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From: James Lu <jamtlu@gmail.com>
Date: Wed, 26 Jan 2022 23:20:40 -0500
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To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
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Subject: Re: [bitcoin-dev] TXHASH + CHECKSIGFROMSTACKVERIFY in lieu of CTV
	and ANYPREVOUT
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What if OP_TXHASH is a no op except for the purpose of emulating CTV and
APO?

On Wed, Jan 26, 2022 at 5:16 PM Jeremy via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hi Russell,
>
> Thanks for this email, it's great to see this approach described.
>
> A few preliminary notes of feedback:
>
> 1) a Verify approach can be made to work for OP_TXHASH (even with CTV
> as-is) E.g., suppose a semantic added for a single byte stack[-1] sighash
> flag to read the hash at stack[-2], then the hash can be passed in instead
> of put on the stack. This has the disadvantage of larger witnesses, but the
> advantage of allowing undefined sighash flags to pass for any hash type.
> 2) using the internal key for APO covenants is not an option because it
> makes transaction construction interactive and precludes contracts with a
> NUMS point taproot key. Instead, if you want similar savings, you should
> advocate an OP_GENERATOR which puts G on the stack. Further, an untagged
> APO variant which has split R and S values would permit something like
> <sig> OP_GENERATOR OP_GENERATOR CHECKSIGAPO, which would be only 2 more
> bytes than CTV.
> 3) I count something like 20 different flags in your proposal. As long as
> flags are under 40 bytes (and 32 assuming we want it to be easy) without
> upgrading math this should be feasible to manipulate on the stack
> programmatically. This is ignoring some of the more flexible additions you
> mention about picking which outputs/inputs are included. However, 20 flags
> means that for testing we would want comprehensive tests and understanding
> for ~1 million different flag combos and the behaviors they expose. I think
> this necessitates a formal model of scripting and transaction validity
> properties. Are there any combinations that might be undesirable?
> 4) Just hashing or not hashing isn't actually that flexible, because it
> doesn't natively let you do things like (for example) TLUV. You really do
> need tx operations for directly manipulating the data on the stack to
> construct the hash if you want more flexible covenants. This happens to be
> compatible with either a Verify or Push approach, since you either
> destructure a pushed hash or build up a hash for a verify.
> 5) Flexible hashing has the potential for quadratic hashing bugs. The
> fields you propose seem to be within similar range to work you could cause
> with a regular OP_HASH256, although you'd want to be careful with some of
> the proposed extensions that you don't create risk of quadratic hashing,
> which seems possible with an output selecting opcode unless you cache
> properly (which might be tricky to do). Overall for the fields explicitly
> mentioned, seems safe, the "possibles" seem to have some more complex
> interactions. E.g., CTV with the ability to pick a subset of outputs would
> be exposed to quadratic hashing.
> 6) Missing field: covering the annex or some sub-range of the annex
> (quadratic hashing issues on the latter)
> 7) It seems simpler to, for many of these fields, push values directly (as
> in OP_PUSHTXDATA from Johnson Lau) because the combo of flags to push the
> hash of a single output's amount to emulate OP_AMOUNT looks 'general but
> annoying'. It may make more sense to do the OP_PUSHTXDATA style opcode
> instead. This also makes it simpler to think about the combinations of
> flags, since it's really N independent multi-byte opcodes.
>
>
> Ultimately if we had OP_TXHASH available "tomorrow", I would be able to
> build out the use cases I care about for CTV (and more). So I don't have an
> opposition on it with regards to lack of function.
>
> However, if one finds the TXHASH approach acceptable, then you should also
> be relatively fine doing APO, CTV, CSFS, TXHASH acceptable in any order
> (whenever "ready"), unless you are particularly sensitive to "technical
> debt" and "soft fork processes". The only costs of doing something for CTV
> or APO given an eventual TXHASH is perhaps a wasted key version or the 32
> byte argument of a NOP opcode and some code to maintain.
>
> Are there other costs I am missing?
>
> However, as it pertains to actual rollout:
>
> - OP_TXHASH+CSFSV doesn't seem to be the "full" set of things needed (we
> still need e.g. OP_CAT, Upgraded >=64 bit Math, TLUV or OP_TWEAK
> OP_TAPBRANCH OP_MANIPULATETAPTREE, and more) to full realize covenanting
> power it intends to introduce.
> - What sort of timeline would it take to ready something like TXHASH (and
> desired friends) given greater scope of testing and analysis (standalone +
> compared to CTV)?
> - Is there opposition from the community to this degree of
> general/recursive covenants?
> - Does it make "more sense" to invest the research and development effort
> that would go into proving TXHASH safe, for example, into Simplicity
> instead?
>
> Overall, *my opinion *is that:
>
> - TXHASH is an acceptable theoretical approach, and I am happy to put more
> thought into it and maybe draft a prototype of it.
