[bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[Jeremy Rubin]

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I've seen some discussion of what the Annex can be used for in Bitcoin. For
example, some people have discussed using the annex as a data field for
something like CHECKSIGFROMSTACK type stuff (additional authenticated data)
or for something like delegation (the delegation is to the annex). I think
before devs get too excited, we should have an open discussion about what
this is actually for, and figure out if there are any constraints to using
it however we may please.

The BIP is tight lipped about it's purpose, saying mostly only:

*What is the purpose of the annex? The annex is a reserved space for future
extensions, such as indicating the validation costs of computationally
expensive new opcodes in a way that is recognizable without knowing the
scriptPubKey of the output being spent. Until the meaning of this field is
defined by another softfork, users SHOULD NOT include annex in
transactions, or it may lead to PERMANENT FUND LOSS.*

*The annex (or the lack of thereof) is always covered by the signature and
contributes to transaction weight, but is otherwise ignored during taproot
validation.*

*Execute the script, according to the applicable script rules[11], using
the witness stack elements excluding the script s, the control block c, and
the annex a if present, as initial stack.*

Essentially, I read this as saying: The annex is the ability to pad a
transaction with an additional string of 0's that contribute to the virtual
weight of a transaction, but has no validation cost itself. Therefore,
somehow, if you needed to validate more signatures than 1 per 50 virtual
weight units, you could add padding to buy extra gas. Or, we might somehow
make the witness a small language (e.g., run length encoded zeros) such
that we can very quickly compute an equivalent number of zeros to 'charge'
without actually consuming the space but still consuming a linearizable
resource... or something like that. We might also e.g. want to use the
annex to reserve something else, like the amount of memory. In general, we
are using the annex to express a resource constraint efficiently. This
might be useful for e.g. simplicity one day.

Generating an Annex: One should write a tracing executor for a script, run
it, measure the resource costs, and then generate an annex that captures
any externalized costs.

-------------------

Introducing OP_ANNEX: Suppose there were some sort of annex pushing opcode,
OP_ANNEX which puts the annex on the stack as well as a 0 or 1 (to
differentiate annex is 0 from no annex, e.g. 0 1 means annex was 0 and 0 0
means no annex). This would be equivalent to something based on <annex
flag> OP_TXHASH <has annex flag> OP_TXHASH.

Now suppose that I have a computation that I am running in a script as
follows:

OP_ANNEX
OP_IF
    `some operation that requires annex to be <1>`
OP_ELSE
    OP_SIZE
    `some operation that requires annex to be len(annex) + 1 or does a
checksig`
OP_ENDIF

Now every time you run this, it requires one more resource unit than the
last time you ran it, which makes your satisfier use the annex as some sort
of "scratch space" for a looping construct, where you compute a new annex,
loop with that value, and see if that annex is now accepted by the program.

In short, it kinda seems like being able to read the annex off of the stack
makes witness construction somehow turing complete, because we can use it
as a register/tape for some sort of computational model.

-------------------

This seems at odds with using the annex as something that just helps you
heuristically guess  computation costs, now it's somehow something that
acts to make script satisfiers recursive.

Because the Annex is signed, and must be the same, this can also be
inconvenient:

Suppose that you have a Miniscript that is something like: and(or(PK(A),
PK(A')), X, or(PK(B), PK(B'))).

A or A' should sign with B or B'. X is some sort of fragment that might
require a value that is unknown (and maybe recursively defined?) so
therefore if we send the PSBT to A first, which commits to the annex, and
then X reads the annex and say it must be something else, A must sign
again. So you might say, run X first, and then sign with A and C or B.
However, what if the script somehow detects the bitstring WHICH_A WHICH_B
and has a different Annex per selection (e.g., interpret the bitstring as a
int and annex must == that int). Now, given and(or(K1, K1'),... or(Kn,
Kn')) we end up with needing to pre-sign 2**n annex values somehow... this
seems problematic theoretically.

Of course this wouldn't be miniscript then. Because miniscript is just for
the well behaved subset of script, and this seems ill behaved. So maybe
we're OK?

But I think the issue still arises where suppose I have a simple thing
like: and(COLD_LOGIC, HOT_LOGIC) where both contains a signature, if
COLD_LOGIC and HOT_LOGIC can both have different costs, I need to decide
what logic each satisfier for the branch is going to use in advance, or
sign all possible sums of both our annex costs? This could come up if
cold/hot e.g. use different numbers of signatures / use checksigCISAadd
which maybe requires an annex argument.



------------

It seems like one good option is if we just go on and banish the OP_ANNEX.
Maybe that solves some of this? I sort of think so. It definitely seems
like we're not supposed to access it via script, given the quote from above:

*Execute the script, according to the applicable script rules[11], using
the witness stack elements excluding the script s, the control block c, and
the annex a if present, as initial stack.*

If we were meant to have it, we would have not nixed it from the stack, no?
Or would have made the opcode for it as a part of taproot...

But recall that the annex is committed to by the signature.

So it's only a matter of time till we see some sort of Cat and Schnorr
Tricks III the Annex Edition that lets you use G cleverly to get the annex
onto the stack again, and then it's like we had OP_ANNEX all along, or
without CAT, at least something that we can detect that the value has
changed and cause this satisfier looping issue somehow.

Not to mention if we just got OP_TXHASH



-----------

Is the annex bad? After writing this I sort of think so?

One solution would be to... just soft-fork it out. Always must be 0. When
we come up with a use case for something like an annex, we can find a way
to add it back.  Maybe this means somehow pushing multiple annexes and
having an annex stack, where only sub-segments are signed for the last
executed signature? That would solve looping... but would it break some
aggregation thing? Maybe.


Another solution would be to make it so the annex is never committed to and
unobservable from the script, but that the annex is always something that
you can run get_annex(stack) to generate the annex. Thus it is a hint for
validation rules, but not directly readable, and if it is modified you
figure out the txn was cheaper sometime after you execute the scripts and
can decrease the value when you relay. But this sounds like something that
needs to be a p2p only annex, because consensus we may not care (unless
it's something like preallocating memory for validation?).

-----------------------

Overall my preference is -- perhaps sadly -- looking like we should
soft-fork it out of our current Checksig (making the policy that it must 0
a consensus rule) and redesign the annex technique later when we actually
know what it is for with a new checksig or other mechanism. But It's not a
hard opinion! It just seems like you can't practically use the annex for
this worklimit type thing *and* observe it from the stack meaningfully.



Thanks for coming to my ted-talk,

Jeremy


--
@JeremyRubin <https://twitter.com/JeremyRubin>

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Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[ZmnSCPxj]

Good morning Jeremy,

Umm `OP_ANNEX` seems boring ....


> It seems like one good option is if we just go on and banish the OP_ANNEX. Maybe that solves some of this? I sort of think so. It definitely seems like we're not supposed to access it via script, given the quote from above:
>
> Execute the script, according to the applicable script rules[11], using the witness stack elements excluding the script s, the control block c, and the annex a if present, as initial stack.
> If we were meant to have it, we would have not nixed it from the stack, no? Or would have made the opcode for it as a part of taproot...
>
> But recall that the annex is committed to by the signature.
>
> So it's only a matter of time till we see some sort of Cat and Schnorr Tricks III the Annex Edition that lets you use G cleverly to get the annex onto the stack again, and then it's like we had OP_ANNEX all along, or without CAT, at least something that we can detect that the value has changed and cause this satisfier looping issue somehow.

... Never mind I take that back.

Hmmm.

Actually if the Annex is supposed to be ***just*** for adding weight to the transaction so that we can do something like increase limits on SCRIPT execution, then it does *not* have to be covered by any signature.
It would then be third-party malleable, but suppose we have a "valid" transaction on the mempool where the Annex weight is the minimum necessary:

* If a malleated transaction has a too-low Annex, then the malleated transaction fails validation and the current transaction stays in the mempool.
* If a malleated transaction has a higher Annex, then the malleated transaction has lower feerate than the current transaction and cannot evict it from the mempool.

Regards,
ZmnSCPxj

Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[Christian Decker]

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We'd have to be very carefully with this kind of third-party malleability,
since it'd make transaction pinning trivial without even requiring the
ability to spend one of the outputs (which current CPFP based pinning
attacks require).

