Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation

[Bram Cohen via bitcoin-dev]

On Mon, Mar 26, 2018 at 11:34 PM, Anthony Towns  wrote:

> On Wed, Mar 21, 2018 at 05:47:01PM -0700, Bram Cohen via bitcoin-dev wrote:
> > [...] Most unused opcodes should be reclaimed as RETURN_VALID,
> > but there should still be one OP_NOP and there should be a 'real'
> RETURN_VALID,
> > which (a) is guaranteed to not be soft forked into something else in the
> > future, and (b) doesn't have any parsing weirdness.
>
> What's the reason for those? I could see an argument for RETURN_VALID, I
> guess:
>
>   confA IF condB IF condC IF [pathA] RETURN_VALID ENDIF ENDIF ENDIF [pathB]
>
> is probably simpler and saves 3 bytes compared to:
>
>   1 condA IF condB IF condC IF [pathA] NOT ENDIF ENDIF ENDIF IF [pathB]
> ENDIF
>
> but that doesn't seem crazy compelling?


Mostly yes it's for that case and also for:

   condA IF RETURN_VALID ENDIF condb IF RETURN_VALID ENDIF condc

Technically that can be done with fewer opcodes using OP_BOOLOR but maybe
in the future there will be some incentive for short circuit evaluation

But there's also the general principle that it's only one opcode and if
there are a lot of things which look like RETURN_VALID there should be one
thing which actually is RETURN_VALID


> I don't see a reason to just keep one OP_NOP though.
>

Mostly based on momentum because there are several of them there right now.
If noone else wants to defend it I won't either.


> > By far the most expedient option is (e) cause a RETURN_VALID at parse
> time.
> > There's even precedent for this sort of behavior in the other direction
> with
> > disabled opcodes causing failure at parse time even if they aren't being
> > executed.
>
> You're probably right. That still doesn't let you implement intercal's
> COMEFROM statement as a new opcode, of course. :)
>

That can be in the hardfork wishlist :-)


> > A lot can be said about all the options, but one thing I feel like
> snarking
> > about is that if you get rid of IFs using MAST, then it's highly unclear
> > whether OP_DEPTH should be nuked as well. My feeling is that it should
> and that
> > strict parsing should require that the bottom thing in the witness gets
> > referenced at some point.
>
> I guess when passing the script you could perhaps check if each witness
> item could have been replaced with OP_FALSE or OP_1 and still get the
> same result, and consider the transaction non-standard if so?
>

Essentially all opcodes including OP_PICK make clear at runtime how deep
they go and anything below the max depth can be safely eliminated (or used
as grounds for rejecting in strict mode). The big exception is OP_DEPTH
which totally mangles the assumptions. It's trivial to make scripts which
use OP_DEPTH which become invalid with things added below the stack then go
back to being valid again with more things added even though the individual
items are never even accessed.


>
> > Hacking in a multisig opcode isn't a horrible idea, but it is very stuck
> > specifically on m-of-n and doesn't support more complex formulas for how
> > signatures can be combined, which makes it feel hacky and weird.
>
> Hmm? The opcode I suggested works just as easily with arbitrary formulas,
> eg, "There must be at least 1 signer from pka{1,2,3}, and 3 signers all
> up, except each of pkb{1,2,3,4,5,6} only counts for half":
>
>   0 pkb6 pkb5 pkb4 pkb3 pkb2 pkb1 pka3 pka2 pka1 9 CHECK_AGGSIG_VERIFY
> (declare pubkeys)
>   0b111 CHECK_AGG_SIGNERS VERIFY
> (one of pka{1,2,3} must sign)
>   0b001 CHECK_AGG_SIGNERS
>   0b010 CHECK_AGG_SIGNERS ADD
>   0b100 CHECK_AGG_SIGNERS ADD
>   DUP ADD
> (pka{1,2,3} count double)
>   0b01000 CHECK_AGG_SIGNERS ADD
>   0b1 CHECK_AGG_SIGNERS ADD
>   0b00010 CHECK_AGG_SIGNERS ADD
>   0b00100 CHECK_AGG_SIGNERS ADD
>   0b01000 CHECK_AGG_SIGNERS ADD
>   0b1 CHECK_AGG_SIGNERS ADD
> (pkb{1..6} count single)
>   6 EQUAL
> (summing to a total of 3 doubled)
>
> Not sure that saves it from being "hacky and weird" though...
>

That is very hacky and weird. Doing MAST on lots of possibilities is always
reasonably elegant, and it only gets problematic when the number of
possibilities is truly massive.