> - I prefer CTV as a first step for pragmatic engineering and availability
> timeline reasons.
> - If TXHASH were to take, optimistically, 2 years to develop and review,
> and then 1 year to activate, the "path dependence of software" would put
> Bitcoin in a much better place were we to have CTV within 1 year and
> applications (that are to be a subset of TXHASH later) being built over the
> next few years enhanced in the future by TXHASH's availability.
> - There is an element of expediency meritted for something like CTV
> insofar as it provides primitives to tackle time sensitive issues around
> privacy, scalability, self custody, and decentralization. The
> aforementioned properties may be difficult to reclaim once given away (with
> the exception of perhaps scalability).
> - Bringing CTV to an implemented state of near-unanimous "we could do
> this, technically" is good for concretely driving the process of review for
> any covenant proposals forward, irrespective of if we ultimately activate.
> (I.e., if there were a reason we could not do CTV safely, it would likely
> have implications for any other future covenant)
>
> Concretely, I'm not going to stop advocating for CTV based on the above,
> but I'm very happy to have something new in the mix to consider!
>
> Best,
>
> Jeremy
>
>
> --
> @JeremyRubin <https://twitter.com/JeremyRubin>
> <https://twitter.com/JeremyRubin>
>
>
> On Wed, Jan 26, 2022 at 9:23 AM Russell O'Connor via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Recapping the relationship between CTV and ANYPREVOUT::
>>
>> It is known that there is a significant amount of overlap in the
>> applications that are enabled by the CTV and ANYPREVOUT proposals despite
>> the fact that their primary applications (congestion control for CTV and
>> eltoo lightning channels for ANYPREVOUT) are quite distinct.
>> In particular, ANYPREVOUT can enable most of the applications of CTV,
>> albeit with a higher cost.  The primary functionality of CTV is to allow a
>> scriptPubKey to make a commitment to its spending transaction's hash with
>> the input's TXID excluded from the hash.  This exclusion is necessary
>> because the scriptPubKey is hashed into the input's TXID, and including the
>> TXID would cause a cycle of hash commitments, which is impossible to
>> construct.  On the other hand, ANYPREVOUT defines a signature hash mode
>> that similarly excludes the inputs TXID for its purpose of rebindable
>> signatures.
>>
>> This means that ANYPREVOUT can mimic most of the properties of CTV by
>> committing both a public key along with an ANYPREVOUT signature inside
>> scriptPubKey.  In fact, the only reason Bitcoin doesn't have covenants
>> today is due to this cycle between scriptPubKeys and the TXIDs that occur
>> in all the sighash modes.
>>
>> The major differences between simulating CTV via ANYPREVOUT and the
>> actual CTV proposal is: (1) The cost of simulating CTV.  With CTV the
>> spending transaction is committed using a hash of 32 bytes, while
>> simulating it with ANYPREVOUT requires 64 bytes for a signature, and 32
>> bytes for some public key, plus a few more bytes for various flags.  Some
>> of that cost could be reduced by using the inner public key (1 byte
>> representation) and, if we had CAT, maybe by assembling the signature from
>> reusable pieces (i.e. setting the nonce of the commited signature equal to
>> the public key).
>>
>> The other major difference is: (2) CTV's transaction hash covers values
>> such as the number of inputs in the transaction and their sequence numbers,
>> which ANYPREVOUT does not cover.  CTV's hash contains enough information so
>> that when combined with the missing TXIDs, you can compute the TXID of the
>> spending transaction.  In particular if the number of inputs is committed
>> to being 1, once the scriptpubkey's transaction id is known and committed
>> to the blockchain, the TXID of its spending transaction is deducible.  And
>> if that transaction has outputs that have CTV commitments in them, you can
>> deduce their spending TXIDs in turn.  While this is a pretty neat feature,
>> something that ANYPREVOUT cannot mimic, the main application for it is
>> listed as using congestion control to fund lightning channels, fixing their
>> TXIDs in advance of them being placed on chain.  However, if ANYPREVOUT
>> were used to mimic CTV, then likely it would be eltoo channels that would
>> be funded, and it isn't necessary to know the TXIDs of eltoo channels in
>> advance in order to use them.
>>
>>
>>
>> An Alternative Proposal::
>>
>> Given the overlap in functionality between CTV and ANYPREVOUT, I think it
>> makes sense to decompose their operations into their constituent pieces and
>> reassemble their behaviour programmatically.  To this end, I'd like to
>> instead propose OP_TXHASH and OP_CHECKSIGFROMSTACKVERIFY.
>>
>> OP_TXHASH would pop a txhash flag from the stack and compute a (tagged)
>> txhash in accordance with that flag, and push the resulting hash onto the
>> stack.
>> OP_CHECKSIGFROMSTACKVERIFY would pop a pubkey, message, and signature
>> from the stack and fail if the signature does not verify on that message.