Cheers,
Christian

On Sat, 5 Mar 2022, 00:33 ZmnSCPxj via bitcoin-dev, <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Good morning Jeremy,
>
> Umm `OP_ANNEX` seems boring ....
>
>
> > It seems like one good option is if we just go on and banish the
> OP_ANNEX. Maybe that solves some of this? I sort of think so. It definitely
> seems like we're not supposed to access it via script, given the quote from
> above:
> >
> > Execute the script, according to the applicable script rules[11], using
> the witness stack elements excluding the script s, the control block c, and
> the annex a if present, as initial stack.
> > If we were meant to have it, we would have not nixed it from the stack,
> no? Or would have made the opcode for it as a part of taproot...
> >
> > But recall that the annex is committed to by the signature.
> >
> > So it's only a matter of time till we see some sort of Cat and Schnorr
> Tricks III the Annex Edition that lets you use G cleverly to get the annex
> onto the stack again, and then it's like we had OP_ANNEX all along, or
> without CAT, at least something that we can detect that the value has
> changed and cause this satisfier looping issue somehow.
>
> ... Never mind I take that back.
>
> Hmmm.
>
> Actually if the Annex is supposed to be ***just*** for adding weight to
> the transaction so that we can do something like increase limits on SCRIPT
> execution, then it does *not* have to be covered by any signature.
> It would then be third-party malleable, but suppose we have a "valid"
> transaction on the mempool where the Annex weight is the minimum necessary:
>
> * If a malleated transaction has a too-low Annex, then the malleated
> transaction fails validation and the current transaction stays in the
> mempool.
> * If a malleated transaction has a higher Annex, then the malleated
> transaction has lower feerate than the current transaction and cannot evict
> it from the mempool.
>
> Regards,
> ZmnSCPxj
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[Anthony Towns]

On Fri, Mar 04, 2022 at 11:21:41PM +0000, Jeremy Rubin via bitcoin-dev wrote:
> I've seen some discussion of what the Annex can be used for in Bitcoin. 

https://www.erisian.com.au/meetbot/taproot-bip-review/2019/taproot-bip-review.2019-11-12-19.00.log.html

includes some discussion on that topic from the taproot review meetings.

The difference between information in the annex and information in
either a script (or the input data for the script that is the rest of
the witness) is (in theory) that the annex can be analysed immediately
and unconditionally, without necessarily even knowing anything about
the utxo being spent.

The idea is that we would define some simple way of encoding (multiple)
entries into the annex -- perhaps a tag/length/value scheme like
lightning uses; maybe if we add a lisp scripting language to consensus,
we just reuse the list encoding from that? -- at which point we might
use one tag to specify that a transaction uses advanced computation, and
needs to be treated as having a heavier weight than its serialized size
implies; but we could use another tag for per-input absolute locktimes;
or another tag to commit to a past block height having a particular hash.

It seems like a good place for optimising SIGHASH_GROUP (allowing a group
of inputs to claim a group of outputs for signing, but not allowing inputs
from different groups to ever claim the same output; so that each output
is hashed at most once for this purpose) -- since each input's validity
depends on the other inputs' state, it's better to be able to get at
that state as easily as possible rather than having to actually execute
other scripts before your can tell if your script is going to be valid.

> The BIP is tight lipped about it's purpose

BIP341 only reserves an area to put the annex; it doesn't define how
it's used or why it should be used.

> Essentially, I read this as saying: The annex is the ability to pad a
> transaction with an additional string of 0's 

If you wanted to pad it directly, you can do that in script already
with a PUSH/DROP combo.

The point of doing it in the annex is you could have a short byte
string, perhaps something like "0x010201a4" saying "tag 1, data length 2
bytes, value 420" and have the consensus intepretation of that be "this
transaction should be treated as if it's 420 weight units more expensive
than its serialized size", while only increasing its witness size by
6 bytes (annex length, annex flag, and the four bytes above). Adding 6
bytes for a 426 weight unit increase seems much better than adding 426
witness bytes.

The example scenario is that if there was an opcode to verify a
zero-knowledge proof, eg I think bulletproof range proofs are something
like 10x longer than a signature, but require something like 400x the
validation time. Since checksig has a validation weight of 50 units,
a bulletproof verify might have a 400x greater validation weight, ie
20,000 units, while your witness data is only 650 bytes serialized. In
that case, we'd need to artificially bump the weight of you transaction
up by the missing 19,350 units, or else an attacker could fill a block
with perhaps 6000 bulletproofs costing the equivalent of 120M signature
operations, rather than the 80k sigops we currently expect as the maximum
in a block. Seems better to just have "0x01024b96" stuck in the annex,
than 19kB of zeroes.

> Introducing OP_ANNEX: Suppose there were some sort of annex pushing opcode,
> OP_ANNEX which puts the annex on the stack

I think you'd want to have a way of accessing individual entries from
the annex, rather than the annex as a single unit.

> Now suppose that I have a computation that I am running in a script as
> follows:
> 
> OP_ANNEX
> OP_IF
>     `some operation that requires annex to be <1>`
> OP_ELSE
>     OP_SIZE
>     `some operation that requires annex to be len(annex) + 1 or does a
> checksig`
> OP_ENDIF
> 
> Now every time you run this,

You only run a script from a transaction once at which point its
annex is known (a different annex gives a different wtxid and breaks
any signatures), and can't reference previous or future transactions'
annexes...

> Because the Annex is signed, and must be the same, this can also be
> inconvenient:

The annex is committed to by signatures in the same way nVersion,
nLockTime and nSequence are committed to by signatures; I think it helps
to think about it in a similar way.

> Suppose that you have a Miniscript that is something like: and(or(PK(A),
> PK(A')), X, or(PK(B), PK(B'))).
> 
> A or A' should sign with B or B'. X is some sort of fragment that might
> require a value that is unknown (and maybe recursively defined?) so
> therefore if we send the PSBT to A first, which commits to the annex, and
> then X reads the annex and say it must be something else, A must sign
> again. So you might say, run X first, and then sign with A and C or B.
> However, what if the script somehow detects the bitstring WHICH_A WHICH_B
> and has a different Annex per selection (e.g., interpret the bitstring as a
> int and annex must == that int). Now, given and(or(K1, K1'),... or(Kn,
> Kn')) we end up with needing to pre-sign 2**n annex values somehow... this
> seems problematic theoretically.

Note that you need to know what the annex will contain before you sign,
since the annex is committed to via the signature. If "X" will need
entries in the annex that aren't able to be calculated by the other
parties, then they need to be the first to contribute to the PSBT, not A.

I think the analogy to locktimes would be "I need the locktime to be at
least block 900k, should I just sign that now, or check that nobody else
is going to want it to be block 950k or something? Or should I just sign
with nLockTime at 900k, 910k, 920k, 930k, etc and let someone else pick
the right one?" The obvious solution is just to work out what the
nLockTime should be first, then run signing rounds. Likewise, work out
what the annex should be first, then run the signing rounds.

CLTV also has the problem that if you have one script fragment with
CLTV by time, and another with CLTV by height, you can't come up with
an nLockTime that will ever satisfy both. If you somehow have script
fragments that require incompatible interpretations of the annex, you're
likewise going to be out of luck.

Having a way of specifying locktimes in the annex can solve that
particular problem with CLTV (different inputs can sign different
locktimes, and you could have different tags for by-time/by-height so
that even the same input can have different clauses requiring both),
but the general problem still exists.

(eg, you might have per-input by-height absolute locktimes as annex
entry 3, and per-input by-time absolute locktimes as annex entry 4,
so you might convert:

 "900e3 CLTV DROP" -> "900e3 3 PUSH_ANNEX_ENTRY GREATERTHANOREQUAL VERIFY"

 "500e6 CLTV DROP" -> "500e6 4 PUSH_ANNEX_ENTRY GREATERTHANOREQUAL VERIFY"

for height/time locktime checks respectively)

> Of course this wouldn't be miniscript then. Because miniscript is just for
> the well behaved subset of script, and this seems ill behaved. So maybe
> we're OK?

The CLTV issue hit miniscript:

https://medium.com/blockstream/dont-mix-your-timelocks-d9939b665094

> But I think the issue still arises where suppose I have a simple thing
> like: and(COLD_LOGIC, HOT_LOGIC) where both contains a signature, if
> COLD_LOGIC and HOT_LOGIC can both have different costs, I need to decide
> what logic each satisfier for the branch is going to use in advance, or
> sign all possible sums of both our annex costs? This could come up if
> cold/hot e.g. use different numbers of signatures / use checksigCISAadd
> which maybe requires an annex argument.

Signatures pay for themselves -- every signature is 64 or 65 bytes,
but only has 50 units of validation weight. (That is, a signature check
is about 50x the cost of hashing 520 bytes of data, which is the next
highest cost operation we have, and is treated as costing 1 unit, and
immediately paid for by the 1 byte that writing OP_HASH256 takes up)

That's why the "add cost" use of the annex is only talked about in
hypotheticals, not specified -- for reasonable scripts with today's
opcodes, it's not needed.

If you're doing cross-input signature aggregation, everybody needs to
agree on the message they're signing in the first place, so you definitely
can't delay figuring out some bits of some annex until after signing.

> It seems like one good option is if we just go on and banish the OP_ANNEX.
> Maybe that solves some of this? I sort of think so. It definitely seems
> like we're not supposed to access it via script, given the quote from above:

How the annex works isn't defined, so it doesn't make any sense to
access it from script. When how it works is defined, I expect it might
well make sense to access it from script -- in a similar way that the
CLTV and CSV opcodes allow accessing nLockTime and nSequence from script.

