BIP: 118 Layer: Consensus (soft fork) Title: SIGHASH_ANYPREVOUT for Taproot Scripts Author: Christian Decker <firstname.lastname@example.org> Anthony Towns <email@example.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 Requires: 340, 341, 342
This BIP describes a new type of public key for tapscript (BIP 342) transactions. It allows signatures for these public keys to not commit to the exact UTXO being spent. This enables dynamic binding of transactions to different UTXOs, provided they have compatible scripts.
This document is licensed under the 3-clause BSD license.
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 respond to a given transaction being seen on-chain with a predetermined reaction in the form of another transaction. Often the same reaction is desired for a variety of different transactions that may be seen on-chain, but because the input signatures in the response transaction commit to the exact transaction that is being reacted to, this means a new signature must be created for every possible transaction one wishes to be able to react to.
This proposal introduces a new public key type that modifies the behavior of the transaction digest algorithm used in the signature creation and verification, by excluding the commitment to the previous output (and, optionally, the witness script and value ). Removing this commitment allows dynamic rebinding of a signed transaction to another previous output that requires authorisation by the same key.
The dynamic rebinding is opt-in due to using a separate public key type, and the breadth of transactions the signature can be rebound to can be further restricted by using different keys, committing to the script being spent in the signature, using different amounts between UTXOs, using different nSequence values in the spending transaction, or using the codeseparator opcode to commit to the position in the script.
This BIP modifies the behaviour of the BIP
342 signature opcodes (
CHECKSIGADD) for public keys that have a length
of 33 bytes and a first byte of
0x01 or the public key which is
precisely the single byte vector
0x01. These keys are termed
BIP 118 public keys.
Rules for signature opcodes
The BIP 342 rules for signature opcodes
are modified by removing keys with the first byte
0x01 and length of
either 1-byte or 33-bytes from the list of unknown public key types, and
adding the following rule prior to the handling of unknown public key
- If the public key is the single byte
0x01, or if the public key is 33 bytes and the first byte of the public key is
0x01, it is considered to be a BIP 118 public key:
- If the signature is not the empty vector, the signature is
validated according to the BIP
341 signing validation rules
with the public key, allowable
hash_typevalues, and transaction digest modified as defined below.
- If the signature is not the empty vector, the signature is validated according to the BIP 341 signing validation rules with the public key, allowable
To convert a 33-byte BIP 118 public key for use with BIP
340, remove the
0x01 prefix and use
the remaining 32 bytes.
We define the functions Msg118(hash_type) and Ext118(hash_type) which compute the message being signed as a byte array.
The parameter hash_type is an 8-bit unsigned value, reusing values
defined in BIP 341, with the addition
that the values
also valid for BIP 118 public keys.
We define the following constants using bits 6 and 7 of
SIGHASH_ANYPREVOUT = 0x40
SIGHASH_ANYPREVOUTANYSCRIPT = 0xc0
The following restrictions apply and cause validation failure if violated:
- Using any undefined hash_type (not 0x00, 0x01, 0x02, 0x03, 0x41, 0x42, 0x43, 0x81, 0x82, 0x83, 0xc1, 0xc2, or 0xc3).
SIGHASH_SINGLEwithout a "corresponding output" (an output with the same index as the input being verified).
If these restrictions are not violated, Msg118(hash_type) evaluates as follows.
If hash_type & 0x40 == 0, then Msg118(hash_type) = SigMsg(hash_type, 1), where SigMsg is as defined in BIP 341.
If hash_type & 0x40 != 0, then Msg118(hash_type) is the concatenation of the following data, in order (with byte size of each item listed in parentheses). Numerical values in 2, 4, or 8-byte items are encoded in little-endian.
- hash_type (1).
- Transaction data:
- nVersion (4): the nVersion of the transaction.
- nLockTime (4): the nLockTime of the transaction.
- If hash_type & 3 does not equal
- sha_outputs (32): the SHA256 of the serialization of all
- sha_outputs (32): the SHA256 of the serialization of all outputs in
- Data about this input:
- spend_type (1): equal to 2 if no annex is present, or 3 otherwise (the original witness stack has two or more witness elements, and the first byte of the last element is 0x50)
- If hash_type & 0xc0 is
- amount (8): value of the previous output spent by this input.
- scriptPubKey (35): scriptPubKey of the previous output
spent by this input, serialized as script inside
CTxOut. Its size is always 35 bytes.
- nSequence (4): nSequence of this input.
- If an annex is present (the lowest bit of spend_type is set):
- sha_annex (32): the SHA256 of (compact_size(size of annex) || annex), where annex includes the mandatory 0x50 prefix.
- Data about this output:
- If hash_type & 3 equals
- sha_single_output (32): the SHA256 of the corresponding
- sha_single_output (32): the SHA256 of the corresponding output in
- If hash_type & 3 equals
Similarly, Ext118(hash_type) evaluates to the concatenation of the following data, in order:
- If hash_type & 0xc0 is not
- tapleaf_hash (32): the tapleaf hash as defined in BIP 341
- key_version (1): a constant value 0x01 representing that this is a signature for a BIP 118 public key.