>>
>> CTV and TXHASH have roughly equivalent functionality.  'CTV DROP' can be
>> simulated by '<ctv_style_flag> TXHASH EQUALVERIFY'.  The reverse is also
>> true where '<ctv_style_flag> TXHASH' can be simulated by CTV by
>> '<ctv-result-from-witness-stack> CTV', however, as you can see, simulating
>> TXHASH from CTV is much more expensive than the other way around, because
>> the resulting 32-byte hash result must be included as part of the witness
>> stack.
>>
>> '<anyprevout-pubkey> CHECKSIGVERIFY can be simulated by '<apo_style_flag>
>> TXHASH <pubkey> CHECKSIGFROMSTACKVERIFY'.  Here we see the advantage of
>> pushing the hash value onto the stack.  APO can be simulated without
>> needing to include a copy of the resulting txhash inside the witness data.
>>
>> In addition to the CTV and ANYPREVOUT applications, with
>> CHECKSIGFROMSTACKVERIFY we can verify signatures on arbitrary messages
>> signed by oracles for oracle applications.  This is where we see the
>> benefit of decomposing operations into primitive pieces.  By giving users
>> the ability to program their own use cases from components, we get more
>> applications out of fewer op codes!
>>
>>
>>
>> Caveats::
>>
>> First, I acknowledge that replicating the behaviour of CTV and ANYPREVOUT
>> does cost a few more bytes than using the custom purpose built proposals
>> themselves.  That is the price to be paid when we choose the ability to
>> program solutions from pieces.  But we get to reap the advantages of being
>> able to build more applications from these pieces.
>>
>> Unlike CTV, TXHASH is not NOP-compatable and can only be implemented
>> within tapscript.  In particular, bare CTV isn't possible with this
>> proposal.  However, this proposal doesn't preclude the possibility of
>> having CTV added to legacy script in while having TXHASH added to tapscript.
>>
>> For similar reasons, TXHASH is not amenable to extending the set of
>> txflags at a later date.  In theory, one could have TXHASH
>> abort-with-success when encountering an unknown set of flags.  However,
>> this would make analyzing tapscript much more difficult. Tapscripts would
>> then be able to abort with success or failure depending on the order script
>> fragments are assembled and executed, and getting the order incorrect would
>> be catastrophic.  This behavior is manifestly different from the current
>> batch of OP_SUCCESS opcodes that abort-with-success just by their mere
>> presence, whether they would be executed or not.
>>
>> I believe the difficulties with upgrading TXHASH can be mitigated by
>> designing a robust set of TXHASH flags from the start.  For example having
>> bits to control whether (1) the version is covered; (2) the locktime is
>> covered; (3) txids are covered; (4) sequence numbers are covered; (5) input
>> amounts are covered; (6) input scriptpubkeys are covered; (7) number of
>> inputs is covered; (8) output amounts are covered; (9) output scriptpubkeys
>> are covered; (10) number of outputs is covered; (11) the tapbranch is
>> covered; (12) the tapleaf is covered; (13) the opseparator value is
>> covered; (14) whether all, one, or no inputs are covered; (15) whether all,
>> one or no outputs are covered; (16) whether the one input position is
>> covered; (17) whether the one output position is covered; (18) whether the
>> sighash flags are covered or not (note: whether or not the sighash flags
>> are or are not covered must itself be covered).  Possibly specifying which
>> input or output position is covered in the single case and whether the
>> position is relative to the input's position or is an absolute position.
>>
>> That all said, even if other txhash flag modes are needed in the future,
>> adding TXHASH2 always remains an option.
>>
>>
>>
>> Interactions with potential future opcodes::
>>
>> We should give some consideration on how these opcodes may interact with
>> future opcodes such as CAT, rolling SHA256 opcodes, or how it might
>> interface with other covenant opcodes that may do things like, directly
>> push input or output amounts onto the stack for computation purposes,
>> opcodes which have been added to the Elements project.
>>
>> With CAT and/or rolling SHA256 opcodes and/or existing SHA256 opcodes,
>> the CHECKSIGFROMSTACKVERIFY could verify signatures on programmatically
>> assembled messages.  Also, in combination with multiple calls to TXHASH,
>> could be used to create signatures that commit to complex subsets of
>> transaction data.
>>
>> If new opcodes are added to push parts of the transaction data direction
>> onto the stack, e.g. OP_INSPECTOUTPUTVALUE, there is perhaps concern that
>> they would obsolete TXHASH, since, in the presence of rolling SHA256
>> opcodes, TXHASH could be simulated.  However, given that TXHASH can
>> compactly create a hash of large portions of transaction data, it seems
>> unlikely that TXHASH would fall into disuse.  Also, a combination of TXHASH
>> and transaction introspection opcodes can be used to build "*subtractive
>> covenants*".