To expand on that: the logic to prevent a transaction confirming too
early occurs by looking at nLockTime and nSequence, but script can
ensure that an attempt to use "bad" values for those can never be a
valid transaction; likewise, consensus may look at the annex to enforce
new conditions as to when a transaction might be valid (and can do so
without needing to evaluate any scripts), but the individual scripts can
make sure that the annex has been set to what the utxo owner considered
to be reasonable values.

> One solution would be to... just soft-fork it out. Always must be 0. When
> we come up with a use case for something like an annex, we can find a way
> to add it back.

The point of reserving the annex the way it has been is exactly this --
it should not be used now, but when we agree on how it should be used,
we have an area that's immediately ready to be used.

(For the cases where you don't need script to enforce reasonable values,
reserving it now means those new consensus rules can be used immediately
with utxos that predate the new consensus rules -- so you could update
offchain contracts from per-tx to per-input locktimes immediately without
having to update the utxo on-chain first)

Cheers,
aj

Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[Jeremy Rubin]

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On Sat, Mar 5, 2022 at 5:59 AM Anthony Towns <aj@erisian.com.au> wrote:

> On Fri, Mar 04, 2022 at 11:21:41PM +0000, Jeremy Rubin via bitcoin-dev
> wrote:
> > I've seen some discussion of what the Annex can be used for in Bitcoin.
>
>
> https://www.erisian.com.au/meetbot/taproot-bip-review/2019/taproot-bip-review.2019-11-12-19.00.log.html
>
> includes some discussion on that topic from the taproot review meetings.
>
> The difference between information in the annex and information in
> either a script (or the input data for the script that is the rest of
> the witness) is (in theory) that the annex can be analysed immediately
> and unconditionally, without necessarily even knowing anything about
> the utxo being spent.
>

I agree that should happen, but there are cases where this would not work.
E.g., imagine OP_LISP_EVAL + OP_ANNEX... and then you do delegation via the
thing in the annex.

Now the annex can be executed as a script.



>
> The idea is that we would define some simple way of encoding (multiple)
> entries into the annex -- perhaps a tag/length/value scheme like
> lightning uses; maybe if we add a lisp scripting language to consensus,
> we just reuse the list encoding from that? -- at which point we might
> use one tag to specify that a transaction uses advanced computation, and
> needs to be treated as having a heavier weight than its serialized size
> implies; but we could use another tag for per-input absolute locktimes;
> or another tag to commit to a past block height having a particular hash.
>

Yes, this seems tough to do without redefining checksig to allow partial
annexes. Hence thinking we should make our current checksig behavior
require it be 0, future operations should be engineered with specific
structured annex in mind.



>
> It seems like a good place for optimising SIGHASH_GROUP (allowing a group
> of inputs to claim a group of outputs for signing, but not allowing inputs
> from different groups to ever claim the same output; so that each output
> is hashed at most once for this purpose) -- since each input's validity
> depends on the other inputs' state, it's better to be able to get at
> that state as easily as possible rather than having to actually execute
> other scripts before your can tell if your script is going to be valid.
>

I think SIGHASH_GROUP could be some sort of mutable stack value, not ANNEX.
you want to be able to compute what range you should sign, and then the
signature should cover the actual range not the argument itself.

Why sign the annex literally?

Why require that all signatures in one output sign the exact same digest?
What if one wants to sign for value and another for value + change?



>
> > The BIP is tight lipped about it's purpose
>
> BIP341 only reserves an area to put the annex; it doesn't define how
> it's used or why it should be used.
>
>
It does define how it's used, Checksig must commit to it. Were there no
opcodes dependent on it I would agree, and that would be preferable.




> > Essentially, I read this as saying: The annex is the ability to pad a
> > transaction with an additional string of 0's
>
> If you wanted to pad it directly, you can do that in script already
> with a PUSH/DROP combo.
>

You cannot, because the push/drop would not be signed and would be
malleable.

The annex is not malleable, so it can be used to this as authenticated
padding.



>
> The point of doing it in the annex is you could have a short byte
> string, perhaps something like "0x010201a4" saying "tag 1, data length 2
> bytes, value 420" and have the consensus intepretation of that be "this
> transaction should be treated as if it's 420 weight units more expensive
> than its serialized size", while only increasing its witness size by
> 6 bytes (annex length, annex flag, and the four bytes above). Adding 6
> bytes for a 426 weight unit increase seems much better than adding 426
> witness bytes.
>
>
Yes, that's what I say in the next sentence,

*> Or, we might somehow make the witness a small language (e.g., run length
encoded zeros) such that we can very quickly compute an equivalent number
of zeros to 'charge' without actually consuming the space but still
consuming a linearizable resource... or something like that.*

so I think we concur on that.



> > Introducing OP_ANNEX: Suppose there were some sort of annex pushing
> opcode,
> > OP_ANNEX which puts the annex on the stack
>
> I think you'd want to have a way of accessing individual entries from
> the annex, rather than the annex as a single unit.
>

Or OP_ANNEX + OP_SUBSTR + OP_POVARINTSTR? Then you can just do 2 pops for
the length and the tag and then get the data.


>
> > Now suppose that I have a computation that I am running in a script as
> > follows:
> >
> > OP_ANNEX
> > OP_IF
> >     `some operation that requires annex to be <1>`
> > OP_ELSE
> >     OP_SIZE
> >     `some operation that requires annex to be len(annex) + 1 or does a
> > checksig`
> > OP_ENDIF
> >
> > Now every time you run this,
>
> You only run a script from a transaction once at which point its
> annex is known (a different annex gives a different wtxid and breaks
> any signatures), and can't reference previous or future transactions'
> annexes...
>
>
In a transaction validator, yes. But in a satisfier, no.

And it doesn't break the signatures if we add the ability to only sign over
a part of an annex either/multiple annexes, since the annex could be
mutable partially.


Not true about accessing previous TXNs annexes. All coins spend from
Coinbase transactions. If you can get the COutpoint you're spending, you
can get the parent of the COutpoint... and iterate backwards so on and so
forth. Then you have the CB txn, which commits to the tree of wtxids. So
you get previous transactions annexes comitted there.


For future transactions, you can, as a miner with decent hashrate you could
promise what your Coinbase transaction would be for a future block and what
the Outputs would be, and then you can pop open that as well... but you
can't show valid PoW for that one so I'm not sure that's different than
authenticated data. But where it does have a use is that you could, if you
had OP_COUTPOINTVERIFY, say that this coin is only spendable if a miner
mines the specific block that you want at a certain height (e.g., with only
your txn in it?) and then they can claim the outpoint in the future... so
maybe there is something there bizzare that can happen with that
capability....



> > Because the Annex is signed, and must be the same, this can also be
> > inconvenient:
>
> The annex is committed to by signatures in the same way nVersion,
> nLockTime and nSequence are committed to by signatures; I think it helps
> to think about it in a similar way.
>

nSequence, yes, nLockTime is per-tx.

BTW i think we now consider nSeq/nLock to be misdesigned given desire to
vary these per-input/per-tx....\

so if the annex is like these perhaps it's also misdesigned.

>
> > Suppose that you have a Miniscript that is something like: and(or(PK(A),
> > PK(A')), X, or(PK(B), PK(B'))).
> >
> > A or A' should sign with B or B'. X is some sort of fragment that might
> > require a value that is unknown (and maybe recursively defined?) so
> > therefore if we send the PSBT to A first, which commits to the annex, and
> > then X reads the annex and say it must be something else, A must sign
> > again. So you might say, run X first, and then sign with A and C or B.
> > However, what if the script somehow detects the bitstring WHICH_A WHICH_B
> > and has a different Annex per selection (e.g., interpret the bitstring
> as a
> > int and annex must == that int). Now, given and(or(K1, K1'),... or(Kn,
> > Kn')) we end up with needing to pre-sign 2**n annex values somehow...
> this
> > seems problematic theoretically.
>
> Note that you need to know what the annex will contain before you sign,
> since the annex is committed to via the signature. If "X" will need
> entries in the annex that aren't able to be calculated by the other
> parties, then they need to be the first to contribute to the PSBT, not A.
>
> I think the analogy to locktimes would be "I need the locktime to be at
> least block 900k, should I just sign that now, or check that nobody else
> is going to want it to be block 950k or something? Or should I just sign
> with nLockTime at 900k, 910k, 920k, 930k, etc and let someone else pick
> the right one?" The obvious solution is just to work out what the
> nLockTime should be first, then run signing rounds. Likewise, work out
> what the annex should be first, then run the signing rounds.
>


Yes, my point is this is computationally hard to do sometimes.

>
> CLTV also has the problem that if you have one script fragment with
> CLTV by time, and another with CLTV by height, you can't come up with
> an nLockTime that will ever satisfy both. If you somehow have script
> fragments that require incompatible interpretations of the annex, you're
> likewise going to be out of luck.
>
>
Yes, see above. If we don't know how the annex will be structured or used,
this is the point of this thread....

We need to drill down how to not introduce these problems.



