Add BIP for OP_TXHASH and OP_CHECKTXHASHVERIFY

bip-txhash.mediawiki

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+<pre>
+  BIP: tbd
+  Layer: Consensus (soft fork)
+  Title: OP_TXHASH and OP_CHECKTXHASHVERIFY
+  Author: Steven Roose <[email protected]>
+  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-tbd
+  Status: Draft
+  Type: Standards Track
+  Created: 2023-09-03
+  License: BSD-3-Clause
+</pre>
+
+==Abstract==
+
+This BIP proposes two new opcodes, <code>OP_CHECKTXHASHVERIFY</code>, to be activated
+as a change to the semantics of <code>OP_NOP4</code> in legacy script, segwit and tapscript;
+and OP_TXHASH, to be activated as a change to the semantics of <code>OP_SUCCESS189</code>
+in tapscript only.
+
+These opcodes provide a generalized method for introspecting certain details of
+the spending transaction, which enables non-interactive enforcement of certain
+properties of the transaction spending a certain UTXO.
+
+The constructions specified in this BIP also open up the way for other
+potential updates; see Motivation section for more details.
+
+
+==Summary==
+
+===OP_CHECKTXHASHVERIFY===
+
+The first new opcode, <code>OP_CHECKTXHASHVERIFY</code>, redefines the <code>OP_NOP4</code> opcode (<code>0xb3</code>) as a soft fork upgrade.
+
+It has the following semantics:
+
+* There is at least one element on the stack, fail otherwise.
+* The element on the stack is at least 32 bytes long, fail otherwise.
+* The first 32 bytes are interpreted as the TxHash and the remaining suffix bytes specify the TxFieldSelector.
+* If the TxFieldSelector is invalid, fail.
+* The actual TxHash of the transaction at the current input index, calculated using the given TxFieldSelector must be equal to the first 32 bytes of the element on the stack, fail otherwise.
+
+
+===OP_TXHASH===
+
+The second new opcode, <code>OP_TXHASH</code>, redefines the <code>OP_SUCCESS189</code> tapscript opcode (<code>0xbd</code>) as a soft fork upgrade.
+
+It has the following semantics:
+
+* There is at least one element on the stack, fail otherwise.
+* The element is interpreted as the TxFieldSelector and is popped off the stack.
+* If the TxFieldSelector is invalid, fail.
+* The 32-byte TxHash of the transaction at the current input index, calculated using the given TxFieldSelector is pushed onto the stack.
+
+===TxFieldSelector===
+
+The TxFieldSelector has the following semantics. We will give a brief conceptual
+summary, followed by a reference implementation of the CalculateTxHash function.
+
+* There are two special cases for the TxFieldSelector:
+** the empty value, zero bytes long: it is set equal to <code>TXFS_SPECIAL_TEMPLATE</code>, the de-facto default value which means everything except the prevouts and the prevout scriptPubkeys.
+
+Special case <code>TXFS_SPECIAL_TEMPLATE</code> is 4 bytes long, as follows:
+*** 1. <code>TXFS_ALL</code>
+*** 2. <code>TXFS_INPUTS_TEMPLATE | TXFS_OUTPUTS_ALL</code>
+*** 3. <code>TXFS_INOUT_NUMBER | TXFS_INOUT_SELECTION_ALL</code>
+*** 4. <code>TXFS_INOUT_NUMBER | TXFS_INOUT_SELECTION_ALL</code>
+
+** the <code>0x00</code> byte: it is set equal to <code>TXFS_SPECIAL_ALL</code>, which means "ALL" and is primarily useful to emulate <code>SIGHASH_ALL</code> when <code>OP_TXHASH</code> is used in combination with <code>OP_CHECKSIGFROMSTACK</code>.