>>
>> The usual way of building a covenant, which we will call "*additive *
>> *covenants*", is to push all the parts of the transaction data you would
>> like to fix onto the stack, hash it all together, and verify the resulting
>> hash matches a fixed value.  Another way of building covenants, which we
>> will call "*subtractive covenants*", is to push all the parts of the
>> transaction data you would like to remain free onto the stack.  Then use
>> rolling SHA256 opcodes starting from a fixed midstate that commits to a
>> prefix of the transaction hash data. The free parts are hashed into that
>> midstate.  Finally, the resulting hash value is verified to match a value
>> returned by TXHASH.  The ability to nicely build subtractive covenants
>> depends on the details of how the TXHASH hash value is constructed,
>> something that I'm told CTV has given consideration to.
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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<div dir=3D"auto">What if OP_TXHASH is a no op except for the purpose of em=
ulating CTV and APO?</div><div><br><div class=3D"gmail_quote"><div dir=3D"l=
tr" class=3D"gmail_attr">On Wed, Jan 26, 2022 at 5:16 PM Jeremy via bitcoin=
-dev &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-d=
ev@lists.linuxfoundation.org</a>&gt; wrote:<br></div><blockquote class=3D"g=
mail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left-width:1px;border-=
left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div =
dir=3D"ltr"><div class=3D"gmail_default" style=3D"font-family:arial,helveti=
ca,sans-serif;font-size:small;color:rgb(0,0,0)">Hi Russell,</div><div class=
=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif;font-siz=
e:small;color:rgb(0,0,0)"><br></div><div class=3D"gmail_default" style=3D"f=
ont-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">Tha=
nks for this email, it&#39;s great to see this approach described.</div><di=
v class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif;f=
ont-size:small;color:rgb(0,0,0)"><br></div><div class=3D"gmail_default" sty=
le=3D"font-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,=
0)">A few preliminary notes of feedback:</div><div class=3D"gmail_default" =
style=3D"font-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0=
,0,0)"><br></div><div class=3D"gmail_default" style=3D"font-family:arial,he=
lvetica,sans-serif;font-size:small;color:rgb(0,0,0)">1) a Verify approach c=
an be made to work for OP_TXHASH (even with CTV as-is) E.g., suppose a sema=
ntic added for a single byte stack[-1] sighash flag to read the hash at sta=
ck[-2], then the hash can be passed in instead of put on the stack. This ha=
s the disadvantage of larger witnesses, but the advantage=C2=A0of allowing =
undefined sighash flags to pass for any hash type.</div><div class=3D"gmail=
_default" style=3D"font-family:arial,helvetica,sans-serif;font-size:small;c=
olor:rgb(0,0,0)">2) using the internal key for APO covenants is not an opti=
on because it makes transaction construction interactive and precludes cont=
racts with a NUMS point taproot key. Instead, if you want similar savings, =
you should advocate an OP_GENERATOR which puts G on the stack. Further, an =
untagged APO variant which has split R and S values would permit something =
like &lt;sig&gt; OP_GENERATOR OP_GENERATOR CHECKSIGAPO, which would be only=
 2 more bytes than CTV.</div><div class=3D"gmail_default" style=3D"font-fam=
ily:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">3) I count=
 something like 20 different flags in your proposal. As long as flags are u=
nder 40 bytes (and 32 assuming we want it to be easy) without upgrading mat=
h this should be feasible to manipulate on the stack programmatically. This=
 is ignoring some of the more flexible additions you mention about picking =
which outputs/inputs are included. However, 20 flags means that for testing=
 we would want comprehensive tests and understanding for ~1 million differe=
nt flag combos and the behaviors they expose. I think this necessitates a f=
ormal model of scripting and transaction validity properties. Are there any=
 combinations that might be undesirable?</div><div class=3D"gmail_default" =
style=3D"font-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0=
,0,0)">4) Just hashing or not hashing isn&#39;t actually that flexible, bec=
ause it doesn&#39;t natively let you do things like (for example) TLUV. You=
 really do need tx operations for directly manipulating the data on the sta=
ck to construct the hash if you want more flexible covenants. This happens =
to be compatible with either a Verify or Push approach, since you either de=
structure a pushed hash or build up a hash for a verify.</div><div class=3D=
"gmail_default" style=3D"font-family:arial,helvetica,sans-serif;font-size:s=
mall;color:rgb(0,0,0)">5) Flexible hashing has the potential for quadratic =
hashing bugs. The fields you propose seem to be within similar range to wor=