> Having a way of specifying locktimes in the annex can solve that
> particular problem with CLTV (different inputs can sign different
> locktimes, and you could have different tags for by-time/by-height so
> that even the same input can have different clauses requiring both),
> but the general problem still exists.
>
> (eg, you might have per-input by-height absolute locktimes as annex
> entry 3, and per-input by-time absolute locktimes as annex entry 4,
> so you might convert:
>
>  "900e3 CLTV DROP" -> "900e3 3 PUSH_ANNEX_ENTRY GREATERTHANOREQUAL VERIFY"
>
>  "500e6 CLTV DROP" -> "500e6 4 PUSH_ANNEX_ENTRY GREATERTHANOREQUAL VERIFY"
>
> for height/time locktime checks respectively)
>
> > Of course this wouldn't be miniscript then. Because miniscript is just
> for
> > the well behaved subset of script, and this seems ill behaved. So maybe
> > we're OK?
>
> The CLTV issue hit miniscript:
>
> https://medium.com/blockstream/dont-mix-your-timelocks-d9939b665094


Maybe the humour didn't hit -- we can only define well behaved as best we
know, and the solution was to re-define miniscript to only be the well
defined subset of miniscript once the bug in the spec was found.



>
> > It seems like one good option is if we just go on and banish the
> OP_ANNEX.
> > Maybe that solves some of this? I sort of think so. It definitely seems
> > like we're not supposed to access it via script, given the quote from
> above:
>
> How the annex works isn't defined, so it doesn't make any sense to
> access it from script. When how it works is defined, I expect it might
> well make sense to access it from script -- in a similar way that the
> CLTV and CSV opcodes allow accessing nLockTime and nSequence from script.
>

That's false: CLTV and CSV expressly do not allow accessing it from script,
only lower bounding it (and transitively proving that it was not of the
other flavour).

So you can't actually get the exact nLockTime / Sequence on the stack
(exception: if you use the maximum allowable value, then there are no other
values...)


Given that it's not defined at all, that's why I'm skeptical about signing
it at all presently.

If theres a future upgrade, it would be compatible as we can add new
sighash flags to cover that.


> > One solution would be to... just soft-fork it out. Always must be 0. When
> > we come up with a use case for something like an annex, we can find a way
> > to add it back.
>
> The point of reserving the annex the way it has been is exactly this --
> it should not be used now, but when we agree on how it should be used,
> we have an area that's immediately ready to be used.


> (For the cases where you don't need script to enforce reasonable values,
> reserving it now means those new consensus rules can be used immediately
> with utxos that predate the new consensus rules -- so you could update
> offchain contracts from per-tx to per-input locktimes immediately without
> having to update the utxo on-chain first)
>

I highly doubt that we will not need new sighash flags once it is ready to
allow partial covers of the annex, e.g. like the structured ones described
above.

We're already doing a soft fork for the new annex rules, so this isn't a
big deal...

Legacy outputs can use these new sighash flags as well, in theory (maybe
I'll do a post on why we shouldn't...)



Cheers,

Jeremy

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Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[Anthony Towns]

On Sat, Mar 05, 2022 at 12:20:02PM +0000, Jeremy Rubin via bitcoin-dev wrote:
> On Sat, Mar 5, 2022 at 5:59 AM Anthony Towns <aj@erisian.com.au> wrote:
> > The difference between information in the annex and information in
> > either a script (or the input data for the script that is the rest of
> > the witness) is (in theory) that the annex can be analysed immediately
> > and unconditionally, without necessarily even knowing anything about
> > the utxo being spent.
> I agree that should happen, but there are cases where this would not work.
> E.g., imagine OP_LISP_EVAL + OP_ANNEX... and then you do delegation via the
> thing in the annex.
> Now the annex can be executed as a script.

You've got the implication backwards: the benefit isn't that the annex
*can't* be used as/in a script; it's that it *can* be used *without*
having to execute/analyse a script (and without even having to load the
utxo being spent).

How big a benefit that is might be debatable -- it's only a different
ordering of the work you have to do to be sure the transaction is valid;
it doesn't reduce the total work. And I think you can easily design
invalid transactions that will maximise the work required to establish
the tx is invalid, no matter what order you validate things.

> Yes, this seems tough to do without redefining checksig to allow partial
> annexes.

"Redefining checksig to allow X" in taproot means "defining a new pubkey
format that allows a new sighash that allows X", which, if it turns out
to be necessary/useful, is entirely possible.  It's not sensible to do
what you suggest *now* though, because we don't have a spec of how a
partial annex might look.

> Hence thinking we should make our current checksig behavior
> require it be 0,

Signatures already require the annex to not be present. If you personally
want to do that for every future transaction you sign off on, you
already can.

> > It seems like a good place for optimising SIGHASH_GROUP (allowing a group
> > of inputs to claim a group of outputs for signing, but not allowing inputs
> > from different groups to ever claim the same output; so that each output
> > is hashed at most once for this purpose) -- since each input's validity
> > depends on the other inputs' state, it's better to be able to get at
> > that state as easily as possible rather than having to actually execute
> > other scripts before your can tell if your script is going to be valid.
> I think SIGHASH_GROUP could be some sort of mutable stack value, not ANNEX.

The annex is already a stack value, and the SIGHASH_GROUP parameter
cannot be mutable since it will break the corresponding signature, and
(in order to ensure validating SIGHASH_GROUP signatures don't require
hashing the same output multiple times) also impacts SIGHASH_GROUP
signatures from other inputs.

> you want to be able to compute what range you should sign, and then the
> signature should cover the actual range not the argument itself.

The value that SIGHASH_GROUP proposes putting in the annex is just an
indication of whether (a) this input is using the same output group as
the previous input; or else (b) how many outputs are in this input's
output group. The signature naturally commits to that value because it's
signing all the outputs in the group anyway.

> Why sign the annex literally?

To prevent it from being third-party malleable.

When there is some meaning assigned to the annex then perhaps it will
make sense to add some more granular way of accessing it via script, but
until then, committing to the whole thing is the best option possible,
since it still allows some potential uses of the annex without having
to define a new sighash.

Note that signing only part of the annex means that you probably
reintroduce the quadratic hashing problem -- that is, with a script of
length X and an annex of length Y, you may have to hash O(X*Y) bytes
instead of O(X+Y) bytes (because every X/k bytes of the script selects
a different Y/j subset of the annex to sign).

> Why require that all signatures in one output sign the exact same digest?
> What if one wants to sign for value and another for value + change?

You can already have one signature for value and one for value+change:
use SIGHASH_SINGLE for the former, and SIGHASH_ALL for the latter.
SIGHASH_GROUP is designed for the case where the "value" goes to
multiple places.

> > > Essentially, I read this as saying: The annex is the ability to pad a
> > > transaction with an additional string of 0's
> > If you wanted to pad it directly, you can do that in script already
> > with a PUSH/DROP combo.
> You cannot, because the push/drop would not be signed and would be
> malleable.

If it's a PUSH, then it's in the tapscript and committed to by the
scriptPubKey, and not malleable.

There's currently no reason to have padding specifiable at spend time --
you know when you're writing the script whether the spender can reuse
the same signature for multiple CHECKSIG ops, because the only way to
do that is to add DUP/etc opcodes -- so if you're doing that, you can
add any necessary padding at the same time.

> The annex is not malleable, so it can be used to this as authenticated
> padding.

The reason that the annex is not third-party malleable is that its
content is committed to by signatures.

> > The point of doing it in the annex is you could have a short byte
> > string, perhaps something like "0x010201a4" saying "tag 1, data length 2
> > bytes, value 420" and have the consensus intepretation of that be "this
> > transaction should be treated as if it's 420 weight units more expensive
> > than its serialized size", while only increasing its witness size by
> > 6 bytes (annex length, annex flag, and the four bytes above). Adding 6
> > bytes for a 426 weight unit increase seems much better than adding 426
> > witness bytes.
> Yes, that's what I say in the next sentence,
> *> Or, we might somehow make the witness a small language (e.g., run length
> encoded zeros)

If you're doing run-length encoding, you might as well just use gzip at
the p2p and storage layers; you don't need to touch consensus at all.
That's not an extensible or particularly interesting idea.

> > > Introducing OP_ANNEX: Suppose there were some sort of annex pushing
> > opcode,
> > > OP_ANNEX which puts the annex on the stack
> > I think you'd want to have a way of accessing individual entries from
> > the annex, rather than the annex as a single unit.
> Or OP_ANNEX + OP_SUBSTR + OP_POVARINTSTR? Then you can just do 2 pops for
> the length and the tag and then get the data.

If you want to make things as inconvenient as possible, sure, I guess?

> > > Now every time you run this,
> > You only run a script from a transaction once at which point its
> > annex is known (a different annex gives a different wtxid and breaks
> > any signatures), and can't reference previous or future transactions'
> > annexes...
> In a transaction validator, yes. But in a satisfier, no.

In a satisfier you don't "run" a script, you provide a solution to
the script...