It's also the case that BLS can support complex key agreement schemes
without even giving away that it isn't a simple single signature. Just
saying.
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Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation

[Anthony Towns via bitcoin-dev]

On Wed, Mar 21, 2018 at 05:47:01PM -0700, Bram Cohen via bitcoin-dev wrote:
> [...] Most unused opcodes should be reclaimed as RETURN_VALID,
> but there should still be one OP_NOP and there should be a 'real' 
> RETURN_VALID,
> which (a) is guaranteed to not be soft forked into something else in the
> future, and (b) doesn't have any parsing weirdness.

What's the reason for those? I could see an argument for RETURN_VALID, I guess:

  confA IF condB IF condC IF [pathA] RETURN_VALID ENDIF ENDIF ENDIF [pathB]

is probably simpler and saves 3 bytes compared to:

  1 condA IF condB IF condC IF [pathA] NOT ENDIF ENDIF ENDIF IF [pathB] ENDIF

but that doesn't seem crazy compelling? I don't see a reason to just keep
one OP_NOP though.

> The parsing weirdness of
> all the unclaimed opcodes is interesting. Because everything in an IF clause
> needs to be parsed in order to find where the ELSE is, you have a few options
> for dealing with an unknown opcode getting parsed in an unexecuted section of
> code. They are (a) avoid the problem completely by exterminating IF and 
> MASTing
> (b) avoid the problem completely by getting rid of IF and adding IFJUMP,
> IFNJUMP, and JUMP which specify a number of bytes (this also allows for script
> merkleization) (c) require all new opcodes have fixed length 1, even after
> they're soft forked, (d) do almost like (c) but require that on new soft forks
> people hack their old scripts to still parse properly by avoiding the OP_ELSE
> in inopportune places (yuck!) (e) make it so that the unknown opcodes case a
> RETURN_VALID even when they're parsed, regardless of whether they're being
> executed.

I was figuring (c), fwiw, and assuming that opcodes will just be about
manipulating stack values and marking the script as invalid, rather than,
say, introducing new flow control ops.

> By far the most expedient option is (e) cause a RETURN_VALID at parse time.
> There's even precedent for this sort of behavior in the other direction with
> disabled opcodes causing failure at parse time even if they aren't being
> executed.

You're probably right. That still doesn't let you implement intercal's
COMEFROM statement as a new opcode, of course. :)

> A lot can be said about all the options, but one thing I feel like snarking
> about is that if you get rid of IFs using MAST, then it's highly unclear
> whether OP_DEPTH should be nuked as well. My feeling is that it should and 
> that
> strict parsing should require that the bottom thing in the witness gets
> referenced at some point.

I guess when passing the script you could perhaps check if each witness
item could have been replaced with OP_FALSE or OP_1 and still get the
same result, and consider the transaction non-standard if so?

> Hacking in a multisig opcode isn't a horrible idea, but it is very stuck
> specifically on m-of-n and doesn't support more complex formulas for how
> signatures can be combined, which makes it feel hacky and weird.