k you could cause with a regular OP_HASH256, although you&#39;d want to be =
careful with some of the proposed extensions that you don&#39;t create risk=
 of quadratic hashing, which seems possible with an output selecting opcode=
 unless you cache properly (which might be tricky to do). Overall for the f=
ields explicitly mentioned, seems safe, the &quot;possibles&quot; seem to h=
ave some more complex interactions. E.g., CTV with the ability to pick a su=
bset of outputs would be exposed to quadratic hashing.</div><div class=3D"g=
mail_default" style=3D"font-family:arial,helvetica,sans-serif;font-size:sma=
ll;color:rgb(0,0,0)">6) Missing field: covering the annex or some sub-range=
 of the annex (quadratic hashing issues on the latter)</div><div class=3D"g=
mail_default" style=3D"font-family:arial,helvetica,sans-serif;font-size:sma=
ll;color:rgb(0,0,0)">7) It seems simpler to, for many of these fields, push=
 values directly (as in OP_PUSHTXDATA from Johnson Lau) because the combo o=
f flags to push the hash of a single output&#39;s amount to emulate OP_AMOU=
NT looks &#39;general but annoying&#39;. It may make more sense to do the O=
P_PUSHTXDATA=C2=A0style opcode instead. This also makes it simpler to think=
 about the combinations of flags, since it&#39;s really N independent multi=
-byte opcodes.</div><div class=3D"gmail_default" style=3D"font-family:arial=
,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)"><br></div><div clas=
s=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif;font-si=
ze:small;color:rgb(0,0,0)"><br></div><div class=3D"gmail_default" style=3D"=
font-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">Ul=
timately if we had OP_TXHASH available &quot;tomorrow&quot;, I would be abl=
e to build out the use cases I care about for CTV (and more). So I don&#39;=
t have an opposition on it with regards to lack of function.</div><div clas=
s=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif;font-si=
ze:small;color:rgb(0,0,0)"><br></div><div class=3D"gmail_default" style=3D"=
font-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)"><d=
iv class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif"=
>However, if one finds the TXHASH approach acceptable, then you should also=
 be relatively fine doing APO, CTV, CSFS, TXHASH acceptable in any order (w=
henever &quot;ready&quot;), unless you are particularly sensitive to &quot;=
technical debt&quot; and &quot;soft fork=C2=A0processes&quot;. The only cos=
ts of doing something for CTV or APO given an eventual TXHASH=C2=A0is perha=
ps a wasted key version or the 32 byte argument of a NOP opcode and some co=
de to maintain.</div><div class=3D"gmail_default" style=3D"font-family:aria=
l,helvetica,sans-serif"><br></div><div class=3D"gmail_default" style=3D"fon=
t-family:arial,helvetica,sans-serif">Are there other costs I am missing?</d=
iv><div class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-s=
erif"></div></div><div class=3D"gmail_default" style=3D"font-family:arial,h=
elvetica,sans-serif;font-size:small;color:rgb(0,0,0)"><br></div><div class=
=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif;font-siz=
e:small;color:rgb(0,0,0)">However, as it pertains to actual rollout:</div><=
div class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif=
;font-size:small;color:rgb(0,0,0)"><br></div><div class=3D"gmail_default" s=
tyle=3D"font-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0,=
0,0)">- OP_TXHASH+CSFSV doesn&#39;t seem to be the &quot;full&quot; set of =
things needed (we still need e.g. OP_CAT, Upgraded &gt;=3D64 bit Math, TLUV=
 or OP_TWEAK OP_TAPBRANCH OP_MANIPULATETAPTREE, and more) to full realize c=
ovenanting power it intends to introduce.=C2=A0</div><div class=3D"gmail_de=
fault" style=3D"font-family:arial,helvetica,sans-serif;font-size:small;colo=
r:rgb(0,0,0)">- What sort of timeline would it take to ready something like=
 TXHASH=C2=A0(and desired friends) given greater scope of testing and analy=
sis (standalone=C2=A0+ compared to CTV)?</div><div class=3D"gmail_default" =
style=3D"font-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0=
,0,0)">- Is there opposition from the community to this degree of general/r=
ecursive covenants?</div><div class=3D"gmail_default" style=3D"font-family:=
arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">- Does it make=
 &quot;more sense&quot; to invest the research and development effort that =
would go into proving=C2=A0TXHASH safe, for example, into Simplicity instea=
d?</div><div class=3D"gmail_default" style=3D"font-family:arial,helvetica,s=
ans-serif;font-size:small;color:rgb(0,0,0)"><br></div><div class=3D"gmail_d=
efault" style=3D"font-family:arial,helvetica,sans-serif;font-size:small;col=
or:rgb(0,0,0)">Overall, <i style=3D"font-family:arial,helvetica,sans-serif"=
>my opinion </i>is that:</div><div class=3D"gmail_default" style=3D"font-fa=
mily:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)"><br></div=