+
+Special case <code>TXFS_SPECIAL_TEMPLATE</code> is 4 bytes long, as follows:
+*** 1. <code>TXFS_ALL</code>
+*** 2. <code>TXFS_INPUTS_ALL | TXFS_OUTPUTS_ALL</code>
+*** 3. <code>TXFS_INOUT_NUMBER | TXFS_INOUT_SELECTION_ALL</code>
+*** 4. <code>TXFS_INOUT_NUMBER | TXFS_INOUT_SELECTION_ALL</code>
+
+* The first byte of the TxFieldSelector has its 8 bits assigned as follows, from lowest to highest:
+** 1. version (<code>TXFS_VERSION</code>)
+** 2. locktime (<code>TXFS_LOCKTIME</code>)
+** 3. current input index (<code>TXFS_CURRENT_INPUT_IDX</code>)
+** 4. current input control block (or empty) (<code>TXFS_CURRENT_INPUT_CONTROL_BLOCK</code>)
+** 5. current script last <code>OP_CODESEPARATOR</code> position (or 0xffffffff) (<code>TXFS_CURRENT_INPUT_LAST_CODESEPARATOR_POS</code>)
+** 6. inputs (<code>TXFS_INPUTS</code>)
+** 7. outputs (<code>TXFS_OUTPUTS</code>)
+
+* The last (highest) bit of the first byte (<code>TXFS_CONTROL</code>), we will call the "control bit", and it can be used to control the behavior of the opcode. For <code>OP_TXHASH</code> and <code>OP_CHECKTXHASHVERIFY</code>, the control bit is used to determine whether the TxFieldSelector itself has to be included in the resulting hash. (For potential other uses of the TxFieldSelector (like a hypothetical <code>OP_TX</code>), this bit can be repurposed.)
+
+* If either "inputs" or "outputs" is set to 1, expect another byte with its 8 bits assigning the following variables, from lowest to highest:
+** Specifying which fields of the inputs will be selected:
+*** 1. prevouts (<code>TXFS_INPUTS_PREVOUTS</code>)
+*** 2. sequences (<code>TXFS_INPUTS_SEQUENCES</code>)
+*** 3. scriptSigs (<code>TXFS_INPUTS_SCRIPTSIGS</code>)
+*** 4. prevout scriptPubkeys (<code>TXFS_INPUTS_PREV_SCRIPTPUBKEYS</code>)
+*** 5. prevout values (<code>TXFS_INPUTS_PREV_VALUED</code>)
+*** 6. taproot annexes (<code>TXFS_INPUTS_TAPROOT_ANNEXES</code>)
+** Specifying which fields of the outputs will be selected:
+*** 7. scriptPubkeys (<code>TXFS_OUTPUTS_SCRIPTPUBKEYS</code>)
+*** 8. values (<code>TXFS_OUTPUTS_VALUES</code>)
+
+//TODO(stevenroose) check that the 7 and 8 render correctly
+
+* We define as follows:
+** <code>TXFS_ALL = TXFS_VERSION | TXFS_LOCKTIME | TXFS_CURRENT_INPUT_IDX | TXFS_CURRENT_INPUT_CONTROL_BLOCK | TXFS_CURRENT_INPUT_LAST_CODESEPARATOR_POS | TXFS_INPUTS | TXFS_OUTPUTS | TXFS_CONTROL</code>
+** <code>TXFS_INPUTS_ALL = TXFS_INPUTS_PREVOUTS | TXFS_INPUTS_SEQUENCES | TXFS_INPUTS_SCRIPTSIGS | TXFS_INPUTS_PREV_SCRIPTPUBKEYS | TXFS_INPUTS_PREV_VALUES | TXFS_INPUTS_TAPROOT_ANNEXES</code>
+** <code>TXFS_INPUTS_TEMPLATE = TXFS_INPUTS_SEQUENCES | TXFS_INPUTS_SCRIPTSIGS | TXFS_INPUTS_PREV_VALUES | TXFS_INPUTS_TAPROOT_ANNEXES</code>
+** <code>TXFS_OUTPUTS_ALL = TXFS_OUTPUTS_SCRIPTPUBKEYS | TXFS_OUTPUTS_VALUES</code>
+
+For both inputs and then outputs, do the following:
+
+* If the "in/outputs" field is set to 1, another additional byte is expected:
+** The highest bit (<code>TXFS_INOUT_NUMBER</code>) indicates whether the "number of in-/outputs" should be committed to.
