[bitcoin-dev] BIP141 segwit consensus rule update: extension of witness program definition

[bitcoin-dev] BIP141 segwit consensus rule update: extension of witness program definition

[Johnson Lau]

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Please note that the segregated witness (BIP141) consensus rule is updated. Originally, a witness program is a scriptPubKey or redeemScript that consists of a 1-byte push opcode (OP_0 to OP_16) followed by a data push between 2 and 32 bytes. The definition is now extended to 2 to 40 bytes:
https://github.com/bitcoin/bips/commit/d1b52cb198066d4e515e8a50fc3928c5397c3d9b https://github.com/bitcoin/bitcoin/pull/7910/commits/14d4d1d23a3cbaa8a3051d0da10ff7a536517ed0


Why?
----------
BIP141 defines only version 0 witness program: 20 bytes program for P2WPKH and 32 bytes program for P2WSH. Versions 1 to 16 are not defined, and are considered as anyone-can-spend scripts, reserved for future extension (e.g. the proposed BIP114). BIP141 also requires that only a witness program input may have witness data. Therefore, before this update, an 1-byte push opcode followed by a 33 bytes data push was not considered to be a witness program, and no witness data is allowed for that.

This may be over-restrictive for a future witness program softfork. When 32-byte program is used, this leaves only 16 versions for upgrade, and any “sub-version” metadata must be recorded in the witness field. This may not be compatible with some novel hashing functions we are exploring.

By extending the maximum length by 8 bytes, it allows up to 16 * 2 ^ 64 versions for future upgrades, which is enough for any foreseeable use.


Why not make it even bigger, e.g. 75 bytes?
----------
A 40 bytes witness program allows a 32-byte hash with 8-byte metadata. For any scripts that are larger than 32 bytes, they should be recorded in the witness field, like P2WSH in BIP141, to reduce the transaction cost and impact on UTXO set. Since SHA256 is already used everywhere, it is very unlikely that we would require a larger witness program (e.g. SHA512) without also a major revamp of the bitcoin protocol.

In any case, since scripts with a 1-byte push followed by a push of >40 bytes remain anyone-can-spend, we always have the option to redefine them with a softfork.


What are affected?
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As defined in BIP141, a version 0 witness program is valid only with 20 bytes (P2WPKH) or 32 bytes (P2WSH). Before this update, an OP_0 followed by a data push of 33-40 bytes was not a witness program and considered as anyone-can-spend. Now, such a script will fail due to incorrect witness program length.

Before this update, no witness data was allowed for a script with a 1-byte push followed by a data push of 33-40 bytes. This is now allowed.



Actions to take:
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If you are running a segnet node, or a testnet node with segwit code, please upgrade to the latest version at https://github.com/bitcoin/bitcoin/pull/7910

If you have an alternative implementation, please make sure your consensus code is updated accordingly, or your node may fork off the network.

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Re: [bitcoin-dev] BIP141 segwit consensus rule update: extension of witness program definition

[Luke Dashjr]

On Wednesday, June 08, 2016 5:57:36 AM Johnson Lau via bitcoin-dev wrote:
> Why not make it even bigger, e.g. 75 bytes?

I don't see a sufficient answer to this question. Pieter explained why >75 
would be annoying, but 75 seems like it should be fine.

> In any case, since scripts with a 1-byte push followed by a push of >40
> bytes remain anyone-can-spend, we always have the option to redefine them
> with a softfork.

It's not that simple, since this is preventing use of the witness field for 
such scripts. With this limit in place, any such a softfork would suddenly 
require either two different witness commitments, or disabling the previous 
witness transaction format.

Luke

Re: [bitcoin-dev] BIP141 segwit consensus rule update: extension of witness program definition

[Johnson Lau]

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> On 8 Jun 2016, at 15:29, Luke Dashjr <luke@dashjr.org> wrote:
> 
> On Wednesday, June 08, 2016 5:57:36 AM Johnson Lau via bitcoin-dev wrote:
>> Why not make it even bigger, e.g. 75 bytes?
> 
> I don't see a sufficient answer to this question. Pieter explained why >75
> would be annoying, but 75 seems like it should be fine.
> 
>> In any case, since scripts with a 1-byte push followed by a push of >40
>> bytes remain anyone-can-spend, we always have the option to redefine them
>> with a softfork.
> 
> It's not that simple, since this is preventing use of the witness field for
> such scripts. With this limit in place, any such a softfork would suddenly
> require either two different witness commitments, or disabling the previous
> witness transaction format.
> 
> Luke

This is exactly why I proposed to extend the definition. My initial proposal was extending it to 33 bytes to effectively allow 16*256 new script versions, assuming we will keep using 32 bytes program hash.

If someday 32 bytes hash is deemed to be unsafe, the txid would also be unsafe and a hard fork might be needed. Therefore, I don’t see how a witness program larger than 40 bytes would be useful in any case (as it is more expensive and takes more UTXO space). I think Pieter doesn’t want to make it unnecessarily lenient.

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Re: [bitcoin-dev] BIP141 segwit consensus rule update: extension of witness program definition

[Luke Dashjr]

On Wednesday, June 08, 2016 8:23:51 AM Johnson Lau wrote:
> If someday 32 bytes hash is deemed to be unsafe, the txid would also be
> unsafe and a hard fork might be needed. Therefore, I don’t see how a
> witness program larger than 40 bytes would be useful in any case (as it is
> more expensive and takes more UTXO space). I think Pieter doesn’t want to
> make it unnecessarily lenient.

There is no harm in being lenient, but it limits the ability to do softfork 
upgrades in the future. I appreciate Pieter's concern that we'd need to do 
more development and testing to go to this extreme, which is why I am only 
asking the limit raised to 75 bytes.

Luke

Re: [bitcoin-dev] BIP141 segwit consensus rule update: extension of witness program definition

[Pieter Wuille]

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On Jun 8, 2016 18:46, "Luke Dashjr via bitcoin-dev" <
bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> On Wednesday, June 08, 2016 8:23:51 AM Johnson Lau wrote:
> > If someday 32 bytes hash is deemed to be unsafe, the txid would also be
> > unsafe and a hard fork might be needed. Therefore, I don’t see how a
> > witness program larger than 40 bytes would be useful in any case (as it
is
> > more expensive and takes more UTXO space). I think Pieter doesn’t want
to
> > make it unnecessarily lenient.
>
> There is no harm in being lenient, but it limits the ability to do
softfork
> upgrades in the future. I appreciate Pieter's concern that we'd need to do
> more development and testing to go to this extreme, which is why I am only
> asking the limit raised to 75 bytes.

No strong opinion, but I'd rather not change it anymore, as I don't see the
point. Any data you would want to encode there can be moved to the witness
at 1/4 the cost and replaced by a 256-bit hash. If the data is 43 bytes or
higher, that is even cheaper. The only thing that cannot be in the hash is
metadata to indicate what hashing/rule scheme itself is used. I think 68
bits (OP_n + 8 bytes) for that is plenty.

-- 
Pieter

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