><div class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-ser=
if;font-size:small;color:rgb(0,0,0)">- TXHASH=C2=A0is an acceptable theoret=
ical approach, and I am happy to put more thought into it and maybe draft a=
 prototype of it.</div><div class=3D"gmail_default" style=3D"font-family:ar=
ial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">- I prefer CTV a=
s a first step for pragmatic engineering and availability timeline reasons.=
</div><div class=3D"gmail_default" style=3D"font-family:arial,helvetica,san=
s-serif;font-size:small;color:rgb(0,0,0)">- If TXHASH were to take, optimis=
tically, 2 years to develop and review, and then 1 year to activate, the &q=
uot;path dependence of software&quot; would put Bitcoin in a much better pl=
ace were we to have CTV within 1 year and applications (that are to be a su=
bset of TXHASH=C2=A0later) being built over the next few years enhanced in =
the future by TXHASH&#39;s availability.=C2=A0</div><div class=3D"gmail_def=
ault" style=3D"font-family:arial,helvetica,sans-serif;font-size:small;color=
:rgb(0,0,0)">- There is an element of expediency meritted for something lik=
e CTV insofar as it provides primitives to tackle time sensitive issues aro=
und privacy, scalability, self custody, and decentralization. The aforement=
ioned properties may be difficult to reclaim once given away (with the exce=
ption of perhaps scalability).</div><div class=3D"gmail_default" style=3D"f=
ont-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">- B=
ringing CTV to an implemented state of near-unanimous &quot;we could do thi=
s, technically&quot; is good for concretely driving the process of review f=
or any covenant proposals forward, irrespective of if we ultimately activat=
e. (I.e., if there were a reason we could not do CTV safely, it would likel=
y have implications for any other future covenant)</div><div class=3D"gmail=
_default" style=3D"font-family:arial,helvetica,sans-serif;font-size:small;c=
olor:rgb(0,0,0)"><br></div><div class=3D"gmail_default" style=3D"font-famil=
y:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">Concretely, =
I&#39;m not going to stop advocating for CTV based on the above, but I&#39;=
m very happy to have something new in the mix to consider!</div><div class=
=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif;font-siz=
e:small;color:rgb(0,0,0)"><br></div><div class=3D"gmail_default" style=3D"f=
ont-family:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">Bes=
t,</div><div class=3D"gmail_default" style=3D"font-family:arial,helvetica,s=
ans-serif;font-size:small;color:rgb(0,0,0)"><br></div><div class=3D"gmail_d=
efault" style=3D"font-family:arial,helvetica,sans-serif;font-size:small;col=
or:rgb(0,0,0)">Jeremy</div><div class=3D"gmail_default" style=3D"font-famil=
y:arial,helvetica,sans-serif;font-size:small;color:rgb(0,0,0)"><br></div><d=
iv class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif;=
font-size:small;color:rgb(0,0,0)"><br></div><div><div dir=3D"ltr" data-smar=
tmail=3D"gmail_signature"><div dir=3D"ltr">--<br><a href=3D"https://twitter=
.com/JeremyRubin" target=3D"_blank">@JeremyRubin</a><a href=3D"https://twit=
ter.com/JeremyRubin" target=3D"_blank"></a></div></div></div><br></div><br>=
<div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Wed, Ja=
n 26, 2022 at 9:23 AM Russell O&#39;Connor via bitcoin-dev &lt;<a href=3D"m=
ailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">bitcoin-dev@=
lists.linuxfoundation.org</a>&gt; wrote:<br></div><blockquote class=3D"gmai=
l_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left-width:1px;border-lef=
t-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div dir=
=3D"ltr"><div>Recapping the relationship between CTV and ANYPREVOUT::<br></=
div><div><br></div><div>It is known that there is a significant amount of o=
verlap in the applications that are enabled by the CTV and ANYPREVOUT propo=
sals despite the fact that their primary applications (congestion control f=
or CTV and eltoo lightning channels for ANYPREVOUT) are quite distinct.</di=
v><div>In particular, ANYPREVOUT can enable most of the applications of CTV=
, albeit with a higher cost.=C2=A0 The primary functionality of CTV is to a=
llow a scriptPubKey to make a commitment to its spending transaction&#39;s =
hash with the input&#39;s TXID excluded from the hash.=C2=A0 This exclusion=
 is necessary because the scriptPubKey is hashed into the input&#39;s TXID,=
 and including the TXID would cause a cycle of hash commitments, which is i=
mpossible to construct.=C2=A0 On the other hand, ANYPREVOUT defines a signa=
ture hash mode that similarly excludes the inputs TXID for its purpose of r=
ebindable signatures.</div><div><br></div><div>This means that ANYPREVOUT c=
an mimic most of the properties of CTV by committing both a public key alon=
g with an ANYPREVOUT signature inside scriptPubKey.=C2=A0 In fact, the only=
