<pre>
  BIP: 118
  Layer: Consensus (soft fork)
  Title: SIGHASH_NOINPUT
  Author: Christian Decker <decker.christian@gmail.com>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0118
  Status: Draft
  Type: Standards Track
  Created: 2017-02-28
  License: BSD-3-Clause
</pre>

== Abstract ==
This BIP describes a new signature hash flag (<tt>sighash</tt>-flag) for
segwit transactions. It removes any commitment to the output being
spent from the signature verification mechanism. This enables dynamic
binding of transactions to outputs, predicated solely on the
compatibility of output scripts to input scripts.

== Motivation ==
Off-chain protocols make use of transactions that are not yet
broadcast to the Bitcoin network in order to renegotiate the final
state that should be settled on-chain.
In a number of cases it is desirable to react to a given transaction
being seen on-chain with a predetermined reaction in the form of
another transaction.
Often the reaction is identical, no matter which transaction is seen
on-chain, but the application still needs to create many identical
transactions.
This is because signatures in the input of a transaction uniquely
commit to the hash of the transaction that created the output being
spent.

This proposal introduces a new sighash flag that modifies the behavior
of the transaction digest algorithm used in the signature creation and
verification, to exclude the previous output commitment.
By removing the commitment we enable dynamic rebinding of a signed
transaction to outputs whose <tt>witnessProgram</tt> and value match the ones
in the <tt>witness</tt> of the spending transaction.

The dynamic binding is opt-in and can further be restricted by using
unique <tt>witnessProgram</tt> scripts that are specific to the application
instance, e.g., using public keys that are specific to the off-chain
protocol instance.

== Specification ==
<tt>SIGHASH_NOINPUT</tt> is a flag with value <tt>0x40</tt> appended to a signature
so that the signature does not commit to any of the inputs, and
therefore to the outputs being spent. The flag applies solely to the
verification of that single signature.

The <tt>SIGHASH_NOINPUT</tt> flag is only active for segwit scripts with
version 1 or higher. Should the flag be used in a non-segwit script or
a segwit script of version 0, the current behavior is maintained and
the script execution MUST abort with a failure.

The transaction digest algorithm from BIP 143 is used when verifying a
<tt>SIGHASH_NOINPUT</tt> signature, with the following modifications:

    2. hashPrevouts (32-byte hash) is 32 0x00 bytes
    3. hashSequence (32-byte hash) is 32 0x00 bytes
    4. outpoint (32-byte hash + 4-byte little endian) is
       set to 36 0x00 bytes
    5. scriptCode of the input is set to an empty script
       0x00

The <tt>value</tt> of the previous output remains part of the transaction
digest and is therefore also committed to in the signature.

The <tt>NOINPUT</tt> flag MAY be combined with the <tt>SINGLE</tt> flag in which
case the <tt>hashOutputs</tt> is modified as per BIP
143<ref>https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki</ref>: it
only commits to the output with the matching index, if such output exists, and
is a <tt>uint256</tt> <tt>0x0000......0000</tt> otherwise.

Being a change in the digest algorithm, the <tt>NOINPUT</tt> flag applies to
all segwit signature verification opcodes, specifically it applies to:

* <tt>OP_CHECKSIG</tt>

* <tt>OP_CHECKSIGVERIFY</tt>

* <tt>OP_CHECKMULTISIG</tt>

* <tt>OP_CHECKMULTISIGVERIFY</tt>

== Binding through scripts ==
Using <tt>NOINPUT</tt> the input containing the signature no longer
references a specific output.
Any participant can take a transaction and rewrite it by changing the
hash reference to the previous output, without invalidating the
signatures.
This allows transactions to be bound to any output that matches the
value committed to in the signature and whose <tt>witnessProgram</tt>,
combined with the spending transaction's <tt>witness</tt> returns <tt>true</tt>.

Previously, all information in the transaction was committed in the
signature itself, while now the relationship between the spending
transaction and the output being spent is solely based on the
compatibility of the <tt>witnessProgram</tt> and the <tt>witness</tt>.

This also means that particular care has to be taken in order to avoid
unintentionally enabling this rebinding mechanism. <tt>NOINPUT</tt> MUST NOT
be used, unless it is explicitly needed for the application, e.g., it
MUST NOT be a default signing flag in a wallet
implementation. Rebinding is only possible when the outputs the
transaction may bind to all use the same public keys. Any public key
that is used in a <tt>NOINPUT</tt> signature MUST only be used for outputs
that the input may bind to, and they MUST NOT be used for transactions
that the input may not bind to. For example an application SHOULD
generate a new key-pair for the application instance using <tt>NOINPUT</tt>
signatures and MUST NOT reuse them afterwards.

== Deployment ==
The <tt>NOINPUT</tt> sighash flag is to be deployed during a regular segwit
script update.

== Backward compatibility ==
As a soft fork, older software will continue to operate without
modification. Non-upgraded nodes, however, will not verify the
validity of the new sighash flag and will consider the transaction
valid by default. Being only applicable to segwit transactions,
non-segwit nodes will see an anyone-can-spend script and will consider
it valid.

== Acknowledgments ==

The <tt>NOINPUT</tt> sighash flag was first proposed by Joseph Poon in
February 2016<ref>https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-February/012460.html</ref>, after being mentioned in the original
Lightning paper<ref>http://lightning.network/lightning-network.pdf</ref>. A formal proposal was however
deferred until after the activation of segwit. This proposal is a
continuation of this discussion and attempts to formalize it in such a
way that it can be included in the Bitcoin protocol. As such we'd like
acknowledge Joseph Poon and Thaddeus Dryja as the original inventors
of the <tt>NOINPUT</tt> sighash flag, and its uses in off-chain protocols.

== References ==

<references/>

== Copyright ==

This document is licensed under the BSD 3 Clause license.