Hmm? The opcode I suggested works just as easily with arbitrary formulas,
eg, "There must be at least 1 signer from pka{1,2,3}, and 3 signers all
up, except each of pkb{1,2,3,4,5,6} only counts for half":

  0 pkb6 pkb5 pkb4 pkb3 pkb2 pkb1 pka3 pka2 pka1 9 CHECK_AGGSIG_VERIFY
(declare pubkeys)
  0b111 CHECK_AGG_SIGNERS VERIFY
(one of pka{1,2,3} must sign)
  0b001 CHECK_AGG_SIGNERS
  0b010 CHECK_AGG_SIGNERS ADD
  0b100 CHECK_AGG_SIGNERS ADD
  DUP ADD
(pka{1,2,3} count double)
  0b01000 CHECK_AGG_SIGNERS ADD
  0b1 CHECK_AGG_SIGNERS ADD
  0b00010 CHECK_AGG_SIGNERS ADD
  0b00100 CHECK_AGG_SIGNERS ADD
  0b01000 CHECK_AGG_SIGNERS ADD
  0b1 CHECK_AGG_SIGNERS ADD
(pkb{1..6} count single)
  6 EQUAL
(summing to a total of 3 doubled)

Not sure that saves it from being "hacky and weird" though...

(There are different ways you could do "CHECK_AGG_SIGNERS": for
instance, take a bitmask of keys and return the bitwise-and with the
keys that signed, or take a bitmask and just return the number of keys
matching that bitmask that signed, or take a pubkey index and return a
boolean whether that key signed)

Cheers,
aj

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Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation

[Bram Cohen via bitcoin-dev]

Regarding the proposed segwit v2 with reclaiming most things as
RETURN_VALID, the net result for what's being proposed in the near future
for supporting aggregated signatures in the not-so-near future is to punt.
A number of strategies are possible for how to deal with new opcodes being
added later on, and the general strategy of making unused opcodes be
RETURN_VALID for now and figuring out how to handle it later works for all
of them. I think this is the right approach, but wanted to clarify that it
is in fact the approach being proposed.

That said, there are some subtleties to getting it right which the last
message doesn't really cover. Most unused opcodes should be reclaimed as
RETURN_VALID, but there should still be one OP_NOP and there should be a
'real' RETURN_VALID, which (a) is guaranteed to not be soft forked into
something else in the future, and (b) doesn't have any parsing weirdness.
The parsing weirdness of all the unclaimed opcodes is interesting. Because
everything in an IF clause needs to be parsed in order to find where the
ELSE is, you have a few options for dealing with an unknown opcode getting
parsed in an unexecuted section of code. They are (a) avoid the problem
completely by exterminating IF and MASTing (b) avoid the problem completely
by getting rid of IF and adding IFJUMP, IFNJUMP, and JUMP which specify a
number of bytes (this also allows for script merkleization) (c) require all
new opcodes have fixed length 1, even after they're soft forked, (d) do
almost like (c) but require that on new soft forks people hack their old
scripts to still parse properly by avoiding the OP_ELSE in inopportune
places (yuck!) (e) make it so that the unknown opcodes case a RETURN_VALID
even when they're parsed, regardless of whether they're being executed.

By far the most expedient option is (e) cause a RETURN_VALID at parse time.
There's even precedent for this sort of behavior in the other direction
with disabled opcodes causing failure at parse time even if they aren't
being executed.

A lot can be said about all the options, but one thing I feel like snarking
about is that if you get rid of IFs using MAST, then it's highly unclear
whether OP_DEPTH should be nuked as well. My feeling is that it should and
that strict parsing should require that the bottom thing in the witness
gets referenced at some point.

Hacking in a multisig opcode isn't a horrible idea, but it is very stuck
specifically on m-of-n and doesn't support more complex formulas for how
signatures can be combined, which makes it feel hacky and weird.

Also it may make sense to seriously consider BLS signatures, which have a
lot of practical benefits starting with them being noninteractively
aggregatable so you can always assume that they're aggregated instead of
requiring complex semantics to specify what's aggregated with what. My team
is working on an implementation which has several advantages over what's
currently in the published literature but it isn't quite ready for public
consumption yet. This should probably go on the pile of reasons why it's
premature to finalize a plan for aggregation at this point.
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Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation

[ZmnSCPxj via bitcoin-dev]

Good morning aj,




​Sent with ProtonMail Secure Email.​

‐‐‐ Original Message ‐‐‐

On March 21, 2018 7:21 PM, Anthony Towns  wrote:

> On Wed, Mar 21, 2018 at 03:53:59AM -0400, ZmnSCPxj wrote:
> 
> > Good morning aj,
> 
> Good evening Zeeman!
> 
> [pulled from the bottom of your mail]
> 
> > This way, rather than gathering signatures, we gather public keys for 
> > aggregate signature checking.
> 
> Sorry, I probably didn't explain it well (or at all): during the script,
> 
> you're collecting public keys and messages (ie, BIP 143 style digests)
> 
> which then go into the signing/verification algorithm to produce/check
> 
> the signature.