- codesep_pos (4): the opcode position of the last executed
OP_CODESEPARATORbefore the currently executed signature opcode, with the value in little endian (or 0xffffffff if none executed). The first opcode in a script has a position of 0. A multi-byte push opcode is counted as one opcode, regardless of the size of data being pushed.
- If hash_type & 0xc0 is not
To verify a signature sig for a BIP 118 public key p:
- If the sig is 64 bytes long, return Verify(p, hashTapSigHash(0x00 || Msg118(0x00) || Ext118(0x00)), sig)
- If the sig is 65 bytes long, return sig ≠ 0x00 and Verify(p, hashTapSighash(0x00 || Msg118(sig) || Ext118(sig)), sig[0:64]).
- Otherwise, fail.
Verify is as defined in BIP 340.
The key differences from BIP 342 signature verification are:
- In all cases,
key_versionis set to the constant value
SIGHASH_ANYPREVOUTis set, the digest is calculated as if
SIGHASH_ANYONECANPAYwas set, except
outpointis not included in the digest.
SIGHASH_ANYPREVOUTANYSCRIPTis set, the digest is calculated as if
SIGHASH_ANYONECANPAYwas set, except
tapleaf_hashare not included in the digest.
introduce additional potential for signature replay (that is they allow
the same signature to be reused on a different transaction) when
SIGHASH_ANYONECANPAY signatures prevent
signature replay by committing to one or more inputs, so replay of the
signature is only possible if the same input can be spent multiple
times, which is not possible on the Bitcoin blockchain (due to
enforcement of BIP 30). With
SIGHASH_ANYPREVOUT signature replay is possible for different UTXOs
with the same
scriptPubKey and the same value, while with
SIGHASH_ANYPREVOUTANYSCRIPT signature replay is possible for any UTXOs
that reuse the same BIP 118 public key in one of their potential
As a consequence, implementors MUST ensure that BIP 118 public keys are only reused when signature replay cannot cause loss of funds (eg due to other features of the protocol or other constraints on the transaction), or when such a loss of funds is acceptable.
introduce additional malleability vectors.
In particular, a transaction authenticated using only ANYPREVOUT signatures is malleable to anyone able to provide an alternate input satisfied by the signature -- an input changed in this way would produce a new, valid transaction paying the same recipient, but with a different txid. Depending on the changes to the inputs, this might conflict with the original transaction (if some inputs remain shared) or might result in a double-payment to the recipient (if they do not).
Further, for a chain of transactions using the same
value, and only authenticated via ANYPREVOUT signatures (as envisioned
in eltoo for failure cases), it may be possible for any third party to
malleate the transactions (and their txids) without having access to any
of the private keys, particularly by omitting intermediate transactions.
This form of malleation can be dealt with by the child transactions also using ANYPREVOUT signatures -- when a parent transaction is malleated, its children can be adjusted to reference the new txid as the input and the ANYPREVOUT signatures remain valid.
However child transactions that are authorised by a
SIGHASH_ANYONECANPAY signature will need new signatures if their
inputs are malleated in this way. This risk may be mitigated somewhat by
using BIP 68/BIP
112 relative time locks before spending
a UTXO that had been authorised via an ANYPREVOUT signature with
SIGHASH_ANYONECANPAY: a relative timelock can ensure
that the inputs have enough confirmations that they can only be replaced
in the event of a large block reorg. Note that this approach has
drawbacks: relative timelocks prevent fee-bumping via
child-pays-for-parent, and have the obvious drawback of making the funds
temporarily unusable until the timelock expires.
It is expected that ANYPREVOUT signatures will only be rarely used in practice. Protocol and wallet designers should aim to have their transactions use Taproot key path spends whenever possible, both for efficiency reasons due to the lower transaction weight, but also for privacy reasons to avoid third parties being able to distinguish their transactions from those of other protocols.
Transactions that do use ANYPREVOUT signatures will therefore reveal information about the transaction, potentially including that cooperation was impossible, or what protocol or software was used (due to the details of the script).
In order to maximise privacy, it is therefore recommended that protocol designers only use BIP 118 public keys in scripts that will be spent using at least one ANYPREVOUT signature, and either use key path spends or alternate scripts in the taproot merkle tree for any spends that can be authorised without ANYPREVOUT signatures. Following this recommendation may require additional script branches, which may mean disregarding this recommendation may result in a better tradeoff between cost and privacy in some circumstances.
As a soft fork, older software will continue to operate without modification. Nodes that have not upgraded to support BIP 341 will see all taproot witness programs as anyone-can-spend scripts, and nodes that have upgraded to support BIP 341 and BIP 342 but not BIP 118 will simply treat any non-empty signature against a BIP 118 public key as valid. As such, nodes are strongly encourage to upgrade in order to fully validate signatures for the new public key type.
