Bitcoin Covenants Part 4: OP_CHECKSIGFROMSTACK and OP_CAT
The passage describes two proposed Bitcoin opcodes as covenant building blocks: **OP_CHECKSIGFROMSTACK (OP_CSFS)** and **OP_CAT**. OP_CSFS can verify signatures over arbitrary messages from the stack, enabling oracle-style scripts that validate off-chain events, but it does not by itself bind those messages to a spending transaction. OP_CAT concatenates stack items, and combined with OP_CSFS it can assemble transaction fields into a signed message, creating covenant-like introspection and enforcing predefined transaction templates, similar to OP_CTV. It also explains a separate Taproot-only technique using **OP_CAT plus Schnorr signature quirks**. By fixing signature components in advance, OP_CHECKSIG can be repurposed to check that a transaction’s data matches a target pattern rather than proving key ownership. This works only for **SegWit v1/Taproot**, not SegWit v0, because it relies on BIP-341 sighash rules and Schnorr signatures.