Yes, I think this is indeed what OP_CHECK_AGG_SIG really does.

What I propose is that we have two places where we aggregate public keys: one 
at the script level, and one at the transaction level.  OP_ADD_AGG_PUBKEY adds 
to the script-level aggregate, then OP_CHECK_AGG_SIG adds the script-level 
aggregate to the transaction-level aggregate.

Unfortunately it will not work since transaction-level aggregate (which is 
actually what gets checked) is different between pre-fork and post-fork nodes.

It looks like signature aggregation is difficult to reconcile with script...

Regards,
ZmnSCPxj
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Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation

[ZmnSCPxj via bitcoin-dev]

Good morning aj,

I am probably wrong, but could solution 2 be simplified by using the below 
opcodes for aggregated signatures?

OP_ADD_AGG_PUBKEY - Adds a public key for verification of an aggregated 
signature.

OP_CHECK_AGG_SIG[VERIFY] - Check that the gathered public keys matches the 
aggregated signature.

Then:

 pubkey1 OP_ADD_AGG_PUBKEY
 OP_IF
   pubkey2 OP_ADD_AGG_PUBKEY
 OP_ELSE
   cond OP_CHECKCOVENANT
 OP_ENDIF
 OP_CHECK_AGG_SIG

(omitting the existence of buckets)

I imagine that aggregated signatures, being linear, would allow pubkey to be 
aggregated also by adding the pubkey points (but note that I am not a 
mathematician, I only parrot what better mathematicians say) so 
OP_ADD_AGG_PUBKEY would not require storing all public keys, just adding them 
linearly.

The effect is that in the OP_CHECKCOVENANT case, pre-softfork nodes will not 
actually do any checking.

OP_CHECK_AGG_SIG might accept the signature on the stack (combined signature of 
pubkey1 and pubkey2 and from other inputs), or the bucket the signature is 
stored in.

We might even consider using the altstack: no more OP_ADD_AGG_PUBKEY (one less 
opcode to reserve!), just push pubkeys on the altstack, and OP_CHECK_AGG_SIG 
would take the entire altstack as all the public keys to be used in aggregated 
signature checking.

This way, rather than gathering signatures, we gather public keys for aggregate 
signature checking.  OP_RETURN_TRUE interacts with that by not performing 
aggregate signature checking at all if we encounter OP_RETURN_TRUE first (which 
makes sense: old nodes have no idea what OP_RETURN_TRUE is really doing, and 
would fail to understand all its details).


I am very probably wrong but am willing to learn how to break the above, 
though.  I am probably making a mistake somewhere.

Regards,
ZmnSCPxj

​Sent with ProtonMail Secure Email.​

‐‐‐ Original Message ‐‐‐

On March 21, 2018 12:06 PM, Anthony Towns via bitcoin-dev 
 wrote:

> Hello world,
> 
> There was a lot of discussion on Schnorr sigs and key and signature
> 
> aggregation at the recent core-dev-tech meeting (one relevant conversation
> 
> is transcribed at \[0\]).
> 
> Quick summary, with more background detail in the corresponding footnotes:
> 
> signature aggregation is awesome \[1\], and the possibility of soft-forking
> 
> in new opcodes via OP\_RETURN\_VALID opcodes (instead of OP_NOP) is also
> 
> awesome \[2\].
> 
> Unfortunately doing both of these together may turn out to be awful.
> 
> RETURN_VALID and Signature Aggregation
> 
> 
> -
> 
> Bumping segwit script versions and redefining OP_NOP opcodes are
> 
> fairly straightforward to deal with even with signature aggregation,
> 
> the straightforward implementation of both combined is still a soft-fork.
> 
> RETURN_VALID, unfortunately, has a serious potential pitfall: any
> 
> aggregatable signature operations that occur after it have to go into
> 
> separate buckets.
> 
> As an example of why this is the case, imagine introducing a covenant
> 
> opcode that pulls a potentially complicated condition from the stack
> 
> (perhaps, "an output pays at least 5 satoshi to address xyzzy"),
> 
> checks the condition against the transaction, and then pushes 1 (or 0)
> 
> back onto the stack indicating compliance with the covenant (or not).
> 
> You might then write a script allowing a single person to spend the coins
> 
> if they comply with the covenant, and allow breaking the covenant with
> 
> someone else's sign-off in addition. You could write this as:
> 
> pubkey1 CHECKSIGVERIFY
> 
> cond CHECKCOVENANT IFDUP NOTIF pubkey2 CHECKSIG ENDIF
> 
> If you pass the covenant, you supply "SIGHASHALL|BUCKET_1" and aggregate
> 
> the signature for pubkey1 into bucket1 and you're set; otherwise you supply
> 
> "SIGHASHALL|BUCKET\_1 SIGHASHALL|BUCKET\_1" and aggregate signatures for both
> 
> pubkey1 and pubkey2 into bucket1 and you're set. Great!
> 
> But this isn't a soft-fork: old nodes would see this script as:
> 
> pubkey1 CHECKSIGVERIFY
> 
> cond RETURN_VALID IFDUP NOTIF pubkey2 CHECKSIG ENDIF
> 
> which it would just interpret as:
> 
> pubkey1 CHECKSIGVERIFY cond RETURN_VALID
> 
> which is fine if the covenant was passing; but no good if the covenant
> 
> didn't pass -- they'd be expecting the aggregted sig to just be for
> 
> pubkey1 when it's actually pubkey1+pubkey2, so old nodes would fail the
> 
> tx and new nodes 

Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation

[Andrew Poelstra via bitcoin-dev]

On Wed, Mar 21, 2018 at 02:06:18PM +1000, Anthony Towns via bitcoin-dev wrote:
> 
> That leads me to think that interactive signature aggregation is going to
> take a lot of time and work, and it would make sense to do a v1-upgrade
> that's "just" Schnorr (and taproot and MAST and re-enabling opcodes and
> ...) in the meantime. YMMV.
>

Unfortunately I agree. Another complication with aggregate signatures is
that they complicate blind signature protocols such as [1]. In particular
they break the assumption "one signature can spend at most one UTXO"
meaning that a blind signer cannot tell how many coins they're authorizing
with a given signature, even if they've ensured that the key they're using
only controls UTXOs of a fixed value.

This seems solvable with creative use of ZKPs, but the fact that it's even
a problem caught me off guard, and makes me think that signature aggregation
is much harder to think about than e.g. Taproot which does not change
signature semantics at all.


Andrew



[1] 
https://github.com/jonasnick/scriptless-scripts/blob/blind-swaps/md/partially-blind-swap.md



-- 
Andrew Poelstra
Mathematics Department, Blockstream
Email: apoelstra at wpsoftware.net
Web:   https://www.wpsoftware.net/andrew

"A goose alone, I suppose, can know the loneliness of geese
 who can never find their peace,
 whether north or south or west or east"
   --Joanna Newsom



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Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation

[Anthony Towns via bitcoin-dev]

On Wed, Mar 21, 2018 at 03:53:59AM -0400, ZmnSCPxj wrote:
> Good morning aj,

Good evening Zeeman!

[pulled from the bottom of your mail]
> This way, rather than gathering signatures, we gather public keys for 
> aggregate signature checking.  

Sorry, I probably didn't explain it well (or at all): during the script,
you're collecting public keys and messages (ie, BIP 143 style digests)
which then go into the signing/verification algorithm to produce/check
the signature.

