Native Introspection Opcodes

Summary

This CHIP adds virtual machine operations that let a contract read details of the transaction spending it: output values, recipients, input bytecode and more. It does this “without increasing transaction validation costs,” because nodes already hold all of that data while validating.

Motivation

Covenants (contracts that check how their coins are spent) became possible on BCH with OP_CHECKDATASIG in 2018. But they used a workaround: check the same signature with both OP_CHECKSIG and OP_CHECKDATASIG, then parse a copy of the transaction’s signing data pushed by the spender.

The CHIP says this “doubles or triples the size of even the simplest covenant transactions,” and advanced covenants hit the 1,650-byte standard unlocking script limit. Native opcodes remove the duplication.

What it specifies

Fourteen opcodes at codepoints 0xc0–0xcd:

The CHIP recommended deploying it with Bigger Script Integers, since values can exceed 32-bit range.

Current status

Activated on 15 May 2022. The CHIP records acceptance by Bitcoin Unlimited, Bitcoin Cash Node, Knuth, Bitcoin Verde and Bitauth. Its statements section includes support from Flowee’s founder Tom Zander and from Rosco Kalis, who committed to supporting all the opcodes in CashScript. General Protocols’ early statement said the CHIP needed more analysis before it could take a strong position, while committing resources to support it.