You can certainly create scripts where it's not possible to provide
valid solutions, eg:

    DUP EQUAL NOT VERIFY

or where it's theoretically possible but in practice extremely difficult
to provide solutions, eg:

    DUP 2 <P> <Q> 2 CHECKMULTISIG
    2DUP EQUAL NOT VERIFY SHA256 SWAP SHA256 EQUAL

or where the difficulty is known and there really isn't an easier way
of coming up with a solution than doing multiple guesses and validating
the result:

    SIZE 80 EQUAL NOT VERIFY HASH256 0 18 SUBSTR 0 NUMEQUAL

But if you don't want to make life difficult for yourself, the answer's
pretty easy: just don't do those things. Or, at a higher level, don't
design new opcodes where you have to do those sorts of things.

> Not true about accessing previous TXNs annexes. All coins spend from
> Coinbase transactions. If you can get the COutpoint you're spending, you
> can get the parent of the COutpoint... and iterate backwards so on and so
> forth. Then you have the CB txn, which commits to the tree of wtxids. So
> you get previous transactions annexes comitted there.

None of that information is stored in the utxo database or accessible at
validation time. Adding that information would make the utxo database
much larger, increasing the costs of running a node, and increasing
validation time for each transaction/block.

> For future transactions,

(For future transactions, if you had generic recursive covenants and
a opcode to examine the annex, you could prevent spending without a
particular value appearing in the annex; that doesn't let you "inspect"
a future annex, though)

> > > Because the Annex is signed, and must be the same, this can also be
> > > inconvenient:
> > The annex is committed to by signatures in the same way nVersion,
> > nLockTime and nSequence are committed to by signatures; I think it helps
> > to think about it in a similar way.
> nSequence, yes, nLockTime is per-tx.

nVersion is also per-tx not per-input. You still need to establish all
three of them before you start signing things.

> BTW i think we now consider nSeq/nLock to be misdesigned given desire to
> vary these per-input/per-tx....\

Since nSequence is per-input, you can obviously vary that per-input; and
you can vary all three per-tx.

> > > Suppose that you have a Miniscript that is something like: and(or(PK(A),
> > > PK(A')), X, or(PK(B), PK(B'))).
> Yes, my point is this is computationally hard to do sometimes.

Sometimes, what makes things computationally hard is that you've got
the wrong approach to looking at the problem.

> > CLTV also has the problem that if you have one script fragment with
> > CLTV by time, and another with CLTV by height, you can't come up with
> > an nLockTime that will ever satisfy both. If you somehow have script
> > fragments that require incompatible interpretations of the annex, you're
> > likewise going to be out of luck.
> Yes, see above. If we don't know how the annex will be structured or used,

If you don't know how the annex will be structured or used, don't use
it. That's exactly how things are today, because no one knows how it
will be structured or used.

> this is the point of this thread....
> We need to drill down how to not introduce these problems.

From where I sit, it looks like you're drawing hasty conclusions based
on a lot of misconceptions. That's not the way you avoid introducing
problems...

I mean, having the misconceptions is perfectly reasonable; if anyone
knew exactly how annex things should work, we'd have a spec already. It's
leaping straight to "this is the only way it can work, it's a dumb way,
and therefore we should throw this out immediately" that I don't really
see the humour in.

> > > It seems like one good option is if we just go on and banish the
> > OP_ANNEX.
> > > Maybe that solves some of this? I sort of think so. It definitely seems
> > > like we're not supposed to access it via script, given the quote from
> > above:
> > How the annex works isn't defined, so it doesn't make any sense to
> > access it from script. When how it works is defined, I expect it might
> > well make sense to access it from script -- in a similar way that the
> > CLTV and CSV opcodes allow accessing nLockTime and nSequence from script.
> That's false: CLTV and CSV expressly do not allow accessing it from script,
> only lower bounding it

Lower bounding something requires accessing it.

That CLTV/CSV only allows lower-bounding it rather than more arbitrary
manipulation is mostly due to having to be implemented via upgrading an
OP_NOP opcode, rather than any other reason, IMHO.

> Legacy outputs can use these new sighash flags as well, in theory (maybe
> I'll do a post on why we shouldn't...)

Existing outputs can't use new sighash flags introduced by a soft fork --
if they could, then those outputs would have been anyone-can-spend prior
to the soft fork activating, because node software that doesn't support
the soft fork isn't able to calculate the message that the signature
applies to, so can't reject invalid signatures.

Perhaps you mean "we could replace OP_NOPx by OP_CHECKSIGv2 and allow
creating new p2wsh or p2sh addresses that can be spent using the new
flags", but I can't really think why anyone would bring that up at
this point, except as a way of deliberately wasting people's time and
attention...

Cheers,
aj

Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[Christian Decker]

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One thing that we recently stumbled over was that we use CLTV in eltoo not
for timelock but to have a comparison between two committed numbers coming
from the spent and the spending transaction (ordering requirement of
states). We couldn't use a number on the stack of the scriptSig as the
signature doesn't commit to it, which is why we commandeered nLocktime
values that are already in the past.

With the annex we could have a way to get a committed to number we can pull
onto the stack, and free the nLocktime for other uses again. It'd also be
less roundabout to explain in classes :-)

An added benefit would be that update transactions, being singlesig, can be
combined into larger transactions by third parties or watchtowers to
amortize some of the fixed cost of getting them confirmed, allowing
on-path-aggregation basically (each node can group and aggregate
transactions as they forward them). This is currently not possible since
all the transactions that we'd like to batch would have to have the same
nLocktime at the moment.

So I think it makes sense to partition the annex into a global annex shared
by the entire transaction, and one for each input. Not sure if one for
inputs would also make sense as it'd bloat the utxo set and could be
emulated by using the input that is spending it.