You do need to gather signatures from each private key holder when
producing the aggregate signature, but that happens at the wallet/p2p
level, rather than the consensus level.

> I am probably wrong, but could solution 2 be simplified by using the below 
> opcodes for aggregated signatures?
> 
> OP_ADD_AGG_PUBKEY - Adds a public key for verification of an aggregated 
> signature.
> OP_CHECK_AGG_SIG[VERIFY] - Check that the gathered public keys matches the 
> aggregated signature.

Checking the gathered public keys match the aggregated signature is
something that only happens for the entire transaction as a whole, so
you don't need an opcode for it in the scripts, since they're per-input.

Otherwise, I think that's pretty similar to what I was already saying;
having:

   SIGHASH_ALL|BUCKET_1 pubkey OP_CHECKSIG

would be adding "pubkey" and a message hash calculated via the SIGHASH_ALL
hashing rules to the list of things that the signature for bucket 1 verifies.

FWIW, the Bellare-Neven verification algorithm looks something like:

   s*G = R + K   (s,R is the signature)
   K = sum( H(R, L, i, m) * X_i )   for i corresponding to each pubkey X_i
   L = the concatenation of all the pubkeys, X_0..X_n
   m = the concatenation of all the message hashes, m_0..m_n

So the way I look at it is each input puts a public key and a message hash
(X_i, m_i) into the bucket via a CHECKSIG operation (or similar), and once
you're done, you look into the bucket and there's just a single signature
(s,R) left to verify. You can't start verifying any of it until you've
looked through all the scripts because you need to know L and m before
you can do anything, and both of those require info from every part of
the aggregation.

[0] [1]

> The effect is that in the OP_CHECKCOVENANT case, pre-softfork nodes will not 
> actually do any checking.

Pre-softfork nodes not doing any checking doesn't work with cross-input
signature aggregation as far as I can see. If it did, all you would have
to do to steal people's funds is mine a non-standard transaction:

  inputs:
   my-millions:
 pay-to-pubkey pubkey1
 witness=SIGHASH_ALL|BUCKET_1
   your-two-cents:
 pay-to-script-hash script=[1 OP_RETURN_TRUE pubkey2 CHECKSIG]
 witness=SIGHASH_ALL|BUCKET_1

   bucket1: 64-random-bytes
  output:
   all-the-money: you

Because there's no actual soft-fork at this point every node is an "old"
node, so they all see the OP_RETURN_TRUE and stop validating signatures,
accepting the transaction as valid, and giving you all my money, despite
you being unable to actually produce my signature.

Make sense?

Cheers,
aj

[0] For completeness: constructing the signature for Bellare-Neven
requires two communication phases amongst the signers, and looks
roughly like:

 1. each party generates a random variable r_i, and sharing the
corresponding curve point R_i=r_i*G and their sighash choice
(ie, m_i) with the other signers.

 2. this allows each party to calculate R=sum(R_i) and m,
and hence H(R,L,i,m), at which point each party calculates a
partial signature using their respective private key, x_i:

  s_i = r_i + H(R,L,i,m)*x_i

all these s_i values are then communicated to each signer.

 3. these combine to give the final signature (s,R),
with s=sum(s_i), allowing each signer to verify that the signing
protocol completed successfully, and any signer can broadcast
the transaction to the blockchain

[1] muSig differs in the details, but is basically the same.

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[bitcoin-dev] Soft-forks and schnorr signature aggregation

[Anthony Towns via bitcoin-dev]

Hello world,

There was a lot of discussion on Schnorr sigs and key and signature
aggregation at the recent core-dev-tech meeting (one relevant conversation
is transcribed at [0]).

Quick summary, with more background detail in the corresponding footnotes:
signature aggregation is awesome [1], and the possibility of soft-forking
in new opcodes via OP_RETURN_VALID opcodes (instead of OP_NOP) is also
awesome [2].

Unfortunately doing both of these together may turn out to be awful.

RETURN_VALID and Signature Aggregation
--

Bumping segwit script versions and redefining OP_NOP opcodes are
fairly straightforward to deal with even with signature aggregation,
the straightforward implementation of both combined is still a soft-fork.