+** For the remaining bits, there are three exceptional values:
+*** 0x00 (<code>TXFS_INOUT_SELECTION_NONE</code>) means "no in/outputs" (hence only the number of them as <code>0x80</code> (<code>TXFS_INOUT_NUMBER</code>)).
+*** <code>0x40</code> (<code>TXFS_INOUT_SELECTION_CURRENT</code>) means "select only the in/output of the current input index" (it is invalid when current index exceeds number of outputs).
+*** <code>0x3f</code> (<code>TXFS_INOUT_SELECTION_ALL</code>) means "select all in/outputs".
+** The second highest bit (<code>TXFS_INOUT_SELECTION_MODE</code>) is the "specification mode":
+*** Set to 0 it means "leading mode".
+*** Set to 1 it means "individual mode".
+** The third highest bit (<code>TXFS_INOUT_SELECTION_SIZE</code>) is used to indicate the "index size", i.e. the number of bytes will be used to represent in/output indices.
+** In "leading mode",
+*** With "index size" set to 0, the remaining lowest 5 bits of the first byte will be interpreted as the number of leading in/outputs to select.
+*** With "index size" set to 1, the remaining lowest 5 bits of the first byte together with the 8 bits of the next byte will be interpreted as the number of leading in/outputs to select.
+** In "individual mode", the remaining lowest 5 bits of the first byte will be interpreted as `n`, the number of individual in/outputs to select.
+*** With "index size" set to 0, interpret the following `n` individual bytes as the indices of an individual in/outputs to select.
+*** With "index size" set to 1, interpret the next `n` pairs of two bytes as the indices of individual in/outputs to select.
+
+Effectively, this allows a user to select
+* all in/outputs
+* the current input index
+* the leading in/outputs up to 8192
+* up to 32 individually selected in/outputs
+
+The TxFieldSelector is invalid when
+* a byte is expected but missing
+* additional unexpected bytes are present
+* index size is set to 1 while not being necessary
+* a leading number of individual index is selected out of bounds of the in/outputs
+* individual indices are duplicated or not in increasing order
+
+These limitations are to avoid potential TxFieldSelector malleability. It is
+however allowed to use leading mode where it could be "all". This
+is important to allow for optional addition of extra inputs or outputs.
+//TODO(stevenroose) should we disallow individual that could be leading?
+
+
+===Resource limits===
+
+* For legacy scripts and segwit, we don't add any extra resource limitations, with the argumentation that <code>OP_CHECKTXHASHVERIFY</code> already requires the user to provide at least 32 bytes of extra transaction size, either in the input scriptSig, or the witness. Additional more complex hashes require additional witness bytes. Given that <code>OP_CAT</code> is not available in this context, if a malicious user tries to increase the number of TransactionHashes being calculated by using opcodes like <code>OP_DUP</code>, the TxFieldSelector for all these calculations is identical, so the calculation can be cached within the same transaction.
+
+* For tapscript, primarily motivated by the cheaper opcode <code>OP_TXHASH</code> (it doesn't require an additional 32 witness bytes be provided) and the potential future addition of byte manipulation opcodes like <code>OP_CAT</code>, an additional cost is specified per TransactionHash execution. Using the same validation budget ("sigops budget") introduced in BIP-0342, each TransactionHash decreases the validation budget by 10. If this brings the budget below zero, the script fails immediately.
+
+The following considerations should be made:
+
+** All fields that can be of arbitrary size are cachable as TransactionHash always hashes their hashed values.
+** In "individual mode", a user can at most commit 32 inputs or outputs, which we don't consider excessive for potential repeated use.