Cheers,
Christian

On Sat, 5 Mar 2022, 07:33 Anthony Towns via bitcoin-dev, <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> On Fri, Mar 04, 2022 at 11:21:41PM +0000, Jeremy Rubin via bitcoin-dev
> wrote:
> > I've seen some discussion of what the Annex can be used for in Bitcoin.
>
>
> https://www.erisian.com.au/meetbot/taproot-bip-review/2019/taproot-bip-review.2019-11-12-19.00.log.html
>
> includes some discussion on that topic from the taproot review meetings.
>
> The difference between information in the annex and information in
> either a script (or the input data for the script that is the rest of
> the witness) is (in theory) that the annex can be analysed immediately
> and unconditionally, without necessarily even knowing anything about
> the utxo being spent.
>
> The idea is that we would define some simple way of encoding (multiple)
> entries into the annex -- perhaps a tag/length/value scheme like
> lightning uses; maybe if we add a lisp scripting language to consensus,
> we just reuse the list encoding from that? -- at which point we might
> use one tag to specify that a transaction uses advanced computation, and
> needs to be treated as having a heavier weight than its serialized size
> implies; but we could use another tag for per-input absolute locktimes;
> or another tag to commit to a past block height having a particular hash.
>
> It seems like a good place for optimising SIGHASH_GROUP (allowing a group
> of inputs to claim a group of outputs for signing, but not allowing inputs
> from different groups to ever claim the same output; so that each output
> is hashed at most once for this purpose) -- since each input's validity
> depends on the other inputs' state, it's better to be able to get at
> that state as easily as possible rather than having to actually execute
> other scripts before your can tell if your script is going to be valid.
>
> > The BIP is tight lipped about it's purpose
>
> BIP341 only reserves an area to put the annex; it doesn't define how
> it's used or why it should be used.
>
> > Essentially, I read this as saying: The annex is the ability to pad a
> > transaction with an additional string of 0's
>
> If you wanted to pad it directly, you can do that in script already
> with a PUSH/DROP combo.
>
> The point of doing it in the annex is you could have a short byte
> string, perhaps something like "0x010201a4" saying "tag 1, data length 2
> bytes, value 420" and have the consensus intepretation of that be "this
> transaction should be treated as if it's 420 weight units more expensive
> than its serialized size", while only increasing its witness size by
> 6 bytes (annex length, annex flag, and the four bytes above). Adding 6
> bytes for a 426 weight unit increase seems much better than adding 426
> witness bytes.
>
> The example scenario is that if there was an opcode to verify a
> zero-knowledge proof, eg I think bulletproof range proofs are something
> like 10x longer than a signature, but require something like 400x the
> validation time. Since checksig has a validation weight of 50 units,
> a bulletproof verify might have a 400x greater validation weight, ie
> 20,000 units, while your witness data is only 650 bytes serialized. In
> that case, we'd need to artificially bump the weight of you transaction
> up by the missing 19,350 units, or else an attacker could fill a block
> with perhaps 6000 bulletproofs costing the equivalent of 120M signature
> operations, rather than the 80k sigops we currently expect as the maximum
> in a block. Seems better to just have "0x01024b96" stuck in the annex,
> than 19kB of zeroes.
>
> > Introducing OP_ANNEX: Suppose there were some sort of annex pushing
> opcode,
> > OP_ANNEX which puts the annex on the stack
>
> I think you'd want to have a way of accessing individual entries from
> the annex, rather than the annex as a single unit.
>
> > Now suppose that I have a computation that I am running in a script as
> > follows:
> >
> > OP_ANNEX
> > OP_IF
> >     `some operation that requires annex to be <1>`
> > OP_ELSE
> >     OP_SIZE
> >     `some operation that requires annex to be len(annex) + 1 or does a
> > checksig`
> > OP_ENDIF
> >
> > Now every time you run this,
>
> You only run a script from a transaction once at which point its
> annex is known (a different annex gives a different wtxid and breaks
> any signatures), and can't reference previous or future transactions'
> annexes...
>
> > Because the Annex is signed, and must be the same, this can also be
> > inconvenient:
>
> The annex is committed to by signatures in the same way nVersion,
> nLockTime and nSequence are committed to by signatures; I think it helps
> to think about it in a similar way.
>
> > Suppose that you have a Miniscript that is something like: and(or(PK(A),
> > PK(A')), X, or(PK(B), PK(B'))).
> >
> > A or A' should sign with B or B'. X is some sort of fragment that might
> > require a value that is unknown (and maybe recursively defined?) so
> > therefore if we send the PSBT to A first, which commits to the annex, and
> > then X reads the annex and say it must be something else, A must sign
> > again. So you might say, run X first, and then sign with A and C or B.
> > However, what if the script somehow detects the bitstring WHICH_A WHICH_B
> > and has a different Annex per selection (e.g., interpret the bitstring
> as a
> > int and annex must == that int). Now, given and(or(K1, K1'),... or(Kn,
> > Kn')) we end up with needing to pre-sign 2**n annex values somehow...
> this
> > seems problematic theoretically.
>
> Note that you need to know what the annex will contain before you sign,
> since the annex is committed to via the signature. If "X" will need
> entries in the annex that aren't able to be calculated by the other
> parties, then they need to be the first to contribute to the PSBT, not A.
>
> I think the analogy to locktimes would be "I need the locktime to be at
> least block 900k, should I just sign that now, or check that nobody else
> is going to want it to be block 950k or something? Or should I just sign
> with nLockTime at 900k, 910k, 920k, 930k, etc and let someone else pick
> the right one?" The obvious solution is just to work out what the
> nLockTime should be first, then run signing rounds. Likewise, work out
> what the annex should be first, then run the signing rounds.
>
> CLTV also has the problem that if you have one script fragment with
> CLTV by time, and another with CLTV by height, you can't come up with
> an nLockTime that will ever satisfy both. If you somehow have script
> fragments that require incompatible interpretations of the annex, you're
> likewise going to be out of luck.
>
> Having a way of specifying locktimes in the annex can solve that
> particular problem with CLTV (different inputs can sign different
> locktimes, and you could have different tags for by-time/by-height so
> that even the same input can have different clauses requiring both),
> but the general problem still exists.
>
> (eg, you might have per-input by-height absolute locktimes as annex
> entry 3, and per-input by-time absolute locktimes as annex entry 4,
> so you might convert:
>
>  "900e3 CLTV DROP" -> "900e3 3 PUSH_ANNEX_ENTRY GREATERTHANOREQUAL VERIFY"
>
>  "500e6 CLTV DROP" -> "500e6 4 PUSH_ANNEX_ENTRY GREATERTHANOREQUAL VERIFY"
>
> for height/time locktime checks respectively)
>
> > Of course this wouldn't be miniscript then. Because miniscript is just
> for
> > the well behaved subset of script, and this seems ill behaved. So maybe
> > we're OK?
>
> The CLTV issue hit miniscript:
>
> https://medium.com/blockstream/dont-mix-your-timelocks-d9939b665094
>
> > But I think the issue still arises where suppose I have a simple thing
> > like: and(COLD_LOGIC, HOT_LOGIC) where both contains a signature, if
> > COLD_LOGIC and HOT_LOGIC can both have different costs, I need to decide
> > what logic each satisfier for the branch is going to use in advance, or
> > sign all possible sums of both our annex costs? This could come up if
> > cold/hot e.g. use different numbers of signatures / use checksigCISAadd
> > which maybe requires an annex argument.
>
> Signatures pay for themselves -- every signature is 64 or 65 bytes,
> but only has 50 units of validation weight. (That is, a signature check
> is about 50x the cost of hashing 520 bytes of data, which is the next
> highest cost operation we have, and is treated as costing 1 unit, and
> immediately paid for by the 1 byte that writing OP_HASH256 takes up)
>
> That's why the "add cost" use of the annex is only talked about in
> hypotheticals, not specified -- for reasonable scripts with today's
> opcodes, it's not needed.
>
> If you're doing cross-input signature aggregation, everybody needs to
> agree on the message they're signing in the first place, so you definitely
> can't delay figuring out some bits of some annex until after signing.
>
> > It seems like one good option is if we just go on and banish the
> OP_ANNEX.
> > Maybe that solves some of this? I sort of think so. It definitely seems
> > like we're not supposed to access it via script, given the quote from
> above:
>
> How the annex works isn't defined, so it doesn't make any sense to
> access it from script. When how it works is defined, I expect it might
> well make sense to access it from script -- in a similar way that the
> CLTV and CSV opcodes allow accessing nLockTime and nSequence from script.
>
> To expand on that: the logic to prevent a transaction confirming too
> early occurs by looking at nLockTime and nSequence, but script can
> ensure that an attempt to use "bad" values for those can never be a
> valid transaction; likewise, consensus may look at the annex to enforce
> new conditions as to when a transaction might be valid (and can do so
> without needing to evaluate any scripts), but the individual scripts can
> make sure that the annex has been set to what the utxo owner considered
> to be reasonable values.
>
> > One solution would be to... just soft-fork it out. Always must be 0. When
> > we come up with a use case for something like an annex, we can find a way
> > to add it back.
>
> The point of reserving the annex the way it has been is exactly this --
> it should not be used now, but when we agree on how it should be used,
> we have an area that's immediately ready to be used.
>
> (For the cases where you don't need script to enforce reasonable values,
> reserving it now means those new consensus rules can be used immediately
> with utxos that predate the new consensus rules -- so you could update
> offchain contracts from per-tx to per-input locktimes immediately without
> having to update the utxo on-chain first)
>
> Cheers,
> aj
>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[Jeremy Rubin]

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Hi Christian,

For that purpose I'd recommend having a checksig extra that is

<data> <n> <sig> <pk> checksigextra that allows N extra data items on the
stack in addition to the txn hash. This would allow signers to sign some
addtl arguments, but would not be an annex since the values would not have
any consensus meaning (whereas annex is designed to have one)


I've previously discussed this for eltoo with giving signatures an explicit
extra seqnum, but it can be generalized as above.



W.r.t. pinning, if the annex is a pure function of the script execution,
then there's no issue with letting it be mutable (e.g. for a validation
cost hint). But permitting both validation cost commitments and stack
readability is asking too much of the annex IMO.

On Sun, Mar 6, 2022, 1:13 PM Christian Decker <decker.christian@gmail.com>
wrote:

> One thing that we recently stumbled over was that we use CLTV in eltoo not
> for timelock but to have a comparison between two committed numbers coming
> from the spent and the spending transaction (ordering requirement of
> states). We couldn't use a number on the stack of the scriptSig as the
> signature doesn't commit to it, which is why we commandeered nLocktime
> values that are already in the past.
>
> With the annex we could have a way to get a committed to number we can
> pull onto the stack, and free the nLocktime for other uses again. It'd also
> be less roundabout to explain in classes :-)
>
> An added benefit would be that update transactions, being singlesig, can
> be combined into larger transactions by third parties or watchtowers to
> amortize some of the fixed cost of getting them confirmed, allowing
> on-path-aggregation basically (each node can group and aggregate
> transactions as they forward them). This is currently not possible since
> all the transactions that we'd like to batch would have to have the same
> nLocktime at the moment.
>
> So I think it makes sense to partition the annex into a global annex
> shared by the entire transaction, and one for each input. Not sure if one
> for inputs would also make sense as it'd bloat the utxo set and could be
> emulated by using the input that is spending it.
>
> Cheers,
> Christian
>
> On Sat, 5 Mar 2022, 07:33 Anthony Towns via bitcoin-dev, <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> On Fri, Mar 04, 2022 at 11:21:41PM +0000, Jeremy Rubin via bitcoin-dev
>> wrote:
>> > I've seen some discussion of what the Annex can be used for in Bitcoin.
>>
>>
>> https://www.erisian.com.au/meetbot/taproot-bip-review/2019/taproot-bip-review.2019-11-12-19.00.log.html
>>
>> includes some discussion on that topic from the taproot review meetings.
>>
>> The difference between information in the annex and information in
>> either a script (or the input data for the script that is the rest of
>> the witness) is (in theory) that the annex can be analysed immediately
>> and unconditionally, without necessarily even knowing anything about
>> the utxo being spent.
>>
>> The idea is that we would define some simple way of encoding (multiple)
>> entries into the annex -- perhaps a tag/length/value scheme like
>> lightning uses; maybe if we add a lisp scripting language to consensus,
>> we just reuse the list encoding from that? -- at which point we might
>> use one tag to specify that a transaction uses advanced computation, and
>> needs to be treated as having a heavier weight than its serialized size
>> implies; but we could use another tag for per-input absolute locktimes;
>> or another tag to commit to a past block height having a particular hash.
>>
>> It seems like a good place for optimising SIGHASH_GROUP (allowing a group
>> of inputs to claim a group of outputs for signing, but not allowing inputs
>> from different groups to ever claim the same output; so that each output
>> is hashed at most once for this purpose) -- since each input's validity
>> depends on the other inputs' state, it's better to be able to get at
>> that state as easily as possible rather than having to actually execute
>> other scripts before your can tell if your script is going to be valid.
>>
>> > The BIP is tight lipped about it's purpose
>>
>> BIP341 only reserves an area to put the annex; it doesn't define how
>> it's used or why it should be used.
>>
>> > Essentially, I read this as saying: The annex is the ability to pad a
>> > transaction with an additional string of 0's
>>
>> If you wanted to pad it directly, you can do that in script already
>> with a PUSH/DROP combo.
>>
>> The point of doing it in the annex is you could have a short byte
>> string, perhaps something like "0x010201a4" saying "tag 1, data length 2
>> bytes, value 420" and have the consensus intepretation of that be "this
>> transaction should be treated as if it's 420 weight units more expensive
>> than its serialized size", while only increasing its witness size by
>> 6 bytes (annex length, annex flag, and the four bytes above). Adding 6
>> bytes for a 426 weight unit increase seems much better than adding 426
>> witness bytes.
>>
>> The example scenario is that if there was an opcode to verify a
>> zero-knowledge proof, eg I think bulletproof range proofs are something
>> like 10x longer than a signature, but require something like 400x the
>> validation time. Since checksig has a validation weight of 50 units,
>> a bulletproof verify might have a 400x greater validation weight, ie
>> 20,000 units, while your witness data is only 650 bytes serialized. In
>> that case, we'd need to artificially bump the weight of you transaction
>> up by the missing 19,350 units, or else an attacker could fill a block
>> with perhaps 6000 bulletproofs costing the equivalent of 120M signature
>> operations, rather than the 80k sigops we currently expect as the maximum
>> in a block. Seems better to just have "0x01024b96" stuck in the annex,
>> than 19kB of zeroes.
>>
>> > Introducing OP_ANNEX: Suppose there were some sort of annex pushing
>> opcode,
>> > OP_ANNEX which puts the annex on the stack
>>
>> I think you'd want to have a way of accessing individual entries from
>> the annex, rather than the annex as a single unit.
>>
>> > Now suppose that I have a computation that I am running in a script as
>> > follows:
>> >
>> > OP_ANNEX
>> > OP_IF
>> >     `some operation that requires annex to be <1>`
>> > OP_ELSE
>> >     OP_SIZE
>> >     `some operation that requires annex to be len(annex) + 1 or does a
>> > checksig`
>> > OP_ENDIF
>> >
>> > Now every time you run this,
>>
>> You only run a script from a transaction once at which point its
>> annex is known (a different annex gives a different wtxid and breaks
>> any signatures), and can't reference previous or future transactions'
>> annexes...
>>
>> > Because the Annex is signed, and must be the same, this can also be
>> > inconvenient:
>>
>> The annex is committed to by signatures in the same way nVersion,
>> nLockTime and nSequence are committed to by signatures; I think it helps
>> to think about it in a similar way.
>>
>> > Suppose that you have a Miniscript that is something like: and(or(PK(A),
>> > PK(A')), X, or(PK(B), PK(B'))).
>> >
>> > A or A' should sign with B or B'. X is some sort of fragment that might
>> > require a value that is unknown (and maybe recursively defined?) so
>> > therefore if we send the PSBT to A first, which commits to the annex,
>> and
>> > then X reads the annex and say it must be something else, A must sign
>> > again. So you might say, run X first, and then sign with A and C or B.
>> > However, what if the script somehow detects the bitstring WHICH_A
>> WHICH_B
>> > and has a different Annex per selection (e.g., interpret the bitstring
>> as a
>> > int and annex must == that int). Now, given and(or(K1, K1'),... or(Kn,
>> > Kn')) we end up with needing to pre-sign 2**n annex values somehow...
>> this
>> > seems problematic theoretically.
>>
>> Note that you need to know what the annex will contain before you sign,
>> since the annex is committed to via the signature. If "X" will need
>> entries in the annex that aren't able to be calculated by the other
>> parties, then they need to be the first to contribute to the PSBT, not A.
>>
>> I think the analogy to locktimes would be "I need the locktime to be at
>> least block 900k, should I just sign that now, or check that nobody else
>> is going to want it to be block 950k or something? Or should I just sign
>> with nLockTime at 900k, 910k, 920k, 930k, etc and let someone else pick
>> the right one?" The obvious solution is just to work out what the
>> nLockTime should be first, then run signing rounds. Likewise, work out
>> what the annex should be first, then run the signing rounds.
>>
>> CLTV also has the problem that if you have one script fragment with
>> CLTV by time, and another with CLTV by height, you can't come up with
>> an nLockTime that will ever satisfy both. If you somehow have script
>> fragments that require incompatible interpretations of the annex, you're
>> likewise going to be out of luck.
>>
>> Having a way of specifying locktimes in the annex can solve that
>> particular problem with CLTV (different inputs can sign different
>> locktimes, and you could have different tags for by-time/by-height so
>> that even the same input can have different clauses requiring both),
>> but the general problem still exists.
>>
>> (eg, you might have per-input by-height absolute locktimes as annex
>> entry 3, and per-input by-time absolute locktimes as annex entry 4,
>> so you might convert:
>>
>>  "900e3 CLTV DROP" -> "900e3 3 PUSH_ANNEX_ENTRY GREATERTHANOREQUAL VERIFY"
>>
>>  "500e6 CLTV DROP" -> "500e6 4 PUSH_ANNEX_ENTRY GREATERTHANOREQUAL VERIFY"
>>
>> for height/time locktime checks respectively)
>>
>> > Of course this wouldn't be miniscript then. Because miniscript is just
>> for
>> > the well behaved subset of script, and this seems ill behaved. So maybe
>> > we're OK?
>>
>> The CLTV issue hit miniscript:
>>
>> https://medium.com/blockstream/dont-mix-your-timelocks-d9939b665094
>>
>> > But I think the issue still arises where suppose I have a simple thing
>> > like: and(COLD_LOGIC, HOT_LOGIC) where both contains a signature, if
>> > COLD_LOGIC and HOT_LOGIC can both have different costs, I need to decide
>> > what logic each satisfier for the branch is going to use in advance, or
>> > sign all possible sums of both our annex costs? This could come up if
>> > cold/hot e.g. use different numbers of signatures / use checksigCISAadd
>> > which maybe requires an annex argument.
>>
>> Signatures pay for themselves -- every signature is 64 or 65 bytes,
>> but only has 50 units of validation weight. (That is, a signature check
>> is about 50x the cost of hashing 520 bytes of data, which is the next
>> highest cost operation we have, and is treated as costing 1 unit, and
>> immediately paid for by the 1 byte that writing OP_HASH256 takes up)
>>
>> That's why the "add cost" use of the annex is only talked about in
>> hypotheticals, not specified -- for reasonable scripts with today's
>> opcodes, it's not needed.
>>
>> If you're doing cross-input signature aggregation, everybody needs to
>> agree on the message they're signing in the first place, so you definitely
>> can't delay figuring out some bits of some annex until after signing.
>>
>> > It seems like one good option is if we just go on and banish the
>> OP_ANNEX.
>> > Maybe that solves some of this? I sort of think so. It definitely seems
>> > like we're not supposed to access it via script, given the quote from
>> above:
>>
>> How the annex works isn't defined, so it doesn't make any sense to
>> access it from script. When how it works is defined, I expect it might
>> well make sense to access it from script -- in a similar way that the
>> CLTV and CSV opcodes allow accessing nLockTime and nSequence from script.
>>
>> To expand on that: the logic to prevent a transaction confirming too
>> early occurs by looking at nLockTime and nSequence, but script can
>> ensure that an attempt to use "bad" values for those can never be a
>> valid transaction; likewise, consensus may look at the annex to enforce
>> new conditions as to when a transaction might be valid (and can do so
>> without needing to evaluate any scripts), but the individual scripts can
>> make sure that the annex has been set to what the utxo owner considered
>> to be reasonable values.
>>
>> > One solution would be to... just soft-fork it out. Always must be 0.
>> When
>> > we come up with a use case for something like an annex, we can find a
>> way
>> > to add it back.
>>
>> The point of reserving the annex the way it has been is exactly this --
>> it should not be used now, but when we agree on how it should be used,
>> we have an area that's immediately ready to be used.
>>
>> (For the cases where you don't need script to enforce reasonable values,
>> reserving it now means those new consensus rules can be used immediately
>> with utxos that predate the new consensus rules -- so you could update
>> offchain contracts from per-tx to per-input locktimes immediately without
>> having to update the utxo on-chain first)
>>
>> Cheers,
>> aj
>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>

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Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

[Antoine Riard]

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Hi Jeremy,

> I've seen some discussion of what the Annex can be used for in Bitcoin.
For
> example, some people have discussed using the annex as a data field for
> something like CHECKSIGFROMSTACK type stuff (additional authenticated
data)
> or for something like delegation (the delegation is to the annex). I think
> before devs get too excited, we should have an open discussion about what
> this is actually for, and figure out if there are any constraints to using
> it however we may please.

