From: Johnson Lau <jl2012@xbt.hk>
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Date: Wed, 8 Jun 2016 13:57:36 +0800
Message-Id: <A7E9BC23-6860-4B31-9D4E-11F771A5E581@xbt.hk>
To: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
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Subject: [bitcoin-dev] BIP141 segwit consensus rule update: extension of
	witness program definition
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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/d1b52cb198066d4e515e8a50fc3928c5397=
c3d9b =
https://github.com/bitcoin/bitcoin/pull/7910/commits/14d4d1d23a3cbaa8a3051=
d0da10ff7a536517ed0


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 =E2=80=9Csub-version=E2=80=9D 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?
----------
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:
----------
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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