+** In "prefix mode", a caching strategy can be used where the SHA256 context is stored every N in/outputs so that multiple executions of the TransactionHash function can use the caches and only have to hash an additional N-1 items at most.
+
+
+==Motivation==
+
+This BIP specifies a basic transaction introspection primitive that is useful
+to either reduce interactivity in multi-user protocols or to enforce some basic
+constraints on transactions.
+
+Additionally, the constructions specified in this BIP can lay the groundwork for
+some potential future upgrades:
+* The TxFieldSelector construction would work well with a hypothetical opcode <code>OP_TX</code> that allows for directly introspecting the transaction by putting the fields selected on the stack instead of hashing them together.
+* The TransactionHash obtained by <code>OP_TXHASH</code> can be combined with a hypothetical opcode <code>OP_CHECKSIGFROMSTACK</code> to effectively create an incredibly flexible signature hash, which would enable constructions like <code>SIGHASH_ANYPREVOUT</code>.
+
+===Comparing with some alternative proposals===
+
+* This proposal strictly generalizes BIP-119's <code>OP_CHECKTEMPLATEVERIFY</code>, as the default mode of our TxFieldSelector is effectively the same (though not  byte-for-byte identical) as what <code>OP_CTV</code> acomplishes, without costing any additional bytes. Additionally, using <code>OP_CHECKTXHASHVERIFY</code> allows for more flexibility which can help in the case for
+** enabling adding fees to a transaction without breaking a multi-tx protocol;
+** multi-user protocols where users are only concerned about their own inputs and outputs.
+
+* Constructions like <code>OP_IN_OUT_VALUE</code> used with <code>OP_EQUALVERIFY</code> can be emulated by two <code>OP_TXHASH</code> instances by using the TxFieldSelector to select a single input value first and a single output value second and enforcing equality on the hashes. Neither of these alternatives can be used to enforce small value differencials without the use of 64-bit arithmetic.
+
+* Like mentioned above, <code>SIGHASH_ANYPREVOUT</code> can be emulated using <code>OP_TXHASH</code> when combined with <code>OP_CHECKSIGFROMSTACK</code>: <code><txfs> OP_TXHASH <pubkey> OP_CHECKSIGFROMSTACK` effectively emulates <code>SIGHASH_ANYPREVOUT</code></code>.
+
+
+
+
+==Detailed Specification==
+
+A reference implementation in Rust is provided attached as part of this BIP
+together with a JSON file of test vectors generated using the reference
+implementation.
+
+
+==Implementation==
+
+* A proposed implementation for Bitcoin Core is available here: https://github.com/bitcoin/bitcoin/pull/29050
+* A proposed implementation for rust-bitcoin is available here: https://github.com/rust-bitcoin/rust-bitcoin/pull/2275
+
+
+==Acknowledgement==
+
+Credit for this proposal mostly goes to Jeremy Rubin for his work on BIP-119's
+OP_CHECKTEMPLATEVERIFY and to Russell O'Connor for the original idea of
+generalizing CTV into OP_TXHASH.
+
+Additional thanks to Andrew Poelstra, Greg Sanders, Rearden Code, Rusty Russell
+and others for their feedback on the specification.
+

bip-txhash/ref-impl/Cargo.toml

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+[package]
+name = "txhash-ref"
+version = "0.0.0"
+edition = "2021"
+
+[dependencies]
+bitcoin = { version = "0.31.0", features = [ "serde" ] }
+serde_json = "1.0.108"
+
+# until bitcoin-io is released and https://github.com/rust-bitcoin/rust-bitcoin/pull/2274 is merged
+[patch.crates-io]
+bitcoin = { git = "https://github.com/stevenroose/rust-bitcoin.git", branch = "txhash", features = [ "serde" ] }
+bitcoin_hashes = { git = "https://github.com/stevenroose/rust-bitcoin.git", branch = "txhash" }
+bitcoin-io = { git = "https://github.com/stevenroose/rust-bitcoin.git", branch = "txhash" }