic domain of the inter=
pretation of Bitcoin operations in new ways that complicate their specifica=
tion.=C2=A0 In the case of SIGHASH_ANYPREVOUT, the semantic domain is exten=
ded by adding new counters to track the use of various v0 and v2 signature =
types.=C2=A0 In the case of OP_CHECKOUTPUTHASHVERIFY, it employs a new cont=
ext-sensitive operation that peeks at the value of surrounding opcodes.</di=
v><div><br></div><div>Instead, I propose that, for the time being, we simpl=
y implement OP_CAT and OP_CHECKSIGFROMSTACKVERIFY.=C2=A0 OP_CAT pops two by=
te arrays off the stack and pushes their concatenation back onto the stack.=
=C2=A0 OP_CHECKSIGFROMSTACKVERIFY pops a signature, message, and pubkey off=
 the stack and performs a bip-schnorr verification on the SHA256 hash of th=
e message.<br></div><div><br></div><div>In concert, these two operations en=
able:</div><div><br></div><div>* Oracle signature verification, including d=
iscrete log contracts.</div><div>* Amortized secure multiparty computations=
 (see &quot;Amortizing Secure Computation with Penalties&quot; by Kumaresan=
 and Bentov).</div><div>* Transaction introspection including:<br></div><di=
v><a class=3D"gmail_plusreply" id=3D"gmail-plusReplyChip-0">+</a>=C2=A0Simu=
lated SIGHASH_ANYPREVOUT, which are necessarily chaperoned simply by the na=
ture of the construction.<br></div><div><a class=3D"gmail_plusreply" id=3D"=
gmail-plusReplyChip-1">+</a> Decide if a transaction has exactly one input =
or not. (etc.)</div><div>+ Weak covenants, which can verify output scripts =
to see if they are among a set of predefined values or verify the output ha=
sh.<br></div><div><br></div><div>and presumably more applications as well.<=
br></div><div><br></div><div>For better or for worse, without an OP_PUBKEYT=
WEEK operation available, the more interesting recursive-covenants remain l=
argely out of reach, with the exception of a recursive covenant that is onl=
y able to send back to its own address, possibly abusing its own TXO value =
as a state variable.</div><div><br></div><div>All this is accomplished by t=
wo straightforward opcodes whose semantics are pure computational operation=
s on stack values.=C2=A0 The only semantic side-effect is that OP_CHECKSIGF=
ROMSTACKVERIFY would count towards the existing &#39;sigops_passed&#39; cou=
nt.=C2=A0 Moreover, I feel that adding these operations does not preclude a=
dding more specialized opcodes in the future as an optimization for whateve=
r popular constructions come up, once we know what those are.<br></div><div=
><br></div><div>I feel that this style of generic building blocks truly emb=
odies what is meant by &quot;programmable money&quot;.<br></div></div></div=
></div></div></div>

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