Non-upgraded wallets can receive and send bitcoin from non-upgraded and upgraded wallets using SegWit version 0 programs, traditional pay-to-pubkey-hash, etc. Depending on the implementation, non-upgraded wallets may be able to send to SegWit version 1 programs if they support sending to BIP350 Bech32m addresses and do not prevent the transaction from being broadcast due to considering the outputs non-standard.
Apart from being based on Taproot rather than SegWit v0, the main differences to prior revisions of this BIP are:
- The sighash flag has been renamed from "NOINPUT" to "ANYPREVOUT" to reflect that while any prevout may potentially be used with the signature, some aspects of the input are still committed to, namely the input nSequence value, and (optionally) the spending conditions and amount.
- Previously NOINPUT would have worked for direct public key spends
(assuming deployment was fleshed out in a way similar to BIP 141
P2WPKH and P2WSH), however this proposal only applies to signatures
via tapscript, and not direct key path spends. This means that
addresses must opt-in to the ability to be spent by a
SIGHASH_ANYPREVOUTANYSCRIPTsignature by including an appropriate tapscript path when the address is created.
- NOINPUT signatures do not commit to the output's spending conditions
scriptPubKeyor the redeem/witness script. This behaviour is preserved when
SIGHASH_ANYPREVOUTANYSCRIPTis used, but when
SIGHASH_ANYPREVOUTis used, the signature now commits to
scriptPubKeyand the tapscript.
- NOINPUT signatures did commit to the input's amount. This behaviour
is preserved when
SIGHASH_ANYPREVOUTis used, but not when
OP_CODESEPARATORin script will affect both
SIGHASH_ANYPREVOUTANYSCRIPTsignatures, whereas it would not have in the previous draft.
SIGHASH_NOINPUT flag was first proposed by Joseph Poon in
after being mentioned in the original Lightning
paper by Joseph
Poon and Thaddeus Dryja. This document is the result of discussions with
many people and had direct input from Greg Maxwell, Jonas Nick, Pieter
Wuille and others.
- Why a new public key type? New public key types for tapscript can be introduced in a soft fork by specifying new rules for unknown public key types as specified in BIP 342, as this only requires adding restrictions to the pre-existing signature opcodes. Possible alternative approaches would be to define new script opcodes, to use a different taproot leaf version, or to use a different set of SegWit outputs than those specified by BIP 341; however all of these approaches are more complicated, and are better reserved for other upgrades where the additional flexibility is actually needed. In this case, we specify a new transaction digest, but retain the same elliptic curve and signature algorithm (ie, secp256k1 and BIP 340).
- Why (and why not) commit to the witness script? The
eltoo paper provides an example
of why committing to the witness script is not always appropriate.
It uses script and the transaction
nLockTimeto make signatures asymmetric, so that a transaction with an earlier signature can be spent by a transaction with a later signature, but a transaction with a later signature cannot be spent by a transaction with an earlier signature. As a result, a single signature for a third, even later transaction must be able to spend both the prior transactions, even though they have a different tapscript. On the other hand, these cases also provide a good reason to have the option to commit to the script: because each transaction has a new script, committing to the script allows you to produce a signature that applies to precisely one of these transactions. In the eltoo case, this allows you to have a signature for an update transaction that can be applied to any prior update, and a signature for a settlement transaction that applies only to the corresponding update transaction, while using the same key for both, which in turn allows for a more compact script.
- Why (and why not) commit to the input value? Committing to the input value may provide additional safety that a signature can't be maliciously reused to claim funds that the signer does not intend to spend, so by default it seems sensible to commit to it. However, doing so prevents being able to use a single signature to consolidate a group of UTXOs with the same spending condition into a single UTXO which may be useful for some protocols, such as the proposal for layered commitments with eltoo.
- What about key path spends? This proposal only supports ANYPREVOUT signatures via script path spends, and does not support ANYPREVOUT signatures for key path spends. This is for two reasons: first, not supporting key path spends allows this proposal to be independent of the core changes included in BIP 341 and BIP 342; second, it allows addresses to opt-in or opt-out of ANYPREVOUT support while remaining indistinguishable prior to being spent.
- Use of 0x01 public key type Because
OP_0leaves an empty vector on the stack it would not satisfy BIP 342's rules for unknown public key types. As such, it is convenient to use one of
OP_1NEGATEas a way to reference the taproot internal key. To keep things as simple as possible, we use the first of these, and add the same byte as a prefix to allow ANYPREVOUT signatures for explicitly specified keys.
- Why change key_version? Changing
key_versionensures that if the same private key is used to generate both a BIP 342 key and a BIP 118 public key, that a signature for the BIP 342 key is not also valid for the BIP 118 public key (and vice-versa).