 reason Bitcoin doesn&#39;t have covenants today is due to this cycle betwe=
en scriptPubKeys and the TXIDs that occur in all the sighash modes.<br></di=
v><div><br></div><div>The major differences between simulating CTV via ANYP=
REVOUT and the actual CTV proposal is: (1) The cost of simulating CTV.=C2=
=A0 With CTV the spending transaction is committed using a hash of 32 bytes=
, while simulating it with ANYPREVOUT requires 64 bytes for a signature, an=
d 32 bytes for some public key, plus a few more bytes for various flags.=C2=
=A0 Some of that cost could be reduced by using the inner public key (1 byt=
e representation) and, if we had CAT, maybe by assembling the signature fro=
m reusable pieces (i.e. setting the nonce of the commited signature equal t=
o the public key).<br></div><div><br></div><div>The other major difference =
is: (2) CTV&#39;s transaction hash covers values such as the number of inpu=
ts in the transaction and their sequence numbers, which ANYPREVOUT does not=
 cover.=C2=A0 CTV&#39;s hash contains enough information so that when combi=
ned with the missing TXIDs, you can compute the TXID of the spending transa=
ction.=C2=A0 In particular if the number of inputs is committed to being 1,=
 once the scriptpubkey&#39;s transaction id is known and committed to the b=
lockchain, the TXID of its spending transaction is deducible.=C2=A0 And if =
that transaction has outputs that have CTV commitments in them, you can ded=
uce their spending TXIDs in turn.=C2=A0 While this is a pretty neat feature=
, something that ANYPREVOUT cannot mimic, the main application for it is li=
sted as using congestion control to fund lightning channels, fixing their T=
XIDs in advance of them being placed on chain.=C2=A0 However, if ANYPREVOUT=
 were used to mimic CTV, then likely it would be eltoo channels that would =
be funded, and it isn&#39;t necessary to know the TXIDs of eltoo channels i=
n advance in order to use them.</div><div><br></div><div><br></div><div><br=
></div><div>An Alternative Proposal::<br></div><div><br></div><div>Given th=
e overlap in functionality between CTV and ANYPREVOUT, I think it makes sen=
se to decompose their operations into their constituent pieces and reassemb=
le their behaviour programmatically.=C2=A0 To this end, I&#39;d like to ins=
tead propose OP_TXHASH and OP_CHECKSIGFROMSTACKVERIFY.</div><div><br></div>=
<div>OP_TXHASH would pop a txhash flag from the stack and compute a (tagged=
) txhash in accordance with that flag, and push the resulting hash onto the=
 stack.<br></div><div>OP_CHECKSIGFROMSTACKVERIFY would pop a pubkey, messag=
e, and signature from the stack and fail if the signature does not verify o=
n that message.</div><div><br></div><div>CTV and TXHASH have roughly equiva=
lent functionality.=C2=A0 &#39;CTV DROP&#39; can be simulated by &#39;&lt;c=
tv_style_flag&gt; TXHASH EQUALVERIFY&#39;.=C2=A0 The reverse is also true w=
here &#39;&lt;ctv_style_flag&gt; TXHASH&#39; can be simulated by CTV by &#3=
9;&lt;ctv-result-from-witness-stack&gt; CTV&#39;, however, as you can see, =
simulating TXHASH from CTV is much more expensive than the other way around=
, because the resulting 32-byte hash result must be included as part of the=
 witness stack.</div><div><br></div><div>&#39;&lt;anyprevout-pubkey&gt; CHE=
CKSIGVERIFY can be simulated by &#39;&lt;apo_style_flag&gt; TXHASH &lt;pubk=
ey&gt; CHECKSIGFROMSTACKVERIFY&#39;.=C2=A0 Here we see the advantage of pus=
hing the hash value onto the stack.=C2=A0 APO can be simulated without need=
ing to include a copy of the resulting txhash inside the witness data.<br><=
/div><div><br></div><div>In addition to the CTV and ANYPREVOUT applications=
, with CHECKSIGFROMSTACKVERIFY we can verify signatures on arbitrary messag=
es signed by oracles for oracle applications.=C2=A0 This is where we see th=
e benefit of decomposing operations into primitive pieces.=C2=A0 By giving =
users the ability to program their own use cases from components, we get mo=
re applications out of fewer op codes!</div><div><br></div><div><br></div><=
div><br></div><div>Caveats::</div><div><br></div><div>First, I acknowledge =
that replicating the behaviour of CTV and ANYPREVOUT does cost a few more b=
ytes than using the custom purpose built proposals themselves.=C2=A0 That i=
s the price to be paid when we choose the ability to program solutions from=
 pieces.=C2=A0 But we get to reap the advantages of being able to build mor=
e applications from these pieces.<br></div><div><br></div><div>Unlike CTV, =
TXHASH is not NOP-compatable and can only be implemented within tapscript.=
=C2=A0 In particular, bare CTV isn&#39;t possible with this proposal.=C2=A0=
 However, this proposal doesn&#39;t preclude the possibility of having CTV =
added to legacy script in while having TXHASH added to tapscript.</div><div=
><br></div><div>For similar reasons, TXHASH is not amenable to extending th=
e set of txflags at a later date.=C2=A0 In theory, one could have TXHASH ab=
