Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Clarify exactly which scripts are witness outputs #1537

Merged
merged 1 commit into from
Apr 26, 2024
Merged

Clarify exactly which scripts are witness outputs #1537

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Jump to
Jump to file
Failed to load files.
Diff view
Diff view
10 changes: 7 additions & 3 deletions bip-0141.mediawiki
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -83,19 +83,23 @@ If all transactions in a block do not have witness data, the commitment is optio

=== Witness program ===

A <code>scriptPubKey</code> (or <code>redeemScript</code> as defined in BIP16/P2SH) that consists of a select subset of opcodes (<code>OP_0,OP_1,OP_2,...,OP_16</code>) followed by a data push between 2 and 40 bytes gets a new special meaning. The value of the first push is called the "version byte". The following byte vector pushed is called the "witness program".
A <code>scriptPubKey</code> (or <code>redeemScript</code> as defined in BIP16/P2SH) that consists of a 1-byte push opcode (one of <code>OP_0,OP_1,OP_2,...,OP_16</code>) followed by a direct data push between 2 and 40 bytes gets a new special meaning. The value of the first push is called the "version byte". The following byte vector pushed is called the "witness program".
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I'm not sure about the added word "direct" here. Perhaps "canonical" or "minimal" would be clearer?

Copy link
Member Author

@sipa sipa Apr 26, 2024 •
edited

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

These are the terms as I use them. Bitcoin script knows 3 types of pushes:

  • The OP_n opcodes (0x4f through 0x60, inclusive)
  • The direct pushes (0x01 through 0x4b)
  • The OP_PUSHDATA opcodes (0x4c through 0x4e)

By "minimal push" I understand "whichever of those 3 types that is applicable and minimal in encoded size". It happens to be the case that in this context (for pushes of 2 to 40 bytes) the direct push is always the minimal one, but that seems like a very indirect way of describing it.

It's true that there isn't any well-established terminology for what I call direct pushes here, but given that it's clarified explicitly immediately after, is that an issue?

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Thank you for the clarification, the word "direct" makes complete sense now.

In more detail, this means a <code>scriptPubKey</code> or <code>redeemScript</code> which consists of (in order):
* First, byte 0x00 (<code>OP_0</code>) or any byte between 0x51 (<code>OP_1</code>) and 0x60 (<code>OP_16</code>) inclusive (the version byte).
* Then, a byte ''L'' between 0x02 (push of 2 bytes) and 0x28 (push of 40 bytes) inclusive.
* Finally, ''L'' arbitrary bytes (the witness program).

There are two cases in which witness validation logic are triggered. Each case determines the location of the witness version byte and program, as well as the form of the scriptSig:
# Triggered by a <code>scriptPubKey</code> that is exactly a push of a version byte, plus a push of a witness program. The scriptSig must be exactly empty or validation fails. (''"native witness program"'')
# Triggered when a <code>scriptPubKey</code> is a P2SH script, and the BIP16 <code>redeemScript</code> pushed in the <code>scriptSig</code> is exactly a push of a version byte plus a push of a witness program. The <code>scriptSig</code> must be exactly a push of the BIP16 <code>redeemScript</code> or validation fails. (''"P2SH witness program"'')

If the version byte is 0, and the witness program is 20 bytes:
If the version byte is 0, and the witness program is 20 bytes (''L = 20''):
* It is interpreted as a pay-to-witness-public-key-hash (P2WPKH) program.
* The witness must consist of exactly 2 items (≤ 520 bytes each). The first one a signature, and the second one a public key.
* The HASH160 of the public key must match the 20-byte witness program.
* After normal script evaluation, the signature is verified against the public key with CHECKSIG operation. The verification must result in a single TRUE on the stack.

If the version byte is 0, and the witness program is 32 bytes:
If the version byte is 0, and the witness program is 32 bytes (''L = 32''):
* It is interpreted as a pay-to-witness-script-hash (P2WSH) program.
* The witness must consist of an input stack to feed to the script, followed by a serialized script (<code>witnessScript</code>).
* The <code>witnessScript</code> (≤ 10,000 bytes) is popped off the initial witness stack. SHA256 of the <code>witnessScript</code> must match the 32-byte witness program.
Expand Down