RETURN_VALID, unfortunately, has a serious potential pitfall: any
aggregatable signature operations that occur after it have to go into
separate buckets.

As an example of why this is the case, imagine introducing a covenant
opcode that pulls a potentially complicated condition from the stack
(perhaps, "an output pays at least 5 satoshi to address xyzzy"),
checks the condition against the transaction, and then pushes 1 (or 0)
back onto the stack indicating compliance with the covenant (or not).
You might then write a script allowing a single person to spend the coins
if they comply with the covenant, and allow breaking the covenant with
someone else's sign-off in addition. You could write this as:

  pubkey1 CHECKSIGVERIFY
  cond CHECKCOVENANT IFDUP NOTIF pubkey2 CHECKSIG ENDIF

If you pass the covenant, you supply "SIGHASHALL|BUCKET_1" and aggregate
the signature for pubkey1 into bucket1 and you're set; otherwise you supply
"SIGHASHALL|BUCKET_1 SIGHASHALL|BUCKET_1" and aggregate signatures for both
pubkey1 and pubkey2 into bucket1 and you're set. Great!

But this isn't a soft-fork: old nodes would see this script as:

  pubkey1 CHECKSIGVERIFY
  cond RETURN_VALID IFDUP NOTIF pubkey2 CHECKSIG ENDIF

which it would just interpret as:

  pubkey1 CHECKSIGVERIFY cond RETURN_VALID

which is fine if the covenant was passing; but no good if the covenant
didn't pass -- they'd be expecting the aggregted sig to just be for
pubkey1 when it's actually pubkey1+pubkey2, so old nodes would fail the
tx and new nodes would accept it, making it a hard fork.

Solution 0a / 0b


There are two obvious solutions here:

 0a) Just be very careful to ensure any aggregated signatures that
 are conditional on an redefined RETURN_VALID opcode go into later
 buckets, but be careful about having separate sets of buckets every
 time a soft-fork introduces a new redefined opcode. Probably very
 complicated to implement correctly, and essentially doubles the
 number of buckets you have to potentially deal with every time you
 soft fork in a new opcode.

 0b) Alternatively, forget about the hope that RETURN_VALID
 opcodes could be converted to anything, and just reserve OP_NOP
 opcodes and convert them to CHECK_foo_VERIFY opcodes just as we
 have been doing, and when we can't do that bump the segwit witness
 version for a whole new version of script. Or in twitter speak:
 "non-verify upgrades should be done with new script versions" [3]

I think with a little care we can actually salvage RETURN_VALID though!

Solution 1
--

You don't actually have to write your scripts in ways that can cause
this problem, as long as you're careful. In particular, the problem only
occurs if you do aggregatable CHECKSIG operations after "RETURN_VALID"
-- if you do all the CHECKSIGs first, then all nodes will be checking
for the same signatures, and there's no problem. So you could rewrite
the script above as:

  pubkey1 CHECKSIGVERIFY
  IF pubkey2 CHECKSIG ENDIF
  cond CHECKCOVENANT OR

which is redeemable either by:

  sig1 0[and covenant is met]
  sig1 1 sig2   [covenant is not checked]

The witness in this case is essentially committing to the execution path
that would have been taken in the first script by a fully validating node,
then the new script checks all the signatures, and then validates that the
committed execution path was in fact the one that was meant to be taken.

If people are clever enough to write scripts this way, I believe you
can make RETURN_VALID soft-fork safe simply by having every soft-forked
RETURN_VALID operation set a state flag that makes every subsequent
CHECKSIG operation require a non-aggregated sig.

The drawback of this approach is that if the script is complicated
(eg it has multiple IF conditions, some of which are nested), it may be
difficult to write the script to ensure the signatures are checked in the
same combination as the later logic actually requires -- you might have
to store the flag indicating whether you checked particular signatures
on the altstack, or use DUP and PICK/ROLL to organise it on the stack.

Solution 2
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We could make that simpler for script