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Luke Dashjr edited this page Dec 23, 2016 · 5 revisions

Should be harmless and eliminates some technical debt. Recommended. --Luke Dashjr, 2016-12-23

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BIP: 119
Layer: Consensus (soft fork)
Title: CHECKTEMPLATEVERIFY
Author: Jeremy Rubin [email protected]
Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0119
Status: Draft
Type: Standards Track
Created: 2020-01-06
License: BSD-3-Clause

==Abstract==

This BIP proposes a new opcode, OP_CHECKTEMPLATEVERIFY, to be activated as a change to the semantics of OP_NOP4.

==Summary==

OP_CHECKTEMPLATEVERIFY uses opcode OP_NOP4 (0xb3) as a soft fork upgrade.

OP_CHECKTEMPLATEVERIFY does the following:

  • There is at least one element on the stack, fail otherwise
  • The element on the stack is 32 bytes long, NOP otherwise
  • The DefaultCheckTemplateVerifyHash of the transaction at the current input index is equal to the element on the stack, fail otherwise

The DefaultCheckTemplateVerifyHash commits to the serialized version, locktime, scriptSigs hash (if any non-null scriptSigs), number of inputs, sequences hash, number of outputs, outputs hash, and currently executing input index.

The recommended standardness rules additionally:

  • Reject non-32 byte as SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS.

==Motivation==

This BIP introduces a transaction template, a simple spending restriction that pattern matches a transaction against a hashed transaction specification. OP_CHECKTEMPLATEVERIFY reduces many of the trust, interactivity, and storage requirements inherent with the use of pre-signing in applications. For more details on applications, please see the references.

==Detailed Specification==

The below code is the main logic for verifying CHECKTEMPLATEVERIFY, described in pythonic pseudocode. The canonical specification for the semantics of OP_CHECKTEMPLATEVERIFY as implemented in C++ in the context of Bitcoin Core can be seen in the reference implementation.

The execution of the opcode is as follows:

def execute_bip_119(self): # Before soft-fork activation / failed activation # continue to treat as NOP4 if not self.flags.script_verify_default_check_template_verify_hash: # Potentially set for node-local policy to discourage premature use if self.flags.script_verify_discourage_upgradable_nops: return self.errors_with(errors.script_err_discourage_upgradable_nops) return self.return_as_nop() 
# CTV always requires at least one stack argument
if len(self.stack) < 1:
    return self.errors_with(errors.script_err_invalid_stack_operation)

# CTV only verifies the hash against a 32 byte argument
if len(self.stack[-1]) == 32:
    # Ensure the precomputed data required for anti-DoS is available,
    # or cache it on first use
    if self.context.precomputed_ctv_data == None:
        self.context.precomputed_ctv_data = self.context.tx.get_default_check_template_precomputed_data()

    # If the hashes do not match, return error
    if stack[-1] != self.context.tx.get_default_check_template_hash(self.context.nIn, self.context.precomputed_ctv_data):
        return self.errors_with(errors.script_err_template_mismatch)

    return self.return_as_nop()

# future upgrade can add semantics for this opcode with different length args
# so discourage use when applicable
if self.flags.script_verify_discourage_upgradable_nops:
    return self.errors_with(errors.script_err_discourage_upgradable_nops)
else:
    return self.return_as_nop()

The computation of this hash can be implemented as specified below (where self is the transaction type). Care must be taken that in any validation context, the precomputed data must be initialized to prevent Denial-of-Service attacks. Any implementation must cache these parts of the hash computation to avoid quadratic hashing DoS. All variable length computations must be precomputed including hashes of the scriptsigs, sequences, and outputs. See the section "Denial of Service and Validation Costs" below. This is not a performance optimization.

def ser_compact_size(l): r = b"" if l < 253: # Serialize as unsigned char r = struct.pack("B", l) elif l < 0x10000: # Serialize as unsigned char 253 followed by unsigned 2 byte integer (little endian) r = struct.pack("<BH", 253, l) elif l < 0x100000000: # Serialize as unsigned char 254 followed by unsigned 4 byte integer (little endian) r = struct.pack("<BI", 254, l) else: # Serialize as unsigned char 255 followed by unsigned 8 byte integer (little endian) r = struct.pack("<BQ", 255, l) return r

def ser_string(s): return ser_compact_size(len(s)) + s

class CTxOut: def serialize(self): r = b"" # serialize as signed 8 byte integer (little endian) r += struct.pack("<q", self.nValue) r += ser_string(self.scriptPubKey) return r

def get_default_check_template_precomputed_data(self): result = {} # If there are no s`**

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