ort-with-success when encountering an unknown set of flags.=C2=A0 However, =
this would make analyzing tapscript much more difficult. Tapscripts would t=
hen be able to abort with success or failure depending on the order script =
fragments are assembled and executed, and getting the order incorrect would=
 be catastrophic.=C2=A0 This behavior is manifestly different from the curr=
ent batch of OP_SUCCESS opcodes that abort-with-success just by their mere =
presence, whether they would be executed or not.</div><div><br></div><div>I=
 believe the difficulties with upgrading TXHASH can be mitigated by designi=
ng a robust set of TXHASH flags from the start.=C2=A0 For example having bi=
ts to control whether (1) the version is covered; (2) the locktime is cover=
ed; (3) txids are covered; (4) sequence numbers are covered; (5) input amou=
nts are covered; (6) input scriptpubkeys are covered; (7) number of inputs =
is covered; (8) output amounts are covered; (9) output scriptpubkeys are co=
vered; (10) number of outputs is covered; (11) the tapbranch is covered; (1=
2) the tapleaf is covered; (13) the opseparator value is covered; (14) whet=
her all, one, or no inputs are covered; (15) whether all, one or no outputs=
 are covered; (16) whether the one input position is covered; (17) whether =
the one output position is covered; (18) whether the sighash flags are cove=
red or not (note: whether or not the sighash flags are or are not covered m=
ust itself be covered).=C2=A0 Possibly specifying which input or output pos=
ition is covered in the single case and whether the position is relative to=
 the input&#39;s position or is an absolute position.</div><div><br></div><=
div>That all said, even if other txhash flag modes are needed in the future=
, adding TXHASH2 always remains an option.<br></div><div><br></div><div><br=
></div><div><br></div><div>Interactions with potential future opcodes::<br>=
</div><div><div><br></div><div>We should give some consideration on how the=
se opcodes may interact with future opcodes such as CAT, rolling SHA256 opc=
odes, or how it might interface with other covenant opcodes that may do thi=
ngs like, directly push input or output amounts onto the stack for computat=
ion purposes, opcodes which have been added to the Elements project.</div><=
div><br></div><div>With CAT and/or rolling SHA256 opcodes and/or existing S=
HA256 opcodes, the CHECKSIGFROMSTACKVERIFY could verify signatures on progr=
ammatically assembled messages.=C2=A0 Also, in combination with multiple ca=
lls to TXHASH, could be used to create signatures that commit to complex su=
bsets of transaction data.</div><div><br></div><div>If new opcodes are adde=
d to push parts of the transaction data direction onto the stack, e.g. OP_I=
NSPECTOUTPUTVALUE, there is perhaps concern that they would obsolete TXHASH=
, since, in the presence of rolling SHA256 opcodes, TXHASH could be simulat=
ed.=C2=A0 However, given that TXHASH can compactly create a hash of large p=
ortions of transaction data, it seems unlikely that TXHASH would fall into =
disuse.=C2=A0 Also, a combination of TXHASH and transaction introspection o=
pcodes can be used to build &quot;<i>subtractive covenants</i>&quot;.</div>=
<div><br></div><div>The usual way of building a covenant, which we will cal=
l &quot;<i>additive </i><i>covenants</i>&quot;, is to push all the parts of=
 the transaction data you would like to fix onto the stack, hash it all tog=
ether, and verify the resulting hash matches a fixed value.=C2=A0 Another w=
ay of building covenants, which we will call &quot;<i>subtractive covenants=
</i>&quot;, is to push all the parts of the transaction data you would like=
 to remain free onto the stack.=C2=A0 Then use rolling SHA256 opcodes start=
ing from a fixed midstate that commits to a prefix of the transaction hash =
data. The free parts are hashed into that midstate.=C2=A0 Finally, the resu=
lting hash value is verified to match a value returned by TXHASH.=C2=A0 The=
 ability to nicely build subtractive covenants depends on the details of ho=
w the TXHASH hash value is constructed, something that I&#39;m told CTV has=
 given consideration to.<br></div></div></div>
_______________________________________________<br>
bitcoin-dev mailing list<br>
<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">=
bitcoin-dev@lists.linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer" target=3D"_blank">https://lists.linuxfoundation.org/mail=
man/listinfo/bitcoin-dev</a><br>
</blockquote></div>
_______________________________________________<br>
bitcoin-dev mailing list<br>
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bitcoin-dev@lists.linuxfoundation.org</a><br>
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rel=3D"noreferrer" target=3D"_blank">https://lists.linuxfoundation.org/mail=
man/listinfo/bitcoin-dev</a><br>
</blockquote></div></div>

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