I think one interesting purpose of the annex is to serve as a transaction
field extension, where we assign new consensus validity rules to the annex
payloads.

One could think about new types of locks, e.g where a transaction inclusion
is constrained before the annex payload value is superior to the chain's
`ChainWork`. This could be useful in case of contentious forks, where you
want your transaction to confirm only when enough work is accumulated, and
height isn't a reliable indicator anymore.

Or a relative-timelock where the endpoint is the presence of a state number
encumbering the spent transaction. This could be useful in the context of
payment pools, where the user withdraw transactions are all encumbered by a
bip68 relative-timelock, as you don't know which one is going to confirm
first, but where you don't care about enforcement of the timelocks once the
contestation delay has played once  and no higher-state update transaction
has confirmed.

Of course, we could reuse the nSequence field for some of those new types
of locks, though we would lose the flexibility of combining multiple locks
encumbering the same input.

Another use for the annex is locating there the SIGHASH_GROUP group count
value. One advantage over placing the value as a script stack item could be
to have annex payloads interdependency validity, where other annex payloads
are reusing the group count value as part of their own semantics.

Antoine

Le ven. 4 mars 2022 à 18:22, Jeremy Rubin via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> a écrit :

> I've seen some discussion of what the Annex can be used for in Bitcoin.
> For example, some people have discussed using the annex as a data field for
> something like CHECKSIGFROMSTACK type stuff (additional authenticated data)
> or for something like delegation (the delegation is to the annex). I think
> before devs get too excited, we should have an open discussion about what
> this is actually for, and figure out if there are any constraints to using
> it however we may please.
>
> The BIP is tight lipped about it's purpose, saying mostly only:
>
> *What is the purpose of the annex? The annex is a reserved space for
> future extensions, such as indicating the validation costs of
> computationally expensive new opcodes in a way that is recognizable without
> knowing the scriptPubKey of the output being spent. Until the meaning of
> this field is defined by another softfork, users SHOULD NOT include annex
> in transactions, or it may lead to PERMANENT FUND LOSS.*
>
> *The annex (or the lack of thereof) is always covered by the signature and
> contributes to transaction weight, but is otherwise ignored during taproot
> validation.*
>
> *Execute the script, according to the applicable script rules[11], using
> the witness stack elements excluding the script s, the control block c, and
> the annex a if present, as initial stack.*
>
> Essentially, I read this as saying: The annex is the ability to pad a
> transaction with an additional string of 0's that contribute to the virtual
> weight of a transaction, but has no validation cost itself. Therefore,
> somehow, if you needed to validate more signatures than 1 per 50 virtual
> weight units, you could add padding to buy extra gas. Or, we might somehow
> make the witness a small language (e.g., run length encoded zeros) such
> that we can very quickly compute an equivalent number of zeros to 'charge'
> without actually consuming the space but still consuming a linearizable
> resource... or something like that. We might also e.g. want to use the
> annex to reserve something else, like the amount of memory. In general, we
> are using the annex to express a resource constraint efficiently. This
> might be useful for e.g. simplicity one day.
>
> Generating an Annex: One should write a tracing executor for a script, run
> it, measure the resource costs, and then generate an annex that captures
> any externalized costs.
>
> -------------------
>
> Introducing OP_ANNEX: Suppose there were some sort of annex pushing
> opcode, OP_ANNEX which puts the annex on the stack as well as a 0 or 1 (to
> differentiate annex is 0 from no annex, e.g. 0 1 means annex was 0 and 0 0
> means no annex). This would be equivalent to something based on <annex
> flag> OP_TXHASH <has annex flag> OP_TXHASH.
>
> Now suppose that I have a computation that I am running in a script as
> follows:
>
> OP_ANNEX
> OP_IF
>     `some operation that requires annex to be <1>`
> OP_ELSE
>     OP_SIZE
>     `some operation that requires annex to be len(annex) + 1 or does a
> checksig`
> OP_ENDIF
>
> Now every time you run this, it requires one more resource unit than the
> last time you ran it, which makes your satisfier use the annex as some sort
> of "scratch space" for a looping construct, where you compute a new annex,
> loop with that value, and see if that annex is now accepted by the program.
>
> In short, it kinda seems like being able to read the annex off of the
> stack makes witness construction somehow turing complete, because we can
> use it as a register/tape for some sort of computational model.
>
> -------------------
>
> This seems at odds with using the annex as something that just helps you
> heuristically guess  computation costs, now it's somehow something that
> acts to make script satisfiers recursive.
>
> Because the Annex is signed, and must be the same, this can also be
> inconvenient:
>
> Suppose that you have a Miniscript that is something like: and(or(PK(A),
> PK(A')), X, or(PK(B), PK(B'))).
>
> A or A' should sign with B or B'. X is some sort of fragment that might
> require a value that is unknown (and maybe recursively defined?) so
> therefore if we send the PSBT to A first, which commits to the annex, and
> then X reads the annex and say it must be something else, A must sign
> again. So you might say, run X first, and then sign with A and C or B.
> However, what if the script somehow detects the bitstring WHICH_A WHICH_B
> and has a different Annex per selection (e.g., interpret the bitstring as a
> int and annex must == that int). Now, given and(or(K1, K1'),... or(Kn,
> Kn')) we end up with needing to pre-sign 2**n annex values somehow... this
> seems problematic theoretically.
>
> Of course this wouldn't be miniscript then. Because miniscript is just for
> the well behaved subset of script, and this seems ill behaved. So maybe
> we're OK?
>
> But I think the issue still arises where suppose I have a simple thing
> like: and(COLD_LOGIC, HOT_LOGIC) where both contains a signature, if
> COLD_LOGIC and HOT_LOGIC can both have different costs, I need to decide
> what logic each satisfier for the branch is going to use in advance, or
> sign all possible sums of both our annex costs? This could come up if
> cold/hot e.g. use different numbers of signatures / use checksigCISAadd
> which maybe requires an annex argument.
>
>
>
> ------------
>
> It seems like one good option is if we just go on and banish the OP_ANNEX.
> Maybe that solves some of this? I sort of think so. It definitely seems
> like we're not supposed to access it via script, given the quote from above:
>
> *Execute the script, according to the applicable script rules[11], using
> the witness stack elements excluding the script s, the control block c, and
> the annex a if present, as initial stack.*
>
> If we were meant to have it, we would have not nixed it from the stack,
> no? Or would have made the opcode for it as a part of taproot...
>
> But recall that the annex is committed to by the signature.
>
> So it's only a matter of time till we see some sort of Cat and Schnorr
> Tricks III the Annex Edition that lets you use G cleverly to get the annex
> onto the stack again, and then it's like we had OP_ANNEX all along, or
> without CAT, at least something that we can detect that the value has
> changed and cause this satisfier looping issue somehow.
>
> Not to mention if we just got OP_TXHASH
>
>
>
> -----------
>
> Is the annex bad? After writing this I sort of think so?
>
> One solution would be to... just soft-fork it out. Always must be 0. When
> we come up with a use case for something like an annex, we can find a way
> to add it back.  Maybe this means somehow pushing multiple annexes and
> having an annex stack, where only sub-segments are signed for the last
> executed signature? That would solve looping... but would it break some
> aggregation thing? Maybe.
>
>
> Another solution would be to make it so the annex is never committed to
> and unobservable from the script, but that the annex is always something
> that you can run get_annex(stack) to generate the annex. Thus it is a hint
> for validation rules, but not directly readable, and if it is modified you
> figure out the txn was cheaper sometime after you execute the scripts and
> can decrease the value when you relay. But this sounds like something that
> needs to be a p2p only annex, because consensus we may not care (unless
> it's something like preallocating memory for validation?).
>
> -----------------------
>
> Overall my preference is -- perhaps sadly -- looking like we should
> soft-fork it out of our current Checksig (making the policy that it must 0
> a consensus rule) and redesign the annex technique later when we actually
> know what it is for with a new checksig or other mechanism. But It's not a
> hard opinion! It just seems like you can't practically use the annex for
> this worklimit type thing *and* observe it from the stack meaningfully.
>
>
>
> Thanks for coming to my ted-talk,
>
> Jeremy
>
>
> --
> @JeremyRubin <https://twitter.com/JeremyRubin>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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